Building a 24V battery charger

[bezza7]

Before I start I am not a trained electrician so please don't be too harsh on me.

I was interested in building a small solar charger to top up the batteries on my van since it is sometimes parked up for up to a month without use and the background electrics such as alarm/immobiliser slowly drain the battery- the batteries are 50% discharged in about 2-3 weeks.

The van runs off 24V - 2 x12V batteries and I decided to purchase a couple of small 12V solar panels which I wired together in series and then ran through a regulator to the batteries. The regulator shows that the panel is charging the batteries but in actual fact the batteries actually appear to discharge quite quickly- much more quickly than if the panels aren't connected.

Could anyone make a suggestion about what the problem might be here. I was thinking that the batteries might be discharging to the panel in low light but surely the regulator should stop this. Should I ditch the regulator and replace it with a blocking diode?

Any suggestions appreciated.

Thanks Mike

 

[Lighthouse Phil]

First thing to look at -- have you wired in the panels in reverse to the batts.? ie, pos of panels to neg of batts ? Easy mistake to make, put a meter across the output from the panels, have meter set to read DC volts,, if the red lead from your meter is on the negative of the DC supply (solar panel), you'll see the minus symbol on the meter read out..so what you thought was pos is neg. Another thing...all the electronics in your van could be drawing more current from the van battery then the panels are putting in. What size output are the panels? I've worked with panels with very small outputs (2W) ..if your vans electronics are pulling 0.5A at 24V, your 2W panels will not replenish the battery and it will soon die (RIP).

 

[solarfred]

Agree - what are the panel specs and what is the controller? Then we can have an opinion.

 

[bezza7]

Hi thanks very much for the replies.

The leads are on the right way round I double checked it with my voltmeter. It is also showing as charging the batteries on the regulator which is a one of these

Wholesale New 10A Solar Panel Charge Controller Regulator 12V/24V Autoswitch Express, Free shipping, $13.24-17.0/Piece | DHgate

Before I put the panel in I checked the current drain from the van's electrics which was working out at about 0.5A. I then isolated where the drain was coming from by taking fuses from the box and worked out most of the drain was coming from the cab electrics plus the diesel coil. So when the van is parked up I take these fuses out which leaves the drain at about 0.1A. I have wired two 12V 10W panels together in series which i think should pull in about a peak of 0.4-0.5A in bright sunshine and maybe 25% of that in shade. Obviously its not great but I was expecting it to at least keep my van batteries topped up and in good nick.

Agree - what are the panel specs and what is the controller? Then we can have an opinion.

 

[Richard Burns]

The charge controller you link to states in the specs that the supply voltage is 12V, although it says it has an auto switch and you say it is charging so should be OK.

The supply to the van should then be taken from the output of the charge controller however unless you have rewired the van this is unlikely so the charge controller is not supplying power to van circuits and the monitoring system will not know the output voltage of the battery.
With the supply you have you could probably remove the charge controller.
The panels should (but check) have an internal blocking diode to prevent discharge through the panel in darkness.

 

[bezza7]

Ok so I can just ditch the charge controller and not worry about potentially overcharging the batteries bearing in mind I am running such a small current?

The charge controller you link to states in the specs that the supply voltage is 12V, although it says it has an auto switch and you say it is charging so should be OK.

The supply to the van should then be taken from the output of the charge controller however unless you have rewired the van this is unlikely so the charge controller is not supplying power to van circuits and the monitoring system will not know the output voltage of the battery.
With the supply you have you could probably remove the charge controller.
The panels should (but check) have an internal blocking diode to prevent discharge through the panel in darkness.

 

[solarfred]

I am confused? Surely OP is simply charging the battery to keep it topped up. No need to re wire anything. Just wire the controller to the battery and panels. You must connect the battery to the controller first for it to recognise the voltage. Otherwise it may default to 12 volts. Fuse between battery and controller.
In any shade or dull weather you will not get 25% of the rated maximum output. I would have bigger panels or a 24 volt panel. On an average day the output is tiny as watts = volts x amps. There are inherent losses in the system also. I don't bother stocking anything under 30 watts for battery trickle charging.
More power!!

 

[Richard Burns]

I would say that you do not need the charge controller as most of your charge (if not all) will be consumed by the drain and so it would take a significant time to overcharge the battery, normally you would not need a charge controller on such a small loading.

I was probably a bit OTT with the not connecting the load to the output of the charge controller, sorry.