I mean those Nickel-Cadmium and Nikel-Metal Hydride rechargable batteries that we use at home, not car.

## Batteries

NiMH cells are more common than NiCd nowadays. One of the example of NiMH is composed of nickel oxyhyrodxide, aqueous potassium hydroxide solution, and metal hydride. The chemical equations are:

Positive (+0.52V): $\textrm{NiO(OH)} + \textrm{H}_2\textrm{O} + \textrm{e}^- \to \textrm{Ni(OH)}_2 + \textrm{OH}^-$

Negative (-0.83V): $\textrm{MH} + \textrm{OH}^- \to \textrm{M} + \textrm{H}_2\textrm{O} + \textrm{e}^-$

The metal that used on the negative electrode varies (see Wikipedia). Although the chemistry measured at 1.35V, the charging voltage shall be 1.4-1.6V.

The rechargable cells are subject to memory effect or called voltage depression. This means the battery carries less charge after a series of regular partial discharge-partial recharge process (e.g. satellite using solar power for recharging). This result from the active chemical that did not discharged before recharging will undergo some physical change and increased the cell’s internal resistance. NiMH usually does not have this phenonmenon although it is well-known for NiCd. A deep discharge and recharge can cure this.

Rechargable batteries will self-discharge. It (NiMH) discharges around 5-10% energy on the first day and 0.5-1% per subsequent day at room temperature. Hence the shelf life of NiMH batteries is normally a few months. Some cells have longer shelf life due to new technology, and named as “ready-to-use” or “pre-charged batteries”. An example is my GP ReCyko batteries. It claims to have 80% capacity after one year in shelf at room temperature.

## Chargers

Charging rechargables is not as trivial as it seems to be. There are different ways to charge. Also, in order to conserve the battery, it needs to know when to stop charging.

To recharge batteries, we can provide a constant DC voltage to the reversed polarity. This works for Pb-H2SO4 car batteries. For NiMH and NiCd, we apply constant current.

More sophisticated ways of recharging exists. For example, we may use pulsed charging which charge for 1 sec and cut off the circuit for 20-30 msec to let the cell stablize (i.e. let the recharged active chemical diffuse away from the electrode) and then repeat. A variation of pulsed charging is negative pulse charging, which draws current (double of the charge current) out of the battery for a short time (e.g. 5 msec) instead of using open circuit for stablization. This is claimed to be able to eliminate the bubbles at electrodes.

There are several types of chargers for the batteries: slow charger (0.1C, takes 14-16 hours), quick charger (0.3C, takes 3-6 hours) and fast charger (1C, takes 1 hour). The capacity measured in ampere-hour (Ah) is called the C. If the battery has 2000mAh, then the 0.1C current used in slow charger is 200mA. Usually chargers will have only up to 80% efficiency.

The way to know when to terminate depends on the types of battery. Common safety measure is the temperature cut-off to prevent overheat during over-charging. For NiCd batteries, continuous charging will make the cell voltage drop after the peak is attained, due to the polarization and oxygen build-up. Hence the negative delta V (NDV) is a method for termination, but only works for larger charge current.

$dT/dt$ is the way for NiMH which does not have NDV. It measures the temperature change per unit time and stop charging when the temperature rises too fast. NiMH batteries can also measure the $dV/dt$, which levels off when the batteries are full.

## Memory Effect

In NiCd batteries, memory effect can occur. It means that, when a battery is discharged to $X$ and fully recharged, the battery refuse to further deliver power when it discharged to the previous level $X$ again.

This is due to crystalline formation at anode (cadmium hydroxide). A new battery shall have the crystalline of size of 1um, but when memory effect occur, the crystalline grows to 100um. This decreases the surface area for reaction. Crystalline formation can also occur by overcharging the battery without deep discharge for a prolonged time. Therefore periodic discharge is needed (i.e. exercising the battery).

A way to break the crystalline is to recondition the battery: Slowly drain the current until the cell is in 0.6-0.4V.