I've installed a 12V 80W solar panel plus controller and attached it to a 12V 105Ah leisure battery. At first it (appeared to?) work. The charge status on the solar charge controller was at max and it stayed that way even though I used power.

After a few months of no usage (but connected to the solar panel) I found the battery empty (the solar charge controller indicated overdischarge).

Recharging with an external charger brought the battery back up to 12.46 V, which I researched to be 85 % (I didn't measure before charging unfortunately), but the solar charge controller still only shows it at 25 % full (first LED lit as per image)

Solar charge controller

I can't figure out what this tells me - is the battery weak (it was new before the solar installation), did my solar set up damage it? Is the controller showing the wrong info?

UPDATE: Here's the manual and specs of the controller: enter image description here enter image description here

  • \$\begingroup\$ Something is VERY wrong. If you bought the battery and controller from Jaycar document what happened clearly and take them back - they probably need to replace both. If the battery came from elsewhere the Jaycar controller MAY have damaged it but they may be reluctant to be held responsible. | You seem to be in NZ. Ak or ??? I'm in Auckland - I can discuss further if desired. See my prile for email address. \$\endgroup\$
    – Russell McMahon
    May 8, 2019 at 9:44
  • \$\begingroup\$ Thanks Russell. Battery is not from jaycar. Hmmm sounds no good... . Okay thanks, I'll mail you tomorrow. \$\endgroup\$
    – Peete
    May 8, 2019 at 11:16
  • \$\begingroup\$ The 80% figure at 12.46V is about right for the open circuit terminal voltage of the battery after allowing the surface charge to dissipate ( batteryuniversity.com/index.php/learn/article/… ). 12.46V while charging would be a much lower state of charge - that's likely what the controller is basing its indication on. I'd suspect that your battery has an internal leakage or damaged cell. \$\endgroup\$
    – Phil G
    May 8, 2019 at 13:53

1 Answer 1


You have two problems.

  1. The MPPT charge controller is not charging the battery.
  2. The battery is not reaching 100%

I would start by giving the battery an Equalizing Charge. The battery may be okay.

First thing to do is measure the output voltage of the PV while the MPPT is (or should be) charging. Do this periodically throughout a sunny (or mostly sunny) day.

What I would be expecting is the voltage to remain fairly constant when there is sufficient sunlight (10:00am to 4:00pm). I would expect the voltage to be around 13 V.

What happens to the PV voltage when you change the angle of the panel?

If possible use a shunt resistor to measure the PV current as well.

With an 80 W panel you may get 3 amps at midday assuming the PV voltage is about 13V. Typically it will take days to charge the battery. About 20 Ah (250 Wh) per mostly sunny day.

Check the specs on the MPPT for the minimum PV current.

What you do next depends upon the results of the above. I will monitor this post to look for your results.

What I am testing above is how well the MPPT regulates the PV voltage. The MPPT will decrease the battery charge current when the PV voltage drops below the target PV voltage (typically PV ≈ 13 V). If the PV voltage rises above the target PV voltage the MPPT will increase the charge current. If the PV voltage is closer to 20 V then it is likely the battery is not being charged.

If the PV voltages look good then then next step is to monitor the charge current and voltage. The charge characteristic should look something like this:

enter image description here

enter image description here

  • \$\begingroup\$ Thanks for the detailed instructions! I hope for fine weather on the weekend to do those measurements. \$\endgroup\$
    – Peete
    May 9, 2019 at 2:50
  • \$\begingroup\$ The weather is a big part of solar. I'm in Tampa FL, for the "Sunshine State" there are a lot of clouds here. I added the forecast for Tampa to my post. There won't be a sunny day until Tuesday. \$\endgroup\$ May 9, 2019 at 3:45
  • \$\begingroup\$ I've measured the Voltage at 11.8 V, solar connected on a cloudy day. Solar's been connected all the time since the original post but no load on the battery at all. Seems grim. I have not yet had a chance to do the equalizing charge. The Panel is fixed, can't change the angle. I'll do more measuring! Thanks. \$\endgroup\$
    – Peete
    May 13, 2019 at 22:46
  • \$\begingroup\$ Many MPPT can only charge to 1 V below the maximum PV voltage. You must measure will full sunlight. When cloudy many MPPT will not charge. What is the PV voltage when the panel not connected to anything (open voltage)? \$\endgroup\$ May 13, 2019 at 22:58
  • \$\begingroup\$ Not many sunny days currently in the southern hemisphere... I'll update once I can measure well. Please stay tuned. Thanks! \$\endgroup\$
    – Peete
    May 15, 2019 at 0:52

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