Fix Clock/Background

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In understanding the fix clock problem, it is helpful to understand the power dynamic between the system, the coin cell battery, and the RTC chip.

The XO must maintain power to the RTC chip at all times, even when the system is powered off. When power is lost, the time and date will be forgotten, resulting in the problems described on this page. This is especially important to consider for XOs in storage, or XOs that will not be used for an extended period of time.

The coin cell battery acts only as a backup: when available, the main power source (AC power or the main system battery) is used to keep the clock ticking. When there is no other power available, the coin cell battery takes over.

The coin-cell battery is somewhat rechargeable. On the XO-1, it will recharge from the main power source while the laptop is powered on and fully awake (not suspended). The XO-1.5 and XO-1.75 will charge the coin cell battery when the laptop is powered on and fully awake, and also when it is suspended. When recharging an empty battery, it is expected that the battery will reach normal voltage levels after around 30 minutes of charging, but will require potentially 24 hours for a complete recharge. However, the battery is only marginally rechargeable - it may stop working completely after 30 deep discharges.

A voltmeter can be used to perform some basic checks on the health of the battery. After 30 minutes of charging from empty, a reading of around 2.7v would be reasonable. After 24 hours of charging, when the battery should be fully charged, a reading of around 3.1v would be expected. However, these checks are inconclusive, 'bad' batteries may produce such readings. A real test would need to measure battery capacity, which is non-trivial.

When no other power is available, the coin cell battery is only able to power the RTC chip for a limited time before running out of power. Assuming the RTC battery is fully charged, the following table indicates the expected lifetime of the coil cell battery before it is unable to keep the RTC chip powered.

Laptop model Coin-cell battery life
XO-1 4 months
XO-1.5 2.5 months
XO-1.75 12 months (expected, needs testing)

When placing XOs in storage or some other situation where they will not be used for a significant period of time, it may be worth considering inserting the main laptop battery and ensuring it has a high amount of charge, so that the RTC chip remains powered for an extended amount of time before the coin-cell backup takes over. According to tests done in Uruguay, the following table provides an indication of how long the main battery can keep the RTC chip powered before running out of charge, in the case where the battery initially had 80% charge:

Laptop model Main battery longevity
XO-1 73 days
XO-1.5 174 days
XO-1.75 164 days

Note that the above numbers should be used as indications only; the real numbers will vary based on environmental conditions such as the temperature of the storage area, the exact model of the laptop and the RAM included, firmware version, etc.

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