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Hi. I hope someone can give me some info .I have just upgraded my solar regulator to a votronic mpp 250 and temperature sensor. All seems to be working as I expect it to exept the AES. I have connected this to the wire from my fridge (it took all morning removing the fridge to find the correct wire) the light often comes on but the fridge still runs on gas. Am I expecting to much? or do I have a problem?


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It depends on how the Fridge and the Motorhome are wired, some also require a separate relay to be triggered in the Power controller.

However, unless you have calculated that your Solar set-up can deliver the necessary 13 - 19 amps required, you might be expecting too much?


The documents often talk about the fridge being able to run on 'excess Solar power', but don't explain that the excess must be at least that required by the Fridge to operate on 12v.



For a Fridge to run efficiently on '12v', the main '12v' Element has got to receive 13v and around 13 - 17 amps.

When running from a good Alternator supply, a typical modern Fridge/Freezer will be fed 14.4v and around 15amps, depending on the Fridge. We have seen some Fridge Freezers drawing 17amps.


Obviously supplying a Fridge with a trigger S+/D+ voltage but then only putting 3 amps into the Fridge 'Cooling' Element, might get it to switch from Gas to 12v, but it clearly isn't going to get it to 'cool' at all.


So when these documents talk about feeding 'excess' power to the Fridge they sometimes use the habitation battery to make up the shortfall. This is based on the assumption you have already worked out that your Solar Panels will be big enough to run the 12v Fridge Element so any shortfall will be minimal.


However, what tends to happen, typically, is that the calculations are not done and the habitation battery provides the bulk of the power. Battery provides 12amps and the Solar 1 amp and as a result the battery discharges.



Or that the system fails to trigger at all because the 'excess Solar' is actually very few amps and volts, once recharging the battery to put back the previous nights draw is taken into account, - TV, Satellite, lights, Laptop, phone, etc.



If you were to rig the Fridge so it properly runs only off 'excess Solar' you will need about 400watts of Solar, 300watts to provide 15amps to run the Fridge and 100watts to put back the power drawn the night before.

Even then, obviously, the Solar will only run the Fridge efficiently during the strongest daylight hours and during the longest days of the Summer months. Even in these limited circumstances, probably only in Spain, etc.?



But is it worth the effort anyway?


A typical Fridge uses little Gas.

Most Gas consumption is taken by Heating, hot Water, Oven, Hob, etc. Not the Fridge.

A Fridge is very efficient on Gas because the thermostat turns it off for at least 50% of the time once it reaches temperature.


A lengthy seasons Fridge gas consumption is unlikely to be more than a single gas bottle, so spending £500 to save £29 worth of gas will take quite a long time to become a sound investment. Possibly 17 years?


But remember almost all Fridges, and especially Fridge/Freezers, are pretty poor on 12v. They almost always operate much more efficiently on gas, Also bear in mind that on many Fridge units, the 12v side does not include a thermostat. Generally only the 230v and Gas side have a thermostat as part of the system.


See this post for more info : http://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Thetford-N145-and-Votronic-mpp350/49059/



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Hi and thanks for your prompt replies.

I was not expecting miracles but I suppose not expecting the regulator aes light to be on if not enough power was available to operate the connected device. You have put my mind at ease as I thought I had maybe connected something up wrong.

Thanks again for the advice.


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  • 2 months later...

Since posting the above I have had a few people persevere with using 'Free' Solar to run the Fridge on 12v.


One such example has led to the batteries running low, even though he has 'adequate' spare current to run a 15 amp Fridge. This is because the battery bank is larger than the charging systems can support.

Latest email, followed by my reply -



"Hi again Allan

Sorry to keep bothering you but i fitted the Votronic regulator and meter and set regulator to AGM2 14.7 volts and is only charging to 13.5 volts can you please tell me if i have done something wrong this is the second regulator i have had and the other one was the same





"Hello, I would say that is about right for the batteries you have installed.

A battery that is heavily discharged, overlarge, getting a bit tired and/or has a big load on it (like 15amps for a Fridge Freezer) will place a massive load on the Solar regulator.

That power drain may be so high it stops the charger (this applies to Alternator and 230v mains as well) from reaching 14.0v, let alone 14.7v.


If you have a big battery bank that exceeds the capability of the charger, it's charge voltage may drop, resulting in slower less complete charging.

Your 13.5v is so low, it is effectively a 'trickle' charge rate which won't get a big battery bank to anywhere near full charge, especially if the batteries are not perfect

Typically an overlarge battery bank often only gets 80% charged, hence our battery webpages saying keep it within the capability of the charger. That applies to all battery chargers. Solar is no different.


As an example, turn the fridge off and disconnect all load to the 12v. Start your engine and monitor the habitation battery voltage as it is charged by the alternator.

Then turn on the Fridge and every 12v appliance in the vehicle, and watch the voltage drop.

On a German vehicle the voltage drop is small, about 0.2v, but drop it does.

That shows that even a 140amp alternator with 14.4v output can't maintain 14.4v when the load is high, so what chance will a 15amp Solar charger have?

It is working as designed, you are just asking too much"




Therefore anyone thinking of running the Fridge on Solar needs to be aware that the voltage for charging may be lower than without the 12v Fridge load.

This will obviously affect the Solar chargers ability to put back the load taken out the night before.

A voltage of 13.5v is fine to run a Fridge, but will have a major impact on the ability to charge up the battery.

In D's case above, his big AGM battery bank which must have 14.7v, will probably deteriorate fast with only a 13.5v charge rate. Four x 100Ah batteries every two years at £200 each, total £800, will be one heck of a 2 year running cost.

Once again, not making it worth saving £29 a year on a Gas bottle.



If you do go down this Solar Fridge' route, avoid AGM batteries at all cost. Flooded batteries will be the most tolerant to the lower voltages you are likely to experience, unless your Solar installation is in megawatts.







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I have a Votronic MPP350, works a treat, fridge runs on an off for a couple of hours a day when plenty of solar. I have 3 x 80a/h gels and 300 watts of solar. The Votronic does it in ½ hour blocks so as not to drain the batteries too much.


The only downside is the stupid Dometic fridge it selects 12v solar as the first energy source. So if you are on EHU the Votronic see the batteries as fully charged and solar available so the AES output is on therefore the stupid fridge selects 12v operation.

I have fitted a switch inline with the regulator AES output and the fridge S+ , when I get around to it I will fit a mains powered relay, no urgency as we rarely use EHU.

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Lenny, that is a nice set-up, but it runs off the battery, hence it needing to pause to stop the battery discharging too low.


We were talking above about a fully Solar supported set-up that doesn't deplete the batteries.

Few Solar chargers have the sophistication of the Votronic and can cripple a battery, because by the time they detect it is too low to run the Fridge the battery is already heavily discharged and approaching 'damage territory'.


Few will have the size of Solar Array you have, one person we were contact by, had a 200 watt panel, which in 3 months time, when the Sun gets lower in the sky and the days get shorter will not have any useful 'excess' capacity at all..



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