Blog - Chargers and charging

Q: I am using a new laboratory Power Supply (PS). This PS is able to charge to a max. of 30 Volts and a current of max. 10 Amp. What I do is setup the PS to 4 Volts and 3 Amp. At the end of the charging, the PS automatically reduces the current to 0.01 Amp.

This is 100% correct.  For large capacity cells (40AH and more) you can even charge with the full power (of 10A current), there is no need to reduce the current to 3 Amp.

During the final stage of the charging, when the cells is “full” it does not absorb any more energy and the current will decrease to a very small value.   The charging can be stopped when the current is 1% or less of the nominal capacity (1% of 40AH cell is 0.4A).

The voltage of 4V should not be kept at the cell terminals after the current has already dropped; the power source needs to be disconnected. After the charging is finished and the PS is disconnected, the voltage of the cell will drop back to some 3.40V ~ 3.20V after some period of time.

Q: Please explain how you can charge a cell to 4.00V volt. And why some people suggest charging to 3.65 Volt.

We recommend to reach 4.0V for the initial charge. The 3.65V is a usual charging level for the regular operation of the cells.  This means after you make the initial charge to 4.00V you continue to use the cells and charge to 3.65V (or 3.80V or some similar level) for the regular operation.  Simply said charging to a lower level will extend the cell’s total life span (in number of cycles).

Q: What is the maximal allowable voltage difference between the cells, to see it was fully balanced?

There is not any difference to measured. All cells must be charged to the same level (e.g. 4.0V or 3.8V or 3.65V) after they reach this voltage level and stop absorbing the current (the 1% limit), they are fully charged.

After the charge voltage disconnection, the cells drop the voltage back to the nominal level 3.2V. Each cell may go its own way (speed), but after starting to be under load, they should match the voltage approximately again.

For large quantity of cells, you may wish to charge many cells at the same time using high current chargers.

See an example here:
http://gwl-power.tumblr.com/post/3297893710/the-charging-12-pcs-of-20ah-cells-the-initial

You can also charge single cells with a charger that allows charging the single cells with 3.65V
http://www.ev-power.eu/Chargers-6V-to-36V/Charger-3-6V-6A-for-LiFePO4-cells-1-cell.html