September 4, 2016

Solar Cluster: Load test using the power controller

So, last night I started doing some light testing of the power controller. I installed a 5A fuse and hooked it up to a 10Ah LiFePO₄ battery and a homebrew 3A charger (LM2576-based with a 16V IBM laptop PSU as mains source) set to its maximum voltage (~15V).

The controller came to life and immediately started flashing its “high voltage” and “low temperature” LEDs, indicating that:

  • It thought the temperature was high enough to warrant a fan turning slowly (above ~20°C)
  • It thought the battery voltage was too high for comfort (IPMI complains when the voltage gets much about 13.6V.)

In order to get the battery to discharge, I plugged in an old Icom IC-706MkII G transceiver, set it on receive mode. I didn’t have an antenna attached, so this would have represented a very light load for what would be production use.

The battery started discharging, and after a few tens of minutes, the high voltage warning LED had stopped flashing and instead the “good voltage” LED was staying constantly on. So battery was in a range the controller was happy with.

It was going to take a long time though, for that set to drain a 10Ah battery in receive mode.

This morning, I got out the big guns. I plugged the actual cluster in and fired it up. After about 30 minutes of run time, the battery had drained sufficiently that the charger started flashing the “good voltage” LED, indicating battery was getting low. It had also turned on the MOSFET controlling the charger I had plugged in, and the charger was desperately trying to keep up with the ~5A load that the cluster was drawing. (Did I mention this was a 3A charger?)

So far so good. I powered off the cluster and unplugged it. It continued to let the charger do its job, and after another short while, the battery had regained some charge. It kept the charger on until momentarily, it peaked over the “high voltage” threshold. The “high voltage” LED blinked a few times, then the MOSFET turned off and the “good voltage” LED remained solid.

The battery was sitting at 13V. A little short of my 13.5V set-point, but close enough given it was on a fairly weak charger. So that ATTiny24A is doing exactly what I intended.

Maybe the high set-point could do with some adjustment, or a turn-off delay added (with a separate “high critical” set-point for immediate shut-off), but so far, so good.

It might be worth me getting some PCBs fabricated and shrinking the prototype board down using SMD parts, but this controller works so far.