In mid April 2017 we tested our first LiFePO4 (LFP) batteries. They are Drypower 1500mAh 18650 cells in a 4S holder, fully charged and ready to go.
The cells were purchased from a local eBay supplier, along with a charger, cell holders, and other odds and ends from various Chinese eBay sellers. Everything arrived by mid April so we began some simple drainage tests.
The initial test was to drain the batteries through a 6W LED GU5.3 12V AC light, which also works fine with DC. Back of the envelop maths shows this light would drain 500mA and so the 4S pack should last 3h. I was sceptical.
Drypower’s DataSheet for these cells claims the voltage range is 3.3V down to 2.0V. As I was about to do the first test of these cells I didn’t want to push things down to these low voltage levels so I decided to cut the test off when the cell voltage reached 2.9V, being a 4S pack voltage of 11.6V.
The first couple of tests were cut short for a variety of reasons, mostly to do with getting the test equipment set up correctly.
The next test ran to completion, 11.6V. Here’s the voltage graph.
Taking you through this graph, first you see a short, gentle ski slope, which is the cells self discharging because they are straight out of the charger, then a sharp drop down when I attached the 6W light. Following this the cells powered the light for 3 hours, at which point I disconnected the light and the cells recovered somewhat.
What I take away from this test is that the LFP’s have a fairly linear drop in voltage for quite a long time and then display the typical sudden drop in voltage as their capacity heads towards the lower limit. Doing this same test on NiMH cells, for example, would show a steep drop at the beginning of the test, a relatively flat middle, and a sudden drop at the knee.