Battery voltage

Post by **bmcecosse** » Thu Dec 03, 2009 9:57 pm

My battery is on charge - via one of the little solar panels from Maplins. This gives a whopping 1 watt of power - so just under 100mA. Today when off the battery it was showing 23 volts on open circuit - although in bright sun I have seen well over 30 volts. The battery is sitting at 12.57 volts (my meter is a 'good' one - although not recently calibrated!) - I was a bit disappointed at this - had been hoping for nearer 13 volts, although it seems 2.1 volts is the correct 'off charge' voltage for a fully charged cell. Anyone care to comment ? Anyone else using these chargers ?

Post by **MColes** » Thu Dec 03, 2009 10:10 pm

Should have taken it to sunny Tenerife to charge

Post by **nigelr2000** » Thu Dec 03, 2009 10:15 pm

Well this time of year we get maybe 8 to 10 hours of usuble sunlight if we are lucky so if we assume 10 hours thats 1 amp of charge per day, not a lot or put another way its charging at 1/10th of an amp hour so a 50 amp hour battery would in theory assuming no losses and 10 hours of usable sunlight a day take 50 days to charge from flat. In reality it would probably take a lot longer than that. Have you tried an ammeter wired in series with it to see what the actual charge supplied is ??

Post by **bmcecosse** » Thu Dec 03, 2009 11:23 pm

No - I haven't tried an ammeter - not sure if my meter will go that low! I'm only trying to 'maintain' this battery - which in all honesty is in my TR7, which I don't like to start-up unnecessarily. Although I am considering buying another solar charger for the Minor - which does get started regularly. Of course - it has the advantage of the electric pump - and so starts quickly, the TR7 has to be churned for ages to pump up fuel before it will start.

Post by **RogerRust** » Fri Dec 04, 2009 8:03 am

I don't if this info is of any use.

I have recently made a trickle charger from a 1 amp 12 PSU unit. I adjusted it to provide 13.6 volts off load. At that voltage it floated the charge around 1 amp when I first fitted it dropping to 50 milliamp over a few days. I plan to leave it on all winter if I'm not using the car.

Post by **bmcecosse** » Fri Dec 04, 2009 11:18 am

The other way of doing it is to have a higher voltage, and a large resistance in line. Thus giving a 'constant' current no matter what voltage the battery throws back. I have a couple of units I built like that - to recharge Ni/Cd battery packs I have in my railway lamps!
What volts is your battery sitting at now Roger ?

Post by **RogerRust** » Fri Dec 04, 2009 2:25 pm

It drops to 13.2 when the charger is swithched off for 24hrs. I don't want to cook the battery.

Post by **crossword** » Fri Dec 04, 2009 4:10 pm

Roy,
There is a FAQ very similar to yours on Maplins website under solar battery chargers . The response from Maplin is that a measured 24 V is normal open circuit . I suspect that there is a series resistor to protect the panel in case of either short circuit or a very discharged battery . I guesstimate that to be about 120 Ohms. if you have a resistor of that value available , you could load the panel with that resistor & see if the voltage drops to 12 volts approx

Regards Andy

Post by **bmcecosse** » Fri Dec 04, 2009 4:21 pm

I've no problem with the panel - just a little surprised to 'only' get 12.57 volts at the battery - but according to 'Wiki' - 2.1 volts is correct for a fully charged cell 'off charge' - and the solar panel is so puny, I guess it is effectively off charge! As long as it holds at that through the winter - I'll be happy enough.