12V solar panel battery charging

[jj4091]

I bought but never used a 12V car solar panel charger so warranty is no longer valid.
Recently decided to set it up in the garage to charge spare 12v 40Ah lead acid car battery.
Connected it all up but have never been able to get it to actually charge the battery
Voltage from solar panel = 18+ vdc in hazy sun
Voltage at battery 11.8 volts after a few days usage but was always powerful enough to crank the engine
Thinking controller faulty bought a new one.
Same problem so thought I would bypass the controller, and read the same individual voltages before connecting solar panel to the batteery.
After direct connection of panel to the battery V drops to 11.3 V measured at the battery and doesn't go any higher.
Doide and polarity are ok.
Does this suggest panel is faulty and breaking down under load. Many thanks

 

[flameport]

Voltage with battery connected will be pretty much the same as the battery without anything connected.

To check if the battery is charging requires that the current is measured.

How big is this panel?

 

[Collectors]

I would mesure them indevidualy 1st & not being conected together & charge the battery with a normal charger for a good 8 hours, then disconect & see what the voltage is. 12.6-12.8 fully chaged, 12.4v 75%, 12.2v needs charging & anything below 12v could be nackered.

 

[jj4091]

Voltage with battery connected will be pretty much the same as the battery without anything connected.

To check if the battery is charging requires that the current is measured.

How big is this panel?

Yes I thought it should read which ever was supplying the highest voltage but it doesn't it drops when the panel is connected.
With a mains charger connected to the battery the voltage reads 14+ volts and I would have expected similar from the panel
Won't the voltage increase as it charges, where would you need to measure the current, between the panel and the battery?
Not a big panel approx. 12" x 15" but sold as being suitable to charge a 12v lead acid battery.
Would the current drop and voltage increase if it was charging, as that is the only thing I haven't checked..

 

[jj4091]

I would mesure them indevidualy 1st & not being conected together & charge the battery with a normal charger for a good 8 hours, then disconect & see what the voltage is. 12.6-12.8 fully chaged, 12.4v 75%, 12.2v needs charging & anything below 12v could be nackered.

That is the measurements I have given.
As I said battery is fine only recently removed from scrapped car and been used to power led strip light
Charging it fully first would seem to defeat the object

 

[Collectors]

Try an inline amp reading on just the negitive side & compair with what the panels output should be.

 

[jj4091]

Try an inline amp reading on just the negitive side & compair with what the panels output should be.

Thanks I will try that but have no idea what it should be. Will the current drawn vary with the Ah of the battery being charged rather than the panel.

 

[Johntheo5]

Don't some of these solar trickle chargers have unregulated voltage?, ie, 18/21DCV open circuit, then, depending on the battery condition, right down to 11V ish or so, these might only produce 10/15 watts.

 

[pcaouolte]

Charging it fully first would seem to defeat the object

I believe that the very best you can hope for is that the solar panel will replace the energy used while the car is standing idle. It isn't going to charge a battery from flat unless you live on the equator.

 

[ericmark]

I used one in a caravan to stop self discharge, it was very simple, a diode to stop current flowing battery to solar panel, and a zenor diode to stop it over charging.

My smart battery charger has 5 fixed charge levels, 2.8 amp, 2 amp, 0.8 amp and 0.1 amp and zero. And is controlled by voltage, once the battery hits 14.4 volts, it alternates between 0.8 amp and 0.1 amp and zero. So if it hits 14.4 volts at 0.8 amp, it will drop to 0.1 amp charge, and if it drops to 12.8 amps, it will switch back to 0.8 amp charge, it has never with a car battery hit 14.4 volts with a 0.1 amp charge, but if it did, it would go to zero.

So to maintain a 12 volt car battery we are looking at between 0.1 amps and 0.8 amps, a 200mm x 300mm panel is rated at 300 watt (2.2 amp) maximum. So, yours around ½ that size, so around 1 amp maximum. So a 40 Ah battery, at around 2 Ah per day, which using my own solar panels as a referance seems about all you can hope for, it would take 20 days to charge.

My car battery when car is not used, will last around 6 weeks before it will fail to start the car if not used, so in real terms this time of the year, with panel not in shade, it should with a 12 watt lamp for around an hour a day. As winter draws in that will reduce to zero with the panel only about keeping the battery from self discharge.

But

After direct connection of panel to the battery V drops to 11.3 V measured at the battery and doesn't go any higher.

That makes me think there is no diode in circuit to stop back feed, and the panel is actually draining the battery.

I note my garden lights solar powered work well in the summer, but by this time of year, seems no sooner turned on, and battery is discharged.

 

[jj4091]

I used one in a caravan to stop self discharge, it was very simple, a diode to stop current flowing battery to solar panel, and a zenor diode to stop it over charging.


So to maintain a 12 volt car battery we are looking at between 0.1 amps and 0.8 amps, a 200mm x 300mm panel is rated at 300 watt (2.2 amp) maximum. So, yours around ½ that size, so around 1 amp maximum. So a 40 Ah battery, at around 2 Ah per day, which using my own solar panels as a referance seems about all you can hope for, it would take 20 days to charge.



But

That makes me think there is no diode in circuit to stop back feed, and the panel is actually draining the battery.

Basically I would be happy if it maintained the charge but as it is dropping the voltage across the battery I am thinking it is going to drain it instead.

My panel is as I said approx 12" x15" so actually bigger than 200 x 300mm ( 8" x 12" )

As I also said the diode to stop feedback is ok and I have fitted a switch that can isolate when no output. The overcharging would have been prevented by the controller if it worked but I don't think there is a Zener built into the panel

I'll try measuring the current and fully charge it from the mains and see if it will then maintain the voltage

 

[jj4091]

I believe that the very best you can hope for is that the solar panel will replace the energy used while the car is standing idle. It isn't going to charge a battery from flat unless you live on the equator.

It's not connected to a car just an LED strip light which is used occasionally and drops the voltage over several days. I wasn't expecting it to charge from flat but did think it would charge from 11.5 V not make it discharge further.

 

[jj4091]

Don't some of these solar trickle chargers have unregulated voltage?, ie, 18/21DCV open circuit, then, depending on the battery condition, right down to 11V ish or so, these might only produce 10/15 watts.

That is what appears to be happening. In which case I wouldn't consider it fit for purpose as you were supposed to be able to charge either a leisure battery or the main battery in a camper van without the use of mains hook up or the alternator running
So my original thought that the panel was no good seems to be correct as I am not sure what useful purpose it would realistically serve

 

[pcaouolte]

I wasn't expecting it to charge from flat but did think it would charge from 11.5 V

A car battery at 11.5V is flat.

 

[Johntheo5]

A car battery at 11.5V is flat.
View attachment 422756

Volts	Capacity %		volts/cell
11.51	10		1.918
11.66	20		1.943
11.81	30		1.968
11.96	40		1.993
12.10	50		2.017
12.24	60		2.040
12.37	70		2.062
12.50	80		2.083
12.62	90		2.103
12.73	100		2.122