Connecting different size Battery

[Anthony N]

My question i want to connect in series a 7s 92p 18650 diy battery to a
7s 74p 18650 battery in series to make a single 14s 48 volt battery is this workable ???
Or will the smaller 74p be over charged /discharged in use and charging? they are all same age batteries tesla cell

ok the rest of the story i have added one 74p to a tesla cell to bring up the voltage from 25.2 to 29.4
and i have built a 92p 7 s 29.4 volt im hoping to connect in series to 48 volts it will be charged by a Growatt 6000 wt with charge controller to 57 volts
or i can split it into 2 7s with 2 smaller units with 2 charge controllers Thanks for the helppppppp...!!!

 

[gnubie]

Batteries in series should have the same capacity. If they are different you'll get voltage problems across them during charging etc. All things considered if I was DIYing a battery and needed 48V nominal, I'd build a 48V battery rather than two 24V (or any other voltage) in series. A single battery avoids having to control the state of charge of two separate batteries with an over-arching balancer or the like.

That's just my opinion, not necessarily the best

 

[DThames]

Consider this simple example. Connect 2p (3 ahr each cell) in series with 6p (3 ahr each cell) to 7.4v nominal. Put a 3 amp load on it. In two hours the 2p "half" of the battery will be depleted while the 6p "half" will have 2/3 of its energy left.

Your suggestion of 74p and 92p is not such an extreme case but you can see the issue. If you carefully monitored the voltage of the least part of the series string of batteries, you can make this work. But it would be a last choice type of solution.

 

[Mike Jordan]

There is another thread here within the past few days using the prismatic cells. A couple of different folks connected 280ah and 90ah batts in parallel and some ran tests with data reporting BMSes and data reporting shunts. If worked completely opposite of what I thought. The loads pulled from the batts, and the charge pushed to the batts at a differential matched almost perfectly by the differential in capacity. I would have bet big money against that. Their BMSes had good LVD and HVD set up. Which is certainly a requirement to make that work

I don't remember what the thread was called. But I think it had 280ah and 90ah in the title if you want to try to search

 

[DThames]

There is another thread here within the past few days using the prismatic cells. A couple of different folks connected 280ah and 90ah batts in parallel and some ran tests with data reporting BMSes and data reporting shunts. If worked completely opposite of what I thought. The loads pulled from the batts, and the charge pushed to the batts at a differential matched almost perfectly by the differential in capacity. I would have bet big money against that. Their BMSes had good LVD and HVD set up. Which is certainly a requirement to make that work

I don't remember what the thread was called. But I think it had 280ah and 90ah in the title if you want to try to search

If you find the thread, let me know. I have been planning a similar test. Each battery will have an equivalent circuit. The current will flow based on the combined equivalent circuits. My plan is to have a 7s Li-ion and an 8s LiFePo4 in parallel (for my test). With the proper discharge and charge range, I am pretty sure those batteries could work just fine together. I want to test with similar capacities and dis-similar capacities.

 

[Mike Jordan]

If you find the thread, let me know. I have been planning a similar test. Each battery will have an equivalent circuit. The current will flow based on the combined equivalent circuits. My plan is to have a 7s Li-ion and an 8s LiFePo4 in parallel (for my test). With the proper discharge and charge range, I am pretty sure those batteries could work just fine together. I want to test with similar capacities and dis-similar capacities.

Combining disparate chemistries seams like a really bad idea. But post how it goes