November 14, 2001

Preparing the batteries:

The Electric Imp's development battery pack is made up of 180 2.14 volt 25 amp/hr Cyclon BC batteries. These batteries will be wired in series, meaning the negative of the first battery will be connected to the positive of the second battery. The voltages are added 2.14 volts + 2.14 volts. The end result is a 385 volt, 25 amp/hr pack.

These 180 batteries were not made at the same time and have seen different usage. This means they have aged differently and no longer have identical capacities. This is not the optimum set up for a battery pack. It is fine for testing software and charging methods. It is a pack we can make mistakes on.

When charging batteries in series, it is best to start with all the batteries at the same state of charge. Otherwise, some batteries will reach full charge and start to overcharge, while others in the series will not yet be full. The simplest method is to start with batteries all at the same state of charge.

Many of the batteries are at different state of charge. A Cyclon battery should read 2.14 volts when fully charged. The voltage method of checking will give the state of charge (SOC) within 20 % if the battery has been resting for 24 hours before testing, or within 5% if the batteries have been resting over 5 days. Some of the batteries are reading less than one volt.

To bring back the really low batteries, I connect them in parallel to a constant voltage source. This means that each battery will see between 2.45 and 2.50 volts and will draw exactly the amp/hours each need to recharge.

For a constant voltage source, we use an adjustable constant power source.

recharging the low batteries

This is a very slow method since there is a practical limit of 6 batteries on the charger at once and each set of batteries needs around 48 hours to recharge.

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