How to use nickel-metal hydride batteries correctly?

Problem No. 1: Using rechargeable batteries right out of the box

We strongly recommend that Ni-MH rechargeable batteries be fully charged before first use.

It is true that most rechargeable batteries are described as Ready to Use. However, we must be aware that no battery will be 100% charged at the factory. The most capacious rechargeable batteries usually do not exceed 50% of the factory charge, and even branded rechargeable batteries with a small capacity offer about 70-80% of the initial charge.

In addition, there are uneven sets and some cells may be factory-charged to different degrees. If we use an uneven set in our device, we intensify the later negative consequences:

• the device lasts much shorter,
• we risk discharging the weakest links from the set too deeply,
• The weakest of the batteries, despite the fact that it was originally technically efficient, may never reach its full, nominal parameters again.

Problem No. 2: Using the Lowest Charge Currents

If we have the opportunity, we should avoid chargers with low charging current, which result in charging times exceeding 5-6 hours.

Most users are still convinced that the lower the charging current, the better for the battery. This thesis is not entirely wrong, but here the key role is played by the charger used. With a low current of 100-300 mA, we can charge all AA/AAA batteries with almost impunity, provided that we additionally control the charging time and charge batteries that were previously completely discharged.

If we have an automatic charger, then the charging current cannot be too low - with a low charging current, due to the characteristics of the voltage of Ni-MH batteries, it is impossible to precisely determine the moment of full charging of the battery. In these situations, even the best charger can have problems. If the current in the automatic charger is too low (and this will be the value of 200-300 mA for most currently manufactured batteries), the batteries are incompletely charged or significantly overcharged.

Problem No. 3: Additional discharge of the battery in the charger before each subsequent charge

This is an absolutely bad habit that we should avoid with Ni-MH batteries.

We have already described this issue quite extensively - link. The most important argument is the fact (confirmed on the basis of official tests of Eneloop batteries) that a battery that is completely discharged each time has a service life about 4 times lower than a battery discharged to about 50%. This means that a battery that is not completely discharged each time can give off up to 100% more energy during its lifetime. The life of rechargeable batteries decreases by another 50% if we regularly discharge them below the safe voltage of 1.0V.

In the case of Ni-MH rechargeable batteries, you should not be afraid of the memory effect or the lazy battery effect. Even if such an effect occurs, it will be less problematic than the effects of synthetic, excessive wear of batteries through their additional discharge in the charger.

Problem No. 4: Incorrect (too low) measurements of the capacity of the new battery. Measuring and interpreting the battery capacity in charging mode

Not always too low a result returned by the charger will indicate a defect in the battery.

Ni-MH batteries require at least a few charging cycles to form properly. We should not expect the full performance of the battery after just 1-3 charges. The capacity during the first dozen or so charging cycles on a good quality cell is constantly slightly increasing. In addition, such a battery shapes its target voltage characteristics with subsequent charging cycles. For example, it is typical to lower the maximum voltage achieved (even by 0.05V) for completely new, unformed batteries.

Another mistake is estimating the capacity of a new cell based on the mAh value used to charge such a battery. Depending on the situation, a Ni-MH battery can take from about 100 mAh to even a multiple of its nominal capacity each time it tries to charge - this does not have to have anything to do with its actual capacity.

Problem No. 5: Problems with charging new batteries or those that have not been recharged for a long time. Overcharging, overheating, undercharging of rechargeable batteries

For the first charging cycles, some rechargeable batteries require special attention.

New batteries, as well as those that we have not put in the charger for a long time, can be chimerical during the first attempts to charge them.

Even if we choose the charging current properly, it happens that either the charger turns off charging prematurely or leads to overcharging/significant heating of such batteries. In the case of completely new batteries, try to supervise the charging process - controlling its time and cell temperature. When the batteries become very warm (where it is difficult for us to keep our finger on the cell), stop the charging process.

Problem No. 6: Choosing the most capacious rechargeable batteries for all devices

The battery with the highest capacity will not always be suitable for our application.

The capacity value expressed in mAh is often the main criterion for choosing a power supply for our devices. After all, a higher mAh capacity means longer operating time - but is it really?

Rechargeable batteries with the highest capacities, such as 2500 mAh for AA, or 1000 mAh for AAA, are highly strenuous and at the same time very technologically advanced. They have a high capacitance that is very close to the maximum technological limitation for Ni-MH technology. Usually, such batteries are also characterized by low resistance and high current efficiency.

Problem No. 7: Using rechargeable batteries as a direct replacement for disposable batteries in all devices

In many devices, the use of rechargeable batteries brings virtually no benefits, neither economic nor ecological.

Supporters of the use of rechargeable batteries instead of disposable batteries very often cite longer battery life and environmental considerations as the main argument. However, both arguments are not always confirmed in practice. Rechargeable batteries will not always work optimally and longer in a given device. The carbon footprint of a Ni-MH battery is several times higher than that of a typical alkaline battery. It is completely pointless to install rechargeable batteries (especially the most capacious ones) in devices that do not require battery replacement for a year or more.

After 2-3 years of operation (regardless of its nature), almost all the most capacious Ni-MH batteries turn out to be very unreliable and unpredictable.

Problem No. 8: Your charger needs to charge the batteries in pairs

This is a strong simplification and, in my opinion, a design error of some chargers - if we still use such a charger, we should consider replacing it with one with a sufficiently high charging current and independent charging channels.

There are still charger models available on the market that require the batteries to be installed in pairs. Such chargers should be avoided, because we usually do not have equal batteries, with exactly the same degree of charge/discharge and installing an unequal pair in such a charger will always result in suboptimal charging. In the long run, this will mean a significant reduction in the service life of our cells.

In addition, such chargers often have very low charging currents, which makes it even more difficult to assess their full charge (according to point 2).

Problem No. 9: Using different rechargeable batteries in one device

Avoid this if you care about the life of the batteries.

Installing different batteries or uneven ones with different degrees of charge usually results in too deep discharge of the weakest cell and significantly shortened operating time of the entire device. As a result, this extremely discharged battery will be unnecessarily and permanently destroyed in such situations - similarly as we described it in point 1.