How long to charge rechargeable batteries and other rules for using Ni-MH rechargeable batteries

Remember that Ni-MH batteries particularly dislike:

• Extremely low discharge - even multiple discharges of batteries below 1.0V and their subsequent storage in that state leads to their rapid and permanent destruction - you should monitor the charge level of your batteries and avoid storing them completely discharged.
• Very slow discharge - using batteries for a long period (many months), e.g., in remote controls for RTV equipment, leads to an increase in their internal resistance – as resistance increases, the efficiency of energy delivery by the battery decreases, which consequently leads to its premature wear. Pay attention to the product lines dedicated to such devices, e.g., Eneloop for DECT devices, or everActive Infinity Line

What has a particularly negative impact on Ni-MH batteries?

• Using and storing at extremely high (above 60°C) and low (below -20°C) temperatures. This leads to their much faster wear.

Even if the described technology related to the use of nickel-metal hydride batteries is not completely foreign to us, there is a good chance that we have not avoided some basic mistakes that led to the faster wear of our cells. We provide examples of stereotypes and check popular theses that have significantly lost their value:

• Does charging Ni-MH batteries more slowly benefit their lifespan?
  This is currently largely a false statement, as most modern chargers for Ni-MH batteries actually require a suitably high charging current to precisely determine the moment of full charge. The minimum value of optimal charging current for popular AA and the most capacious AAA batteries is about 400 mA. Currents of 200 mA in the case of automatic, microprocessor-controlled chargers are optimal only for low-capacity batteries.
• Does complete discharge of the battery before charging ensure that batteries maintain their full efficiency for longer?
  Absolutely not - this advice was very valuable several decades ago when Ni-Cd (nickel-cadmium) batteries, which had a very noticeable memory effect, were still commonly used. Such actions with Ni-MH batteries will lead to a significant reduction in the lifespan of these cells. We will discuss this topic in more detail in a separate article.

So how long should we charge these batteries?

If we do not have a processor-controlled battery charger that selects the optimal charging time and voltage for us, we must calculate the necessary charging time ourselves.
Unlike car batteries, which are charged at a constant voltage, Ni-Cd and Ni-MH batteries are charged at a constant current (constant intensity). The simplest method of charging is simply connecting the charged cell to a power source with a current intensity of C/10 for about 14-16 hours. C here represents the capacity of the battery (measured in milliampere-hours - mAh) - let’s recall that a battery with a capacity of 1000mAh (or one Ah - ampere-hour) is one that can deliver a current of 1A for one hour (or a current of 2A for half an hour, 500mA for two hours, etc.) Therefore, a completely discharged battery with a capacity of 2000mAh should, according to this method, be charged for 15 hours with a current of 200mA.

The described method is quite simple and safe - the charging current is low enough that it does not pose drastic side effects in case of keeping the battery in the charger for too long (the battery does not overheat excessively). The simplicity of the method - as well as the scheme allowing to build a charger using it - has made it so that this is how most timer-controlled chargers operate. They simply apply a constant current for a specified duration, after which they disconnect the current (or switch to a trickle charge mode, with a very low current, aimed at preventing the battery from self-discharging). Since the charging time and current are usually set "rigidly" here, this method means that in timer-controlled chargers we should charge batteries of capacities for which those chargers were designed. Unfortunately, this method has several disadvantages - it is inefficient (assumes significant energy losses during charging), and since we have no control over how much energy has actually been delivered to the battery, we do not know if the battery has been significantly overcharged, etc.

When to disconnect the current?

The method described above worked well in times when all batteries had more or less the same small capacity. Currently, with the development of all kinds of portable equipment, increasingly capacious batteries have also been created (e.g., Panasonic 2500 mAh) - which, when charged in the traditional way, simply would not utilize their full capacity. To address this problem (and, as we will see, to speed up the charging process without negative effects on the lifespan of the batteries), processor chargers have been invented.

The basic question that the charger must "answer" is - "when is the battery already fully charged?" Unfortunately, obtaining an answer is not at all simple. As we have seen, we can apply the "eyeball" method here - the battery has more or less that capacity, if we charge it for more or less that time, it should not be overcharged too much. Let’s consider other possibilities - first, let’s look at the voltage changes of a single charged cell during the charging process.

The voltage on the charged battery does not depend linearly on its charge level (by the way - this causes serious problems when, for example, we want to find out how much our battery is discharged - the voltage for 10% capacity is almost identical to that for 60%). Fortunately, however, at the end of charging, the voltage begins to rise quite sharply (this increase is, however, much smaller for Ni-MH batteries compared to older Ni-Cd) - to gently taper off at the moment when the battery is fully charged. Therefore, to determine when to stop charging the battery (or better - when to switch to trickle charging) "it is enough" to monitor the voltage on the charged cell - and disconnect the current when it starts to drop. The solution seems ideal! Unfortunately, there is one problem - the voltage drop is not very large (it is proportional to the value of the charging current) - it is usually about 10mV for Ni-Cd cells, and about 2-3mV for Ni-MH cells. Measuring such a small voltage difference (additionally of uncertain size - taking into account different battery designs, their usage histories, external disturbances) is not an easy task - for this, the "processor" in processor chargers is precisely needed!

Of course, it is hard to trust such an uncertain measurement as the only source of knowledge about when to end charging. Therefore, charger manufacturers have also looked at the temperature characteristics of the cell during the charging process.
As charging progresses, the temperature of the charged cell also rises - and additionally, the plotted characteristic is already somewhat more linear than that we dealt with in the case of voltage. This characteristic is also used by better processor chargers to determine the moment to end charging - the current can be disconnected based on the increase in temperature above a certain threshold, as well as when the rate of temperature increase (measured in degrees Celsius per unit of time) exceeds a certain limit. One should never forget about the old good timer mechanism "just in case" - disconnecting the charging current if it drags on too long.

How to charge batteries faster?

The answer to this question is simple - charge with a higher current! Unfortunately, we will quickly find out that even at currents of C/2 and higher, our cells heat up very quickly to high temperatures - which in extreme cases may even threaten the destruction of the battery or its leakage. Again, processors placed in chargers come to our aid - we can entrust them with the control of the charging current so that it lasts faster - without damaging our batteries. Below we present several charging scenarios in different chargers. Before each charging, we discharged the battery - this guarantees that the charging always takes place under the same conditions - from completely discharged cells. During normal operation, there is no need to discharge the Ni-MH battery each time as it shortens its lifespan (discharge is equivalent to normal use of the battery).
The patient was the everActive Silver Line AA battery with a minimum capacity of 1900 mAh.

• charging with a current of 1C

Indicates full charging of the battery in just one hour, however, the cell heats up very quickly under such conditions - this phase of charging ends immediately when the rate of temperature increase of the cell exceeds 1 degree Celsius per minute, or when a drop in voltage on the battery is detected in the final phase of charging

• charging with a current of C/5

Ends when a drop in voltage on the battery is noted (additional criteria may also include temperature increase as well as charging time)

• charging with a current of C/10 or lower

Under such charging conditions, the voltage drop in the final phase of charging is completely invisible (the voltage slowly increases all the time), which is why most chargers will have trouble correctly automatically detecting the full charge of the battery - under such conditions, it is necessary to primarily monitor the charging time. This is the least efficient and simplest method of charging used in most of the cheapest chargers, often charging only ends when the battery is removed from the charger. This method also carries the greatest risk of regular overcharging of the battery, which can result in its faster wear

• trickle charging with a current of C/100 - ending only when the battery is removed from the charger - aims to keep it in a state of permanent full charge (it should be remembered that, for example, batteries left "on the shelf" can lose up to 3% of their charge daily (except for new generation batteries with low self-discharge, e.g., Eneloop, which lose capacity very slowly when not used) - this means they can completely discharge within a month!)

We recommend chargers:

everActive UC-4200 charger for cylindrical batteries Li-ion, Li-FePO4, Li-HV, Ni-MH, and 9V

• professional, universal microprocessor charger for Li-ion, Li-FePO4, Li-HV, Ni-MH cells and 6F22/9V,
• discharge, refreshing and testing function for cells,
• large and clear display with backlight,
• supported sizes: R6 AA, R03 AAA, R14 C, SC, R20 D, 10440, 14500, 14650, 17500, 17670, 18350, 18500, 18650, 20700, 21700 - also protected, 22650, 25500, 26500, 26650, 32650, 33600, 16340 R-CR123e, 6F22 / 9V
• charging current: from 200mA up to 2000mA for Li-ion / Li-FePO4, from 200mA to 1000mA for Ni-MH, 60/120mA for 9V,
• very precise control of charging regardless of the type of battery.
• 0V cell reactivation

40,00 € gross 40,00 € net

High stock

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Ni-MH rechargeable battery charger everActive NC-3000

• the most advanced Ni-MH battery charger from everActive
• processor-controlled with discharge and capacity measurement and unique internal resistance test of the cell,
  refreshing, maintenance, and conditioning functions for batteries,
• min. charging time for 4 batteries 2500mAh - 3h.
• supported batteries: Ni-MH, Ni-Cd, 1-4x R6/AA, R03/AAA, 1-2x R14/C, R20/D using an optional adapter
• input voltage 12V DC - includes AC power adapter, optional car adapter available

18,90 € gross 18,90 € net

High stock

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everActive UC-4000 charger for Li-ion and Ni-MH cylindrical batteries

• professional, universal processor charger for Li-ion, Li-FePO4, Ni-MH cells,
• function of discharging and refreshing cells,
• unique function of measuring and reviewing two capacity values for each battery - Capacity Review,
• supported sizes: R6 AA, R03 AAA, R14 C, R20 D, 10440, 14500, 14650, 17500, 17670, 18350, 18500, 18650, 20700, 21700 - unsecured only, 22650, 25500, 26500, 26650, 32650, 33600, 16340 R-CR123e,
• charging current: 500 mA, 1000 mA for Li-ion / Li-FePO4, 500 mA for Ni-MH,
• Very accurate charging control regardless of the type of battery.

28,60 € gross 28,60 € net

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Author: Michał Serediński
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