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I am trying to recover the use of an old cordless drill by replacing the Ni-Cad batteries with Li-ion. I have removed the 10 x 1.2V Ni-Cad cells that were wired in series with 4 x 3.7V 4200mAh high drain Li-ion cells wired in series.

The original charger arrangement is through a transformer and a protection circuit built into a charging adaptor that fits on the battery pack (you have to remove the battery pack from the drill in order to fit the charging adaptor). The output of the adaptor is 15.5Vdc to 15.7Vdc at 500mA. I have charged the battery pack once and stopped when the battery pack voltage reached 15V. The drill appears to work well - there is plenty of torque but I'm not certain that the speed of the drill is as fast as it was previously.

Now the question: am I likely to cause any damage to the batteries by charging them in series (as opposed to using a balanced charger in parallel) and do you have any tips to get the best out of these cells i.e. what voltage to charge to, for how long, when to top charge during periods of non-use, etc.

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Lithium-Ion series battery packs are pretty much always charged in series. For high discharge / fast charge applications, there are wires connected between each of the cells that are brought out to a balancing circuit that eliminates any imbalance between the cells during/after the charging cycle. If these imbalances are not corrected, subsequent charge/discharge cycles make the imbalance worse and worse until one or more cells gets overcharged or overdischarged and are destroyed.

If you are not very familiar with Lithium-Ion batteries and safety, it is critical that you use a properly designed and purpose built Lithium-Ion charger to charge the battery. Inexpensive ones with built-in balancing circuits are available for model aircraft/quadrotor battery charging from all the usual vendors.

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