XW6048 for whole house battery backup

dBeau

Re: XW6048 for whole house battery backup

RCinFLA wrote: »

For a EU6500is you would like to be able to consume 4000 watts for charging. For a 48 v battery system that would be 4000 x 0.92 for XW inverter efficiency or about 3700 watts into batteries. 3700w/52 vdc = 71 amps of battery charging capability. If that is 15% of battery AH for safe charging, then battery bank would be 475 AH.

Ah, I like what you did there. In order to minimize generator runtime you figured the smallest battery bank size that can accept the brunt of the generator's output. The only "magic" you used was the 15% rule. Now the specs for the UB121000 say that they can take 20amps into it's 100AH capacity, so that could imply that 20% might be a good rule for them or perhaps AGM in general? So at 20%, I'd end up 355AH of AGM. Or, is there really no practical difference in the charge rates of the two technologies?

Now with FLA, T105's say, my budget would allow for two strings of 8 batteries or 450AH. So at 15%, that'd be 68A*52V=3500w or 3850w at the generator. Or, with AGM I'd be looking at 3 strings of 4 UB12100 for 300AH. At 20% that'd be 60A*52V=3100w or 3400w at the generator. But if the AGMs can only take 15%... 45A*52V=2340w or 2543w at the generator.

RCinFLA wrote: »

You have to also look at your load expectations and how long you want to go between charging. A 475 AH battery, utilizing 50% capacity, yields 238 AH available.

(50vdc x 238 AH) x 0.92 efficiency yields about 11 kWH's available between charging.

I'm expecting a "worst case" average load of 2kw and at least 2 hours between charging. But unless you are suggesting taking 70% from the batteries I only see that we have 30% to play with... that is, the first discharge when the grid is out will draw 50% then we can bulk charge back to 80% then discharge to 50% and repeat a few times before having to charge all the way back to 100%. Hopefully, the grid will be back by then. To get 50% utilization the first discharge would go to 30% with bulk charge back to 80% then discharge to 30% again. Am I being too conservative by only going down to 50%?

So...

FLA at 15% charge rate, 30% utilization: 450AH*30% = 135AH, 135AH*50v = 6.7KWH or 3.35 hours at 2kw. Charging at 68 amps to 80% takes 2 hours yielding 37% generator runtime with 4.5 cycles per 24 hours.

AGM at 15% charge rate, 30% utilization: 300AH*30% = 90AH, 90AH*50v = 4.5KWH or 2.25 hours at 2kw. Charging at 45 amps to 80% takes 2 hours yielding 47% generator runtime with 5.6 cycles per 24 hours.

AGM at 20% charge rate, 30% utilization: 300AH*30% = 90AH, 90AH*50v = 4.5WH or 2.25 hours at 2kw. Charging at 60amps to 80% takes 1.5 hours yielding 40% generator runtime with 6.5 cycles per 24 hours.

After all that, it's pretty clear (assuming the math is close and performing maintenance on the FLA bank is not an issue) that the two strings T105's are the clear winner... better use of the generator's capacity and fewer cycles during extended outages. I guess I'll be learning how to care for FLA batteries. It is interesting to note too that all three configurations do meet my design goals.