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I wish to start a lithium-ion battery bank but want to add additional Li-ion cells once I establish my power needs. So can I add additional Li-ion cells after say six month to a year?

Davide Andrea
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john Done
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  • No, you should not mix old and new batteries in general, and especially not lithium ones. – Hearth Aug 22 '21 at 05:57
  • hmm. I assume these are not just plain lithium batteries but something with some circuitry in it to manage the charging. I dont see why it wont work. I'm interested to see what others say. – kalyanswaroop Aug 22 '21 at 06:03
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    @kalyanswaroop never assume anything – jsotola Aug 22 '21 at 06:55
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    You haven’t said how the batteries are connected. You could have a bank of unconnected batteries that you have to manually swap. – HandyHowie Aug 22 '21 at 07:03

2 Answers2

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You can do this, but be aware that as a lithium-ion battery ages its internal resistance increases.

When you wire an old and new cell in parallel the new cell will initially supply more of the load current. This causes it to discharge faster until both cells share the load equally. When load current is reduced the old cell will charge the new cell for a while until they equalize again. When charging a similar effect occurs.

How much the old cell's resistance increases depends on how it is used. For lowest degradation it should be maintained at ~50% charge and the temperature should be kept below 25 °C. Charge/discharge cycles should be shallow, eg. 80% to 20% rather than 100% to 0%. If these rules are followed the resistance should not increase much in 6 months to 1 year.

So long as the cell resistances are similar, the current is not too high (ie. less than the charge/discharge ratings of a single cell), and the battery is not discharged too far (eg. no less than 3.5 V), this temporary imbalance should not be a problem. However it could be a problem if you intend to draw more current from the larger battery.

Bruce Abbott
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Assuming that you did the right thing and protected the battery with a BMS (Battery Management System), then, yes, you can mix new and old Li-ion cells because the BMS will ensure that no single cell is operated outside its Safe Operating Area. Just make sure that:

  1. When you connect cells in parallel, they are at the same voltage, and the used cannot disconnect and reconnect cells at will
  2. If you have cells in both parallel and series, that you start by making a block of cells in parallel, then connect a number of these blocks in series ("Parallel first arrangement")

But just because you can, that doesn't mean you should. Using well-matched cells will give your battery the highest effective capacity and the longest life.

Davide Andrea
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