Charging NiMH and NiCd Batteries

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The charging characteristics of these types of battery are very similar; the recommended method is constant current charging. This is where the charger provides a fixed charging current to the battery. The variant in these types of charger is the rate in which they provide this charging current and for nickel-based generally slow (trickle), quick (rapid) and fast current chargers. Within these different types of charger you will find varying levels of quality generally related to price. For detailed information about the different types of charger see our Choosing the right charger section.

Nickel-based batteries initially have a huge number of tiny crystals (approx 1um) on the surface of the positive plate (see Battery Science for more info) of the battery. Over time, these crystals get replaced by much larger crystals (approx 50 to 100um) presenting a much smaller surface area to the electrolyte and hence reducing battery performance. It is generally considered that fast rate charges reduce the rate of this crystalline growth and are recommended over slow rate charging for nickel based batteries.

A General Formulae

If you cannot afford to buy an expensive ‘intelligent’ charger, or deem it unnecessary for your application, then a common question is how long to leave your battery on charge. A general formula has been produced for you to calculate an estimate for charge time of your battery:

Suitable charge time (hours) = C rating of battery (mA) * 1.4
                                                           Output current of charger (mA)

Where C is a symbol commonly used to designate a charge or discharge rate equivalent to the rated capacity of the battery e.g. charging a battery nominally rated with a capacity of 4Ah at the C rate would be charging at 4A. This does NOT mean that the battery will fully recharge in one hour, as the battery will not have a charging efficiency of 100%. For instance, some of the charging energy is dissipated as heat. Charging efficiency will depend on the cell type, charging rate and temperature, but would typically be between 50 and 75%.
Sub multiples and multiples of "C" are commonly used, such as C/2 or 2C, which for a 4Ah cell would be 2A and 8A respectively.

The above formula is a general rule and should not be taken as the definite value to go by, it also assumes that the battery you are charging is in a fully discharged state when you put it on charge.

The figure of 1.4 in the numerator (top of the equation) comes from the fact that when using a standard constant current charger, you need to put up to 140% of energy into your battery when charging to get 100% out.

When using fast charge techniques, the figure should be reduced by up to 15% i.e. multiply the ‘C rating of battery’ by 1.25 instead of 1.4.

General Charging tips

    * Try to avoid overcharging; this is the main and most common cause of reduction in life in NiMH and NiCd batteries.
    * Nickel-based batteries prefer fast-charging but be you must take care when using this method because overcharging at this rate will very quickly damage the cells.
    * Avoid high temperature during charging. Discontinue the use of chargers that cook batteries.
    * A charger for nickel-metal-hydride can also accommodate nickel-cadmium, but not the other way around. A charger designed for nickel-cadmium would overcharge the nickel-metal-hydride battery.
    * Nickel- and Lithium-based batteries require different charge algorithms. The two chemistries can normally not be interchanged in the same charger.
    * Do not leave your batteries in the charger when not charging.
    * Don’t leave rechargeable batteries unused for too long. If you do have to leave them stored for an extended period, it is recommended that you leave them at approximately 40% charge. Then charge them up again before use.
    * Always charge your batteries before use either when first purchased or if they have been unused for any length of time. (NiMH or NiCd can loose up to 30% of their capacity per month).
    * Always use the correct type of charger. Some chargers are specifically designed for certain types of battery (i.e. NiMH or NiCd). Always check you are using the right charger for your batteries.
    * Try to keep your battery and charger contacts clean. This will ensure good connections between the devices and optimise operation.
    * It is generally considered that fully discharging your battery before charging it up is recommended to prolong performance. Repetitive partial charging and discharging does not really affect your batteries performance due to the so-called ‘Memory Effect’.


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