Couple of DIY battery build questions

[memilanuk]

I've been looking more and more at the DIY 12 v 280 ah builds, and think I may go down that route. I'm sure I'll have lots more questions along the way, but there are a couple I kind of want to get squared away in my mind before I decide to take the leap.

I've seen much discussion about cell balancing prior / during assembly. I've also seen what appeared to be a few references to dealing with cells getting out of balance down the road. Is that a common thing? Is it something the BMS should take care of, or something that the user needs to pull the battery and do manually? How do commercial batteries like Battleborn, Renogy, SOK, etc. handle this?

Second item... is a little more long-winded. Right now I have two 12 vdc FLA batteries, guesstimate of 85 ah each (there literally is nothing on them anywhere to indicate the capacity) - so in theory, about 85 ah of usable storage. If I go to a DIY 12 vdc 4S 280 aH LFP battery, I should have over triple the capacity I have now, even without draining it all the way to zero, which is awesome I'm planning on going with an inverter/charger unit to kill two birds with one stone, as my OEM charger doesn't do lithium. Plus a solar charger, plus a DC-to-DC charger for alternator charging... all of which is going to eat up room I don't have a lot of (slide-in truck camper)... so that's why I'm only looking at one 280 ah battery initially. If I wind up having the room, and want to add a second DIY battery of similar size... is there anything particular I need to be concerned about as far as just plugging the second battery in parallel with the first? Assuming here that the wiring, OCPD, etc. are all appropriately sized.

Thanks,

Monte

 

[John Frum]

Care has to be taken so that the batteries charge/discharge reasonably equally.
This resource will help

Wiring Unlimited

Wiring Unlimited is a great resource that covers many many common topics and questions relevant to DIYers. It is a free E-book from Victron Written by Margreet Leeftink. Check it out! Download

diysolarforum.com

You also have to consider charge current.
You want enough current so that the batteries can charge properly with minimum stress.
You also make sure not too apply too much current for one battery in the event that one is offline.

Say you have 2x 280ah batteries at 12.8 nominal volts.
the Optimal charge current for those batteries is 0.2 to 0.5c which is 56 to 140 amps.
So for 2 you would not want the charge current to exceed 140 amps.
Which is a fine rate for both and not too much for 1.

 

[John Frum]

280amp hours * 3.2 volts * 4 cells = 3584 watt hours
3584 watt hours * .9 @snoobler factor * .8 bandwidth management = 2580.48 practical watt hours per battery.

 

[memilanuk]

Care has to be taken so that the batteries charge/discharge reasonably equally.
This resource will help

Wiring Unlimited

Wiring Unlimited is a great resource that covers many many common topics and questions relevant to DIYers. It is a free E-book from Victron Written by Margreet Leeftink. Check it out! Download

diysolarforum.com

Being fairly Victron-centered, that seems to talk about their 'battery balancer', which looks like it would be appropriate for use with two or more batteries.

Does the BMS in a DIY battery do the same balancing function for the *cells* of that particular battery? I understand they provide protection features; less clear what all other functions they perform.

 

[John Frum]

Being fairly Victron-centered, that seems to talk about their 'battery balancer', which looks like it would be appropriate for use with two or more batteries.

Please disregard whatever you read about "battery balancer".
You want page 18 Parallel battery bank wiring.

Does the BMS in a DIY battery do the same balancing function for the *cells* of that particular battery?

The BMS can usually maintain the balance of a properly top balanced battery.

I understand they provide protection features; less clear what all other functions they perform.

A bms has 2 main functions.
Ensure the battery cells do not deviate from the safe voltage envelope.
Provide cell balancing.
They usually provide over-current protection but it should not be relied upon.
The smart units that are popular here also do current accounting.

 

[memilanuk]

Please disregard whatever you read about "battery balancer".
You want page 18 Parallel battery bank wiring.

Yah, figured I'd go diagonal like the pair of FLA
I have now are.

The BMS can usually maintain the balance of a properly top balanced battery.

A bms has 2 main functions.
Ensure the battery cells do not deviate from the safe voltage envelope.
Provide cell balancing.
They usually provide over-current protection but it should not be relied upon.
The smart units that are popular here also do current accounting.

Gotcha ?

 

[John Frum]

Yah, figured I'd go diagonal like the pair of FLA
I have now are.

For 2 batteries diagonal is fine.
For 3 or more those other methods are better.

 

[memilanuk]

So... circling back around... any concerns about adding the second battery at a later date? With FLA there's always been a concern about batteries being of similar age/vintage. I've gathered that LFP generally doesn't have the same issue, just want to make sure the DIYs were included in that.

 

[time2roll]

So... circling back around... any concerns about adding the second battery at a later date? With FLA there's always been a concern about batteries being of similar age/vintage. I've gathered that LFP generally doesn't have the same issue, just want to make sure the DIYs were included in that.

I would have no concerns. Some might disagree. I would have the expectation that the new battery may have a shorter life and need to be replaced same as when the older battery is replaced.