Is 24v Charging the Same as 12v Charging?

No plans at all. Just thinking that I might be able to get 16.6a worth ( at 8.3a) from that 230w panel with PWM at 24v when it is at 51C (25C ambient) instead of that lousy 13.5a I was getting last summer with MPPT.

I would need some 12v for the rig during the day (very light loads) and go back to 12v just before supper until next morning. The mechanics of that are a separate issue but seem not very difficult.

I said before that I would prefer MPPT in Spring and Fall and PWM in Summer. I thought I was stuck because I have only one 24v panel.

To get out of that by having two 12v panels so I could choose series or parallel, it turns out I can swap the batteries around instead.


So now I am working on how to do all that and figuring if it is worth the bother. I am especially intrigued by the 230w's Isc being higher in proportion to the 130w's Isc That 16.6 is higher than the 14.5 two 12s would get me.

230/130 x 8.2 = 14.5 and 2 x 8.3 = 16.6

I was thinking I could get 14.5 instead of 13.5 but now it looks like I could get 16.6. (ie 16.6 worth by getting 8.3)

Oh the trouble we put ourselves through to gain a few watts . . . er amps.:)

Hi,

I think BFL, being a careful man, is more likely to set up switching to easily and quickly change his wiring to the battery bank from 12 to 24 volts for extracting all the amps (since he is allergic to watts) from the solar system. Small loads (up to 20 amps) could be run from a DC to DC converter for under $20. When he wants to use his (expensive) inverters he can switch the bank back to 12 volts, or for small loads up to 150 watts, he can connect the inverter to the dc to dc converter output.

This will allow him to charge with pwm technology and get the same or better power to the bank than an expensive mppt unit.

BFL13 wrote:
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.

So are you planning a 24 volt battery bank to run a inverter? That normally works great, especially if you are installing this on a MCI or other RV with a 24 volt starter and 24 volt alternator to charge the system while driving. If it is a cabin in the woods, then 48 volts will make sense if you have not already bought the 24 volt output controller and inverters.

With a 24 volt system, the battery is still rated in the same 'amp hours'. Just that you can store twice as much energy in a 24 volt 100 amp hour battery bank compared to a 100 amp hour 12 volt battery. (2400 watts VS 1200 watts).

Your wiring size is smaller on a 24 volt system. AS noted a 230 watt panel is only 8.3 amps - so #`10 wire can handle 2 of them in parallel, or many in series. Many controllers can have a 150 volt or 600 volt 'maximum input limit' - so it would be possible to have 8 -10 solar panels of the same amperage capacity installed in series, then using #10 wire run this high voltage to the MPPT controller, with it's output being limited by it's output amperage maximum and voltage that the battery is rated at.

Some MPPT controllers are auto-adjusting for their input and output voltages. Connect them to a 24 volt battery, and they will regulate at 28.8 volts to shut off. Same controller to a 12 volt battery, and it will shut off at 14.4 volts.

Your battery bank weight will be the same no matter the voltage for a given number of watts stored. So 2 each 100 amp hour 12 volt batteries in parallel will store 1,200 watts in each battery. If in series you will have 100 amp hours at 24 volts, if in parallel you will have 200 amp hours at 12 volts. Both will store 2,400 watts. Normally a 100 amp hour 12 volt battery will be 60 pounds, and installing to many batteries will add a lot of extra weight to a RV, so try to avoid that.

If you will be using a lot of 12 or 24 volt power each day, it would be better to install more solar panels than to install larger batteries. You only need to size a RV battery bank to your night time loads, and let the daytime loads run directly from the solar panels. As you know, a 230 watt solar panel is only about 30 pounds, and will produce about 1,000 watts per day, while the battery will only store power, and not 'produce' anything.

The other disadvantage of a larger battery bank is they lose about 10% of their capacity monthly. So a 200 amp hour battery will lose 20 amp hours, or 240 watts a month, about 8 watts per day lost to in-efficiencies in the system. 20 AH at 24 volts is 480 watts, or 16 watts per day.

Good Luck,


Fred.

wa8yxm wrote:
To summarize
If you are measuring the amp hours capacity of the battery bank then amps = amps no matter what the size....

If you are measuring state o charge of two same capacity (Amp hour) battery banks, then you caharge them at the exact same rate. (Amps) they should charge together, If you charge one at half the rate, it will take twice as long to recover.

If you use WATTS of capacity,, Your math is good.

I think you confused yourself.

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

So two 12-volt batteries in series to produce 24 volts have half the capacity (amp hours) of two 12-volt batteries in parallel.

The equalizer is that when you use amps out of the 24-volt setup you use half as much and the rate (amps) you recharge is also half.

So ignoring losses, 200 amp hours of 12-volt battery bank equals 100 amp hours of 24-volt battery bank.

MEXICOWANDERER wrote:
Go down the line BFL and check the voltage across each positive and negative. 12-volts same as always. Charge amperage acceptance is the same too. Chech the voltage differential in each series pair. Unequal voltage equals problems - fix it so each battery in series reads the same. Check this while charging then again at rest

My two "12s" are actually four 6s. The two "12s" in series look an awful lot like four 6s in series now that I draw a picture of it!

My pairs of 6s have always had slightly different voltages for each 6, which I thought was trivial. Not trivial? Four in series each a tiny bit different in voltage makes a "bad 24"? How bad is bad? They are all at full baseline SG after four years in an ugly series- parallel with uneven links etc.

I figure with my drawing of the string of four 6s:

The charging wires stay at each end of the string on same posts whether 24 or 12.

To swap back and forth between 24 and 12, I just need to remove the neg parallel link's end from Batt #3 and shift the pos link's end from Batt #4 to the neg on Batt #3. That link would be longer than the other two links.

If it matters I could have another link the same length as the other series links dangling from the pos post of Batt #2 ready to go for when in 24 mode. The longer link would be then be dangling until in 12 mode.

------------------

On 24 vs 12 "capacity"-- Thanks. I was thinking something like that where a 100AH capacity bank at 12v is a 50AH capacity bank at 24v.
Their "20 hour rates" would be 5 amps for the 12v and 2.5 amps for the 24v but they would each still do the 20 hours. (To get down to 10.5v for the 12 and down to 21v for the 24) The 50% SOC voltages would be 12.1 for the 12 and 24.2 for the 24 with Trojan Wet 6s.

BFL13 wrote:
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! :) )

Some confusion

If you have 100 amp hours (C/20 Rate) of battery in both the 24 and 12 volt banks, then the answer to the above is NO, 1/2 the amps charging is 1/2 the charge rate.

However if you have say 1,000 WATTS of battery, then your math is good.

Since what we use is volt-amps, which at DC is the same as Watts, we normally would only use 50 amp hours worth of 24 volt batteries to replace that 100 amp hours of 12's (Both banks would thus be 1200 watts, or rounded, 1000)



To summarize
If you are measuring the amp hours capacity of the battery bank then amps = amps no matter what the size....

If you are measuring state o charge of two same capacity (Amp hour) battery banks, then you caharge them at the exact same rate. (Amps) they should charge together, If you charge one at half the rate, it will take twice as long to recover.

If you use WATTS of capacity,, Your math is good.

Looks like I have a lot of work to do once we set up with solar again after mid-April! Oh well, it's all to a good cause :)

Of course you guys on solar now could solve all this ahead of time.

Go down the line BFL and check the voltage across each positive and negative. 12-volts same as always. Charge amperage acceptance is the same too. Chech the voltage differential in each series pair. Unequal voltage equals problems - fix it so each battery in series reads the same. Check this while charging then again at rest

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.