Battery Charging - Quick Charge Function

Charging lithium-based batteries with a quick charge function is not as easy as it would seem. The design of the lithium-ion battery does not permit super-fast charging. However there are some steps which can be taken to permit lithium battery charge time to be reduced. Because of strict rules for charging lithium-ion cells it is important to follow guidelines in charging procedures provided by the manufacturers.

There is an charge technique which permits a faster recharge and XPower1 has implemented this approach in our design. In addition we are working on new designs which can be used in the future which permit batteries to charge in seconds verses hours. This new technique will work with new lithium-ion battery material which is going to be released in 2012..

Lets not wait until 2012 however to improve charging. There is information below regarding implementing steps to a faster charging.

A little background on lithium-ion may also help so we have provide this for your information. We will try not to get too technical. Lithium-ion batteries are a clean battery system and they do not need priming as with other batteries such as nickel-based batteries. The 1st charge is no different to the 2th charge or the 100th charge. No need with Lithium-ion batteries to charge the battery for 8 hours or more for the first time they are used. This just is not required.

Typically cells are charged to 4.20 volts with a tolerance of +/-0.05V/cell. There is a reason for this level of voltage because charging only to 4.10V reduces the capacity by 10% but provides a longer service life. Newer cell are capable of delivering a good cycle count with a charge to 4.20 volts per cell.

There are charge stages of a lithium-ion battery and it is how there stages are implemented that can be used in our technique to charge faster. Increasing the charge current on a lithium-ion charger does not shorten the charge time by much. The voltage peak will be reached quicker with higher current however the topping charge will take a longer period of time to perform.

So what is the typical time it will take to charge a lithium-ion battery used in a cellphone. Most charges take 1 hour for a full charge. These are cellphone, MP3 player type batteries are smaller batteries. The charged is at 1C. Larger batteries such as a 18650 cell used for laptops will take 0.8C or less for a charge. The charge efficiency is 99.9% and the battery remains cool during charge. There is a threshold for voltage which is normally attained at full charge of a battery and once this is reached the current drops to 3% of the rated. So it levels off after the threshold is reached. Increasing the charge current does not shorten the charge time by very much. If the current is increased this the voltage peak is reached quicker but the charge will take longer to complete top off the battery.

To fast charge lithium-ion there is a common technique. The technique is during charging to eliminate one stage ( second stage of charging) and go directly to 'ready' state once the voltage threshold is reached at the end first stage of charging. The charge level at this point is only about 70% of total capacity. To top the battery normally will take twice as long as the initial first charge stage. But 70% during half the charging stage is good and permits a user to make a fast charge and have a high level of capacity quickly. XPower1 uses this technique to charge the battery as fast as possible during the first stage of charging.

Lithium-ion batteries are unable to absorb overcharging so the Xpower1 does not permit this. This overcharging is called a conntinuous trickle charge. Instead, a brief topping charge is provided to compensate for the small self-discharge the battery and its protective circuit consume. Depending on the battery, a topping charge may be repeated once every 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05V/cell and turns off at a high 4.20V/cell.