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24v Solar panels through MPPT to 12v Batteries

T

Tsotup

New Member
Joined
Mar 6, 2023
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Hey, first post on the forum

So I've done a lot of googling and found nothing conclusive but here's my question:

If I ran 24v (about 2kw worth via 3 panels) solar panels on my box truck's roof to charge a 12v battery bank (200Ah 12v, potentially up to x4(800Ah)
through an MPPT would that be a good and efficient idea?

My thoughts are that because 24v panels are more space efficient and run at a lower amperage for the total wattage, I could just use the MPPT
that I would've bought anyway to convert the power with minimal loss back to 12v (or whatever 12v charging voltage I need) and the amperage
almost doubled, as apposed to the amperage being thrown away by a PWM charger. Also the money saving on cables and chargers would be massive.

I'm quite the power user and basically want as much power as possible in the truck I'm designing. These are the panels I'm wanting to get 3 of:

aasolar.co.nz

670W Half Cut PERC Mono PV Solar Panel

670W Half Cut PERC Mono PV Solar Panel, large solar panel perfect for residential and commercial applications. Shipped Nationwide
aasolar.co.nz aasolar.co.nz

And I want to use the 206Ah SOK batteries
 
You will need a good controller, 100v min as those panels are 46VoC and cold weather could push them higher (assuming parallel setup here). Probably need a hefty amp output as well, at least 60-80 amps. Others here are far more knowledgeable, so see what they say.
 
If you are going to use a MPPT charge controller, you should not look at panels as 12 or 24 volt panels. Just try to wire the panels so that the VOC is below the maximum voltage that your charge controller can handle at the coldes tempurture that you expect to see and above the expected battery voltage by 5 volts. Panels wired in series means the voltage is additive. Panels wired in parallel means the voltage stays the same.
 
Agree with @PanelsUpSolar about 12 volt vs 24 volt panels.

With 3 panels your options are 1s3p and 3s1p.
That means 1 string of 3 panels in series or 3 panels in parallel.

For option #1 that means a string open circuit voltage of 138 volts and a short circuit current of 17.45 amps.
You will need an MPPT controller that can handle ~172.5 volts on the pv side.
Since you have ~2000 watts of pv, the system side of the mppt should be rated for ~140 amps.

For option #2 that means a string open circuit voltage of 46 volts and a short circuit current of 52.35 amps.
Option #2 requires thick cables and a combiner box with over-current protection per string.
You can get by with an mppt controller with much lower pv side voltage but you still need ~140 amps on the system side.

If you move up to 24 volts on the system side things get more practical.
Is 24 volts core voltage a possibility?
 
Put simply...
3 * 46 VOC / .8 temperature coefficient headroom = 172.5 volts
and
670 watts * 3 panels / 14 charge volts = 143.571428571 charge amps
 
 
 
Tsotup said:
Wouldn't 40A be fine?
Click to expand...
rodrick said:
670 watts/12 volts=55.8 amps going to batteries
Click to expand...
With panels laying flat on a vehicle they may not reach the full 670W.

And, you'd be charging at closer to 14V so

670W / 14V charging = 47.8A.

A 40A SCC would clip anything over 40A
14V x 40A = 560W max usable watts (is that fine with you?)
 
rodrick said:
670 watts/12 volts=55.8 amps going to batteries
Click to expand...
MisterSandals said:
With panels laying flat on a vehicle they may not reach the full 670W.

And, you'd be charging at closer to 14V so

670W / 14V charging = 47.8A.

A 40A SCC would clip anything over 40A
14V x 40A = 560W max usable watts (is that fine with you?)
Click to expand...
Ahhh I what you mean now. Why on earth would the max amperage not be on the spec sheet?

Anyway thanks for you guys' help. I'm sure I'll be back in another post at some point.

I guess my last question would be, if you were to try get the maximum amount of solar power on the roof of a truck how
would you go about it? 24v down to 12v through mppt(s) or just straight 12v? The truck's roof size is going to be 2.4x4m
(approx 8x13 feet)
 
Tsotup said:
24v down to 12v through mppt(s) or just straight 12v?
Click to expand...
You have to stop thinking 12V and 24V panels. Those are marketing terms from when SCCs were PWM tech. Now with MPPT, the SCC accepts voltage a lot higher, as specified by max input voltage and sometimes an MPPT optimized range.

To get the maximum amount on the roof, I'd first try to get the biggest panels that will fit. If you have smaller areas, then you need to do Tetris to set as many watts on the roof as possible.

Then before buying the panels, make sure they can be wired so that the array voltage is within the range of your preferred SCCs.
Combine in series to add volts. Combine in parallel to add amps. Its far easier with panels with similar specs (volts and amps).

Some people make cardboard cutouts of common/available panel sizes and see how they fit in top.
 
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