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2x200W 24V Parallel Panels to 12V or 24V battery bank? Which is better and why.

Philfr

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I've decided to go with 400W solar made up of 2x200W 24V panels wired in parallel bringing theoretical 24V 16.6A down to the controller and then to a 3-4kilowatt LiPo battery bank & inverter in my small RV camper that has a typical set of existing 12V appliances, 110V outlets, as well as fridge and A/C. I'd like the boon docking freedom and the potential to run my soft start A/C for short periods on occasion. I really don't want to carry a gas & generator.

My question is whether the ease of going with a 12V battery bank set up is better than the efficiency of going with a 24V battery bank into a 24V to 12V converter. Both systems are shown on Will's videos. Thanks - I am just starting this retrofit and I just read another thread on this site where the guy's 24V to 12V converter caught fire maybe because he has the wrong voltage inverter so it's a good reason to be careful and ask you smart folks.
 
Are you planning on charging from the alternator, too? That might dictate bank voltage unless you have a DC-DC charger that will do 12v -> 24v. Otherwise the loads might dictate: using mainly native 12v == 12v bank. Using mainly 120vac would lean toward 24v bank. "Six of one", as they say.

As an aside, if I were running a 24v bank I'd consider running the 24v panels in series to get plenty of MPPT headroom. Some controllers require a pretty good delta (Vbatt + 5v) to get the party started. I think some Victrons act that way, 'though they only need +1v after the controller is running. I run nominal 24v panel -> MPPT -> 12v bank and it works pretty well.
 
For most RV's, a 12v system is less complex, less expensive and easier to understand than a 24v system. Where a 24v system makes sense is if the system is already 24v or you plan to install an inverter that is rated for more than 3000 watts.

The voltage of the panels doesn't matter, the MPPT solar charge controller will convert the panels voltage to whatever the battery's voltage is.

The converter fire you may be referring to was because that was a very incorrect usage of a converter. The converter was feeding an inverter that used way more amps than the converter was rated for. If you stick with 12v, step up/down (aka boost/buck) converters are not necessary.
 
Just as a point of reference, your AC for an hour will eat about an entire day’s worth of solar at 400W. I wouldn’t leave the gen at home just yet— get a propane capable model if you don’t want a gas can.

All other things being equal, 24V vs 12V isn’t likely to matter much in this scenario— you get the same watt hours from your batteries either way. 12V will use bigger cables, 24V will require a DC-DC solution of some sort. (More hassle, another box to troubleshoot when something rattled loose). Agree strongly with running your panels in series with a 24V system.
 
Are you planning on charging from the alternator, too? That might dictate bank voltage unless you have a DC-DC charger that will do 12v -> 24v. Otherwise the loads might dictate: using mainly native 12v == 12v bank. Using mainly 120vac would lean toward 24v bank. "Six of one", as they say.

As an aside, if I were running a 24v bank I'd consider running the 24v panels in series to get plenty of MPPT headroom. Some controllers require a pretty good delta (Vbatt + 5v) to get the party started. I think some Victrons act that way, 'though they only need +1v after the controller is running. I run nominal 24v panel -> MPPT -> 12v bank and it works pretty well.
Thanks- I had planned parallel hookup to minimize shading effects but it’s a small RV so maybe that doesn’t matter. If 24V battery bank then I don’t plan to charge from alternator.
 
Thanks- I had planned parallel hookup to minimize shading effects but it’s a small RV so maybe that doesn’t matter. If 24V battery bank then I don’t plan to charge from alternator.

My two panels are in series. I figure if one is in the shade, so is the other. To avoid shade and still get power, I also have two panels that I deploy on the ground.

Series connection is easier. Parallel requires a Y cable. Small expense, but one that I avoided.
 
Just as a point of reference, your AC for an hour will eat about an entire day’s worth of solar at 400W. I wouldn’t leave the gen at home just yet— get a propane capable model if you don’t want a gas can.

All other things being equal, 24V vs 12V isn’t likely to matter much in this scenario— you get the same watt hours from your batteries either way. 12V will use bigger cables, 24V will require a DC-DC solution of some sort. (More hassle, another box to troubleshoot when something rattled loose). Agree strongly with running your panels in series with a 24V system.
I see your points on complexity and troubleshooting. I was encouraged by a few YouTube vids timing softstart Coleman a/c operation anywhere from 1-3 hours from a +3kW battery bank. Yes replenishment from a 400W system would take a lot of time. I am only considering to 'take the heat off' 10-30min. Not all night.
 
The voltage of the panels doesn't matter, the MPPT solar charge controller will convert the panels voltage to whatever the battery's voltage is.

With some exotic exceptions, MPPT controllers downconvert the panels' voltage to charging voltage. I am suggesting there are common situations in in which 24v panels and 24v bank (or 12v panels and 12v bank. etc) lack sufficient voltage overhead to allow the downconversion, and the bank does not get fully charged.

temperature derating

For the purposes of illustration, let's assume

  • 36Vmp for mono and 35Vmp for poly panels.
  • Pmax/°C == -0.48
  • +1v minimum to sustain PPT function, as with victron
  • 14.4v quasi-Vabs, as with BattleBorn, so 28.8v for a 24v bank, so we need >= 29.8v to charge to manufacturer's recommendations.
  • the controller specs allow wiring the panels in serial

By 80deg F ambient Vmp is depressed ~16%, so our poly is down to 29.4v and cannot charge the bank to Vabs. Mono can hold 29.8v until the low 90F range, after which the controller can't reach Vabs either.

Wiring in series here gives us 70Vmp72Vmp to work with, plenty of excess voltage to avoid incomplete charging due to temperature-related panel derating.

If OP wants to run 24v panels and 24v bank I'd find panels with the highest practical Vmp to give the controller more of a chance in hot ambient conditions.



marginal insolation

Panels typically hold ~normal voltage until irradience drops below 20%, as one finds in dark overcast, rain, etc. In my observation in the wild, my panels put out something like 15% of rated in overcast and 8% of rated power under dark overcast. So we are talking about 32w-60w available if we can keep voltage propped up, vs 0w available if voltage collapses below Vbatt.

In my experience, running 24v panel into 12v bank provides enough excess voltage that even under voltage collapse conditions I am getting the yields described above. The same thing happens at dusk/dawn, but the power available then is so minimal I think it does not factor into the decision.
 
I would (still) argue that the voltage of the panels (24v vs 12v) doesn't matter. The configuration of the panels does matter.

My 320 watt panels are configured in series for ~80 volts. That would be plenty for any system voltage (12v, 24v, 48v).
 
With some exotic exceptions, MPPT controllers downconvert the panels' voltage to charging voltage. I am suggesting there are common situations in in which 24v panels and 24v bank (or 12v panels and 12v bank. etc) lack sufficient voltage overhead to allow the downconversion, and the bank does not get fully charged.

temperature derating

For the purposes of illustration, let's assume

  • 36Vmp for mono and 35Vmp for poly panels.
  • Pmax/°C == -0.48
  • +1v minimum to sustain PPT function, as with victron
  • 14.4v quasi-Vabs, as with BattleBorn, so 28.8v for a 24v bank, so we need >= 29.8v to charge to manufacturer's recommendations.
  • the controller specs allow wiring the panels in serial

By 80deg F ambient Vmp is depressed ~16%, so our poly is down to 29.4v and cannot charge the bank to Vabs. Mono can hold 29.8v until the low 90F range, after which the controller can't reach Vabs either.

Wiring in series here gives us 70Vmp72Vmp to work with, plenty of excess voltage to avoid incomplete charging due to temperature-related panel derating.

If OP wants to run 24v panels and 24v bank I'd find panels with the highest practical Vmp to give the controller more of a chance in hot ambient conditions.



marginal insolation

Panels typically hold ~normal voltage until irradience drops below 20%, as one finds in dark overcast, rain, etc. In my observation in the wild, my panels put out something like 15% of rated in overcast and 8% of rated power under dark overcast. So we are talking about 32w-60w available if we can keep voltage propped up, vs 0w available if voltage collapses below Vbatt.

In my experience, running 24v panel into 12v bank provides enough excess voltage that even under voltage collapse conditions I am getting the yields described above. The same thing happens at dusk/dawn, but the power available then is so minimal I think it does not factor into the decision.
This seems to factor into my question on the solar panel forum asking if anyone knew a source of 100W 24V solar. Then I could series parallel into nominal 48V 8amp going to the controller. I don't think such a beast exists however.
 
I would (still) argue that the voltage of the panels (24v vs 12v) doesn't matter. The configuration of the panels does matter.

Agreed, total string voltage is what matters in relation to battery voltage. ?

Since A) OP didn't mention panels of different voltages and B) your comment followed my input on configuration I (incorrectly) assumed you were suggesting 24v vs 48v string feeding a 24v bank wouldn't make a difference. Mea culpa.
 
Yeah, I should have included more context in my post. If all someone has is a single panel, then panel voltage certainly matters.
thumb_up.gif
 
Solar panels have diodes in them so if one panel is shaded you still get power from the other. I prefer series so you get more charging time in the day.

the 24v system would be very much a pain in the rear for such a small system... I have 700w of solar, 1000w inverter and 7.3KW hour battery at 12vdc 560ah... i wouldn't go to 24v unless you are going to run larger solar (1500w +) or larger inverter (3000w or more). I want to get a 5th wheel and will most likely go 24vdc in that but that will be 2500w solar 20KWH battery and 1 or 2 large inverter

Now on the 24v-12v convertor that you read that could on fire. It was because he was running a 12v inverter on a 24v battery.... you basically can't do that
 
Solar panels have diodes in them so if one panel is shaded you still get power from the other. I prefer series so you get more charging time in the day.

the 24v system would be very much a pain in the rear for such a small system... I have 700w of solar, 1000w inverter and 7.3KW hour battery at 12vdc 560ah... i wouldn't go to 24v unless you are going to run larger solar (1500w +) or larger inverter (3000w or more). I want to get a 5th wheel and will most likely go 24vdc in that but that will be 2500w solar 20KWH battery and 1 or 2 large inverter

Now on the 24v-12v convertor that you read that could on fire. It was because he was running a 12v inverter on a 24v battery.... you basically can't do that
Thanks - my goals are evolving. This started out as RV only system, but since fires and power outages are more commonplace in CA I will also use as emergency power for household appliances. I am starting out with 400W (2x200W 24V Rich Solar, 37.6Vmp, 45.6Voc in series) on the RV but would like to go up to 600W and then 800W later. I really like the MPP Hybrid LV2424 24V 2.4kW, 120V output, 80A MPPT ALL IN ONE - so unfortunately that drives me to 24V. No one makes an all-in-one yet that is 12V output but also has a higher power inverter. So using the MPP Hybrid all in one would make me use 24V (2x12V 170ah in series) battery bank. All of which would require me to use 24V-12V converter for the RV DC power, and disconnect any 12V coming from the car alternator to the RV. I don't consider the latter a big loss though.
 
For most RV's, a 12v system is less complex, less expensive and easier to understand than a 24v system. Where a 24v system makes sense is if the system is already 24v or you plan to install an inverter that is rated for more than 3000 watts.

The voltage of the panels doesn't matter, the MPPT solar charge controller will convert the panels voltage to whatever the battery's voltage is.

The converter fire you may be referring to was because that was a very incorrect usage of a converter. The converter was feeding an inverter that used way more amps than the converter was rated for. If you stick with 12v, step up/down (aka boost/buck) converters are not necessary.
Actually, I find that 12V is more expensive!... E.g., 1) Higher current means thicker($$) wires 2) MPPT controllers need to handle twice the current.
Also, Inverter/Chargers for 12V are less efficient... E.g., Multiplus 12/3000=>120A @ 12V; Multiplus 24/3000=> 70A @ 24V.

Granted, it's certainly not as convenient for 12VDC loads, which would require adding 24V->12v converters.
Still, the cost of additional DC-DC converters is much less than the difference between MPPT controllers... Do the math, and you'll see :).
 
Actually, I find that 12V is more expensive!... E.g., 1) Higher current means thicker($$) wires 2) MPPT controllers need to handle twice the current.
Also, Inverter/Chargers for 12V are less efficient... E.g., Multiplus 12/3000=>120A @ 12V; Multiplus 24/3000=> 70A @ 24V.

Granted, it's certainly not as convenient for 12VDC loads, which would require adding 24V->12v converters.
Still, the cost of additional DC-DC converters is much less than the difference between MPPT controllers... Do the math, and you'll see :).

If your wires are so long that there is a significant savings at 24v, then locating the devices closer together is one solution. In my case, the cost for cables, 12v versus 24v, would be no more than a couple dollars.
 
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