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How to charge your NiCd and NiMH batteries

Batteries are ready to work instantly (provided they are charged of course) and can be used at a relatively wide temperature range. By contrast battery charging has many limitations and as a consumer it’s a good idea to follow the guidelines on how and when to charge. The different battery types have varying chemical compositions, and should therefore be charged differently. In the following we will give you some advice on the do’s and don’ts of battery charging, so you can get the best out of your batteries and maximize battery longevity.


Top three advice on battery charging:

1. Invest in a decent charger

To a large extent the reliability and longevity of a battery depends on the quality of the charger. A decent charger must charge the battery gently and effectively without causing overheating or overcharging. Battery chargers are often given low priority, especially for consumer products. However a good charger, and correct use of this, will protect and significantly prolong the life of your batteries.


2. Avoid high temperatures during charging

Elevated temperatures during charging can lead to overheating and overcharging, and destroy your batteries, or at least significantly shorten the lifespan of the batteries. The battery will heat up a little during charge, and that is ok, for a short period of time. The battery must however cool down when ready. Most new chargers switch off when there is a rise in temperature. Discontinue using a charger that keeps the battery warm on standby.


3. Recharge your batteries fully, without interruption

Repeated, partial charging can generate heat buildup and lead to overcharging of the batteries. Overcharging happens when the batteries are fully charged and the energy supplied is no longer used for the charging process, but instead generates heat and gas inside the batteries. This will damage and reduce the life of your batteries.



More advice if you want to take good care of your batteries and maximize their performance.

Remember to discharge your batteries fully at least every 3 months to avoid “memory”
It is important that you on regular basis remember to discharge your batteries fully. When a normal NiCd or NiMH battery is only partially discharged before recharging, it will eventually “forget” that it has the additional capacity available, and eventually completely stop to discharge fully. This phenomenon is called “memory”, and significantly reduces the battery efficiency.


Take your batteries out of the charger when fully charged
Although most new chargers automatically turn of when the batteries are fully charged, it is still a good idea not to let your batteries stay too long in the charger. Trickle charge can still be harmful to some batteries if exposed to it for longer periods of time.


Charge and discharge your new batteries fully
Charge overnight (slow charge) and let the battery be fully discharged before recharging. A new battery doesn’t reach its full capacity until it has been fully discharged and recharged a few times.


Use the “Cycle function” once in a while (on both new and old batteries)
The cycle function, which is available on many modern chargers, restores the capacity of your battery by automatically discharging and recharging the batteries 2-3 times in a row. If this function is not available on your charger, you can achieve the same effect by performing the charge and discharge cycles manually.


Advantages and disadvantages of slow charging
The advantage of slow charge is that the risk of damaging the battery by overcharging and overheating is very small because the charge current is very low. The disadvantage, however, is that the charging time is relatively long. Slow charge typically takes 14-16 hours at 0.1C (see explanation of what “C” is further down the page)


Advantages and disadvantages of fast charging
The advantage of fast charge is that the charge time is very short. Fast charge takes about 3-6 hours at 0.3C and really fast charge only about 1 hour at 1.0C. The disadvantage is that you must pay extra attention to the battery, so that it won’t overheat or overcharge. It becomes even more important that the charging of the battery is discontinued when the batteries are fully charged. It requires that your battery charger is configured to recognize the battery chemistry of the individual battery and can detect when the battery gets too hot and/or when it is fully charged.


What does C stands for?
C stands for “charge rate” and is the way you measure the charge and discharge current. A charge rate of 1C means you can charge with a charging current, which is 1 x battery capacity (measured in mAh). I.e. a battery of 1000 mAh, can tolerate being charged at 1000 mAh (1000mAh x 1 = 1000 mAh). If there is a charge rate of 2C it would be 2x the battery’s capacity. So 1000 mAh x 2 = 2000 mAh etc. So to put it into other words, the higher charge rate, the higher charging or discharging current you can apply to the battery, and the faster the charge and discharge will be.

(Battery capacity is measured in mAh and the voltage in volts).


Use your batteries regularly
Batteries perform better if they are used regularly. If possible avoid letting your battery sit dormant for long periods of time. Use the cycle function (charge, discharge, recharge) to restore battery performance on batteries, which have been left unused for a long period of time.


For long term storage: Keep your batteries fully charged
If you’re not using your batteries for more than 6 months make sure that they are stored fully charged, and recharge them at least every 12 months. Store at temperatures between 10° C and 30 ° C.