24V battery bank with solar - pros and cons

BFL13 wrote:
You don't double the power. 8 amps on a 24v batt is the same as 16 amps on a 12v batt. Same charging time on same batteries whether wired as 24 or 12.

However the controller is more efficient without having to buck the voltage down to 12v. So you do get a little better that way but not a lot. Fairly trivial in fact.

You might look at these results for 24-12 and 24-24 before going to a lot of trouble

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/28400759.cfm

Nice data - thanks. Now you got me thinking of possibly doing PWM to a 24V battery bank.

Do you remember seeing any data on how PWM does vs. MPPT with shading involved? I remember seeing a real long thread some years ago on this forum about shading and the big lesson I remembered from it was of needing bypass diodes to minimize the impact of partial shading.

smkettner wrote:
I assume you have calculated that you need 1000 watts?
At that level I assume your usage is high such that you may want more battery.

1,000 Watts will be overkill for me when in full sunshine but I camp in sites that are typically partially shaded from trees so the hours of productive sunlight are limited.

And when we do camp in places with more sun then we will be able to use more solar power for things like the microwave, toaster and coffee maker.

I am hoping that 4 large solar panels will generate some power for me even if some of them are in shade as the sun moves. Do you have experience with this scenario?

My previous trailer had a single 140W panel and it was rarely enough to keep up with my 26AH daily use when dry camping due to shady campsites.

You don't double the power. 8 amps on a 24v batt is the same as 16 amps on a 12v batt. Same charging time on same batteries whether wired as 24 or 12.

However the controller is more efficient without having to buck the voltage down to 12v. So you do get a little better that way but not a lot. Fairly trivial in fact.

You might look at these results for 24-12 and 24-24 before going to a lot of trouble

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/28400759.cfm

Here is what happened when trying to charge a 24v bank (four 6s) with a 60 cell panel. Got to 30v (same as 15v with 12s) but stalled out there so would not be able to equalize if that is part of the plan.

----I have not calculated what would happen with four 60 cell panels in series-parallel.

In this case, the 60 cell panel ran out of "overhead" on a 24v battery bank when it got Vbatt to 30v, but the neat thing is to watch the Vmp climb. (Or maybe the controller ran out of Vmp it could go up to while in MPPT?--panel voltage would go to 35v or whatever once in Float)

THE TEST
The bank was full, in Float (as set) at 14.4v as 12v bank, so knocked that down with microwave on inverter for three minutes, disconnected solar, put batts in 24v mode, connected solar controller but not array yet, and set Absorb to 31v (max setting) and Float to 29v (max setting) By now battery holding at 25.6v ready to start, so connected array

I measured Vmp at the controller array terminals and took battery voltage and amps from the controller's display. I did not note the times. Controller was in Bulk and doing MPPT-- So here is how it went with Vmp, V batt, Amps, output watts

29.0, 26.3, 6.0
29.8, 27.4, 5.01, 138
30.1, 27.7, 3.52
30.4, 28.7, 4.64
30.4, 29.3, 4.81, 142
30.7, 29.9, 4.66
31.0, 30.2, 4.52
31.1, 30.3, 4.04, 137

It stayed at 30.3 saw 30.4 but fell back to 30.3 again so stuck there, still; showing Bulk ( not yet to the 31v Vabs setting)

The key seems to be the MPPT finds an ever higher Vmp as Vbatt gets higher, so the amps keep coming.

I assume you have calculated that you need 1000 watts?
At that level I assume your usage is high such that you may want more battery.