On Saturday, November 28, 2015 at 4:13:26 PM UTC-8, Richard wrote:


I agree about the protection circuit, but will take the advice of both Bioenno and Stark Power not to use higher voltage chargers.

Richard
www.craggyaero.com

A good idea.

However, there's a difference between charging a 12 volt battery with a capable 24 volt battery charger and connecting a relatively low power solar panel to that same 12 volt battery - even if the PV panel has a higher Voc (as all the panels we are talking about do).

A typical 36 cell panel, intended for 12 volt systems, might have an Voc (open circuit voltage) of +- 20 volts. But connecting that PV panel to the battery doesn't mean the voltage in the system would be bumped up to 20 volts.. Ever. The PV panel voltage will be instantly "clamped" to battery voltage - and then charging will occur which would slowly raise system voltage, but only if current output of the PV panel is more than the system avionics load is consuming. Normally, with load applied and the small PV panels we are using, there is no net charge to the battery as long as avionics are on.

So, in flight, no issue, even with no solar charge regulator, as long as the avionics continues to use more current than the solar panel output - which again is the case in most current glider PV panel installations. Using a solar charge controller intended for lead acid batteries? Sure, also no problem as long as the glider isn't left in the sun with avionics off and the PV panel still charging.

If planning to leave it parked in the sun with avionics off, then it's important to make sure the solar charge controller max output voltage will not exceed the battery max charging voltage. On my LFP batteries that's 14.2 volts.

That said, it's best to use a solar charge controller intended for LFP batteries. And plan on occasionally using a recommended LiFePO4 battery charger to do a cell balance charge if that's the mfg's recommendation.