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

Remember that Ni-MH batteries do not particularly like:

• Extremely low discharge - even several times discharging batteries below the level of 1.0V and their subsequent storage in such a state leads to their quick and permanent destruction - you should control the level of charge of your batteries and avoid storing them completely discharged.
• Very slow discharge - using batteries for a long period of time (many months), e.g. in remote controls for audio/video equipment, leads to an increase in their internal resistance - as the resistance increases, the efficiency of energy release by the battery decreases, which in turn leads to its premature wear. Take a look at the product line dedicated to this type of device, such as Eneloop for DECT devices, or everActive Infinity Line

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

• Use and storage at extremely high (above 60 degrees C) and low (below -20 degrees C) temperatures. This leads to much faster wear.

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

• Is the slower we charge Ni-MH batteries, the better for their life?
  This is currently a largely wrong statement, because most modern chargers for Ni-MH batteries even require a sufficiently high charging current to precisely determine the moment of full charge. The minimum value of the optimal charging current for popular AA sticks and the most capacious AAA is approx. 400 mA. Currents of 200 mA in the case of automatic chargers, controlled by a microprocessor, are optimal only for low-capacity batteries <800 mAh.
• Does each time the battery is completely discharged before charging, does the battery retain its full capacity for longer?
  Absolutely not - this is advice that was very valuable several years ago, when Ni-Cd (nickel-cadmium) batteries with a very visible memory effect were still widely used. This type of operation with a Ni-MH battery will lead to a significant reduction in the life of these cells. We will discuss this topic in more detail in a separate article.

So how long will it take to finally charge these batteries?

If we do not have a processor battery charger that will select the optimal charging time and current 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 current). The simplest method of charging is simply to connect the cell to a C/10 current source for about 14-16 hours. C stands for the capacity of the battery (calculated in milliampere-hours - mAh) - let us remind you that a battery with a capacity of 1000mAh (i.e. one Ah - ampere-hours) is one that is able to provide current of 1A for an hour (or current of 2A for half an hour, 500mA for two hours, etc.). Therefore, a completely discharged battery with a capacity of 2000mAh would, according to this method, be15 hours with 200mA current.

The described method is quite simple and safe - the charging current is so low that it does not threaten drastic side effects if the battery is kept in the charger for too long (the battery does not overheat excessively). The simplicity of the method - as well as the scheme for building a charger using it - has made this how most timer-controlled chargers work. They simply apply a constant current for a certain period of time, after which they disconnect the current (or switch to trickle charging mode, with a very small current, to prevent the battery from self-discharge). Since the charging time and current are usually set "hard", this method means that in timer-controlled chargers we should charge batteries with the capacities to which these chargers have been adapted. Unfortunately, this method has a number of drawbacks - it is not very effective (it assumes significant energy losses during charging), and because we do not have any control over how much energy has actually been supplied to the battery, we do not know whether the battery has not been significantly overcharged, etc.

When to disconnect the electricity?

The method described above worked well at a time when all rechargeable batteries had more or less the same, small capacity. Nowadays, with the development of all kinds of portable equipment, more and more capacious rechargeable batteries (e.g. Panasonic 2500 mAh) have also been created - which, charged in a traditional way, would simply not use all their capacity. To remedy this problem (and, as we will see, speed up the charging process without negative effects on the life of the batteries), processor chargers were invented.

The basic question that the charger must "answer" is - "when is the battery fully charged?" Unfortunately, getting an answer is not easy at all. As we have seen, the "by eye" method can be used here - the battery has more or less this capacity, if we charge it for about that amount of time, it should not overcharge too much. Let's consider other possibilities – to start, let's look at the voltage changes of a single cell being charged during the charging process.

The voltage on the charged battery does not depend linearly on the level of its charge (by the way - it causes serious problems when, for example, we want to find out how much our battery is discharged - the voltage for 10% of the capacity is almost identical to that for 60%). Fortunately, however, at the end of charging, the voltage begins to increase quite rapidly (this increase, however, is much smaller in the case of Ni-MH batteries compared to older Ni-Cd batteries) - only to gently decrease when the battery is fully charged. Therefore, in order 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 cell being charged - and disconnect the current when it starts to drop. The solution seems perfect! Unfortunately, there is one problem - the voltage drop is not too high (it depends proportionally on the value of the charging current) - it is usually about 10mV for a Ni-Cd cell, and about 2-3mV for a Ni-MH cell. Measuring such a small voltage difference (additionally of an uncertain size - let's take into account the different designs of batteries, their operating history, external interference) is not an easy task - this is what you need this "processor" in processor chargers for!

Of course, it is difficult to believe such an uncertain measurement as the only source of knowledge about when to stop charging. Therefore, charger manufacturers also looked at the temperature characteristics of the cell during the charging process.
As the charging progresses, the temperature of the charged cell also increases - and in addition, the plotted characteristic is slightly more linear than the one we dealt with in the case of voltage. This is also the characteristic that better processor chargers are used to determine the moment of charging completion - the current can be cut off both on the basis of an increase in temperature above a certain threshold and when the temperature exceeds a certain thresholdtemperature (the speed at which it rises, measured in degrees Celsius per unit time) of a certain limit. You should also never forget about the good old clock mechanism "just in case" - disconnecting the charging current if it drags on too much.

How to charge batteries faster?

The answer to this question is simple - charge with more 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 to destroy the battery/leak it. Again, processors placed in chargers come to our aid - we can entrust them with controlling the charging current so that it lasts faster - without destroying our batteries. Below are some scenarios for charging runs on different chargers. Before each charge, we discharged the battery - this guarantees that charging always took 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 this shortens its life (after all, discharging is equivalent to normal use of the battery).
The patient was an everActive Silver Line AA rechargeable battery with a minimum capacity of 1900 mAh.

• 1C current charging
  
  It means that the battery is fully charged in just one hour, but the cell in such conditions heats up very quickly - this phase of charging ends immediately when the speed of the cell's temperature increase exceeds 1 degree Celsius/minute, or when a voltage drop is detected on the battery in the final phase of charging


• C/5 charging
  
  It ends when the voltage on the battery is recorded (additional criteria may also be an increase in temperature as well as charging time)


• charging with current C/10 and lower
  
  In such charging conditions, the voltage drop in the final phase of charging is completely invisible (the voltage is slowly increasing all the time), so most chargers will have a problem with correctly automatically detecting the full charge of the battery - in such conditions, the charging time should be controlled above all. This is the least efficient and simplest method of charging used in most of the cheapest chargers, often the charging ends only 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 faster wear


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

We recommend chargers:
Read also:
What are the differences between rechargeable batteries and alkaline batteries?
Batteries or rechargeable batteries? The choice depends on the device!

Author: Michał Seredziński
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