Strings 3 of 4, or 4 of 3?

zozomike

So I just mounted a dozen SPR-E20-327 panels I got on a deal. ( Voc 64.0, Isc 6.46) The only reasons I wanted more panels are my intentions to move to an electric vehicle next year. Also on those winter storm weeks (kind of rare here) it may help to have 3 arrays at very low output vs 2 at very low. So, I will eventually add one more FM-100 I guess, for these, (Max Voc 275 in a 24 volt system, Max Vmpp 64Adc) I know I am over watting the FM 100 @ 24 volt, but if that seems to be an issue I could drop a string I guess. And I could change CC if something else seems more appropriate? 
So would you wire 3 strings of 4=VOC 260,Isc 18 or 4 strings of 3 VOC 195, Isc 24. That seems more comfortable, but is there anything inappropriate about odd numbers in a string? I hope my math is good? I

mike95490

First this, i can't understand, must be a typo somewhere.
>    (Max Voc 275 in a 24 volt system, Max Vmpp 64Adc)

Second, you are going over 2 strings, no matter what, so you will need a Combiner Box and i would suggest the Midnight Solar, with circuit breakers, so you can evaluate each string on it's own when you need to test them.

Going over 150V requires more expensive circuit breakers ( series ganged 150V breakers ), so weigh that against copper expenses and resistance losses

if you plan to recharge an EV in the future, you will NEED a 48V battery, so go there now.

FM-100 uses a field replaceable cooling fan. 

zozomike

Sorry for the confusion, The maximum the FM 100 can take 275 Vdc in a 24 volt system, per pg 65 of manual

Open Circuit Voltage (Voc)

Open-circuit voltage (Voc) is the unloaded voltage generated by the PV array. The FLEXmax 100

controller can withstand Voc of up to 300 Vdc. However, if the Voc exceeds the following voltages, the

controller will suspend operation to protect the system components. (See pages 38 and 57.)

 24-volt system ≥ 275 Voc

 36-volt system ≥ 283 Voc

 48-volt system ≥ 290 Voc
And per pg 59:Maximum Input Current (short-circuit) 64 Adc
Yes of course I will want a combiner, thanks for the tip on that one Mike I will review it. I have 2 each of the Midnight MNPV6 on my 2 existing arrays. 
I do not understand why 48 volt for EV charging, guess I have a lot to learn. Nor do I understand how to reconfigure my 10 BB24 batteries for 48 volts. I suppose I could cable them into 5 series strings paralleled? Hmm, some head scratching needed....

mike95490

To be able to safely store enough watt-hours locally, have an inverter large enough to properly do the recharge job and to
have enough storage to buffer the inverter input while large loads are applied via the charger, 48V is the minimum .   Now, with
Li batteries still in their infancy, many BMS won't work at 48V.

What's your average car charger going to pull, if you are charging solar noon + - 2 hours only?
 30A @ 240V ? 50A @ 240V     That's 7,200w and 12,000 watt.   At 24V, that battery cable is pushing 300A, even more at 12kw.

or you skip the idea of a solar car charger and run simple grid tie,

zozomike

Yes well we have wandered far from the original question, which was just about string size. But to address the go grid idea, I am about $20,000.00 away from the grid. I can buy a lot more fun stuff with that than an electricity bill. That plan is a year or more away, and those systems are developing fast.