There is a lot on charging methods below.  For the specifics on how to look after your phone battery take a look here.

The notes below discuss general charging methods particularly of NiCd and NiMH but also Lithium for those who are interested.  Although you have no choice when charging phone batteries (the charging method is determined by the phones charging circuit)  they do explain the background.

How can NiMH or NiCd cells be charged?

 There are many methods of charging. In general they fall into one of 3 main groups:-

  • Standard charging - at 1/10th of the batteries capacity for about 15 hours.
  • Rapid Charging - at about 1/3 of the batteries capacity for about 5 hours.
  • Quick Charging (or Delta V Charging) where the battery state is measured and charging terminated when complete - about 1 hour.

A bit on terminology - Battery capacity is often abbreviated as the letter C, so you will see reference to things like 1/20 C or C/20.  When talking of a discharge rate of 1/10 C or C/10 it means one tenth of the battery rated capacity. 

For a 600 mAh hour battery this would be 600/10=60mA.  In theory a 600 mAh  battery will give 600mA for one hour, 60 mA for 10 hours, or 6mA for 100 hours.  In practice you tend to find that at high currents the rated capacity is never quite reached, at low currents it is exceeded.

Similarly for charging, a rate of 1/10 C means a charging current of one tenth of the battery stated capacity.  A trickle charge at 1/10 C is usually safe for any battery.

Standard (or trickle charging).

This involves using a current of about 50 mA (for AA cells) and leaving them on charge for 15 hours. At this current level, oxygen diffusion is more than enough to take care of the excess current once full charge is achieved. Of course, one runs the risk of voltage depression due to overcharge.

Low current charging graph

In the example shown the charging current is maintained at a constant 0.1C for 16 hours.  The rise in cell voltage can be seen on the upper line on the graph.

Note that NiCd and NiMH are always charged at a constant current, unlike lead acid batteries which are charged at constant voltage.

Fast Charging. 

A variation of trickle charging is fast charging.  Here a charging current if 0.3 to 1.0C is used.  It is essential that the battery is discharged fully before charging so such chargers often start with a discharge cycle to get the battery to a known state of charge

High Current Charging Graph

In this example a rate of 1/3 C has been maintained for 4 to 5 hours.  This charging method has a tendency to overheat batteries, especially when charging current of 1 C are used.

 Delta V Charging.

The best method of charging both NiCd and NiMH batteries is the so called delta-V (change in voltage) method.  If one plots the terminal voltage of the cell during a charge with a constant current, it will continue to rise slowly as charging progresses. At the point of full charge, the cell voltage will drop in a fairly short time. The amount of drop is small, about 10 mV/cell for NiCd (lower for NiMH), but is distinctive.  DV charging is nearly always allied with temperature measurement as a backup (and for the truly paranoid really high capacity chargers for big batteries usually incorporate fail safe timers as well).

Delta V charging Graph

Here an initial charge rate of 1 C has been used and when the fully charged state has been reached a maintenance charge of 1/30 to 1/50 C is used to maintain the battery.


.There are circuits built specifically for Delta V charging. The Maxim MAX712 and 713 ICs are ones that come to mind. This method is more expensive than others, but gives good reproducible results.

There is a danger in this though. In a battery with a bad cell this delta - V method may not work, and one may end up destroying all the cells, so one needs to be careful.  If one ends up putting in more than double the charge capacity of the cell, then something is wrong.  

Another cheap way is to measure the cell temperature. The cell temperature will rise steeply as full charge is reached. When the cell temperature rises to 10 C or so above ambient, stop charging, or go into trickle mode. Whatever method one chooses, a fail-safe timer is a requirement with high charge currents.  Don't let more than double the cell capacity of charge current flow, just in case. (i.e. for a 800 mAh cell, no more than 1600 mAh of charge). 

NiMH batteries have particular problems with charging. The delta V is very small (~2mV per cell) and more difficult to detect than with NiCd.  NiMH batteries in phones incorporate temperature sensors as a backup to delta V detection.

One particular problem with many phones is that when used in cars the amount of noise and interference on the car supply defeats or masks the delta V detection and phones are more prone to operating on temperature limiting. While this has no significant effect in occasional use it can lead to a  loss of battery life in vehicles where the phone is permanently connected (e.g. a car kit) and a lot of stop start motoring takes place. Each time the ignition is turned off for a few minutes and then turned back on a new charge sequence is initiated.

So, what's the right charge current?

Depends. If using an unregulated charger (one that doesn't do any detection of full charge) then one must restrict the charge current to the overcharge capacity of your cell. All NiCd cells I have seen can handle C/10 (approx. 50 mA for AA cell) indefinitely without venting. This is not to say that there won't be voltage depression, but rather that you won't destroy the cell(s). All phone chargers have full charge detection.

If one wants to get a bit more aggressive, a C/3 charge will recharge the cells in about 4 hours, and at this rate, most cells will handle a bit of overcharge without too much trouble. That is, if one catches the cells within an hour of full charge, things should be OK. No overcharge is best of course.  Only with automatic means of full charge detection should one use charge currents above C/2. At this current level and above, many cells can be easily damaged by overcharging. Those that have oxygen absorbers may not vent, but will still get quite hot.  With a good charge control circuit, charge currents in excess of C have been used the problem here becomes reduced charge efficiency and internal heating from ohmic losses. Unless one is in a great hurry, avoid rates greater than C.  Very high charge currents are often used by owners of model racing cars for their battery packs and by some tool manufacturers - but at the price of low battery life.

You mentioned cell reversal. What is that, and why is it so bad?

In a battery, not all cells are created equal. One will be weaker than the others. So, as the battery is discharged, the weakest cell will use up all its active material. Now, as discharge continues, the current through the dead cell becomes a charging current, except that it is reversed. So, now reduction is occurring at the positive terminal. As there is no more nickelic hydroxide, it reduces the water, and produces hydrogen. Cell pressure builds, and it vents. The cell has lost water and the life of the cell has been shortened.  This is the big danger of battery cycling to prevent memory.  Invariably, unless one is very careful, one ends up reversing a cell. It does much more harm than the cycling does good. Also, keep in mind that cells have a finite life. Each cycle is a bit of life.

What are battery manufacturers doing to prevent damage from overcharging?

Quite a lot. The demand for rapid charging has lead to a great increase in overcharging abuse. Most NiCd cells can be rapid charged. The trick is to stop charging when it is fully charged. The so called "rapid charge" type of cells just incorporate protection against overcharging at high currents. Most often, this is done with activated carbon inserted in the cell to promote the collection of oxygen and to deliver it to the cathode for recombination. By increasing the rate of oxygen transport, one is increasing the ability of the cell to resist venting. Note however, that heat is still generated.   The price one pays for this is reduced capacity. Everything takes space in the cell, and space for carbon means less space for active material. Also, there have been some indications that carbon can cause the cadmium metal to corrode, possibly leading to a shorter life.

High Temperature Batteries

There are ways to make NiCd cells more resistant to the damaging effects of heat. Mainly, using polypropylene separators and changing the electrolyte to sodium hydroxide makes the cells more durable under high temperatures. However, the cost is higher, and the internal resistance is raised, making high current discharge more difficult. Unless one knows that cells will be used at high temperatures, don't bother learn to take care of the cells to avoid overheating them.

What about those super-high capacity cells?

The manufacturers are in a numbers game. It used to be that AA cells were 450 mAh. Then came 500, then 600 mAh. Now, 700, 800 and even 900 mAh and greater cells are available.  The highest capacity cells use foamy or spongy backing material for their plates.  This allows packing more active material into the plates, but the cost is higher resistance. Recall that one of the great virtues of NiCds is their low internal resistance this allows large discharge currents for transmitting, for example.  For most consumer applications, the internal resistance isn't an issue for high power transmitting (e.g. more than 1A of current), it can be a concern and mobile phones have current pulses greater than this when transmitting.


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