Looking after the battery in a mobile phone

This page tells you how to look after your battery. For an explanation of the main battery types, see the Batteries page.
Initial charge

When your new mobile phone is delivered, the battery will not be fully charged. The instructions will almost certainly tell you to charge the battery for at least 14 hours continuously, or for even longer. You are well advised to do this for the second charge as well.

Yes, this is very frustrating, and will stop you trying your new purchase out, but please do follow the instructions. It can make a huge difference to the performance you'll get from the battery in the future.

Note that the phone will indicate that the battery is fully charged after an hour or two, but for the initial charge, you should ignore this.
Why is this necessary?

A battery is made of several cells wired in series. Although good quality batteries are made with matched cells, there will be some variation between the cells in any battery.

When you charge the battery, some of the cells will be fully charged first. The charge current has to pass through all of the cells, and you have to be sure that the trickle-charging completely fills every cell, even though some of the cells are "full" and therefore the charger control circuits are cutting back the charge current. The way to ensure this is to leave the battery on charge for a long, long time for the first couple of charges.
Establish a routine

Depending on the type of battery you have, and how you use your mobile phone, what you do to keep the battery charged and ready will vary. If you are able to establish a routine, it is much more difficult to end up with a flat battery in your phone when you need to make or take a call. You may prefer to leave the phone switched on all the time, and charge every other night, or to switch the phone off, and charge it once a week. Another method may suit you better.
Fill it up!

The way that most batteries charge, they are 50% full in just a few minutes, but it takes longer and longer to approach the 100% mark. Most chargers switch to trickle-charging at around 90% capacity (see below) but if you leave the battery on charge for longer, it will slowly fill right up. Most people find charging overnight the easiest way to do this.
Controlling the charge

Charging a battery is effectively turning electrical energy into chemical energy, which can be stored and converted back when electrical energy is needed to power the phone.

In general, the charging circuitry that regulates the charge current is built into the phone itself. It detects when the battery is fully charged, then cuts the charge current back to short pulses to keep it topped up. It does this by measuring the voltage of the battery as it is charged.

This voltage rises as the battery fills up, but the rate of rise of the voltage falls away at the point where it is full. Mathematicians recognise this as the delta of the charge voltage going negative, and this negative delta detection is what the charge control circuit uses to know the battery is full.

If the phone is unable to detect that the battery is full, the excess power (which cannot be turned into chemical energy) is turned into heat, and as a safety "back stop" the phone monitors the temperature of the battery, and stops the charging if it overheats. This could be too late, though...
Stay cool

The biggest killer of rechargeable mobile phone batteries is heat. This heat usually comes from overcharging. The charge controller may not be able to tell that the battery is fully charged, possibly because you took it off charge when full, switched it off then on again and then put it back on charge a short time later (maybe taking it out of and back into a car kit). If this happens, the "negative delta" point is already passed, and only the over-temperature limit is left. By the time this cuts in, the cells can have been damaged.
Discharge carefully

You can buy battery conditioners, which claim to be able to revitalise tired batteries by discharging and recharging the cells. they often fail to do this effectively, though. If you use a bulb and wire to discharge a battery, you will probably do more damage than good.
Why? and How?

The cells in a battery (for NiCd and NiMH each gives 1.2volts) are wired in series to give the power needed by the phone. Without being able to connect to each individual cell, a discharger can only guess at the state of the individual cells, and they generally just run the battery down till the total voltage adds up to 1v per cell. Professional dischargers connect directly to each individual cell, but few mobile phone batteries have the connections needed to do that.

If you simply use a bulb to drain the battery, you will run it completely flat. As mentioned above, batteries are made of cells wired in series, and some have more capacity than others.

As you discharge a battery, there will come a point when one cell is empty, but the others have charge remaining. If you continue beyond this point, you will start to reverse-charge the cell, which damages it, reducing its capacity.

Next time the battery is discharged, the weakest cell is even weaker, so the damage gets worse and worse, until you have no choice but to buy a new battery.

"The best way to discharge a mobile phone battery"

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