Solar charging question

[NINOT]

I have a 210W mono solar panel, 2 Varta LFD90 batteries and Votronic MPP 250 Duo. It is not connected to the alternator. The only power usage is Dometic CDF18 fridge. This summer every day around 1pm the batteries showed 13.4V ("battery full" light ON on controller) and in the morning the reading was 12.5V ("MPP" light ON on charge controller) I have Efoy system as a back-up. According to the chart that aandncaravan posted 13.4V is 100% full battery and 12.5v is 50% full. My question is: is my set-up working correctly? Is the drop from 13.4V to 12.5V overnight normal? If there is a cloudy day how much longer would batteries hold up? The Efoy system starts when detecting voltage of 12.3V.

[Agaric]

I have a 150w panel with 1 Varta LFD90, with a Votronic MPP165 Duo and an Efoy unit as backup.

 

I have the Efoy switched off and only use it when needed.

 

How much is your amp drain that will tell you why you have the voltage drop, mine with nothing switched on has a maintenance drain of 0.6amps per hour.

 

Also while the panel is charging you'll get a higher voltage than a resting voltage.

[aandncaravan]

Why don't you fit a £10 Wattmeter, found on eBay, which will track the amphours harvested and the exact consumption.

They include a timer so give the exact picture over a day or a week, etc.

 

It will tell you both power used and the Solar charge put back, also displays volts, amps and watts.

 

[BruceM]

I have a similar setup (Solar panel, Varta LFD90s, Votronic MPP 250 Duo). I believe your fridge is rated at about 0.64Ah at 32C so I’d have thought that if fully charged the 180Ah of Vartas would keep the fridge going for quite some time (about 10 days? – someone better check my arithmetic). Add in the solar top up and that could be extended quite a bit depending on the current output from the solar which, as Alan says, may be measured with a low cost watt meter.

 

Re the voltage readings. Do you disconnect the fridge when you check the voltage? If not then you’ll be measuring the battery voltage under load which might explain the 12.5v . Likewise, if the solar is active you will be measuring the Votronic charging voltage or even a combination of the two.

 

[Agaric]

It's more like 6 days down to 50% of charge.

[Dr Dave]

What you do have to remember that the only way to gauge the state of charge of the batteries by terminal voltage is with no equipment connected and a period of standing. If even a light load IE: just the control panel connected to the batteries will skew the results.

Dave

[NINOT]

Thanks everyone. I will buy the wattmeter, the reading I took was with fridge connected.

Now that travel season is almost over, what happens if I disconnect the fridge. There will not be any power usage, is it ok to leave the solar to only charge the batteries or should there be some sort of switch (to turn OFF) between solar and batteries, which I do not have?

[Charles]

No you can leave the solar connected all the time. The Votronic controller will flash it's LED briefly at night so you know it's waiting for the sun to come up (when my van is in the garage it's flashing all winter, I might pull the fuse this year though, there's no point it being on). Off the top of my head the controllers use about 6ma at night which is very low.

You can probably just switch the fridge off?