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I'm currently building an electric bike for a competition. I know I want a 15s lithium ion NCM battery (max 4.2 volts/cell), but don't have a battery management system or a balance charger than can handle 15s. I don't want to spend money on a battery management system or a charger either, as I want to use as many parts as I already have. I have a charger than can charge up to 4s, so I'm thinking to physically split the 15s battery into three 4s packs and one 3s pack. So far so good. But then I want to connect them in series to get overall 15s. Will I run into any voltage imbalance issues when discharging? Thinking of using multiple connectors leading into one discharge connector.

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If you charge them as individual packs like the picture bellow, you should not run into particular issues.enter image description here

But ! The safe discharging of the 4 batteries that you separately charge will rely on the quality/precision of your battery charger. Since the only way your complete battery is balanced, is by reaching the maximum voltage of each cell to that of the reference voltage of your charger. Thus if your charger charges to 4.2 V +/- 2%, you should be totally fine, if it charges within 10% (which might be the case) you could get up to 3.4V of difference between two of your 4 cells pack if one gets recharged 10%% to much and the other 10% to little.

I have used this strategy for a 12 cells battery that I recharged as two 6 cells packs and never had a problem. But I would encourage you to be careful, and take precautions and measurements to make sure all voltages are close enough to each other before attempting to discharge the battery.

One major disadvantage with this charging scheme is that you ALWAYS HAVE TO charge all batteries to their very maximum voltage and very maximum capacity (since most chargers are not smart enough to charge to a precise and balanced "in-between"), so that you do not end up exhausting weaker cells even quicker due to their reduced capacity and tendency to not fully charge at the same pace to that of good cells, in less ideal charging scenarios.

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