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diy solar

diy solar

Build a Battery … Where to start?

They are all charged within a few 100 mv at 3.65v .. and it’s as tight at the clamps that are used for charging.. I don’t think it will be a problem ..
If they are .100 mv or more off after top balance at 3.65 the cells are not matched very well.
 
If they are .100 mv or more off after top balance at 3.65 the cells are not matched very well.

So you independent charge 9 cells to 3.65 over days and days .. and yours are all perfectly within range of each other? Come on, 100mv is within 1/10 of a volt, there were not BALANCED that that is what connecting them together would do .. they are probably within .0001 now that they have been balanced with each other for like 20 hours or so, not just charged.

And non of it matters anyhow, the BMS will do a balance .. but it’s my first home built and I want to take all the steps..
 
So you independent charge 9 cells to 3.65 over days and days .. and yours are all perfectly within range of each other? Come on, 100mv is within 1/10 of a volt, there were not BALANCED that that is what connecting them together would do .. they are probably within .0001 now that they have been balanced with each other for like 20 hours or so, not just charged.

And non of it matters anyhow, the BMS will do a balance .. but it’s my first home built and I want to take all the steps..
Yes that is what top balance is for, you go to 3.65 until your amps taper to just about zero and all cells should be the same. Mine usually balance to less than .010 and the balancers in the BMS very rarely activate set to .020. I do four to eight cells at a time 72-76 hours for four double that for eight. If your cells are not matched very well they don’t come together at the top.

What are you amps at when finished? What are you using to measure voltage and where?
 
Yes that is what top balance is for, you go to 3.65 until your amps taper to just about zero and all cells should be the same. Mine usually balance to less than .010 and the balancers in the BMS very rarely activate set to .020. I do four to eight cells at a time 72-76 hours for four double that for eight. If your cells are not matched very well they don’t come together at the top.

What are you amps at when finished? What are you using to measure voltage and where?
To be honest I did not measure them.. could have been like this when I connected them …


I charged them cutoff 1a and moved on .. knowing they would get the full treatment with BMS .. more worried about building a battery with bad cell.. so they could have been .001 difference… what I was saying is thee should be less then .1v difference .. would be a few watts and nothing more transferring back and forth ..
 
So you independent charge 9 cells to 3.65 over days and days .. and yours are all perfectly within range of each other? Come on, 100mv is within 1/10 of a volt, there were not BALANCED that that is what connecting them together would do .. they are probably within .0001 now that they have been balanced with each other for like 20 hours or so, not just charged.

And non of it matters anyhow, the BMS will do a balance .. but it’s my first home built and I want to take all the steps..
Nope you just assemble the battery and let the BMS “top balance” them.

With long string charging such as you showed, any minor connection imperfection will have one cell lag behind, with such low current and voltages you a sneeze and a cell is unbalanced.

There’s so much human error at play that there such a big chance of messing it up.

I’ll be curious once you assemble the battery how the cell delta is on your first charge.
 
Nope you just assemble the battery and let the BMS “top balance” them.

With long string charging such as you showed, any minor connection imperfection will have one cell lag behind, with such low current and voltages you a sneeze and a cell is unbalanced.

There’s so much human error at play that there such a big chance of messing it up.

I’ll be curious once you assemble the battery how the cell delta is on your first charge.
I will keep record of everything and post it.. you say that, and I say I am not taking it apart cause I put a bad cell in .. going into a small compartment in the RV and there is no room to work, and I would rather wait now for a few days to make sure no bad cells and a few month to get a replacement then to have to pull it apart and replace a cell after it is mounted.. but, in normal install yep that’s what I would do .
 
OK... suggestions for the best layout?

PXL-20250124-232256542.jpg
PXL-20250124-231916802.jpg


Various thoughts here... there is another plywood lid that will go over the top with more blue foam insulation. It is not attached yet.

The batteries themselves are beneath the hinged shelf visible in the pics.

The big power switch will probably go on the right / outside (shelf hinge side, adjacent to the actual +/- terminals.
I may also consider mounting the I/O board on that outside as well.
The BMS display I will consider trying to mount in the plywood lid.
At bottom left in pic is the thermostat/relay for the heaters. I will likely mount that and the 48->12v buck converter (not pictured) underneath in the battery compartment. Especially since the buck converter reviews say "gets warm under continuous use" which would be of benefit since it will only be used to run the heating pads. lol
While I don't anticipate frequently accessing the battery compartment, obviously we want to minimize the flexing of the wires... although I may be overthinking that part of it if I only go in there a handful of times per year...

So really it comes down to "which orientation for BMS" and "where to put the fuse" and "what am I missing here?"

Thanks in advance for the thoughtful replies! :)
 
48v? Is that a t class fuse? And we have an inverter bms, that IIRC, actually balances better than the older models, just the comms thing.
 
I would rethink your fuse choice, I wouldn’t bet my life on that one.
Yeah I completely forgot about "T-class" when ordering... so re-looking at that today.

What are you planning for separator sheets between the cells?

And for under the cells?
Under, planning to get a sheet of aluminum to spread the heat from the heating pads.

What do you mean "between the cells?"
 
Yeah I completely forgot about "T-class" when ordering... so re-looking at that today.


Under, planning to get a sheet of aluminum to spread the heat from the heating pads.

What do you mean "between the cells?"
Uh oh. This might help with the separator materiel https://diysolarforum.com/threads/lifepo4-cell-separator-material.28023/ The blue cell wrap is thin and can rub through creating a short.

If compressed, the cells expand/contract with the soc.

Some kind of insulator between the bottom of the cells and the al would be nice.

IIRC, the inverter bms balances better than the older design but comms has some issues, and most people buy the inverter board for comms.
 
From my researching, poron is the current favorite for cell separation
And don't use an ANL, they get hot and have a very low AIC. Use a class T
I bought Poron for doing the external compression.
Stopped by West Marine but they didn't have any Class T in stock so I'll be looking online again.

Thanks for the feedback!

Still interested in thoughts on the actual component layout.
 
I bought Poron for doing the external compression.
Stopped by West Marine but they didn't have any Class T in stock so I'll be looking online again.

Thanks for the feedback!

Still interested in thoughts on the actual component layout.
Riedon and DonRowe are the best places I've found for price, quality, and availability, but I think rideon has stopped selling class t fuses, althougn it looks like they still sell shunts.

What voltage and aic for that switch?

I have enough battery that none of the bms are stressed, but if you only have one battery and it's being used heavily, I wonder if the bms might need some air flow? Otherwise, I sort of let the components find where they want to go; large gauge wire has a way of doing that.
 
Riedon and DonRowe are the best places I've found for price, quality, and availability, but I think rideon has stopped selling class t fuses, although it looks like they still sell shunts.
Appreciate the recommendation. :)
What voltage and aic for that switch?
Spec sheet says says up to 60v, 275A continuous / 350A intermittent / 1250A starting

I have enough battery that none of the bms are stressed, but if you only have one battery and it's being used heavily, I wonder if the bms might need some air flow? Otherwise, I sort of let the components find where they want to go; large gauge wire has a way of doing that.
Interesting. I had considered putting a fan in there but have not done it yet; I don't think it's going to be highly stressed; just providing juice with some panels for an off-grid shed. 16x MB31 (314AH) batteries is probably overkill for shed lighting and a well pump. In fact, I'm probably going to set the BMS to report 100% when the batteries are only at 50% because we only get up there for maybe one weekend a month... although we hope to get there more often in the coming year. We may build a little off-grid 2br / 800sqft cottage in which case this battery would become (I think) #1 of 2 to cover the cottage and shed both.
🤷‍♂️
 
I am building 2 x 12.8 314ah batteries - I've choosen 3mm poron for between the cells and a 300amp 4s JBD BMS..

I am still working out if I want to use aluminum or steel plate or wood at the ends.
 
Appreciate the recommendation. :)

Spec sheet says says up to 60v, 275A continuous / 350A intermittent / 1250A starting


Interesting. I had considered putting a fan in there but have not done it yet; I don't think it's going to be highly stressed; just providing juice with some panels for an off-grid shed. 16x MB31 (314AH) batteries is probably overkill for shed lighting and a well pump. In fact, I'm probably going to set the BMS to report 100% when the batteries are only at 50% because we only get up there for maybe one weekend a month... although we hope to get there more often in the coming year. We may build a little off-grid 2br / 800sqft cottage in which case this battery would become (I think) #1 of 2 to cover the cottage and shed both.
🤷‍♂️
Switch. It's nice to know that if you turn a switch off while under heavy load, it will not arc out and catch fire.

You will need to set the charge high enough to activate the balancer. It's best to balance in the 'knees' as this is where the cells start showing imbalances.
 
You will need to set the charge high enough to activate the balancer. It's best to balance in the 'knees' as this is where the cells start showing imbalances.
Yeah that's a good point. Just concerned about keeping the battery up at a higher state of charge. 🤔
 

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