Thursday, June 21, 2012

LiFePO4 ESS update: After bottom balancing for several days, back to charging

LiFePO4 Battery Energy Storage System (ESS or BESS) update

I have been bottom balancing my cells for 5 days now and they are all really close to 2.70V, which is a fully discharged cell

I have been using my bottom balancing device based on a programmable dual relay voltmeter and it is really usefull for this matter
The voltage of the groups of 4 cells tend go back up after leaving them for few yours, not much but they were sometimes back to 2.75V, so I would repeat the process over and over, for all groups

Something important I noticed was after the first night, some groups of cells already bottom balanced in the evening at 2.70V went down to 2.55V or 2.60V when checked the next morning ! After few minutes I realized it could be caused by only one thing: the digital voltmeter that is always ON and connected to the pack through the DC-DC converter: The cells being very low, their tiny consumption, over time, was making the voltage drop quite a bit (we are on the steep part of the discharge curve). I had to push some energy - using a small 12V charger - into these groups of cells to reach 2.70V again.

Since my new LiFePO4 24V 10A charger is still in transit in China, I decided to use the 48V one and it should not be a problem because I am limiting the pack voltage to 29.2V via programmable voltmeter second relay and the automotive relay placed on the charger line

Pack voltage down to 21.6V = 8 x 2.70V

Charger switched ON, and voltage going up already, ampmeter showing 3.5A

Afer few minutes, voltage os going up pretty fast; here 22.3V = 2,78V per cell

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