Building 100 AH LiFePo4 Battery Box

I truly grok the "heat and bug" seasonal limitations... (Sorry to the non-Heinlien fans for the reference).

My friend got out of the hospital a week ago and was back in by Sunday night, so things have been touch & go, but right now, it's looking like they may have sorted him out, and he might be released again next week, depending on the results of the next round of tests.

In the meantime, I've been trying to find out as much info on the communication protocols of the BougeRV 60-amp controllers, and as far as I can tell, there's no provision for them to work together with others, even if they are the same brand & model. That doesn't mean one can't use them in parallel, but one is likely to need to log into each separately to manage them.  It's looking more & more like the BougeRV controller is really intended for systems where it's the only controller in the system, so if one is OK with the limitations of an 800 watt charger (12 Vdc) or a 1600 watt charger (24 Vdc), this is the one to get, but if one has much greater needs, it might be better in the long run to go with the Victron system. I can possibly try to use both of the BougeRV controllers at the same time, but I don't have enough of the same type of solar panels to max out both controllers - I can run 1 with 800 watts of panels and only 400 watts for the other one until/unless I buy another pair of 200 watt panels.

I do have a pair of 36 Vdc rigid panels that are a pretty good match for the Victron 30-amp controller buried in the box, so I can drag those out into the yard and add them to the mix, but I don't think it's a good idea to mix those panels with the 24 Vdc Harbor Freight panels.

Clearly a win for the Victron system, as they can be linked via Bluetooth and you end up with one as the master controller and the others as slaves, so you change settings on one and the changes are propagated automatically to the rest of the network. Their network can also include other devices, like their Smart Shunt (which I have, but was saving for the 24 Vdc battery build).

Should be sunny all weekend, so hopefully I can get some charging data from one of the BougeRV controllers and the group of Victron controllers too, but I'm also trying to cobble together a minimal laptop to load Linux onto, and copy some videos to, so I can deliver it to the hospital and help keep my friend from going stir crazy and being a pain to the staff... It just needs to be able to run Linux Mint, and VLC for this to work.

Finally getting back to this thread now that some of deep summer chores on on the wane...

The 12V build that I described above was tested today in some water pumping and had a somewhat surprise result.  For details on the pumping set-up see my entries here -- https://permies.com/t/179996/Sprinkler-pumps-shallow-pumps-irrigation

So even though I'm mixing brands, I decided to try linking (in parallel) my 100Ah battery build with a recently-purchased 100Ah Ecoworthy 12V LiFePO4 battery.  Bluetooth allows me to connect to each separately for monitoring parameters during operation.  The two batteries are supporting a 0.75 hp 120V electric water pump pulling water for our garden.  The set-up with the 3000W pure sine wave inverter is shown below....tonight's test had the system running for about 30 min.

While all worked well, I was curious about the imbalance between the two batteries in terms of amp draw. Thus, when using two Ecoworthy batteries purchaed all at the same time and starting at the same state of charge, they both equally split the 60A load into 30A to each battery.  In the configuraton shown below, the Ecoworthy was drawing 40+ amps and the home-build around 20A or a bit less.  The BMS in the home-build is set to deliver 100A from the battery if need be, so it doesn't appear to be limiting the amperage output this way.  When I used the battery alone to run a vacuum cleaner some weeks back it delivered 40A no problem, so this "seems" to rule out a connection/wire bottleneck, yes?  Any ideas here regarding the imbalance?  As an aside, the BMS in the home-build was keeping the cells themselves balnced just fine.  Thanks for any suggestions or questions....

I can only guess that the 2 different batteries, even though they have the same voltages, have different resistances, so there's a current imbalance.

While I don't have the equipment to measure battery cell internal resistance, all batteries (& cells) have an internal resistance, and one of the biggest benefits of buying cells from a reputable vendor, is the matching process, so that there's documentation of the specs, & all the cells at least start out contributing nearly equally to the loads, and charging nearly equally from the charging. It's unlikely to be perfect, but balancing takes care of the rest.

If you know what the internal resistance is of the set when you start, you can generally use that number later, to shop for additional cells/batteries to add to the system however, as technology marches on, the trend is toward lower internal resistance, for higher efficiency cells, so at some point, you might be better off just accepting the newer/higher efficiency that also usually comes with a lower price tag too.  It's not the end of the world, but in your example, the more efficient battery will deplete faster, so if you're needing the full name plate capacity of the combined system (200 amps), you'll only have a chance for that while both batteries are contributing (& I wouldn't advise pushing it past the weakest link).

For lighter use, you're fine as long as each bank has its own BMS and is staying balanced.

I have seen Andy on the Off-Grid Garage YouTube channel use such a meter, but I haven't yet felt moved to buy one. I don't *think* they're too expensive, I just haven't gotten around to it.

Well, it wasn't what I would call done yet, but it was pressed into service, and I have learned from the experience, how I wish to alter it going forward.

We have gone to visit Wheaton Labs for a week (as SEPPers), and rented the Allerton Abbey cabin. It has a rudimentary solar system, which was (I'm lead to believe) cobbled together with old parts laying around, etc. It only provides some basic inside lighting for a few hours, but the batteries are old lead acid batteries, that are long past their prime, so even if we shut off those lights before they quit, they are still completely dead by morning.

Stepping back to the beginning of this trip, we elected not to bring the much larger capacity 24 Vdc system, because we were running short of cargo space, and really didn't HAVE TO have 120 VAC, so the smaller DC only box seemed plenty. Turned out, I'd mis-calculated the load of running both the DC refrigerator and the DC freezer, plus the drive ended up being over 2 days, and by the time we arrived, the 4 KWh battery was depleted, and it was dark, so no solar was available to recharge it until morning.

I also discovered in the morning, that I'd forgotten to pack all the PV cables, so I could only connect 400 watts of panels, and it was somewhat cloudy on Monday too, so not much solar harvested. On Tuesday, we went into town, and I found a solar dealer who was willing to sell me some 10 gauge PV wire and an MC4 connector pair, so I could connect the other 400 watts of panels (I only brought 800 watts this trip because our usage wasn't expected to be that much without AC appliances). It turned out that I was also missing some ring terminal connectors too, but those are easy to find everywhere.

I didn't have the Allen wrench to tighten the connector on the circuit breaker terminals, so I bypassed them. It won't pass a code inspection, but it does work to charge the battery. In our best sun this afternoon, we were charging at over 600 watts, and overall we got the batteries charged up to 25% capacity, while still running the DC loads and that should be plenty to make it through the night until we get more solar power.

As far as the learning part of this, I've learned that I really should make sure I have a way to charge the 12 Vdc system with the vehicle, while going down the road, even if it is only a small charge, like 8 amps or less, and I should always bring tools!

They sell these chargers that go from your vehicle battery through the firewall (or underneath if you have a truck and are going to the bed). Mine charges up to 500W but that's also limited by the controller in my power station. It's pretty sweet because 2h of driving will charge 1kWh. They now sell ones with higher capacity too, I'm not sure what the maximum draw on a car battery would be.

If you take this thing on road trips a lot, I'd recommend it. $200 on Amazon or so I think.

Do you have a thread or video on how you built the battery box? That sounds interesting.

M Pearson wrote:They sell these chargers that go from your vehicle battery through the firewall (or underneath if you have a truck and are going to the bed). Mine charges up to 500W but that's also limited by the controller in my power station. It's pretty sweet because 2h of driving will charge 1kWh. They now sell ones with higher capacity too, I'm not sure what the maximum draw on a car battery would be.

If you take this thing on road trips a lot, I'd recommend it. $200 on Amazon or so I think.

Do you have a thread or video on how you built the battery box? That sounds interesting.

Most of the build for this box is in this thread, even though mine is most certainly NOT a 100 Ah battery (314 Ah), because I hadn't finished it enough to be comfortable submitting it as a small DC solar system badge bit yet. It's close, but really the hold up is sorting out where the best places are, to mount the exterior sockets and connections. I even brought my drill with me, to maybe work parts of that out.

As far as taking frequent road trips, this is kind of the maiden voyage of this 2017 Dodge Grand Caravan GT. We bought it a few weeks ago as a cash private sale, got plates and tags, and hit the road. Still learning about the various issues and hopefully minor repairs it needs, but I don't frequently get enough time off to go on many road trips, plus, this van gets pretty poor fuel economy compared to our last one (a 2013 Dodge Grand Caravan that we bought new from a dealer). I've no problem with modding the vehicle, but a trailer hitch is probably the next step, since this trip has proven that it's a challenge with 2 Great Danes, wife, luggage, and food for all (we feed raw, so the 80 qt freezer was mandatory, and 55 qt refrigerator was mostly for human food).

There are other badge bits for expanding capacity and/or adding permanent 12 Vdc electrical upgrades, and I will probably do at least a dedicated permanent DC socket rated for 20-30 amps. The van already has a small capacity inverter for 115 Vac power, but I don't think it's large enough to be terribly useful. I might upgrade that after a trailer hitch? Gotta get some more interior space and luggage capacity first.

Just to make it back home, I'll limit the BMS to a 7-amp charge, & temporarily wire it to one of the 12 volt outlets in the van.

As an aside, Allerton Abbey is a comfortable cozy place to stay, as far as cabin rentals go 🙂 - I highly recommend it if you want to make a pilgrimage to Wheaton Labs!

That sounds like a fun and educational trip, Allen....thanks for posting this.

I think one example of the charger that M. Pearson was referring to is demonstrated here, although it comes as part of an add-on to a portable power unit:   https://www.youtube.com/watch?v=r29jk4OyOJs    

Victron's version is shown here....and I'm immediately intrigued!  
https://www.amazon.com/Victron-Energy-24-Volt-Battery-Charger/dp/B0F43Z9R53/ref=sr_1_1?crid=11SA759Q2P2XE&dib=eyJ2IjoiMSJ9.wQqVjiogszb9YmxEDHxJJwNXheb2W-VzltHjPLEfihAKoiCWMBX_uaHc41TB59AqGrAGhQDWFVvLJZNT3wDa3MnfCIsKDtgycSW5tZDG3Y21bE_kwl00Gs03KbbalLlIA8ZnTPmLRv8QHgjVTfrjQpJaFdaMG_KF1RXGHp4P-HSfDqyq_BpSAdB-aO5lnDQFgczMpo_TLrs1aGn8TBu4RdKQB8Kd6fPSkGPOr2d0u4yhrGBDPkatCJEA5_96gJoNT3q0z5AmSCMgUWIBsWRYhRJ2hnCjkJcnm5cYhGzVLAY.XQScvxBCHoLmgauk0P9b5OBMI5Ds775aYcXGT9a6qj8&dib_tag=se&keywords=alternator+charger+dc+to+dc+bluetooth&qid=1792060582&sprefix=Alternator+charger%2Caps%2C215&sr=8-1

I had not heard of these until now and they look like a more simple, albeit lower power alternative to adding a generator to your engine serpentine belt that I'd also seen advertised.   The alternator charges would make sense for most individuals it seems.

That wasn't really what I had in mind, but thanks for the suggestions.

At least for a 12 Vdc LiFePO4 system, the stock alternator output voltage is high enough to charge the batteries, so as long as the BMS is configured for such, it will charge without blowing a fuse or tripping a breaker.

It is a 1600+ mile trip for us one-way, and a trickle charge for the entire trip will surely be enough to top off the batteries as needed, plus, now our solar is running well enough that it may not be needed at all.

In any case, I'll have some more data on how well it worked by the end of the trip, so we can all learn from the experiment.

Allen Jackson wrote:....

At least for a 12 Vdc LiFePO4 system, the stock alternator output voltage is high enough to charge the batteries, so as long as the BMS is configured for such, it will charge without blowing a fuse or tripping a breaker.

Out of curiosity, how does that work with respect to surges in either amps or voltage with variable engine speed.  I realize that there are voltage regulators in car electronics, but if, say, the amperage is beyond what the BMS 'recommends' for the cells being managed, does it simply shut off the ability for the battery to accept charge and then recover once the amps are back in range?  If so, I could see where a separate charger is less of a concern....the BMS essentially serving the same function.  Or?.....

Thanks!

Since current draw demand is always determined by the load, telling the BMS to limit charging to 8 amps would do just that - until changed, that charging circuit won't draw more than 8 amps. The voltage may surge or ebb, and the current can drop lower than the 8 amps if the supply can't provide 8 amps, but excess current is what would blow a fuse, so the BMS serves that protective role.

The BMS is also the protection from over-voltage, so if the supply voltage exceeds the threshold, the BMS will shut off the charging port.

A solar charge controller is kind of like the voltage regulator for the solar panels, because that voltage can swing widely & wildly multiple times a minute through a much larger range, vs. a car alternator & voltage regulator, that will keep the system voltage within a much narrower range that just happens to be in the acceptable range to charge a 12 Vdc LiFePO4 battery 😊

The only danger is from the BMS deciding that the food source is so yummy that it gets too greedy, too fast, and starts blowing fuses/breakers.

Here's a good example of the Victron networking - I have 2 different size controllers running in this network, but no matter which one I log into, I can view the individual performance of that one AND the total performance of the network overall 🙂

SUPER convenient!

Also, in one day's worth of solar with this setup, my battery charge level went from 5% at the beginning of the day, to 93% by the time the solar power was waning. All the while running both the refrigerator and the freezer, and recharging numerous other devices that used USB charging. I fully expect to be fully charged by mid-morning tomorrow, and pack up the solar panels in the afternoon, to leave the cabin Saturday morning, and being able to trickle charge along the way should keep the system nearly full by the time we get back home, even though there's likely to be an overnight stay in a motel, with the appliances still running at night on battery power - with the vehicle power shut off.

Although it's rather the wrong time of year for the following suggestion, are you using the roof rack of your minivan while traveling?  In a pinch, would strapping down two of your solar panels on that roof supplement in any significant way the trickle charging from your vehicle alternator....during mid-summer of course with the longer days?  I realize this could cause too much drag on the car, but perhaps with some sort of roof spoiler to reduce friction? So nice to see the utility of even these small battery set-ups, the recharging diversity that one can implement, and the utility of the bluetooth monitoring of it all.

Edit for additional tangential question:  Will sodium ion battery cells require a BMS purchase specific for that chemistry or will the same BMS's sold for LiFePO4 be usable for sodium ion?  I could see where it might just be a matter of tweaking the parameters controlled by the BMS to allow its use with that chemistry, due to the different discharge curve and perhaps other idiosyncracies of those cells but don't know for sure.  Just looking ahead to potential future builds.....  Thanks!

M Pearson wrote:Do you have a thread or video on how you built the battery box? That sounds interesting.

I have gone ahead and submitted it for the small DC solar badge bit, so all of the details are in one place.

I think it's ugly as sin, but I can't argue with its proven functionality, as we've been living off it for the past week? The food in the freezer is still frozen hard, and the food in the refrigerator is still cold... Our phones & tablets are charged up, and our rechargeable lights are too. Can't beat that - the wife is content with the camping trip, I'm not in the dog house for dragging them all the way out here, so all around, it's a win😊.

https://permies.com/wiki/14/112937/pep-electricity/Small-DC-solar-system-PEP#3995265

John Weiland wrote:Although it's rather the wrong time of year for the following suggestion, are you using the roof rack of your minivan while traveling?  In a pinch, would strapping down two of your solar panels on that roof supplement in any significant way the trickle charging from your vehicle alternator....during mid-summer of course with the longer days?  I realize this could cause too much drag on the car, but perhaps with some sort of roof spoiler to reduce friction? So nice to see the utility of even these small battery set-ups, the recharging diversity that one can implement, and the utility of the bluetooth monitoring of it all.

Edit for additional tangential question:  Will sodium ion battery cells require a BMS purchase specific for that chemistry or will the same BMS's sold for LiFePO4 be usable for sodium ion?  I could see where it might just be a matter of tweaking the parameters controlled by the BMS to allow its use with that chemistry, due to the different discharge curve and perhaps other idiosyncracies of those cells but don't know for sure.  Just looking ahead to potential future builds.....  Thanks!

Ironically, on our return trip, we will be carrying the solar panels on the roof rack, but they will be folded up, not deployed. It's not worth the bother at this point to try to rig up something to be able to use them going down the road, when they won't need more than the trickle charging I can get from plugging into the 12 Vdc outlet inside. Putting the panels on the roof will free up a bit of interior space to ease the trip home.

As far as sodium batteries, it's my understanding that they have such a different charge/discharge curve, that one would need completely different gear to manage them.

As a follow-up, the vehicle charging cable seems to help a little, but it's not keeping up with the load on the battery. This merely tells me that I probably have to run a dedicated 10-gauge pair with a separate breaker to an XT-60 outlet for this use.

This is not an issue for the remainder of this trip, because there's plenty of juice left to make it home today, and our forecast is for great solar for the rest of the week, so I can place the panels in the back yard, & let them do their thing.

We made it home safely and there's 55% capacity left in the battery. We won't even unload the van tonight, and just let it run until morning

Allen Jackson wrote:We made it home safely and there's 55% capacity left in the battery. We won't even unload the van tonight, and just let it run until morning

Just such an odd coincidence.  I travelled a bit this weekend to visit family.  My brother and his wife pulled up in a "new" (used...1998) conversion van with much of the same intent as yours. They had a Jackery power station, a 12V fridge/freezer combination, stowed solar panels for recharging, a trickle charger from the 12V port inthe van for the Jackery, and a sleeping bunk built in.  As his lifestyle has been pretty mainstream, I'm wondering if this approach is becoming more popular now for being more mobile and off-grid,,,?

Since small DC compressors have really come down in price, I can see how DC refrigerator/freezers become so much more convenient than having to buy ice and deal with the melted mess for any extended trip.

Thank you DARPA! Years ago, they put out a request for designs for a cooling system small enough to be wearable in battle. (Think back to the Desert Storm era), but quickly realized that it was a bad idea to deploy these in a hostile situation. NASA picked up the ball, and integrated the technology into space suits, and there were several other market uses that were found, like portable temperature controlled drug storage, and organ transportation, and as the market grew, the Chinese clones made them a cheaper commodity.

Ladies and gentlemen, I am happy to report that while I didn't get around to unloading the appliances from the minivan until this evening (wife and I were both zonked from the trip back), and had left it running both the refrigerator and the freezer (which needs defrosting now, too), it was only down to 43% capacity, which leads me to believe that I didn't start the trip with it fully charged in the first place.

I also found that my small DC solar badge bit submission was flagged incomplete, and really, I can't complain, because while many of you have been following along, the volunteer review staffers probably haven't and I really need to clean up the submission, so it looks more like a step-by-step journal of the build, which should make it much easier to judge it.

To be clear, no one is questioning my results, they continue to speak for themselves, but my documentation is lacking, so I will try to assemble something more coherent and methodical in a separate thread. It doesn't mean I will abandon this thread, at least not until Eric finishes his build 😉, but I really should post a separate and clear thread of my specific build.

Remember to be kind to the volunteer staff of this freely available platform!

This was educational... I'm finding that when my engine is off (with no loads on the battery box) it's slowly charging my vehicle battery/electrical system.

With no other loads on the battery box, and the engine running, there is a very slight charging going on, where the vehicle is charging the battery box, but it's only about 1.44 amps, which beats a sharp stick in the eye, but not the level of charging I was expecting by any means. The regulated voltage of the vehicle is just not high enough to get a significant amount of charging current, so I'd have to go with some DC to DC booster/charger, if I really want vehicle charging. I'd probably be better off unfolding a few panels on the roof - they're already paid for 😉

On the plus side, if I make sure it's fully charged before leaving home, it will probably run much longer with no other charging for much longer than I am willing to travel?

Allen Jackson wrote: This was educational... I'm finding that when my engine is off (with no loads on the battery box) it's slowly charging my vehicle battery/electrical system.

This is very helpful for many reasons, but also because I may have solved a Christmas present conundrum for my brother, mentioned above.  No doubt he too will be running into the the slow trickle-charge issue....even moreso since I suspect he won't be using solar panels as much as you are.  The DC-DC box between the vehicle battery and the DC power station would be great for him to try and won't break the bank.  

As an aside, the weekend's project will be to remove 6 X 6V legacy (=HEAVY!) lead acid batteries from my farm-yard golf cart...one or more seem wonky or underperforming and it's a good time of year to do some testing. In the meantime, I'm going to try replacing them with 3 and 4 LiFePO4 batteries (100Ah each) to provide the stock 36V or alternatively an upped 48V to the system.  Good rumor has it that the better quality carts of yester-year had components built like a tank....hence motors and controllers that can handle the increased voltage to 48V (power draw is the same....just a slight reduction in current).  The one component I did invest in is the solenoid which some say is best to keep at the same voltage as the battery pack.  The experiment:  Can these "non-locomotion" 12V batteries (100 amp BMS) power the cart at either 36V or 48V?  Because of the current demand at 36V, the BMS might be tripped.....but if the potentially 200-250A draw is distributed *equally* across all 3 batteries, it might be okay--especially as start-up and cruising amps range from ~50 - 150A.  It also could be a complete bust....but that's the excitement of experimentation!   Fun!!...... :-)

I'm struggling with the choice to build something to my needs, vs build to the BB requirements...

As I contemplate building a better version of the "Charge and Carry" lithium battery box BB (because having AC available would be really nice), I realize that I can build to the posted minimum requirements, or I can build something that would be truly useful in a field/off grid/camping experience.

The conundrum comes down to weight/capacity. My biggest issue with my DC 12 volt system, is that it's uncomfortably heavy, especially for that cheap plastic toolbox. Unfortunately, my calculations suggest that for camping with family (not solo), I probably want to have 8 KWh of energy stored, for the various electric appliances/cooking apparatus I'd love to have available to use. The DC box is close to 60+ lbs and the weight would more than double if I made a system with double the battery capacity AND also had an inverter, plus all the AC outlets/breakers, etc.

I suspect that a 150 lb box of anything won't qualify for the BB, and it would surely be a challenge to carry or transport too, so I'm contemplating if making it modular would disqualify it for that BB, or worst-case scenario, I buy some more 105 Ah cells, and build a much lower capacity, but lighter box that only had 2560 Wh (2.5 KWh), but weighed less than the 12 DC system...

If I go the modular route, I can wire it so that the BMS power is disabled until both battery modules are connected, and the BMS automatically measures the resistance of the balancing wires, so I don't have to worry about those either. The AC side would also have to be a separate module too. At what point does it stop qualifying for the BB?

If I just build with 105 Ah cells, I'll need to pony up another wad of cash, to get more of those, because I only have 4 right now, but I expect I'll have to find a new use for them after the BB is completed, because I don't think I'm willing to live with a 2.5 KWh energy cap - isn't that why hand trucks exist???

I'll try asking for guidance in the BB clarification thread, and see where that goes, even though they have not answered my last question about the same BB - where I'd pointed out that the 220 volt requirement was an error - none of the examples they gave can support or provide 220 VAC. (It's a 120 Vac 30 amp RV outlet, NOT a 220 volt outlet)

John Weiland wrote:
As an aside, the weekend's project will be to remove 6 X 6V legacy (=HEAVY!) lead acid batteries from my farm-yard golf cart...one or more seem wonky or underperforming and it's a good time of year to do some testing. In the meantime, I'm going to try replacing them with 3 and 4 LiFePO4 batteries (100Ah each) to provide the stock 36V or alternatively an upped 48V to the system.  Good rumor has it that the better quality carts of yester-year had components built like a tank....hence motors and controllers that can handle the increased voltage to 48V (power draw is the same....just a slight reduction in current).  The one component I did invest in is the solenoid which some say is best to keep at the same voltage as the battery pack.  The experiment:  Can these "non-locomotion" 12V batteries (100 amp BMS) power the cart at either 36V or 48V?  Because of the current demand at 36V, the BMS might be tripped.....but if the potentially 200-250A draw is distributed *equally* across all 3 batteries, it might be okay--especially as start-up and cruising amps range from ~50 - 150A.  It also could be a complete bust....but that's the excitement of experimentation!   Fun!!......

I typed out a detailed reply to this, and it disappeared... Round 2?

The current drawn is exactly equal in all the cells of a series stack, and your BMS shouldn't allow you to abuse your batteries for very long under those conditions.

For that application, you'll be much better off, with something like the 280 Ah cells and a 200 amp BMS.

Allen Jackson wrote:

The current drawn is exactly equal in all the cells of a series stack, and your BMS shouldn't allow you to abuse your batteries for very long under those conditions.

For that application, you'll be much better off, with something like the 280 Ah cells and a 200 amp BMS.

A-ha!....Understood.   For some reason, it was clear to me that the amperage of the array of 3 (36V) or 4 (48V) batteries in series would be 100A, but my brain was incorrectly computing that the amp *draw* would be distributed across, say, the 4 batteries.  Thus, incorrectly thinking that each battery would experience a 25 amp draw.  So now, if I wired 3 batteries in series to provide 36V and a second array of 3 batteries in series to give 36V.....and *then* parallel connected these two banks to result in 36V @ 200A, might this buy me some graces with the higher powers in the universe? ;-)  Because I actually do have 6 batteries just now (all LiFePO4, all 100Ah, although one is homemade, and another an older but exact version of the newer 4)...and could round out the experiment in this way.

In some regards, these efforts mirror the objectives of your own:  Weight and bulk reduction within the equipment. Certainly, if I had a different model-year of golf cart this might be much easier, but mine is pretty locked into GC2 battery dimensions.  So when I start adding Ah to the desired battery, I not only start increasing the weight of each battery, but I also size the battery out of fitment in the cart battery trays.  It's quite possible I may just break down and go with commercial 48V/50Ah batteries (X4) that are GC2-size and call it a day.  Maybe the the weekend's results will provides some clarity.....?

Yes, that 4S2P arrangement would work much better and as long as you didn't slam the system very hard, or at least hard enough to go over 200 amps for long, it should work.

Keep in mind, with the battery packs being different, they won't likely all quit at the same time, & as soon as the first one (the weakest one in the group) goes into protection mode, it will immediately shut down that series string, and the overload on the remaining string should shut down that string shortly thereafter.

It occurs to me, that an inexpensive DC to DC boost converter, I can probably do in-vehicle charging just fine.

For example, if I get a 12 Vdc to 24 Vdc boost converter, like this one:  https://www.amazon.com/YRDZXG-19V10A190W-Converter-Regulated-Waterproof/dp/B0CTFJ8CGH

It's rated to supply up to 360 watts of power, and I can plug the high voltage output into a solar controller solar input, and as long as the input is at least 5 volts higher than the battery voltage, the mppt controller will charge from it. For my uses, I only need about 10 - 12 amps to trickle charge while running both my DC refrigerator/freezers, which by the numbers shouldn't use more than 120 watts, so the leftover surplus should top off the battery...

At any rate, the price is much easier to swallow, and I could conceivably use one on each solar controller for only $26/each, vs the nearly $300 for the really good looking Victron (but expensive) charger.

Re-visiting an earlier idea, I've been pondering the challenge of how to split up a larger battery system, to make it easier to transport, but still effectively still an 8S or more battery, and do it safely enough that the entire system is disabled until everything is correctly plugged in/connected.

My experience with my JK BMS on my 12 Vdc system, it appears that the length of the sensing wires is not an issue, because the system can measure the resistance of each, and compensate for any differences (within reason or limits I haven't tested yet).

As I'm inclined to build a system that will meet my own needs as a higher priority than meeting the requirements of any specific badge bit, I'm going to go ahead with my attempt to make a 24 Vdc system, but make it modular enough to be more practical to transport, and to that end, if I run the sensing wires for the 2nd battery box through a multi-conductor connector, like those weatherpak automotive ones, AND also loop the BMS power lead thru that same connector, the whole system can't even be turned on without first having everything plugged in and connected correctly.

For an 8S battery split into 2 halves, I think that will mean 4 sensing wires, 1 ground wire, and 2 wire spots to loop the power switch thru.

This is all very preliminary and floating around mostly in my head. Another idea that's competing with it, is to just build the entire system into the rolling large tool box I first bought, because we now have a minivan that can carry that large of an object, whereas my car would be hard pressed to fit it in, not to mention lifting that weight up into a back seat or similar. If I went that route, it might even still be an edge case to qualify for the "Charge and Carry" lithium battery box BB, and I don't have to buy any more parts to bring it to life then...

Alas the plans from my most recent post have not come to pass.  With an unusually warm end to October, I was lulled into thinking more time was available for the battery testing, but the colder temps moving in now are a reminder of the winter approaching.  To that end I needed to shift back to a small addition project on an outbuilding that will house, eventually, a rocket mass heater split between the larger 'host' building and the addition being fused to it.  A few trips to the building supply and it seems to be coming along well enough.  The tractor does a great job of substituting for that extra pair of hands so common with contractor teams....and will be super-handy at helping to position the two solar panels I have onto the roof.

In the meantime, a quick question.  I now need to get my LiFePO4 batteries out of the garage building (unheated) and was hoping to just store them in a room in the house.  My plan is to go with some advice off the internet and here to discharge them to around 60% SOC and just monitor them through the winter.  Thoughts?  There are no plans at the moment to use them except for emergencie and was hoping that simply storing them unconnected might be the safest way for them to be inside.  No other buildings on the property are heated at the moment and the very cold extremes I suspect would not be good for them.  Thanks for any advice on this....I hope I'm not being too paranoid about this!

Wish I had some insights or suggestions, Allen, for your build but I'm still several steps behind on the technology it seems.   A part of me wants to move cautiously (financially speaking) into so much of this as I really dread investing too much into something that becomes obsolete the following year due to unexpected advances.  That sentiment is countered by the impetus to simply take some level of 'plunge' so that experience can be gained.  Continued good luck with your build and testing!

I think the important thing is to not store them in a discharged state, as opposed to discharging them down to 60% SOC.

If you're shipping them, then you must discharge them (not sure to what level), but storing shouldn't be as critical. If it were me, & I had the luxury of choice, I'd fully charge them, especially since there's still a level of self-discharge they will have and I don't know when I'd be able to return to the scene of the crime...

I wasn't looking for insights earlier, so much as I was trying to gage the interest level of being able to safely modular-ize an energy storage system to make it have both higher portability and higher capacity.

I have come to realize that the SKIP/PEP programs, while a wonderful idea, are for me, more likely a distraction and a detour from my retirement plans I've been working on for over 30 years. I'm not adverse to permaculture at all, but I am not a purist either and as I review the PEP skill levels & tasks required to achieve them, had I started on that path even 30 years ago I'd be good with it, and well on my way toward succeeding there, but now, realistically, I really should already have my land long before I'll ever qualify for getting into their queue to inherit property.

In my (limited?) experience, the "Otis's" that are looking for someone worthy of taking over their property, aren't generally permaculture purists either, so while they want to find someone "worthy", it's not going to be based on whether that person can build a working door with wooden hinges... The successful "Otis" is a small business owner, who has a respect for their land, but will value things like business savvy, resourcefulness and a deep respect for the land, much higher than many of the skills needed to obtain the higher badge levels.

If they will their property to someone who doesn't understand those concepts, that person is much more likely to be insolvent in 5 years because they didn't understand what it takes to stay above water as a business owner, and then the land ends up going to auction anyway.

Why any of this is relevant to this thread, is that my original concept of retirement involved saving up & buying some land in a remote (Alaskan?) wilderness area, and building a homestead. I've put a lot of thought into how that might look, and how best to actually pull it off. I've revisited and revised my plans many times along the way, in response to either life changes or technological advances. I've been married now for 23 years and I've discussed it with my wife along the way, and she was reluctant, but laid down her conditions for approval. I believe I'm empowered and equipped to meet & exceed her requirements even more now than ever before.

Over the years, I have studied home building as an amateur/outsider looking in, and in particular I had the opportunity to observe first-hand when they were building the rather large/luxury home next door. As I am self-employed and often work from home, I got to observe what tools the crew was using to build this home, from the rough carpentry framing, to the brick outer shell it was finished with. I timed how long they typically used their tools to estimate how much power they used on a daily basis, and came up with a value of something like 12 KWh.

I also realize that there was a crew of 6-8 tradesman working on any given day on that house, and I'm not likely to be able to use that much power for building if I'm working alone or at best, with a partner. So as long as I can reliably recharge whatever I use each day (on average) I'll never have to worry about power at that location again.

Then I tried to figure out how to get 12+ KWh reliably to a remote off-grid location on a daily basis. The cost to achieve this has effectively plummeted in the last 20 years and I can realistically pack up the equipment to do this into a minivan right now, sans the solar panels, and I can go pick up close to 6 Kw of solar panels for under $3K now with that same minivan next week, if I wish.

https://signaturesolar.com/cw-energy-595w-bifacial-solar-panel/

I actually have 80 KWh of batteries, although I might not start out with all of that heading out to a construction site, just because I don't expect to need anywhere close to that during a construction phase.  Adding all of them (& maybe even more) to a finished dwelling is the desired result, as the more battery storage the more days of autonomy and/or the less generator fuel I'll need on cloudy days.

(I have 5 of these same model battery boxes at home)


I followed Mike Reynolds' adventures with Earthship Biotechture back when they were getting started promoting their Earthship concept and it struck a chord with me, that they would show up on a building site, and set up the solar system on a temporary basis, and use the power from it to help build the house they came to build, then install it as the permanent power system for the house. I actually met & briefly talked with Mike Reynolds once when he visited the university of Wisconsin - Madison to give a talk.

I know I don't want to build a pure Earthship to live in, but I will happily borrow the useful concepts from it. Likewise, I don't envision a wolfati being my dream home, but I will shamelessly borrow the best ideas from it too.