I posted a similar question a while ago and got the answer I thought I was looking for but I now have a new multimeter so I'm able to get readings that should be of some use.

I have a solar panel, charge controller and battery. I've left it out for over a week to charge in very good sunlight and the battery hasn't charged. The battery level indicator on the charge controller stays the same. I will try and provide as much info as possible. If someone could point out what is wrong and what I need to do to I'd it, that would be fantastic!

Solar Panel spec:

  • Pmax - 5W
  • Vpm - 17.9V
  • Ipm - 0.28A
  • Voc - 22.41V
  • Isc - 0.3A

Battery spec:

  • Sealed lead-acid
  • 12V
  • 4.5AH
  • constant voltage charge
  • Initial current: less than 1.35A
  • Cycle use: 14.0V-15.0V
  • Standby use: 13.5V-13.8V

Solar Charge Controller spec:

  • Voltage: 12V/24V AUTOMATIC
  • Current: 5A (That's all that's on the sticker)

When I measure the voltage across the solar panel on its own it gives around 17v when in good sunlight which will charge a 12v battery, right? When I hook up the solar panel and battery to the solar charge controller and measure the voltage across the battery terminals it's around 12.2v and across the solar terminals it's around 12.4v. What's happening? I should probably also point out that when the solar panel is out of the sun and therefore producing less than 12v, the battery terminals still show around 12.2v on the multimeter while the voltage across the solar terminals shows less than 12v. I also don't understand why the load light on the charge controller is always on even when there is no load connected and measuring the voltage across the load terminals, it shows 12v. The voltage across the battery after charging is often around 12.3v but the the next da has returned to around 12.2v

I would appreciate at help. Thank You


closed as unclear what you're asking by Leon Heller, Anindo Ghosh, Matt Young, PeterJ, Dave Tweed Aug 26 '13 at 11:35


  • \$\begingroup\$ What current should be charging the battery? Is the controller specified for a panel that is only 5W? \$\endgroup\$ – Andy aka Aug 24 '13 at 18:12
  • \$\begingroup\$ I don't know the wattage that the controller is specified for as it only has voltage and amps on the sticker. Could you explain what you mean by "What current should be charging the battery?" \$\endgroup\$ – user2329585 Aug 24 '13 at 18:27
  • \$\begingroup\$ What is the recommended charging current for the battery. For instance, if you fed the charger from a bench power supply capable of delivering about 15V, what current would the charger push into the battery as a means for charging it. \$\endgroup\$ – Andy aka Aug 24 '13 at 18:34
  • \$\begingroup\$ Ok, well all the info I have on the battery is in the question itself. Is there nothing else that could be causing problems, because it all seems like it should work to me. \$\endgroup\$ – user2329585 Aug 24 '13 at 18:53

Throw out (put it back on the shelf) that charge controller, there isn't enough information about it to make sense of.

Just get a Schottky diode (1A will do. Find one on eBay or whatever, look for 'solar panel blocking diode') and put it between the solar panel and the battery. You will at least get some charging from the panel. If you pop out every evening and disconnect the panel from the battery before dark, you could even skip the diode.

You should measure the current from the panel to the battery. But this might be too likely to destroy your meter or at least blow the fuse in it, as it is nearly 100% certain you will put the current meter probes across the battery.

A quicker way is to see if the battery voltage goes up when you connect the panel to it. First get rid of 'surface charge' by discharging the battery a bit, say, with a light bulb or something. Remove the lightbulb or whatever. Then, measure the voltage (across the battery) before you connect the panel, then right after (with the panel in full sun, of course).

If you just leave the panel connected, with no blocking diode, when the panel is in the dark current will flow back into the panel from the battery, and, depending, may discharge the battery more than it is charged in the day. That is why I said 'if you pop out every evening and disconnect ...'.

Once you get all this sorted out, have worked out some way to measure current, know what condition your battery is in, and have your system doing some work, try the charge controller again. Probably it works just fine, and there is some mis-understanding.

  • \$\begingroup\$ Thanks for your answer. I did what you said and connected the panel directly to the battery and when I measured the voltage, it showed 12.27v. Is that normal? Should I just leave it out and see what the voltage from the battery is after a few days or a week and see if it's risen slightly? \$\endgroup\$ – user2329585 Aug 25 '13 at 8:55
  • \$\begingroup\$ Another question. Is a 1A Schottky Barrier Rectifier alright to use? Thanks \$\endgroup\$ – user2329585 Aug 25 '13 at 10:46
  • \$\begingroup\$ Thank you sir for the edit. I did what you suggested and when I measured the voltage between the + and - on the battery it read 12.2. When I connected it to the solar panel and did the same thing, it read 12.3. Is this a good sign? I also tried placing the multimeters positive to the + on the solar panel and the multimeters negative to the - on the battery and then it read 6v! Is that a bad sign? I hadn't used the multimeter like that before. Thanks again \$\endgroup\$ – user2329585 Aug 25 '13 at 16:43
  • \$\begingroup\$ Yes, that's what I did. I measured the different between the panel and the battery. I take it that isn't what I need to do. Anyway after leaving the solar panel in the sun for around a day and then disconnecting, removing the surface charge with a motor, then leaving it disconnected overnight, in the morning, the voltage had stayed around 12.7. It seems that it's charged. I shall have to see how things go from there. Thanks for your help and apologies for anything that was unclear (as at least 5 people seem to think I was unclear) \$\endgroup\$ – user2329585 Aug 27 '13 at 7:51

Your solar panel may be able to put out 17 V open circuit but it quickly drops when you put a load on it which,apparently, is what your charge controller is doing. Without a data sheet and/or schematic for the charge controller, it is difficult to troubleshoot anything.Given the specifications you have listed, it is almost certain that the solar panel could not do any harm to the battery. Therefore, as suggested by Bennett, discard the charge controller and just connect the panel directly to the battery. The diode is a good suggestion to avoid any possibility of the battery trying to feed power back to the panel.

  • \$\begingroup\$ The diode is essential if left connected when not enough sun to charge. \$\endgroup\$ – Russell McMahon Jul 22 '14 at 23:45
