question about amps vs volts for battery maintenance

elesaver · August 2015

In providing sufficient charging capacity for a battery bank, is it better to configure panels for higher volts (series) or higher amp hours (parallel).

Basically, the question references this scenerio: 24V battery bank with 938 aH; six panels 300W, 8.18 a, 36.73 nominal voltage currently wired as 3 strings
of two into a combiner box. The battery DOD planned is to not exceed 50% although I do have a generator in the event of a problem. The controller is an Midnight
Solar Classic 200 and the inverter is an Outback VFX 3524.

Any thoughts would be of great help.

vtmaps · August 2015

elesaver wrote: »

In providing sufficient charging capacity for a battery bank, is it better to configure panels for higher volts (series) or higher amp hours (parallel).

You are better off with lower voltage from the panels. Your current configuration (three strings, each string with 2 panels in series) is optimal. Why do you have a Classic 200 (rather than a Classic 150)? How far are the panels from the controller?

--vtMaps

inetdog · August 2015

vtmaps wrote: »

You are better off with lower voltage from the panels. Your current configuration (three strings, each string with 2 panels in series) is optimal. Why do you have a Classic 200 (rather than a Classic 150)? How far are the panels from the controller?

--vtMaps

But the reason that the lower voltage is better is simply that the CC will be more efficient when the input voltage is closer to the output voltage. That difference in efficiency may not be enough to overcome other reasons (wire size and voltage drop for a long DC run to the CC) for going to a higher voltage.

elesaver · August 2015

The controller is 60' from the panels with #10 wire. I have the Classic 200 because it was a good price at the time and I thought it would be better to have higher volts than higher amps. As with many things, you can't go back!

The controller shows up to 71+ volts and up to 30 amps when the sun is bright . Of course those numbers vary with the cloud cover, etc. I wasn't certain that 30 amps would be sufficient for adequate keeping this sized battery and/or would it be enough to equalize when needed? As I stated, I do have a Honda eu3000i that I plan to wire to the inverter. I have a mate2 (uninstalled at this moment) and when I can, I want to get the atkinson generator start module to keep things working well. I've looked at the wiring diagram at the atkinson site and it will take some pondering for me to be able to get it working. But, isn't it good to have new challenges? :-)

westbranch · August 2015

A signature line would help us all understand your system and limitations...;)

vtmaps · August 2015

elesaver wrote: »

The controller is 60' from the panels with #10 wire.

1800 watts configured as three strings of two panels in series will have a (combined) Vmp of 73.5 and Imp of 24.5. You will have a 4% voltage drop on 60' of #10 cable. That represents a 74 watt cable loss at full 1800 watt nameplate power (seldom achieved). The power loss goes as the square of the current, so at half power your cable loss would be less than 19 watts.

Your alternatives are to use heavier cable (#6 cable would have a 1.4% voltage drop), or configure your panels at a higher voltage.

If you put the panels in two strings of three panels in series, the voltage drop with the #10 cable is 1.8% and the cable loss is 33 watts at full 1800 watt nameplate power (seldom achieved). At full power (seldom achieved) you have reduced your cable loss by 41 watts, but you have increased the power loss in the controller which will run hotter and die sooner. Midnite has not released efficiency graphs for the Classics, but on their forum one of the engineers said he expects it to be similar to the Outback FM controllers. In that case, most (but not all) of the 41 watts you save in the cable will be lost in the Classic. That is at full power (seldom achieved). At lower powers the efficiency gains in the controller (of having a lower input voltage) will trump the cable loss.

So what to do? First choice: replace #10 cable with #6 cable. Second choice: not sure... probably leave the configuration at 2 in series, but since you have a Classic 200, you could go to 3 panels in series. Three in series will give you less voltage drop (helps controller to know true voltage of batteries), but three in series also makes your controller run hotter and at typical operating conditions (less than full power) your overall system will be less efficient than having two panels in series.

--vtMaps

elesaver · August 2015

Vtmaps, your info was exactly what I was looking for. I had pondered replacing the #10 cable earlier. Now I see it would be a good thing to do. Having a Classic "die early" would be awful. They are not cheap. Thanks for your help.

elesaver · August 2015

As Columbo would say, "And, another thing..." If this system is generating 24.5 Imp, how do I translate that into charging amps put into the battery bank? I only want a maximum of 40% DOD. Yes, I realize that amount of sun is not the same all the time, etc., but as a general rule, is there a formula to be able to know this? Thanks again.

elesaver · August 2015

I would add a signature line but I can't seem to find the location to do that. :-)

westbranch · August 2015

go to the top right corner of this page, and click on your user name then go to user settings, then to the left side of the page, to USER Settings, then ACCOUNT, go down the page and look for some small BLUE print in the Conversation details section

BB. · August 2015

Typical charging current:

430 Watt array * 0.77 panel+controller derating * 1/14.5 volts charging = 22.8 amps "typical" maximum charging current

MPPT vs PWM controllers, the maximum charging current is calculated differently--because they work on entirely different principles. But the above equation is "close enough" for an otherwise correctly designed system.

-Bill

vtmaps · August 2015

BB. wrote: »

Typical charging current:

430 Watt array * 0.77 panel+controller derating * 1/14.5 volts charging = 22.8 amps "typical" maximum charging current

Huh? I thought this was about 1800 watts of panels on a 24 volt system. --vtMaps

vtmaps · August 2015

elesaver wrote: »

If this system is generating 24.5 Imp, how do I translate that into charging amps put into the battery bank?

Your controller is an MPPT. You have watts going in and most of those watts coming out (some loss due to inefficiency). If you have 1800 watts going in, and the battery voltage happens to be 25 volts, you have 1800 ÷ 25 = 72 amps. If your controller and wire losses are 6%, then you have 67.7 amps going into the battery.

As your battery voltage gets higher, the current into the battery gets lower. At 28 volts, the current (before losses) is 1800 ÷ 28 = 64.3 amps. 60.4 amps after losses.

Once the battery voltage reaches the absorb voltage, the current into the battery will taper down to a final value called 'end amps'.

--vtMaps

BB. · August 2015

Was not sure what numbers. Just take the equation and put in your numbers.

-Bill

question about amps vs volts for battery maintenance

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