Connecting different amp hours in parallel

birchfinnigus

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Hi! I tried to research this. But I came across a lot of conflicting information.

I built my first battery, a 60AH 48v Fortune bank w/ 100amp Overkill BMS.

I'm going to build my second battery when parts arrive (if they arrive) 280AH 48v Lishen w/ 2x 8s Overkill BMS (increase the amps).
EDIT: Misunderstanding of amps, now going to use one 200A Daly.

Can I put these in parallel and connect them both to my inverter? When searching for this question I saw a lot of answers relating to lead acid batteries, and how you don't want to do this because the batteries will attempt to equalize putting a lot of pressure on the smallest battery. Then I read if you add a diode you can put different AH batteries in parallel. But it seems like that's all advice for banks that do not use a BMS.

Any insight is very much appreciated. Thanks!
 
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FilterGuy

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I'm going to build my second battery when parts arrive (if they arrive) 240AH 48v Lishen w/ 2x 8s Overkill BMS (increase the amps).
How many cells are you using in that build? Are you using 120Ah Cells?

Can I put these in parallel and connect them both to my inverter?
Yes, you can.... It is not ideal but it does not really hurt anything. @Will Prowse has done it in his videos several times.
 

birchfinnigus

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@FilterGuy First battery is 16S 48v 60ah Fortune Cells. Second battery is 2x 8s24v in series with 280AH Lishen Cells.

I'm doing 2x 8S to get to 200 amps using overkill BMS (we chatted briefly on my other thread)

Follow up question. If one bank can do 200 amps, and the other can only do 100 amps, does that mean when a 200 amp draw occurs the 100 amp will trigger and disconnect?
 

FilterGuy

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@FilterGuy First battery is 16S 48v 60ah Fortune Cells. Second battery is 2x 8s24v in series with 280AH Lishen Cells.

I'm doing 2x 8S to get to 200 amps using overkill BMS (we chatted briefly on my other thread)

Follow up question. If one bank can do 200 amps, and the other can only do 100 amps, does that mean when a 200 amp draw occurs the 100 amp will trigger and disconnect?
I must have miscommunicated somehow. To get double the current, it has to be in parallel. 2x16S
 

birchfinnigus

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I must have miscommunicated somehow. To get double the current, it has to be in parallel. 2x16S
Ok! That makes sense. I doubt you miscommunicated, I probably misheard. So in that case I'm going to switch to a Daly BMS because these batteries will never arrive on a second order.
 

birchfinnigus

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@FilterGuy if I have a 16S 60AH with a 100a BMS, and a 16s 280AH with a 100a BMS, does that mean I have 200a draw and 340AH total?

Thank you for being patient while I struggle with the basics.
 
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FilterGuy

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@FilterGuy if I have a 16S 60AH with a 100a BMS, and a 16s 280AH with a 100a BMS, does that mean I have 200a draw and 340AH total?
First.... this is starting to get outside my experience set, so all I can do is talk theory (as I understand it).

1) I *think* what will happen in practice is that the system will draw an amount of power that is more or less proportional to the battery size.
So.... if you are drawing a total of 100A, the 60AH battery will provide ~ 60/(60+280) x 100 = 17.7A and the 280Ah bank will provide ~82.3A.
(I am basing this on a video from @Will Prowse where he compared current from several batteries of dis-similar sizes that were tied together)
3) At 121.4amps total the current from the 280Ah battery will be ~100A so that would be the max current the system can provide before the BMS would be over-powered.
 

birchfinnigus

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Ok, makes sense. Also seems to be a better idea to repurpose the 60AH (since it's such a large power diff) to a different project and focus on the 280AH for the main house system.
 

time2roll

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As long as the cables are equal length and installed clean and tight, there should not be an issue.
Test at half power and 3/4 power before you go full on. Check the amp flow to each is in proportion.

With 2x BMS on the 280 battery I think it will be worked a bit harder. Please post the results.

BMS will shut down for overload before things get too crazy.
 

Scph9002

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Yes you can paralell the batteries.
Both batteries will charge and discharge equally based on voltage.

A negligible amount of current if any will flow between the batteries when resting.

Adding more batteries in paralell will infact relieve som stress of the initial battery. The more the better
 

Sky-HHI

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I use three different batteries on my system, a 160 AH, a 40 AH and a 280 AH. They each have there own bms and are paralleled to a common busbar. Scph9002 is correct as they all charge and discharge equally based on voltage. Your bms on each battery will disconnect when it reaches it amp rating. So if you have a 100a bms on one battery and a 200a bms on another, then the 100a bms will disconnect its battery at that rate.
 
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