David Baillie

Ahmet Oguz Akyuz wrote:Hi There,

In my setup, I two two strings of panels on east and west side of my roof. Each string has 6 approximately 400W panels serially connected. I would like to ensure that each string gets its own MPPT for optimal performance. I have previously found out that when I directly connected these strings in parallel, I ran into a malfunction in one of the strings (most likely bypass diodes in the strings). So I now want to do things right to avoid a similar future problem.

The issue is that my solar hybrid inverter has a single MPPT module. How can I best add the second MPPT to my system? It seems to be suggested that the second MPPT output should be directly connected to the battery. Doesn't it create a weird configuration where one string goes to an inverter and then to a battery and the other string directly goes to the battery (I mean after the MPPT). I guess if I make this connection, the inverter would be fully unaware of my second string which goes to the battery directly. Wouldn't it cause a problem?

Or would you suggest that using blocking diodes in a combiner box a better option for my system?

Thanks for any insights.

Ahmet, what kind of batteries are you running on your system? Are they lithium and are they in communication with the inverter? I ask because it would affect how you connected the second charge controller. If I was doing it i would hook up the second charge controller not built into the inverter to the solar string that receives sun in the morning. That way as the afternoon sun hits your all in one charge controller which you can monitor better it is able to top off the batteries. If you are using lithium your settings will be very important since the second controller will not be communicating with the batteries and will be relying on battery voltage readings alone which is never a great thing with lithium. Charge at too high voltage and the bms will shut down the bank.  
Cheers, David

William Bronson wrote: So a heat pump works better when the gass or liquid it is stealing heat from is warmer.
A brief search shows that the average temperature of  municipal sewage is 50° to 70  ° F.
I'm gonna guess that the air in the system is a similar temperature.
This gives me some crazy ideas.

-An air sourced heat pump that gets its air source from sewage vents.
To avoid breaking the  liquid seals in the traps we would put in as much air as we extract.
-A liquid sourced heat pump that draws heat from grey water heals  in an insulated tank.
Held greywater tends to turn into to black water, but aeration can  prevent that.
-A liquid sourced heat pump that draws from a counterflow heat exchanger.This could be the least efficient.

I think the air sourced pump could be better because it will get cooler temps overall which will help for cooling, plus no tank of dirty water to deal with.

In a house that has city sewage but doesn't use it, there is way more leeway for such a system.

In house with a septic system, there is already a giant underground container of dirty water.
This could be a place to put a coil.

William,
If you steal heat from a septic system the bacteria that break down the solids into liquids that can easily flow through a tile bed will slow down or stop. They need the heat to stay productive.

Douglas Campbell wrote:Hi David;
The newer inverters can handle the output, but  would need a large  PV array to drive Level 2  @7200 W charging offgrid.

Current EV car batteries are ~~ 75 kWh capacity, so require ~~ 10 h of Level 2 @ 7200 W for complete charging; ~ 10%/h charge to the EV.

A sizable offgrid PV array of 7000 W nominal  might take ~ 10 h of full sun to fully charge an EV battery.

In contrast, Level 1 charging at ~1500 W can often run for ~ 5 h/day from an 7000 W offgrid PV array, in parallel with domestic usage, giving about 2% charge/h or 10% day to the EV.

This all comes from my experience.
At home, with grid tie solar; Level 2 charging generally outruns our instantaneous PV (11400 kW nominal).
At an offgrid place (6700 kW nominal PV) we use Level 1 to gain ~ 10% EV charge daily, once the domestic battery bank is full.

A large offgrid battery bank is ~ 30 kWh capacity, so charging the 75 kWh car from an offgrid  battery bank would drain it in less than a day, but it is useful to even out cloudy patches etc. during charges.

This comes down to use cases, adjusting from 'Drive to the gas station on empty and get 600 km of range for ~$75'.
~ 5 h of Level 1 is sufficient for most users, most days, to get back  ~ 40 km or so.
~ 10 h of Level 2 gives the convenience of a 'fill up' over night or in a day, ~ 400 km or so.
Level 3 is (often) expensive but only used on road trips, ~ 300 km in ~ 30 min or so.
Pee break plus snacks.

Some people drive 100's of km a day and have no /limited charging infrastructure, and large bladders :)
But most people do not.
cheers Doug

Hi Doug,
I am not in anyway doubting your math. I'll stick to off grid and level 2 charging. One of the realities of modern off grid solar is the lowering of the cost of solar arrays and the increase of the size of arrays people are installing. It is not uncommon now for me to quote out a 9600w solar array for an offgrid design that only starts out with 15kWh of energy storage. The arrays are now built to meet all energy needs most of the time even into late fall, early spring.  So on sunny days you sometimes end up even in the winter with huge solar surpluses that you cannot store well. Traditionally the lead acid banks limited excess solar due to slow charge rates and absorb cycles. But now 1.5-2 hrs of sun and the home bank is charged. Some people are upgrading their banks ,and I expect to see more of this, but most are letting this surplus go. So Using a level 2 charger becomes a good option. As you know you can tune them up or down in charge rate to adjust how much power they use per hour. It becomes a good way to absorb all the surplus solar available to you on sunny days in a short time when the home bank is full. The reality of lithium is the array goes from full charging to nothing in a matter of minutes as opposed to the slow decline in wattage you saw in lead acid systems. Level 1 charging makes perfect sense in a lead acid scenario off grid. Level 2 charging becomes a way of increasing your storage potential on sunny days. Most of the off grid inverters I'm installing these days have a way to divert power when the home bank is full to loads such as a hot water tank or say a level 2 charger for the very purpose of surplus absorption. Personally we have not gone the EV route due to costs and existing vehicles. When I kill the work SUV after a long life of good maintenance I hope it will be replaced by a plug in hybrid. When the micra dies, if it ever does, I expect we will be ready to have an all electric replacement for it. My point was just that in an off grid scenario you have huge peaks and valleys in solar production A heavy load like a level 2 would be a great tool for flattening the curve. There are tonnes of conditions to all that of course but the main points hold true.
Cheers,  David Baillie

Douglas Campbell wrote:Interesting discussion.
I charge my EV at home with a Level 2 240V, powered by grid-tied solar (87% of our total household consumption is solar over the year).
Off grid, daytime solar can support Level 1 120V, drawing about 1500 W.  Level 1 is slow, so offgrid feasibility depends upon use-case.
The fix-it-yourself arguments for locally common older cars are strong, but going forward both EV & ICE cars are becoming complicated.
And in my climate, most consumer vehicles older than 15 y rust out including, sadly, my previous Honda CRV.

Thumbs up for a Honda Fit/Jazz, one of our previous favourite cars, although a bit dinky in backroad snow.
I was tempted by the Chev Silverado EV WT, or Ford Lightning F150; quiet power stations on wheels for remote use, but mileage per kW is poor and charging is long.

there are lots of newer grid tied solar inverters coming online now that support high voltage dc charging or at the very least can support a level 2 charger.
Cheers,  David

As a rural tradesperson in Ontario I'm required to move around quite a bit with tools and am usually bringing most material to site. Most people choose trucks in my area I prefer an SUV. If it won't fit in there or is too dirty I have an enclosed trailer. I have a Ford explorer, so fancier than totally necessary, and with regular maintenance it will last at least 15-20 yrs. It's harder on fuel than I would like but it can tow, haul half a soccer team, several hundred pounds of tools and equipment, navigates my rural roads in all seasons and has a dealer within 20 minutes of here. That is an important factor I'm not seeing much on; can you get parts easily for your perfect vehicle? If you are not knowledgeable or equiped to repair it is someone around you able to do it. Older vehicles are simpler but they require repairs as well. Can you do them and get the parts locally to do them? I stick to the common vehicles of the area and am diligent about all the regular low level maintenance I can do. Most maintenance even on new vehicles is mostly brakes, oil, filters, tires, etc. doable by you or just about any mechanic around as long as you don't get too exotic. Our household is fortunate and we have two cars. Number two is a small commuter, a Nissan micra compact, great on fuel, low cost on parts, no bells and whistles.
Cheers,  David

John Weiland wrote:

larry kidd wrote:It got down to about 20f last night and I never insulated or heated the batteries. Lost power about 2:30am took till about noon to get the cells warmed up to about 35f or 2c and got power back online. Spent the better part of the day after that wrapping the cells with heat tape for pipes and put insulation under and over , still need to go back and insulate the sides. Used 30 feet of heat tape with a 90w draw. It has it's own thermostat on at 35 off at 50 if I remember correctly.

Living where we do in the central US just below the Canadian border, an experience like this is what causes me to hesitate on diving into LiFePO4.  I probably will anyway and just keep the investment small to modest.  Wife is still tooling around the farmyard with recent ~10 degree F using lead-acid batteries in a Polaris Ranger EV and we are grateful for the robustness of the time-tested tech, even with the known power deficits of these batteries in cold weather.

There was mention recently of Canada leaning more towards solid-state/sodium ion technology, partially because it may be a less expensive battery to produce, but also in large part due to its greater resiliency to cold temperatures.  Still that battery too will use a battery management system (BMS) and one hopes these don't turn out to be a weak link in the technology.  Larry K, I always wondered if a seedling heating mat would be enough to prevent severe temperature drop in such situations.  CLearly if the location is too cold and the batteries unprotected, the BMS will do best to shut down the battery.  But in situations where the batteries are housed in an insulated container of sorts, a seedling mat seems to be designed to produce low temperature, low wattage heat to the item(s) sitting on the mat.  Perhaps this would be a safe solution for many out there?   Also a question for those having installed LiFePO4 batteries going back a decade or two:  Have you experienced or heard of situations where either the cells or the BMS itself failed causing need for battery or cell replacement? If the BMS goes bad and the cells are otherwise good, can the BMS be replaced (assuming a battery case whose contents can be accessed) fairly easily?  Thanks!

John there is Lithium and then there is lithium... Most of the server rack type assemblies are available with a built in heating mat. It is a little annoying though as it will only power up while the batteries are charging and cannot be used in an off grid discharge only scenario. I have taken to oversizing my insulating boxes by 6 inches on all sides and incorporate a 300 watt heater with a blower. Even with the extra troubles lithium is worth it. The greatest advantage is rate of charge. Traditionally you would limit your array size to match the ideal rate of charge of a lead acid battery since the rest was "wasted". Now we can oversize the array so that you can grab 100 percent of available sun on those days where the sun comes out hard for short amounts of time. Also all that extra time running a generator for absorb charging is also a thing of the past. So the little power you use heating the box is worth it. I would suggest sticking to a company that has distribution and available spare parts like all high cost tech items. I like the units that do closed looped communication with the inverter so you get a real time temp reading and balanced charging. The cheap drop in replacements without comms have not been doing well long term. As to Sodium they are starting to show up but are in their early adopter high cost unknown specs days so I'll wait for now.
Cheers,  David

Jackie Lei wrote:Yes, LiFePO₄ batteries really are a big step up. I’m also planning to upgrade, I’m looking at a 16 kWh LiFePO₄ battery for my home loads. The price is surprisingly low, just a little over $1,200. A friend recommended this battery manufacturer to me since I’m not very familiar with LiFePO₄ products myself.

This is the battery I’m considering. 16KWHCould you help me take a look and let me know if the lifespan can really reach 10 years?

that battery will probably make the cycles it advertises but when you purchase a low cost battery straight from the manufacturer like that you have no way of knowing if it will be supported if something goes wrong. If you live in an area that enforces certification of batteries you might not be able to use that one. As long as you understand those points its specs look good.

David Baillie wrote:

John Weiland wrote:I'm hoping to piece-meal together a small system that would be expandable in the future for more off-grid power.  Initially, I was hoping to school myself by focusing on two essential items of the homestead-- the furnace (propane) for winter and the well pump for water.  As you might expect, non-winter months are not so crucial.  Even if the well becomes inoperative for a period, livestock watering can be done from the river near the house.  

I've already dabbled a bit with 12V-powered inverters for producing low-wattage 120V AC power.  What I'm envisioning for the current project is a 48V inverter/charger (Magnum Energy being one brand of interest) that would keep batteries topped up while grid-power is active, but be able to switch over to powering the furnace motor (120V) and well-pump (220V) if grid-power goes down.  A side angle here is the fact that I'm preparing to convert a 36V golf cart to 48V soon and this likely will involve several (3-4?....more?) 48V/30Ah LiFePO4 batteries.  Clearly one can get larger individual batteries, but I'm interested in keeping individual battery weight as low as possible so that they can be used in the golf cart (solar PV panel roof) in summer and shuttled easily to the basement for winter.

Questions arise around sizing the inverter/charger and battery bank for powering the furnace fan and the well-pump.  The furnace is less of an issue as it should be readily powered by an inverter of 4000-6000W (pure sine wave, peak surge watts nearly double the running watts). If memory serves me, the house well pump was ~2/3 - 3/4 hp submersible running at 220V and while the running amps/watts aren't terrible, the starting amps may be up in the 20s to low 30s.  So I'm more concerned about making sure the well pump won't trigger a system shut-down due to either batteries or inverter (or both) being under-sized.  A parallel string of 4 batteries each at 48V would yield 120Ah with internal BMS's sized for golf-cart amp surges (80 - 100A per battery...typically double that for short spike surges).  As finances allow, I would be integrating solar energy into the system as well as part of the expansion.  Input on this vision and design is most welcomed...  Thanks!

John the magnum is currently discontinued. If you want to stick to the older transformer based units like the magnum then a samlex or victron would do it for you. If you are going lithium choose an inverter meant for them like the lux or sol ark type. I am liking lux these days.

Well John, Michael is right about inrush current. If you are using a 3/4 Hp pump you will want to oversize the inverter. Older transformer units had better surge capability but they have not moved on to the new standards for meeting UL 9540 rules for using Lithium Batteries. In some areas of the world that does not matter. Here in Ontario I have to meet all the latest standards. So, Michael suggested the 6048 which is a good unit but you would want to substitute a 10kw all in one inverter as a subsitute if you wanted to go for the transformerless units. I would also invest in a 3/4 horsepower pump with a soft start like a grundfos pump.
Cheers,  David

John Weiland wrote:I'm hoping to piece-meal together a small system that would be expandable in the future for more off-grid power.  Initially, I was hoping to school myself by focusing on two essential items of the homestead-- the furnace (propane) for winter and the well pump for water.  As you might expect, non-winter months are not so crucial.  Even if the well becomes inoperative for a period, livestock watering can be done from the river near the house.  

I've already dabbled a bit with 12V-powered inverters for producing low-wattage 120V AC power.  What I'm envisioning for the current project is a 48V inverter/charger (Magnum Energy being one brand of interest) that would keep batteries topped up while grid-power is active, but be able to switch over to powering the furnace motor (120V) and well-pump (220V) if grid-power goes down.  A side angle here is the fact that I'm preparing to convert a 36V golf cart to 48V soon and this likely will involve several (3-4?....more?) 48V/30Ah LiFePO4 batteries.  Clearly one can get larger individual batteries, but I'm interested in keeping individual battery weight as low as possible so that they can be used in the golf cart (solar PV panel roof) in summer and shuttled easily to the basement for winter.

Questions arise around sizing the inverter/charger and battery bank for powering the furnace fan and the well-pump.  The furnace is less of an issue as it should be readily powered by an inverter of 4000-6000W (pure sine wave, peak surge watts nearly double the running watts). If memory serves me, the house well pump was ~2/3 - 3/4 hp submersible running at 220V and while the running amps/watts aren't terrible, the starting amps may be up in the 20s to low 30s.  So I'm more concerned about making sure the well pump won't trigger a system shut-down due to either batteries or inverter (or both) being under-sized.  A parallel string of 4 batteries each at 48V would yield 120Ah with internal BMS's sized for golf-cart amp surges (80 - 100A per battery...typically double that for short spike surges).  As finances allow, I would be integrating solar energy into the system as well as part of the expansion.  Input on this vision and design is most welcomed...  Thanks!

John the magnum is currently discontinued. If you want to stick to the older transformer based units like the magnum then a samlex or victron would do it for you. If you are going lithium choose an inverter meant for them like the lux or sol ark type. I am liking lux these days.