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Math Validation

MaikaiLife

MaikaiLife

Solar Enthusiast
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If I have 3x 12v batteries connected in the following configuration.

1: 100 amp
2: 100 amp
3: 70 amp

So, if batteries 2 & 3 (Group A) are put in parallel, it's 12v at 170 amps.

If I put battery group (A) in a series with the 1st battery, I have 24v now. So my math memory recalls that you divide the number by 2 since the voltage is doubled.

270 amp / 2 = 135 amps at 24v.... Am I missing anything?
 
MaikaiLifeDIY said:
If I have 3x 12v batteries connected in the following configuration.

1: 100 amp
2: 100 amp
3: 70 amp

So, if batteries 2 & 3 (Group A) are put in parallel, it's 12v at 170 amps.

If I put battery group (A) in a series with the 1st battery, I have 24v now. So my math memory recalls that you divide the number by 2 since the voltage is doubled.

270 amp / 2 = 135 amps at 24v.... Am I missing anything?
Click to expand...
Yeah...
Anything in series must match exactly...
So, if you want 24v, take the 70Ah out, and JUST use the two 100Ah batteries.
 
Supervstech said:
Yeah...
Anything in series must match exactly...
So, if you want 24v, take the 70Ah out, and JUST use the two 100Ah batteries.
Click to expand...
It still produces 24v, I actually tested them and it's working fine, I just have one battery that is smaller, and rather than buy another or not use they idel battery, I figured why not put it in parallel so it's increasing the overall amperage of the battery bank.

Is there a problem I'm not seeing here?
 
Yes, the series flow will RAPIDLY unbalance between the two sides of the series run, the 100Ah side will drain first, and get hit with voltage sag, resulting in the 170Ah side dipping and you will end up sending 16 or 18v to the 100Ah side when charging...

They have to be matched.
 
Supervstech said:
Yes, the series flow will RAPIDLY unbalance between the two sides of the series run, the 100Ah side will drain first, and get hit with voltage sag, resulting in the 170Ah side dipping and you will end up sending 16 or 18v to the 100Ah side when charging...

They have to be matched.
Click to expand...
Hmm... okay I have a Victron battery balancer on each side, that will balance each 12v side of the bank. Does that make the scenario sound better?
 
What’s the chemistry of the batteries?

Treat it as a 100ah 24v.

What sort loads will this be providing? If you serve very low loads <200w) and never try and go deep into the SOC (nothing lower than 70%) the victron balancer will get a work out.
 
They're all LifePO4, and yes they will rarely discharge below for example 85% SOC and will charge back up to 100% each day via solar.
 
So the 70ah isn’t really doing anything worth while other than sitting on the shelf not being used. It’ll just lope along.
 
I'm gonna steer this thread a little further off-course.

What if mixing different ages of batteries?

Consider my scenario, I have 2 batteries from say 2023 and then I buy 2 more in 2024. They're the same chemistry, size, and different vendors, and certainly different ages. If the batteries have over charge protection and floor discharge protection, how bad is it really to use two new batteries of the same chemistry, size, and voltage to expand a system?
 
MaikaiLifeDIY said:
I'm gonna steer this thread a little further off-course.

What if mixing different ages of batteries?

Consider my scenario, I have 2 batteries from say 2023 and then I buy 2 more in 2024. They're the same chemistry, size, and different vendors, and certainly different ages. If the batteries have over charge protection and floor discharge protection, how bad is it really to use two new batteries of the same chemistry, size, and voltage to expand a system?
Click to expand...
I think Andy from off-grid garage in Australia did a lot of experiments with that... he should have videos on it.
 
This one shows a descent simulation on a discharge, but they're not the same size.


if you never run down the SOC more than say 50% would the charge cycles be the issue, especially when each battery potentially has it's own over charge disconnect?
 
MaikaiLifeDIY said:
Is there anything more relevant to using batteries that are already in a box with a BMS.

For example

2x of these - https://www.amazon.com/Litime-LiFePO4-Battery-Upgraded-Lifespan/dp/B0CKR76TNY/ref=sr_1_2_sspa?c=ts&dib=eyJ2IjoiMSJ9.qHPvGJaQMBx6gVdPr_d8gvpaG7epNLk08QJ1pEhpLLWjW2r0CbjQf4-GoXj0zJ9tnaPcsmcs1YtEIACJGYU6vjuIb3kixPR9mF8aH0pe-RtSHZ7Pwjg5OmeGMVBmxOXSUHzHroyD1TBEFiulgdOGSOH_eCBNM19Ub4n3twTgFOX6sKy1FHoeEdllpREJCjJvxoA_ll7f21YiLILwoiAtaNkADxa9H7BH20jBNdbfIxtek8LhdnJJnMPHPMOjZTaA0PjuCObi1dUnb60-c2qpsCBixibBO6p_grf6RVytNy8.A3m3bK4Umvhc4lL8HxVAqvOe4HpoO3K45mhWONCSpw0&dib_tag=se&keywords=12V+Batteries&qid=1710812609&s=hpc&sr=1-2-spons&ts_id=389574011&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

and

2x of these - https://www.amazon.com/gp/product/B0BL6H1HRM/ref=ox_sc_act_title_2?smid=AS0BKDA9QAEQG&psc=1
Click to expand...
It's the same principle. Whether it's cells or batteries in series. The smallest dictates the overall capacity.
 
The only thing that the 70 will do is throw everything out of balance. And adding the balancer will just exacerbate the situation.
Leave the 70 out.
 
timselectric said:
The only thing that the 70 will do is throw everything out of balance. And adding the balancer will just exacerbate the situation.
Leave the 70 out.
Click to expand...
I've already dumped that idea and moved onto mixing brands and ages of LifePo4 batteries, "same capacity and voltage though".

In my previous post

For example

2x of these - https://www.amazon.com/Litime-LiFeP...389574011&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

and

2x of these - https://www.amazon.com/gp/product/B0BL6H1HRM/ref=ox_sc_act_title_2?smid=AS0BKDA9QAEQG&psc=1
Click to expand...
 
MaikaiLifeDIY said:
I've already dumped that idea and moved onto mixing brands and ages of LifePo4 batteries, "same capacity and voltage though".

In my previous post
Click to expand...
If you are buying, then just buy the proper voltage batteries. You will save yourself a lot of headaches.
If you are trying to Frankenstein what you already have, together. Those will kinda work with a battery balancer.
 
Last edited:
Here is what I'm ultimately going to do, unfortunately it costs another freaking $400, but it will work.

They're the exact same voltage and amps, just different manufactures and about a year difference in age, but the original batteries have not been cycled hard, probably less than 100 cycles, so they're damn near new. With the balancer that will at least handle minor charging variances between them and then I imagine over time they will be fine. Even batteries from the same manufacture can have disimilar resistance so this scenario could be nearly no difference or it could be a little larger, either way, it's what is in the budget and it will work for years.

1710864132472.png
 
I see that you have put both parallel and series jumpers in the middle. (Instead of one or the other) I'm pretty sure that there's a reason to not do that. But for the life of me, I can't remember what it is, right now.

Other than needing a fuse on the parallel jumpers.
Maybe that's the only thing...........
 
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