30V solar panel vs. 12V caravan system

KlausV · July 2015

Hi all

I'm new to this forum and it is the first time I'm working sith solar power systems.

Recently I bought an old caravan which I will use on a festival in Denmark. Then I got the great idea to build up a solar power system to power the refrigerator and to charge our cell phones etc.
The caravan already has a build in 12V system with a 12V battery to power up the loads.

I have installed a 30V, 225W solar panel on the roof and bought a PWM charge controller like this (http://www.phocos.com/products/cx-series-10-40a) I also bought an additional 12V battery so I could make a 24V system as the solar panel would be more efficient to charge a 24V system with a 30V panel.
BUT when hooking up the batteries in series and charging them as a 24V battery bank but only taking the load from one battery (12V) then it empties only the one battery..

So now I have decided to hook the batteries in parallel to get a 12V battery bank and then use the PWM charge controller to charge the bank with 12V. I know that it is not much efficient as the PWM controller will charge with the same current but at lower voltage = less Watts. But it is possible, right?
I should mention that nothing was connected to the charge controllers LOAD terminal - all loads were directly connected to the battery.

Another thing I was wondering about is if it is possible to keep the 24V battery bank and connect the 12V caravan system to the charge controllers LOAD terminal? But is that even possible?
30V solar in --> Charge 24 V battery bank --> Output load 12V (from charge controller)

I don't want to buy a MPPT charge controller as this is just a cheap festival set-up.

The only consuming things in the caravan is:
85W absorption refrigerator
a few lights (will not be used that much)
a 50W car radio with speakers

Then I also have a 12V/230V inverter so we can charge our phones when needed.

I have made a quick drawing of my system set-up :-)
Attachment not found.

Thanks in advance for your help

Best regards from Denmark
Klaus

vtmaps · July 2015

As you realize, you cannot draw from one battery when the two batteries are in series. If you must have 12 volts from a 24 volt bank, you should get a DC to DC converter. But there is another problem with having your batteries in series... The Vmp of a solar panel drops when it is in the hot sun and if you start with a Vmp of 30 volts it will be inadequate for a 24 volt battery. One way to use that panel is to put two panels in series (Vmp = 60) and use a MPPT controller to charge a 24 volt battery. Your other option is to get a boosting controller (Genasun, Solar Converters, and others make them).

Yes, you can put the batteries in parallel and charge them with a PWM controller, but as you realize, that will be very inefficient... you are wasting half your panel.

By the way, does your absorption fridge run on propane? If so, 85 watts is a huge electrical load for a propane fridge. If your fridge draws 85 watts continuously (rather than cycling on and off), you should consider another fridge... your system will never be able to keep up with a continuous draw of 85 watts.

--vtMaps

KlausV · July 2015

Hi vtMaps

Thanks for the quick response!
This is the data sheet for the panel:
Attachment not found.
The rated voltage is about 30V but the open circuit voltage is about 36V. Shouldn't that be sufficient to charge a 24V battery bank?

My absorption fridge is a 3-way type that can run on 24V/230V/propane as the heating element. I actually don't know if it runs in cycles like a compressor fridge. It is an old model from 1979.

But another thing is that I just realized my DC to AC inverter can run on both 12V and 24V circuits and the caravan also has a 230V system.
Will it then be better to keep the 24V battery bank and then use the inverter to power up the 230V system in the caravan? (it is a 1500W inverter)

Thanks

vtmaps · July 2015

KlausV wrote: »

The rated voltage is about 30V but the open circuit voltage is about 36V. Shouldn't that be sufficient to charge a 24V battery bank?

No. Open circuit means you are drawing no power. As soon as you draw power the voltage will drop.

KlausV wrote: »

My absorption fridge is a 3-way type that can run on 24V/230V/propane as the heating element. I actually don't know if it runs in cycles like a compressor fridge. It is an old model from 1979.

Make sure it is running on propane. If you run it on electric it is very very inefficient. Many of those fridges automatically switch to electric when AC power is available. If you use your inverter, make sure that the fridge is not connected to the inverter output.

KlausV wrote: »

But another thing is that I just realized my DC to AC inverter can run on both 12V and 24V circuits and the caravan also has a 230V system.
Will it then be better to keep the 24V battery bank and then use the inverter to power up the 230V system in the caravan? (it is a 1500W inverter)

Yes, use the inverter on 24 volts. But make sure you have a way to charge a 24 volt battery... your current panel and charge controller cannot do the job.

--vtMaps

zoneblue · July 2015

Your 60 cell panel as vt said is not suitable for 24v battery. This is a frequent misunderstanding. Your choices are:
- get a 72 cell panel (or two 36 cell panels) to charge 24v with pwm
- get the wee morningstar Mppt controller and charge 12V with the 60 cell panel
- or use a 36 cell panel to charge 12V.

To explain why a 60 cell panel wont work heres why:
- A PV panel doesnt run at either Voc or Isc. At Voc theres no currnet. At Isc theres no voltage. They run near Vmp.
- Rated Vmp of 29V is in ideal "test" conditions.
- Add some real world heat, then youre down to maybe 28V.
- Add some real world array cable losses and your down to 27.5V.
- Add some real world charge controller losses and youre down to 27V
- Battery needs 28.8V (or so) for the duration of its absorb stage. (FLA needs 29-30V for EQ stage.)
- Early bulk will charge just fine, as battery only at 26V, right near the realisable Vmp.
- But the most problematic time is end bulk and early absorb, when the battery voltage is highest and the charge current is still high.
- What happens then is it goes into a very very slow trickle type process, and at the end of the day its still not charged.
- Not charging properly = sulphation = end of battery.

zoneblue · July 2015

Actually theres one other choice. 60 cell panel plus 8S LiFePO4 battery. These batterys are starting to become more readily available, (boatee and motorbike crowds like them for different reasons. (boats, handle partial SOC for long periods, for bikes: light). They dont need the absorb stage and have been reported to charge fine with 60 cell panels. But imo youll still need short/fat cables.

Shadowcatcher · February 2016

The better choice may be to go with an MPPT controller http://www.morningstarcorp.com/products/sunsaver-mppt which will get the full benefit from your high voltage panel and keep you in the 12V range. This is the one I use with our 60 cell panel.
An EU manufacturer http://http//www.steca.com/index.php?Steca-Solarix-MPPT-en I have one of their PMW controllers.

30V solar panel vs. 12V caravan system

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