Which prevents batteries from being overcharged

Tips for handling rechargeable batteries


I bought 8 batteries with 3800 mAh and put them in a universal charger. After a short diagnosis, 90% is displayed and the red LED lights up for charging. After approx. 20 seconds the red LED (error display) flashes and the display shows 0%. This applies to all 8 batteries! Other batteries with e.g. 2500 mAh work perfectly.

The seller is accommodating and wants to send me 8 new ones right away, but I am not sure that it is really due to the batteries. My question: Can the capacity of the batteries be too high for the charger?

Answer from the author:

I don't think the batteries are defective either. Higher capacity shouldn't necessarily mean a problem either. However, the charger does not seem to fit these batteries. Unfortunately, this happens occasionally. B. because the voltage level is not exactly what the charger expects.

The basic problem is that a charger hardly has the chance to reliably assess the charge status of different types of batteries. Basically, I would therefore not really trust such a display of the charge status - especially if the charger is not optimized for a particular make of battery.


A NiCd AA 1.2 V with 800 mAh was installed in an older hair clipper. I have now exchanged it for a NiMH 1.2 V with 2200 mAh; this is now being sold as a replacement because NiCd batteries are apparently no longer available. Can I continue to use the charger now? How can you tell whether a charger can be used for NiCd or NiMH or both?

Charger data: 6.5 W output, 1.5 V, 1.2 A, 1.8 VA

Answer from the author:

The sale of NiCd batteries is generally prohibited today in order to avoid environmental pollution from the cadmium they contain (if disposed of incorrectly).

The hair clipper will most likely work perfectly with the NiMH battery (since the voltage is quite similar) and even allow a much longer operating time due to the higher capacity. However, it may be that the old charger is not well suited for the NiMH battery, in extreme cases it may even be destroyed by overcharging; the voltage curve during charging is a little different so that the end of charging may not be recognized in time. I would try anyway, but occasionally check whether the battery is getting unusually warm while charging and then stop charging prematurely.


I got rechargeable batteries (AA, 1.2 V, 2300 mAh) and want to charge them with a charger, which only charges when all four batteries are plugged in. There is a regulator that you can slide back and forth between 2.8 V and 5.6 V. How many volts should I set the charger to?

Answer from the author:

If it is the usual NiMH or NiCd batteries, the correct end-of-charge voltage to which you should set the charger would be 4 · 1.45 V = 5.8 V. So it would even be a little more than your device gives.

It is unusual for such an adjustable device to be used as it is then up to the user to set the correct voltage - and this could easily go wrong. Many batteries will be destroyed quickly if the charge is too high.


My e-bike contains a lithium-ion battery that has already lost a lot of its capacity. I would like to connect it to a new battery in order to achieve a long range overall. Can I just connect the two in parallel?

Answer from the author:

That's an interesting question, and I've created a new section on parallel and series connection in the article to answer it. Probably some who use an e-bike or pedelec have already thought about it.

Basically, parallel connection is the only option for this application, and some very important points must be observed. The problem in your case is that you will not easily find a second battery of exactly the same type and that the aging of the original battery will result in significantly different properties. It is particularly important that the minimum and maximum permissible voltage on the battery match as closely as possible. These values ​​are often difficult to find out, especially for an old battery, and no documents are available. Perhaps one would have to measure these voltages - once immediately after full charge and once after full discharge.

Charging would probably be best not done individually, but for the batteries that are permanently connected in parallel - and this with a charging current that would not be too high for a single battery. It could work with the original charger, of course with a correspondingly longer charging time.

It would also have to be checked whether the electronics of the e-bike can handle the combination of two batteries. I would guess that's why it works.

In principle, this combination could be feasible, but in any case not as easy as one might expect.

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