Harm to my AGM Batteries with my max charge set point of my system

[niel]

Just use the grid when its available, including at night. The wear on batteries discharging them at night will result in sooner battery replacement. The amount of grid electric cost saved will not outweight what you will end up spending on replacing batteries more often.

i agree here with rc and only cycle the batteries once every month or 2 just to flex them and be sure all is functional.

[dagelt]

Thanks for everyone's response, I just got my local COOP power company to do true NET metering ( 1 for 1) so I will be switching the way I do run my system but for the last 18 months I have been running like an off grid system. Thanks for the feedback on cycling the batteries each month to exercise them... I may not of thought of that.

Thanks again!

[RCinFLA]

The cycling of the batteries once in a while helps to clear off excessive lead oxide build up on positive plates. Only needs to be 10%- 15% of rated AH used, (85-90% SOC) to do it.

The ripple current results from the single phase AC put out by inverter. Its current profile drawn from battery node is sine squared shaped (looks like full wave rectified waveform).

For 6 kW's from PV controller, the current delivered to battery node is nearly a pure constant 113 amps at 53 vdc. 6 kW's from battery to single phase inverter has a peak current of 160 amps and minimum near zero at the point of AC output voltage is zero crossing. When the inverter needs the peak 160 amps, the difference from the 113 amps, equals 47 amps, must come from battery. It is replaced into battery during the period when AC output voltage is near its minimum, zero crossing point in the sinewave. Average battery goes-in versus goes-out current averages near zero. Bottom line there is 47 + 113 amps, equals 160 amps peak to peak ripple current, going in and out of batteries, 120 times a second.

The batteries have to be big enought, AH wise, to take this ripple current without putting excessive stress on them. Our rule of thumb is about 100 AH minimum battery size for a 48 v system for each 1 kW of PV controller peak power injection. You may want larger AH capacity for off grid power/run time needs. With your approximately 1000 AH battery capacity you should be in good shape to take in 6kW's of solar power.

[dagelt]

Ok RCinFLA,
So if I am understanding everything correctly you are telling me my battery bank is the proper size for my new net metering situation and leaving my inverter set at 53.6 volts and selling anything above it will work fine?

But once a month I should flex my batteries down to around 85% to 90% SOC (10 to 15% DOD) to keep them exercised and in good working condition.

and just use the grid at night since I am able to net meter (get 1 for 1 starting next month)


If I could have lobbied hard enough 18 month ago I would not have installed my battery bank but they were not going to budge.... now I have a lot of extra expense for no reason!!! Well not as much reason....... I guess when my power goes out I will have them to run off of.

again thanks for the advice!