AC vs DC charge rate confusion

[Calihaze]

Hi all-

I’m a little confused when attempting to understand the charge rate for my battery bank, specifically when charging through my VFX3648 Inverter, using my AC 120v Generator. My battery bank is 840ah @ 48v (2 strings of 8, 6 volt @ 420ah). I believe the optimal charge rate is 84a.

My inverter (Outback VFX3648) has a 20a current limit on the AC charger @ 120v. Does that convert to roughly 50a @ 48v DC? I think I am getting lost in this conversion, but if I am not – it means that my charger (inverter) can only charge my bank at a C/17 rate when using the Generator…which would be sad.

Any ideas on where I might be going wrong?

Thanks!

[Mike90250]

Close, but no cigar. 20A @ 120V = 2400 watts / 54v =44A if there were no losses in the charger. So you may have to run the genset a little while longer. But you only need to bulk and do some absorb with the genset, you don't use the genset to get to float, that wastes too much fuel.

[Calihaze]

Thanks for the quick reply Mike, I appreciate it. It may be a newb question, but where do you get the 54v from? I was dividing by 48v...
Also, should I worry about stratification with the low charge rate, or should the EQs keep me safe?

Thanks again!

[Sunking]

54 volts is what 48 volt batteries float at.

[Calihaze]

I don't understand why we would use float instead of 48v in the equation. Does it have something to do with resistance? Regardless, I have my answer now so perhaps I should stop digging deeper. I'm disappointed that I did not catch this mismatch when the vendor sized my system, oh well. Thanks fella's.

[sensij]

If you look at the spec sheet, you'll see that the maximum continuous DC output of that model is 45 A.

The voltage at which the current is delivered will be a function of the battery open circuit voltage, internal resistance, and the losses of the transmission system. If your battery rest voltage is 48 V, your charge will start in the bulk stage at slightly higher voltage than that. If you are nearing the end of bulk, the voltage would be up around 58 V, depending on your settings. If you are in float, it might be 54 V, but it is unlikely you would be pulling all 45 A in that charge state.

With respect to your question on stratification, you might want to check out this thread. The low charge rate won't necessarily hurt you as long as you give it enough charge time to get to 100% SoC before you start to discharge again.

[Calihaze]

ok thanks for the confirmation. After reading that thread, I feel good about my charging habits, despite my mis-matched system. I also have a good understanding on how to upgrade my system so that it matches better, rather than depend on the local vendors who are probably trying to balance a million client needs vs. the "right" way of doing things. I appreciate all the help.

[Sunking]

I don't understand why we would use float instead of 48v in the equation. Does it have something to do with resistance? Regardless, I have my answer now so perhaps I should stop digging deeper. I'm disappointed that I did not catch this mismatch when the vendor sized my system, oh well. Thanks fella's.

Try reading this.

[Mike90250]

There is a minimum amps needed to get the electrolyte bubbling enough to de-stratify it. Too low of charge amps will not be good for tall cells.