Battery Charging

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Battery Charging

Types of Batteries

OLPC is using two battery chemistries: NiMH, and LiFePo4 batteries. The LiFePo4 batteries allow charging at higher temperatures. The NiMH batteries cannot be charged above about 45 degrees C.

The LiFePO4 batteries have labels "Battery sample" on them, and are lighter than NiMH. The LiFePO4 batteries have more capacity because they can be charged to max capacity. In order to achieve 2000 cycles with a NiMH we have to reserve capacity to prevent overcharging. Overcharging significantly reduces the life of a NiMH battery.

Battery Charging Issues

Field reports from the BTest-1 and BTest-2 units have have provided us with information on several different types of battery charging issues. The 2 major symptoms are:

1. Units that will not power up when a battery is inserted even when plugged into AC.
2. Batteries that are not recognized/charged.

Symptom 1 is a hardware problem with BTest-1 systems where a very low battery voltage will prevent the power system from starting up correctly. BTest-2 systems should start up properly.

Symptom 2 is the result of a firmware bug that would corrupt some info stored inside the battery.

In some cases both symptoms may be present at the same time.

Recovery of Dead Batteries

The first step in battery recovery is to upgrade the firmware to B83 or later. Please see Autoreinstallation_image or Upgrading_the_firmware for details on how to update your firmware.

Once you have upgraded then the exact methods depend on what system you have.

BTest-1 Systems

Symptom 1

BTest-1 systems are not capable of trickle charging batteries that have voltages which are very low. They must be externally charged. To Recover these bat on a B1 Please see Enternal Charging

Long Term Storage

If you plan to store a system for many weeks or months, your best strategy is to fully charge the battery and remove the battery from the laptop. Batteries naturally lose charge with time (how fast depends on the battery and its chemistry), and the embedded controller in the laptop, while sipping tiny amounts of power in its low power state, will drain the battery faster. Even though BTest-2 systems should now always be able to recover batteries that are fully discharged (except those noted above), it is a trickle charge process and time consuming.

As you can see above, letting a battery drain all the way will cause problems when recharging, how severe depends on the circumstances.

This strategy to remove batteries is true for all battery powered equipment, from flashlights up. Batteries sometimes leak. And some chemistries of Lithium batteries (not LiFePo) can get into a metastable state where they cannot be recharged.