How to properly use nickel-metal hydride batteries?

Problem 1: Using rechargeable batteries immediately after removing them from the packaging

We definitely recommend charging Ni-MH batteries fully before first use.

Although most batteries are described as ready to use. We must be aware that no battery will be factory charged to 100%. The highest capacity batteries usually do not exceed 50% of factory charge, and even branded batteries with low capacity offer about 70-80% of initial charge.

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

• the device operates much shorter,
• we risk too deep discharging the weakest cells from the set,
• the weakest of the batteries, although originally technically sound, may never achieve full nominal parameters again.

Problem 2: Using the lowest charging currents

If we have the option, avoid chargers with low charging current, where the result is 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 completely wrong, but here the key role is played by the charger used. With a low current of 100-300 mA, we can almost safely charge all AA/AAA batteries, provided we also 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 characteristic voltage curve of Ni-MH batteries, it is impossible to precisely determine the moment of full charging of the battery automatically. In such situations, even the best charger may have problems. With too low a current in an automatic charger (which will be a value of 200-300 mA for most currently produced batteries), there is regularly incomplete charging or significant overcharging of the batteries.

Problem 3: Additional discharging of the battery in the charger before each subsequent charging

This is absolutely a bad habit that we should avoid in the case of Ni-MH batteries.

We have described this issue quite extensively before - link. The most important argument should be the fact (confirmed based on official tests of Eneloop batteries) that a battery that is completely discharged each time shows about 4 times lower lifespan than a battery discharged to about 50%. This means that a battery that is not completely discharged each time can deliver even 100% more energy during its lifetime. The lifespan of batteries drops by another 50% if we regularly allow them to discharge below the safe voltage of 1.0V.

In the case of Ni-MH batteries, it is not worth being afraid of the memory effect or lazy battery effect. Even if such an effect occurs, it will be less problematic than the consequences of synthetic, excessive wear of batteries due to their additional discharging in the charger.

Problem 4: Incorrect (too low) capacity measurements of a new battery. Measurement and interpretation of battery capacity in charging mode

A too low result returned by the charger does not always indicate a defect in the battery.

Ni-MH batteries require at least a few charging cycles to properly form. Do not expect full performance from the battery after just 1-3 charges. The capacity during the first several charging cycles on a good quality cell gradually increases. Additionally, such a battery shapes its target voltage characteristic with each subsequent charging cycle. For example, it is typical to achieve a lower maximum voltage (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. A Ni-MH battery can take from about 100 mAh to even multiples of its nominal capacity during charging attempts, depending on the situation - this does not have to correlate with its actual capacity.

Problem 5: Problems with charging new batteries or those that have not been charged for a long time. Overcharging, overheating, undercharging of batteries

In the case of the first charging cycles, some batteries require special attention.

New batteries, like those that have not been placed in the charger for a long time, can be unpredictable during the first charging attempts.

Even if we choose the charging current appropriately, 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, we should try to supervise the charging process - controlling its time and the temperature of the cells. If the batteries become very warm (where it is difficult to keep a finger on the cell), we should stop the charging process.

Problem 6: Choosing the highest capacity batteries for all devices

A 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 power for our devices. A higher mAh capacity means a longer operating time - but is that really the case?

Batteries with the highest capacities like 2500 mAh for AA or 1000 mAh for AAA are heavily stressed constructions and at the same time very technologically advanced. They have a high capacity that is very close to the maximum technological limit for Ni-MH technology. Usually, such batteries also have low resistance and high current efficiency.

Problem 7: Using rechargeable batteries as direct replacements for disposable batteries in all devices

In many devices, using rechargeable batteries does not bring any benefits, either economic or ecological.

Proponents of using rechargeable batteries instead of disposable batteries often cite longer operating time of rechargeable batteries and ecological reasons as the main argument. However, both arguments do not always have confirmation in practice. Rechargeable batteries do not always operate optimally and longer in a given device. The carbon footprint of producing Ni-MH batteries is several times higher than that of a typical alkaline battery. It is completely pointless to install batteries (especially the highest capacity ones) in devices that do not require battery replacement for a year or longer.

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

Problem 8: The charger requires charging batteries in pairs

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

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

Additionally, such chargers often have very low charging currents, making it even more difficult to assess their full charging (according to point 2).

Problem 9: Using different batteries in one device

Avoid this if we care about the lifespan of the batteries.

Installing different batteries or uneven ones with different levels of charge in a set usually results in too deep discharging 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 damaged in such situations - as we described in point 1.