Solar controller and overcharging battery

[plwsm2000]

Apologies for the long post (the questions are towards the end :-) but I thought I should give a bit of background to this request.

 

Firstly, I would admit that I am particularly nervous about overcharging batteries as I had a bad experience last summer when my new gel battery failed after only 9 months use (it would not hold a charge). I found this out on a bank holiday weekend on Sunday morning on an aire in a remote part of France just as I was trying to put the electric bed away!. The MH had been on a constant EHU over the winter months and there are various "rumours" going about that this may not be a good thing to do for battery reliability. My Schaudt ELB223 gives a constant float charge at 13.7V that I think maybe too high to apply 24/7 and so could be a contributing factor. The battery was replaced under warranty, but overall, it was quite a stressful experience.

 

I am about to fit some solar panels and have been playing around with a Votronic MPP250 Duo controller to see what it will do to my habitation battery. Even though I have a gel battery, I have been looking to use the "Lead acid/AGM1" mode on the solar controller that gives a 14.4V full charge and 13.45V float charge to reduce the possibility of overcharging (the "gel" mode gives 14.3V full charge & 13.8V float charge) .

 

What I have discovered with the solar controller is that if the solar panel voltage drops below about 14V even for a short time (it only needs 18 seconds!), the charger will restart at full charge voltage for at least 30 minutes even with a 100% fully charged battery (BTW, this also switches off the AES output for an hour or so even when it was on before hand). The panels I am looking to use (2x100W not purchased yet) have an open circuit voltage of 20.8V and give max. power at 17.6V. Obviously at night, the solar voltage will drop well below this but what happens to the voltage during the day in full shade? (i.e normal driving through tunnels, near to tall buildings, tree coverage etc.). Would the controller restart each time? Has anyone measured the open circuit voltage in full shade? As the Votronic has a maximum input of 50V apparently, is it worth consider wiring the panels in series (with bypass diodes if needed)?

 

I am also looking to reduce the float charge voltage further to 13.2V when the MH is not being used over the winter (no significant battery drain). The Votronic controller has an input for a battery temperature sensor that I am not planning to use as my battery is located in a double floor and I keep a background heater on in the MH over winter so it should never see extremes of temperature. By adding a 2.2K resistor across the sensor terminals, it "fools" the controller into thinking the battery is at around 40degC and so it reduces the charge voltages to 14.0V full charge, 13.2V float charge. I was thinking of adding a switch to open circuit this resistor when the MH is in normal use. Does anyone have any thoughts on this idea?

 

Phil

[aandncaravan]

Phil, you know your stuff.

 

Your very low float voltage should result in a very healthy battery, even though the Votronic is already about the best in this respect.

 

Yes a good idea with the switch to open circuit the resister to get a higher charge rate when the MH is being used.

 

 

It is normal for most chargers (230v Mains and Solar) to restart the charge program at the beginning if interrupted, even for a second.

Every day when the sun comes up, the Solar charger begins a 'full charge' program, sometimes for 4 hours on some, even though the battery was fully charged when the Sun went down the night before when the Solar Regulator 'shut down'.

 

Your 30 mins at 'full charge' won't cause any harm, especially with the Votronic and especially as it has such a low 13.2v 'trickle' charge for the rest of the time.

 

However you can see the battery life shortening effect that 4 hours at full charge every day for a year can have with a budget Solar Reg. If you then top it off with a 'trickle' at 13.8v output and a battery could suffer heavily.

Especially on an old fashioned Antimony battery.

 

I wouldn't wire the Solar panels in series if it was me, because even if the AES feed stops for a while when driving through tunnels, past trees, etc, the Fridge will stay Cold for that hour the AES is 'off'? I would also expect it to switch to Alternator during that time anyway. I assume the Fridge does auto switch between power sources ok?

 

The problem with series connection means an issue any where in a panel or wiring, results in zero output.

 

When choosing your Solar Panels try and get the ones with the highest Open Circuit voltage, you will see there are small differences with the worst at 19v and the best at 22v, the higher voltage will have quite an effect in Winter.

 

That 3v difference could mean the difference between 11v or 14v at the battery in December. A bit of an extreme example, but you see the potential gain

 

 

 

[plwsm2000]

Thanks for your comments Andy Allan - I feel a bit more confident now.

 

Due to space limitations, I can only fit 2x100W panels and all the ones I have looked at seem to give around 17.5V at max power.(probably due to the number of cells used). I have space for 1x100W and perhaps 1x150W but obviously you cannot mix panels that have different voltages using just one controller. I have restricted myself to ones that use Sunpower cells as these have good reviews and Sunpower have been around for a long time. Maybe the 1x150W is a safer option.

 

I wasn't intending to use the AES function but I thought others may want to know this. It only gives out 0.2A max. so you need an external relay to power the fridge.

 

I am still interested to find out the typical output voltage in full shade (if anyone has measured this).

 

Thanks

Phil

 

(Name edited by Keithl)