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I would like to connect 13S (48V nominal/~25Ah) lithium battery pack in series with a pack of 10 lithium cells (3.7V nominal/~30Ah) in order to get a 14S battery without tearing apart the original pack. I know it is not advised but since the original pack already has a BMS and since I would not charge them together, so I wonder if discharging them together would cause some serious problems? (e.g. heating/fire). If so, I would like to know what is the theoretical understanding behind this. Shouldn't it be alright as discharge rate would be within the max possible range of batteries?

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The concern with series-connected batteries of any type is uneven charge/discharge rates within the string of cells. This can cause overcharging of some cells, which can lead to premature cell failure and it can also lead to reverse charging during discharge which can also lead to cell failure.

The entire purpose of BMS systems is to keep track of the individual cells in the pack and ensure that they are kept balanced at all times. This maximizes cell life and reduces the possibility of cell failure happening.

You should not connect independent battery packs but rather should put together a cell pack you need with an appropriate battery management systems that can control all the cells in the pack.

Can you do what you are proposing? Sure, there is nothing stopping you.

Is it a good idea? Not really since this will almost certainly reduce the lifetime of your battery packs.

Can this cause damage? Yes it can, cells can burst making a mess and possibly causing damage to enclosures or things nearby. It is possible that cells can discharge rapidly causing heating and possibly fire.

It's always best to use LiIon chemistry batteries properly to reduce the possibility of problems.

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  • \$\begingroup\$ Thanks, this is also my understanding. Is there any balancer circuit that could be used to circumvent this problem and make it possible to connect two different battery packs with uneven discharge rates? \$\endgroup\$
    – azad.wolf
    Apr 25, 2021 at 11:38
  • \$\begingroup\$ You can always connect two battery packs in series. The problem is to keep the stronger cells from reverse-biasing the weaker and destroying them. In your case, the thing to do is provide a simple voltage-sensing circuit for each battery pack, and if either pack gets a voltage too low, you MUST turn off power to the load. There is really no alternative. \$\endgroup\$ Apr 25, 2021 at 19:16
  • \$\begingroup\$ @WhatRoughBeast But checking the low voltage limit wouldn't solve the reverse-biasing issue, would it? Also since pack 1 already has a BMS, wouldn't it be doing the same thing? \$\endgroup\$
    – azad.wolf
    Apr 26, 2021 at 16:08

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