Sunday, August 26, 2012

6 Month Capacity Test

So, after 188 daily charge and discharge cycles, what's the score with these Lithium cells?

I decided to find out.

I waited until 3pm on Friday afternoon, when the bank had finished charging and had just gone into float stage and then hit the big red breaker on the PV array.  No charge input.  I then started recording the battery Voltage and AC kWh of energy consumed by the house from the Ofgem AC meter on the output of the inverter.  This meter is calibrated and modified to show down to 100Wh (0.1kWh).

I then did nothing special, other than going about daily life to see how long the battery would last until empty.  It never really gets empty as the protection cuts out the load before it gets below 3.00V per cell.  This leaves enough reserve for the battery to power the charge controllers and the battery monitors.

We did some laundry, which runs the inverter to over 2kW output and we did some vacuum cleaning and the fridge freezer did it's thing.  We even had the folks round for dinner and ran the power hungry video projector for a movie.

After 24 hours, the battery was down from the initial 27.00V to 26.09V and had delivered 5.6kWh of AC power.  So we kept going... And finally to bed.

Woke up the next morning (Sunday) and the power was off.  The logger laptop was still running on its internal battery and showed the inverter had shut down just after 7am.  So the house ran on battery power only with no charge input for just over 40 hours.  The battery was sitting at 24.40V off load and the AC meter read 9.0kWh.

Impressive for a system that I only rate as usable for 80% of the nominal 10.24kWh total capacity (if you take 400Ah x 3.2V x 8 cells = 10.24kWh DC power available).

Quite a bit of power is wasted by the inverter at low power output levels and at best it has an efficiency of about 92%.  So to get 9.0kWh out in terms of AC energy after all the conversion losses is pretty good.

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