First build, I have questions and would like to hear any suggestions.

[OffGridForGood]

Actually a long time back we had an EE on the form Solarod - he was commenting on this exact set up with 2, 3 4 and 5 packs,
I chatted with him about having (then) six packs with two being only 100Ah while the rest are a mix of 280Ah and 304Ah packs. This later lead to pairing up the two 100Ah packs to act like a "200Ah" pack, ie closer to the others, and paying with where in the stack they are set, and what size wire and what length to get the balance 'closer'. This was a good learning experience for me and my ESS - since the vast difference of 100Ah and 304Ah packs was far from a balance before his input lead me to a much improved set up. - just noting this for interest.

 

[robbob2112]

Actually a long time back we had an EE on the form Solarod - he was commenting on this exact set up with 2, 3 4 and 5 packs,
I chatted with him about having (then) six packs with two being only 100Ah while the rest are a mix of 280Ah and 304Ah packs. This later lead to pairing up the two 100Ah packs to act like a "200Ah" pack, ie closer to the others, and paying with where in the stack they are set, and what size wire and what length to get the balance 'closer'. This was a good learning experience for me and my ESS - since the vast difference of 100Ah and 304Ah packs was far from a balance before his input lead me to a much improved set up. - just noting this for interest.

That was a good read and there were links to the math. I think it should be able to calculate the perfect placement of cables on bus bar for balance. And it seems to me that it should be possible to leave the cables at standard intervals and just adjust the length or size of cables to get the same effect.

For instance, we have always said make cable length to bus bars from batteries to bus bars the same size and length. But by going up one or two sizss cable for the unbalanced battery you could make the resistance the same. Could also do the same by making the cables to the batteries charging to quick a few inches longer to add resistance.

Another thought would be to take varying thickness and length of copper and sister it up to the bus bar. Or make an oversized bus bar and file a bit out of the sides of make cuts like they do to calibrate current shunts.

Takes a mico-ohm meter to measure the cable connection resistance and also the internal battery resistance and reactance. There is a meter for that. The it is just plug into some linear algebra tables. Then plug the results into a chart to figure out the right size and length of cables.

This stuff has been bouncing around my head for a week. Can't wait to get the meter and bus bar materials to test.

But, the conclusion is doing 2 , 3, or 4 batteries are easy to make perfect. So if you have more wire them in groups to balance.

Calculation of parallel string battery currents

Hi, everybody! I'm a retired EE. I didn't want my EE skills to fade away after retirment, so I exercise them when I can. Whenever I see a non-trivial circuit analysis problem on one of various forums I frequent, I challenge myself to solve it. Recently I watched Will's video about current...

diysolarforum.com

 

[JWilliams]

Almost finished with the system!!!

 

[JWilliams]

Finished it last week. Very excited to share. Everything is running smoooooth. Haven't really put the batteries to the test yet and not really expecting the best performance after charging them in a parallel train with the 15A charger. They will get above 14V then drop back down to around 13.56V shortly after you take them off. Not sure how much you could really get out of them at night. If anyone has any suggestions please speak them it won't offend me. Thanks for answering my questions.
@timselectric @OffGridForGood @robbob2112 @Hedges @Mattb4

 

[JWilliams]

My focus with this was to keep it as simple as possible. I see a lot of setups that just seem to have too much going on. I wanted a few breaks in the battery cables as possible.

 

[JWilliams]

180V when cold, drops to 171V when really hot. About 4,280W coming in.

 

[robbob2112]

Finished it last week. Very excited to share. Everything is running smoooooth. Haven't really put the batteries to the test yet and not really expecting the best performance after charging them in a parallel train with the 15A charger. They will get above 14V then drop back down to around 13.56V shortly after you take them off. Not sure how much you could really get out of them at night. If anyone has any suggestions please speak them it won't offend me. Thanks for answering my questions.
@timselectric @OffGridForGood @robbob2112 @Hedges @Mattb4

I would go back and line the inside and outside of your battery room with firerock, walls and ceiling. I would remove all the expanded insulation you are using and replace it with rockwool insulation. Nothing in the room that will burn at all. One other suggestion would be to switch to class T fuses on each string. Or keep the MRBF and add class T

If you think I am being falsely critical, go look at this thread.

House burned down

So, after 4 to 5 years of working flawless, the 100 kwh lifepo4 battery caught fire at night and burned our house down. Luckily our family just made in out on time. Inspection of the fire expert revealed that a melt fuse melted, and created an arc between the 2 points wich ultimatly started the...

diysolarforum.com

 

[Hedges]

"I have (12) Fiamm 12flx500 135Ah AGM batteries"

I think that makes it much safer in terms of fire hazard.

 

[JWilliams]

I would go back and line the inside and outside of your battery room with firerock, walls and ceiling. I would remove all the expanded insulation you are using and replace it with rockwool insulation. Nothing in the room that will burn at all. One other suggestion would be to switch to class T fuses on each string. Or keep the MRBF and add class T

If you think I am being falsely critical, go look at this thread.

House burned down

So, after 4 to 5 years of working flawless, the 100 kwh lifepo4 battery caught fire at night and burned our house down. Luckily our family just made in out on time. Inspection of the fire expert revealed that a melt fuse melted, and created an arc between the 2 points wich ultimatly started the...

diysolarforum.com

Yea I read about the class T, I will prob update those. I haven't read it yet, but that thread you linked, the fire was caused by a mrbf fuse? As far as doing more construction, I need a break for a while this was quite the project to do alone.

 

[JWilliams]

"I have (12) Fiamm 12flx500 135Ah AGM batteries"

I think that makes it much safer in terms of fire hazard.

Are you saying the extra string I added is too much and I should go back down to 12? Or that the AGM batteries I'm using are inherently safer than other choices?

 

[JWilliams]

Oh and I will be wiring the output to a breaker box at some point, I just wanted something to plug the AC unit into for the time being. Also finished the last step of grounding the panel frames today into the grounding rod.

 

[Hedges]

Are you saying the extra string I added is too much and I should go back down to 12? Or that the AGM batteries I'm using are inherently safer than other choices?

That AGM is inherently safer.

With excessive over-charging, they can outgas hydrogen. I don't think that is normally an issue. Vent probably helps, and battery temperature sensor should detect, but maybe not if just one battery is experiencing it.

12 or 16 is OK. (I saw the 4 of another color but missed that the number I quoted was different.)
I see you carefully matched cable lengths.

I used 6V rather than 12V, for fewer strings. One system has single string 400 Ah, another 2 strings 250 Ah.

Are those balancers? I've had discussions here about cross-tying or not, and fusing. It was recommended by one brand I bought. Then I have to worry what happens if a battery does go bad. Your balancers should keep all balanced without that problem of current crowding into one parallel battery if another has problems.

I don't see any extra wires from inverter to battery - you should have a temperature sensor and compensated voltage setting for whatever is the charge controller. Lithium doesn't use that, but lead-acid has to (unless your temperature control is sufficiently stable.) Battery will still heat up with charging, so best to use sensor regardless of that A/C.

MRBF on each positive, that is good.

So far looking good.

Did you get all charge voltages/currents/times/temperature coefficients from battery manual and set charge parameters accordingly?

Mine defaulted to 0.6C charge rate (because I had smaller bank than recommended for amount of PV) and I reduced it to 0.2C

 

[timselectric]

If I'm looking at it correctly. Any venting of the room must go through the inverter? This would concern me with any LA type of batteries. AGM are the least likely to vent. But it still is a possibility.

 

[JWilliams]

If I'm looking at it correctly. Any venting of the room must go through the inverter? This would concern me with any LA type of batteries. AGM are the least likely to vent. But it still is a possibility.

Great observation Tim. I will install a small duct fan at the top to recycle the air every 15-30 mins. So is the gas just toxic, or is it combustible?

 

[JWilliams]

That AGM is inherently safer.

With excessive over-charging, they can outgas hydrogen. I don't think that is normally an issue. Vent probably helps, and battery temperature sensor should detect, but maybe not if just one battery is experiencing it.

12 or 16 is OK. (I saw the 4 of another color but missed that the number I quoted was different.)
I see you carefully matched cable lengths.

I used 6V rather than 12V, for fewer strings. One system has single string 400 Ah, another 2 strings 250 Ah.

Are those balancers? I've had discussions here about cross-tying or not, and fusing. It was recommended by one brand I bought. Then I have to worry what happens if a battery does go bad. Your balancers should keep all balanced without that problem of current crowding into one parallel battery if another has problems.

I don't see any extra wires from inverter to battery - you should have a temperature sensor and compensated voltage setting for whatever is the charge controller. Lithium doesn't use that, but lead-acid has to (unless your temperature control is sufficiently stable.) Battery will still heat up with charging, so best to use sensor regardless of that A/C.

MRBF on each positive, that is good.

So far looking good.

Did you get all charge voltages/currents/times/temperature coefficients from battery manual and set charge parameters accordingly?

Mine defaulted to 0.6C charge rate (because I had smaller bank than recommended for amount of PV) and I reduced it to 0.2C

I'm coming back to this once I get home off the road

 

[Hedges]

How explosive is hydrogen gas?

I want to electrolyze water to get oxygen and hydrogen. I want to mix them in a regular balloon and ignite it. How much hydrogen would you need in a 2:1 ratio with air for it to be dangerous? Is this

chemistry.stackexchange.com

Plus, will smell like rotten egg when overcharged. Could corrode electronics.

FLA always outgasses, and things around corrode. Some people here have a plexiglass box with vent, so fumes don't reach electronics above.

AGM is approved for air transport, and is used in racks in computer rooms (I suppose lithium is too, now.)

I think the risk of corrosion damage may be low enough to just let the chips fall where they may.

Only consideration is safety involving hydrogen. I'm not clear how much ventilation is required. Making hydrogen with electrolysis is a slow process. Hydrogen rises. My inclination is that a tiny fan venting top of room is sufficient, but I don't really know.

Do AGM Batteries Need To Be Vented? (Find Out!)

AGM batteries are sealed and produce less hydrogen. So, do they need to be vented like other battery types? Click here for the answers!

batteryknowhow.com

I have my AGM batteries in garage, basement, or under outside deck.
Where I've had problems was unregulated charger on a small motorcycle battery, stank up the place.
With multiple batteries in series, a failed cell or battery could over-charge and outgas.

 

[timselectric]

Great observation Tim. I will install a small duct fan at the top to recycle the air every 15-30 mins. So is the gas just toxic, or is it combustible?

Corrosive

 

[robbob2112]

Yea I read about the class T, I will prob update those. I haven't read it yet, but that thread you linked, the fire was caused by a mrbf fuse? As far as doing more construction, I need a break for a while this was quite the project to do alone.

Considerably different setup but I am paranoid now about anything built inside the house when someone does it.

IMO it was caused by a cell short in a DIY LiFePO4 battery bank - 1 of the 7 in parallel. They vent hydrogen and don't burn because of it, but all it takes is a spark or something over 300c to touch it off. The fuses in question were 125a 58v Mega fuses in flame resistant enclosures. Opinions vary on if I am right, but to little left to know for certain.

Fire investigator says fuse was the start - but that was likely the point of ignition verse root cause.

For your setup with AGM - the MRBF on the post is good enough for each battery string. I would add a Class T on the uplink from your combining bus bar as close to the enclosure as possible.

A simple small bathroom fan that runs at a low speed all the time might be better than just turning it off and on all the time? Probably have to get a speed control to dial it down since the smallest ones run 30CFM.