Battery protection

What measures can I take to assure my batteries do not draw down too far?    I have had solar for years as a backup for years.  I am now planning on using it as my primary source of electricity, so I need to get smarter real fast.

can we get more info please john?

what do you currently have? What is your setup?

Actually I have 2 sets ups.....a primary and a back up.   My primary has about 2500 watts in solar panels, 20 0r so 6v lead acid golf cart batteries, and 2 3000 watt inverters ....yes, I have 2 charge controllers.   My secondary is a 150 watt  panel, a 500 watt inverter, and a wheel chair batter with a controller. The function of the back up is to keep the radio on and a light or two on until I can fix the primary should it go down.  Both systems are 12 v

I have had no problems with drawing dow the batteries .....but, I have not tried to use then as my primary source.

20 batteries is an odd combination for battery strings.  I'm assuming you have a combination of parallel/series strings?  What is the system voltage?  What kind of GC, Trojan T-105s?  Not knowing more information, I'd say that your solar input is inadequate for that size of battery bank.  I can understand why you are concerned about draw down.  I'll make a few assumptions and do the math.  You can tell me how close to right I am.

I think you might have 5 parallel strings of four 6V golf-cart batteries wired in series, or 4S5P, to create a 24V system.  Assuming these are 250Ah batteries and you want to charge at an optimal 1/8 of C, the math works out as....

(250Ah X 5 strings)/8 = 156A

To get 156A charging at ~25V you need 156A X 25V = 3900W of panels.  Add in a 85% inefficiency correction factor, and that becomes 3900W/0.85 = 4588W of panels.  Basically, you need about double the amount of solar you have right now to reach an optimal charge level for that sized bank.  You said that you have two controllers?  Can they individually handle at least 78A?

BTW, that is close to what I started with, though I've upgraded to 5500W two years ago with another 1000W array facing due West.

Shop locally on Craigslist with local pickup instead of shipping.  You save a ton of money.  I just bought some 260W REC panels two months ago for 65$ each.  An extra 2000W would only cost you 520$ at my price.

One one final thing I can tell you is to look at your inverter's operations manual and look for the low-voltage shutdown value.  If your inverter is like mine, it can be programed to shut down at a specific voltage so you batteries do not get drawn to low.

So, Michael seemed to miss that you mentioned that both systems are 12 volts. Back in the old days, 12v was basically the only option, but now most everything is available as 12 or 24, and even 48 is not hard to find. I have a little DC chest freezer that will actually run on 12 or 24. Charge controllers can be programmed to any voltage, so the only real picky piece of hardware is the inverters.

The big problem with 12v systems is how many amps it pulls. You mention having 2 charge controllers, but each one would need to deliver over 100 amps to handle a 2.5kw array. A 3000 watt inverter at 12v would draw 250 amps, which would need cabling the size of your thumb.

Also, with that many batteries in parallel it would be tempting to wire them up in a "ladder" configuration, with 10 sets of 2 in a line, and all the positives and negatives chained together. This saves wiring complexity, but can lead to imbalance, where the cells on the end do more of the work due to the difference in resistance. It could be that they were wired correctly, but it would be quite a complex arrangement. Do you have a picture of the battery bank?

Since lead acid batteries will likely only give you 3 or 4 years of service when cycled daily, I would really suggest thinking about changing to a higher voltage down the road. Since you have so many batteries, you could even split off a portion to make a new separate system. When going completely off grid, redundancy is nice to have. Maxing out the amount of panels that you have is also a good idea, and by upping the voltage you could effectively double the panel wattage without getting any more charge controllers (which are often a big portion of an off grid systems budget).

Anyway, none of that really answered your actual question, which I think I understood to be: "How do I avoid over-discharging my bank?"

So, first off, you did not mention a battery monitor - which is critical. If you do not have one, invest in a monitor that records amp hours in and out. You can also set them up to remind you to equalize your batteries, which you should ideally do every month or so. The newer stuff just has a little clamp that goes over the main power lead, otherwise you will need to wire in a big shunt.

Next, making a point of knowing the cut-off limits of your inverter is helpful, although in my opinion they set the limits so low that a lead acid battery is going to be degraded anyway if you rely on that as your safety measure. Lead acid batteries are pretty terrible. The only thing they really have going for them is that you can buy them at any store on a moments notice. They are no longer cheaper than lithiums when you look at lifecycle cost, so there is really no reason to buy a new bank of lead batteries unless you want something quick and easy and that will handle abusive charging. Anyway, also not really on topic, but to quote Bob Dylan, "Times they are a changing." If you really want to protect your batteries from an inverter, the easiest thing to do is get an inverter with power save mode. The cheapest thing to do is turn the inverter off at night. The most fun thing to do is track down a free starter solenoid (contactor, relay, etc) that can handle a few hundred amps, and wire it up to a simple voltage-based relay controller. Some of the battery monitors may have this function as well. Then you just program it to cut the contact to the inverter once your battery voltage drops to whatever you feel like it ought to be.

Anyway, I sure do love talking about batteries. I just finished installing a 1.5kw array for my cellar today. I built a 7kwh lithium iron phosphate bank using bare cells that ran me 1650$ with the BMS. I fit them all into a milk crate, and can still lift it (although the day is coming where I will need to split it into 2 milk crates to get it back out of there). To get an equivalent amount of storage, you would need over 600 lbs of lead batteries.  I can post some pictures of the setup if anyone is curious.

Yes, over charging is not an issue. My present charge controllers are fine.  The system has been in place for several years. I am planning on giving it some serious use. So my concern is over discharging the battery bank.  

Hi John.  Most of my stuff is 12v too.  I use the charge controller output.  I have one that will crank out 30 amps and the other 60 amps.  These have an adjustable volt control so you can take your batteries down to where you are comfortable.  All battery systems seem to have their own dynamics.  I use a 30f super capacitor with both of these to keep them snappy for our inverters.

Hi Christopher,

I have seen discharge controllers advertised at MWANDS that supposedly shuts down the system when the batteries reach 12.2V.

I believe what you may need is a LVD , Low Voltage Disconnect.
Most inverters have a built in LVD, but not all, some are adjustable, some not
Some charge controllers have  load management (LVD)  built in (Morningstar & others)
LVD can be added between batteries and load(s)
High current LVDs usually use relays or contractors that usually draw extra power and tend to be expensive

I have used LBVDs from Galley Power with great success,
disconnect voltage is adjustable on these and they use very little power internally
My favorite usage is for controlling power to freezers, the freezer will run during the day when voltage is higher  and not at night when voltage is lower so power is available for home lighting , radio etc.  In my situation the freezer runs about the same amount of time, just pulling from abundant daytime solar  power rather than more costly night time power from batteries . Doing this has allowed running a smaller battery bank reducing costs and collateral damage to our environment. This works best when freezers are full with more thermal mass stabilizing the temperatures through the night
It is also possible to run freezers/ refrigerators  array direct, so long as there is enough thermal mass to have stable temperatures through the night.

Running 20 batteries  (10 parallel strings of 2) can and is likely to create issues with self discharge due to imbalances in the individual batteries.
You may consider  going to a 24 or 48 Volt system and/ or using larger batteries with higher amp hour ratings and fewer parallel strings
This factor seems less of an issues when charging is more  frequent or continuous , such as summer time solar or with hydro-electric running 24/7 and more of an issue during the Fall & Winter on  solar / generator  systems. This factor seems to increase as batteries age as  each battery degrades differently than the others causing further imbalance. It is possible to analyze each battery and reconfigure batteries to create more balance, but the process isn't something most get excited about.

Best Wishes

Most higher-end inverters have LowVoltageDisconnect (LVD) settings, which stop the inverter draw from battery when a programmable set point is reached. If using an inverter without such a feature, you'll have to add an LVD device into the mix ... something like this:

https:  //www.amazon.com/Battery-Protection-Undervoltage-Controller-Under-Voltage/dp/B07H4T3LS7

... and sized right for the power draw & cabling in your system. Hopefully, a range of such devices exist, and one will match your power requirements.

Or, bite the bullet, and replace the inverter(s) ... I believe LVD is fairly common in good, pure-sine-wave inverters these days.

John F Dean wrote:What measures can I take to assure my batteries do not draw down too far?    I have had solar for years as a backup for years.  I am now planning on using it as my primary source of electricity, so I need to get smarter real fast.

My questions: Are all the batteries the same age? Are they wet cell? What size of cables are you using? How often do you equalize?

You can get low voltage cut offs designed for off grid systems. Hook it up and clickty click it shuts off at X voltage. Be sure to make sure it can handle the amperage that the inverter pulls.
https://www.amazon.com/Digital-Battery-Low-Voltage-Protection/dp/B07929Y5SZ

I also agree that your system is underpowered for your bank size. Of course that has caveats - like whether or not you're using a PWM or MTTP charge controller, the distance from the panels, size of cables. Typically, you never ever want to go more than three series/parallel in a string at one time or six 6 volt batteries. It has to do with the discharge balance of the bank. You could use a larger number of batteries if you were to disconnect the parallel connections to reduce them to four 6 volt deep cycles in series/parallel and treat them as individual banks to charge them.

there is a limit on the number of lead acid battery strings that can be wired in parallel. Three parallel strings of batteries is the recommended maximum. One or two is more ideal: they will charge and discharge more evenly, which makes them last longer.

https://unboundsolar.com/blog/expand-off-grid-solar-system

Lastly, I started using a fluid product in 2006 in my deep cycles to prevent calcification. I'm going to assume everyone knows that that is. If not, see this link. I have one of my deep cycle batteries I bought in 2006 that still achieves a 90% capacity at full charge that I use for my diesel heater. With the product, I can pull down as much as 40% of the charge (1.550 specific gravity +/-). Normally, I'll hit panic mode at 70% but that's me. The product is called "Battery Equaliser" which is UK product.

I'm so confused at this point, I don't know if my flux capacitor will ever work. (much less, my turbo-encabulator) Guess I'll stick with wood working and composting

Hello Max

Concerning this statement in your last post,

"Lastly, I started using a fluid product in 2006 in my deep cycles to prevent calcification. I'm going to assume everyone knows what that is. If not, see this link",

I went to that link and found it to be just a general frequently asked questions post, with no mention of this product you refer to.  Could you please state what the product is and include a proper link to it?

John Duffy wrote:I'm so confused at this point, I don't know if my flux capacitor will ever work. (much less, my turbo-encabulator) Guess I'll stick with wood working and composting

John, I can assure you that every single person that uses alternative energy has had a leaking flux capacitor.  And the phase detractors of the turbo-encabulator made it a non starter and that was after trying the micro turbo-encabulator and even the retro turbo-encabulator because they used the universal phase detractors.

I started going off grid 20 years ago and even as recently as last year, I discovered the errors of my ways - I know full well that in order to charge a battery bank you need at least 10% of the capacity in amps for X time to charge correctly but I was using half that and wondering why I never reached a full charge all because I fell into the siren call of the automatic charger sham. 432 amp hours = 40+ amps of charge current for X hours, not the 20 I was using.

At least you asked the questions before you lost a boatload of money on ruined deep cycle batteries. Get either a charge controller with a load connection and low voltage cut off or the separate kind. Just be aware that your bank may have issues with load balancing on the discharge side. Equalization is your friend and battery university is the school to attend.

There is so much information that people need to research and learn that a forum thread lacks the depth of that information. Over the years, that has frustrated me to no end to the point that I bailed from giving people advise because it was always after the fact and I would wager just about everyone else who has offered advice to people has gone through the same thing.

At one time the NREL had a free program called HOMER (micro power optimization) where you input your loads, the size of your battery bank, generator, solar panels, wind generation and it would do all the figuring for you. You should be able to download the old versions if you can find it.

What I discovered is that there is no possible way that solar would work for me without spending well into 5 figures to charge $400 worth of batteries because I get a maximum of 5 hours of direct sun. That's why I put my resources into a generator.