With 24v charging on a 24v battery is everything the same as 12s except half the amps? Same charging profile, SOC vs time etc? Is 8.3 amps with 24v the same exactly as 16.6 amps on a 12v bank of the same AH capacity? (Do you express the "AH" as half too?--Please, no Watts here! :) )If so it would be easy to compare results. With my 230w panel and MPPT I was getting 15.5a in the Spring and 13.5a in the Summer. With PWM I would get 14.5a all the time based on my 130w having 8.2 Isc. (which I got all the time no matter Spring or Summer)The Isc of the 230w panel is 8.3 and in the Summer I often saw 8.4 or so. (Isc goes up with heat) Since I get Isc with a 12v panel I should get Isc with the 24v? I notice that the 230w and the 130w have about the same Isc, so it is not twice really. Confusing. Panel brands do vary in Isc per Watt though.I would sure take the 16.6 worth at 12 getting 8.3a on 24v instead of 13.5 on 12 if that is how it works.

Watts makes it easy to compare systems. Yes, 24v charging is half the amps.

Half the amps and exactly twice the voltage.I am curious if Vmp of 30 will be enough in hot conditions.

2oldman wrote:Watts makes it easy to compare systems. Yes, 24v charging is half the amps.So when panel temps get high and the voltage and power drops but the Isc goes up, do you think you are missing a trick by using an MPPT at 24v instead of PWM and Isc at 24v on your 24v bank?Ever measured either way?Of course it means nothing when the MPPT controller goes PWM. We are talking about the buck converter here actually, which is always in place except in "PWM controllers" that don't have one.So a buck converter type controller is vulnerable to a hot panel, but a PWM controller could care less about panel heat. It just keeps getting Isc no matter what.

smkettner wrote:Half the amps and exactly twice the voltage.I am curious if Vmp of 30 will be enough in hot conditions.Define "hot" I have been puzzled by my results at 25C ambient (not that hot at 77F) resulting in panel temp of 51C (10% loss of power from 25C panel rating STC according to the graphs on that)So we have two votes that if I see 8.3a going into a bank of two 12s in series (24v) that is the exact same (just as good) as seeing 16.6a going into the same two 12s in parallel (12v) Right?

BFL13 wrote:So when panel temps get high and the voltage and power drops but the Isc goes up, do you think you are missing a trick by using an MPPT at 24v instead of PWM and Isc at 24v on your 24v bank?I don't pay a lot of attention to MPPT vs PWM, hence I don't know enough to understand what you're asking.At 835w of solar I guess it doesn't make a whole lot of difference.

Is 24v Charging the Same as 12v Charging? | Good Sam Community

24 Replies

BFL13
Explorer II
Feb 09, 2015
red31 wrote:
Does the iv curve show a drop in current ~27v or earlier?
didn't your 12v panel begin the drop ~13.5v?

Yes but not much. IV curves here for similar panels (mine is 230 but they start at 240 now)scroll down a bit.

http://hanwhasolarcanada.com/fileadmin/pdfs/Hanwha_SolarOne_HSC60_Poly_Can-Am_001.pdf

Scroll down to page 2 for the 130w's IV curves

http://files.sharpusa.com/Downloads/Solar/Products/sol_dow_ND130UJF.pdf
red31
Explorer
Feb 09, 2015
Does the iv curve show a drop in current ~27v or earlier?
didn't your 12v panel begin the drop ~13.5v?
BFL13
Explorer II
Feb 09, 2015
jrnymn7 wrote:
Of course, then again, the 24v panel's higher voltage may just wake up the PWM quite early, even at half power.

At 50% panel power (115w), an MPPT would put out about 8.2a @ 14Vcharge (115w/14v), while the PWM would put out about 4.15a (1/2 Isc)... but for how long? Certainly not long enough to make up for 2-3a the rest of the day. So I think your 24v-24v idea may be a good one, provided you don't loose your gains when converting back to usable 12v.

The batteries will be at 24v so it is all full amps for the PWM on the 24v panel. (8.3 is "worth" 16.6) I doubt the PWM controller can run the 24v panel at all on 12v batts (not going to try it either!) I can go panel-direct on 12v batts and get the half-amps worth till Vbat reaches 15v then yank it.
jrnymn7
Explorer
Feb 09, 2015
Of course, then again, the 24v panel's higher voltage may just wake up the PWM quite early, even at half power.

At 50% panel power (115w), an MPPT would put out about 8.2a @ 14Vcharge (115w/14v), while the PWM would put out about 4.15a (1/2 Isc)... but for how long? Certainly not long enough to make up for 2-3a the rest of the day. So I think your 24v-24v idea may be a good one, provided you don't loose your gains when converting back to usable 12v.
BFL13
Explorer II
Feb 09, 2015
The much-vaunted shoulder hours superiority of MPPT for adding to daily AH haul is often raised as a selling point.

My answer is that if shoulder hours with PWM is 2 amps and with MPPT it is 4 amps (!00% gain! Wow! ) that is 2 more amps for however long.
If it is for 8 hours, that is an extra 16AH. Gosh! :)

Besides, I have never seen this where and when we camp using solar. Even if it happened, that would be just another cloudy day that the solar could not "keep up" (Who cares by how much it can't keep up? I don't mind running the gen every so often) and after the batts get down to 50%, out comes the generator and the 100 amp charger. 16AH is ten minutes of gen time before amps taper.

So people will ask if 2 amps is peanuts , then why do you care if you get 16 amps from the panel insteasd of 14? Because! That's why. :)
jrnymn7
Explorer
Feb 09, 2015
I'd be more inclined to look at the overall daily haul. What the MPPT (on hot panels) may loose mid-day, may just more than make up for during the early morning hours; not to mention cloudy/rainy days when Isc takes a cr@p.
BFL13
Explorer II
Feb 08, 2015
smkettner wrote:
Half the amps and exactly twice the voltage.

I am curious if Vmp of 30 will be enough in hot conditions.

I finally maybe got what smk is saying here. Elsewhere, somebody wondered if the Eco-Worthy's 42v limit was enough voltage spread for a 24v battery so that sounds similar. In this case from smk, the panel's voltage above 24v is in question not the controller's. (Vmp does not apply to PWM)

I am going for the Isc, so I am not concerned with the panel's voltage any more than I am when using a 12v panel.

Maybe I should be? 230w Voc when hot was 34v (37v rated) The 130s Voc is 22v and was 20.3v when hot. With PWM as intended to be used, panel voltage is battery voltage, so I don't know how that could affect getting Isc.
BFL13
Explorer II
Feb 07, 2015
The main thing is whether common sense will prevail over amps desire.

I got by just fine with the lousy 13.5 amps so 16.6a worth will not make things better, but it will make me much happier to see that number. ( I guess we all know how that will go!)

The Trimetric's 12v power is another little problem.

EDIT: The Trimetric is good up to 65v so it will work on 24 or 48v battery set-ups. Not sure how it would go when switching back and forth between 24 and 12 for AH count. Probably have to keep a note on a piece of paper of 24v AH then reset. (No way am I going to choose the Watts option)
2oldman
Explorer II
Feb 07, 2015
12thgenusa wrote:
.. 200 amp hours of 12-volt battery bank equals 100 amp hours of 24-volt battery bank.
It does get confusing when thinking in terms of ah!
wa8yxm
Explorer III
Feb 07, 2015
12thgenusa wrote:

I think you confused yourself.

Batteries in series have half the capacity of batteries in parallel whether they are 6-volt or 12-volt.

I did fail to take into account you were re-configuring for use and charge so you were changing the batteries around

In that case, the major difference is less voltage drop in the wires carrying the charging current, Likely you will not notice it, but your calculations are accurate.

Reading more of the thread I figured that out

For charging
6+6+6+6=24

For use
6+6
6+6=12 volt at twice the amp hours

I had assumed
24 volt
v/s
12 volt with the same amp hours

Basically
6+6+6+6

v/s

6+6