Charging NiMH cells and batteries in the right manner is key to maintaining their performance. Knowing how to charge them correctly will ensure a much level of performance and a longer life.
Charging NiMH cells is a little more difficult than their NiCd predecessors because the voltage peak and subsequent fall that was used to detect full charge is very much smaller on NiMH batteries and cells.
Overcharging leads to overheating and damage to the cell resulting in loss of capacity and nickel metal hydride cells are more sensitive to this than NiCds. This means that chargers need to be careful designed to ensure that overcharging does not occur, and users also need to be a little more careful as well.
NiMH charge / discharge characteristics
In operation the NiMH cell has many similar characteristics to the more familiar NiCd. It follows a very similar discharge curve to that of the NiCad allowing for the extra charge it can take. However it is very intolerant of overcharging, suffering a reduced capacity if this occurs. This presents a significant challenge to battery charger designers.
Many intelligent chargers for NiCds sense a small but distinct "bump" in the output voltage when a NiCad is fully charged. However for NiMH cells this increase is very much smaller, making it more difficult to detect. As a result the temperature of the cells is also detected as well, because once fully charged the cell dissipates much of the additional charge as heat. A further complication is that the characteristics of NiMH cells vary significantly from one manufacturer to the next making charge performance more difficult to detect.
It is interesting to note that the charging efficiency of nickel-based batteries of all forms is close to 100% up to about 70% of the full charge. This means that initially there is little temperature rise, but later as the charge level rises, the efficiency drops and heat is generated, raining the temperature of the cell.
NiMH charge methods
There are a number of charging methods that can be used with nickel metal hydride batteries. Unfortunately charging NiMH nickel metal hydride batteries is not as easy to achieve compared to other types of cell or battery.
The NiMH cells, like NiCds require charging using a constant current. The rate of charge is normally specified on the case of the cell and this rate should not be exceeded.
Within the boundary of constant current charging, there are several techniques that can be adopted to prevent overcharge.
- Timer charging: Using time to determine the end of charge is the easiest method to use. Often an electronic timer can be built into the charger, although many basic chargers do not have this facility built in. This approach assumes that the battery or cell is being charged from a known state of charge, e.g. completely discharged.
One of the issues with this method is that if the cell has lost its charge capacity, then a charger expecting to deliver 100% charge according to the time estimated will deliver an overcharge, adding further to the degradation of the cell.
- Thermal detection: Detection of the end of charge of an NiMH cell by detecting the cell temperature also poses difficulties. While the cell will often feel warm when it is being overcharged, sometimes the temperature rise can be difficult to assess accurately as the centre of the cell will be much hotter than the exterior. Also if the cell is being slow-charged, then the temperature rise will be less.
- Negative delta voltage detection: The favoured method for detecting end of charge of a NiCd cell is to use the NDV - negative delta voltage method. This method detects a drop in voltage that appears as the cell becomes full charged. However when charging an NiMH cell it is found that only a small drop in voltage is seen. An NiMH charger must be able to detect a voltage drop of around 5mV per cell. Therefore to reliably detect such a small voltage drop, sufficient noise filtering must be introduced into the NiMH charger to ensure that spurious pickup and other noise does not trigger the end of charge.
- Slow charging NiMH batteries: NiMH battery slow charging is not to be recommended. Using charge rates of between 0.1 and 0.25C do not provide the indicators needed to detect the end of charge. With voltage changes of only about 5mV at full charge rates, the smaller changes arising for a slow charge are virtually impossible to detect. Additionally the temperature ises indicating end of charge are also much lower and not easily detected. Accordingly slow charging NiMH batteries and cells is not to be recommended.
Many modern NiMH chargers combine the three main methods of end of charge detection, NDV, temperature rise detection and then using a timer as a last ditch end of charge termination in case the other effects are masked or pass undetected.
In addition to this, many chargers include a 30-minute topping charge of 0.1C to add a few percentage points of extra charge.
Some advanced NiMH chargers apply an initial fast charge of 1C. After a certain time, or when the cell voltage reaches a certain point, a cool down period is placed into the charge cycle. The charge cycle then proceeds at a lower current. The NiMH charger then applies further current reductions as the charge progresses. This scheme continues until the battery is fully charged. This method of charging is known as the “step-differential charge” method. It is worth noting that it works well for all nickel-based cells and batteries.
However many NiMH cells are charged using simple chargers and as a result, it is very easy for them to be subjected to overcharging. As a result of their intolerance to being overcharged, this can mean that their life is reduced. Many people have found that the average life if NiMH batteries is less than expected. Often this can be attributed to the difficulties with them being overcharged.
NiMH trickle charging
As NiMH batteries are intolerant to overcharge, trickle charging tem has to be undertaken with care.
While NiCd batteries may be trickle charged at a rate of around 0.1C, this is considered too high for NiMH batteries and it is generally accepted that for most NiMH batteries trickle charging should be accomplished at a rate of about 0.05C.
Even when trickle charging at this rate, it is advisable not to leave them trickle charging for too long. It is better not to trickle charge and to replenish any self-discharge before use.
How to charge NiMH batteries: guidelines
In view of the fact that NiMH battery charging must be undertaken in the correct manner, a few guidelines are often helpful.
- Never charge an NiMH cell with incorrect charger: It is never acceptable to charge a battery of any form with a charger that may be unsuitable. NiMH cells cannot be charged with a NiCd charger as end of charge detection will not work.
- Charge at room temperature: NiMH cells do not like being charged at low or high temperatures.
- Check cell temperature : If an NiMH cell becomes hot then the charging should be terminated. Consumer chargers do not always terminate the charge correctly. Remove the batteries when warm to the touch. Stop using a charger that “cooks” batteries.
- Manually check on charge status: With many lower end NiMH battery chargers there is a real possibility of overcharge and there may be no back up timed end of charge. It is therefore wise to manually check whether the battery is likely to be charged and charging should be terminated.
- Check on fast charge capability: Although many NiMH cells can be fast charged it is not appropriate for all. Check on the datasheet before applying a fast charge./li>
- Fast charging: If the NiMH cells can be fast charged, then they are best charged in this mode. It is easier to detect the end of charge point when charged in this way. Up to around 70% charge the charging efficiency is close to 100% and the battery remains cool. It is therefore acceptable to fast charge with most NiMH cells.
- Trickle charging : Ideally do not trickle charge for long periods. Nickel-based batteries must cool down on trickle charge. If warm, trickle charge is too high.
- Use correct charger: Nickel- and lithium-based batteries require very different charge algorithms. Don’t intermix the chargers. A NiMH charger can also charge NiCd; a NiCd charger would overcharge NiMH.
As NiMH cells are more sensitive to the way they are charge when compared to other forms of rechargeable battery, care needs to be taken to adopt the correct NiMH charger and also to use it properly. In this way the cells will last longer and perform better.