I have a 650W off grid solar PV system - 2 x 325W panels in series, a Victron MPPT 100/20 charger and 4 x 12v 95Ah, 800W, Exide AGM batteries wired in series/parallel giving me 24v which powers a Victron Phoenix 800W inverter. I do not allow the batteries to discharge below ~24v, which should be about the 50% discharge point, and re-charge to max. before re-enabling the inverter.
My engineering background tells me I should be able to replace the solar panels with a grid powered 60v 5A current limited power supply and the MPPT charger should find the maximum power point when the current limit just cuts in, thus providing 300W to the charger. I'm considering switching this in overnight during the winter to 'shift' power from our Economy 7 electriciy tarrif to a more useful time of day. 300W should be sufficient to re-charge the batteries from the 50% discharge point in less than seven hours.
The simplest way of switching the power between the panels and PSU is by using two forward biased 10 amp diodes, one in each supply, to select whichever source has the highest voltage. There will not be a clash anyway as I'll use my IOT system to make sure the PSU is on only when it's dark outside.
I don't want to damage the MPPT charger by doing something daft. Am I being too simplistic in my approach or is this a realistic way of achieving my goal ? I know I can add a grid powered charger in paralell with the MPPT charger but the arrangement I'm looking at will be considerably cheaper. Any comments from those who know about these things, or may have done something similar, please ?
Regards
PaulM
