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Issues with Battery Charging from the Solar Panels

Issues with Battery Charging from the Solar Panels

May 16 2016

We seem to have a problem with the battery charging from the solar panels. 

It has been an ongoing question for over a year, as we had low charge in the batteries when we were up on Broome, and then again when we got back home, and it never really got resolved. Last week I was walking past the truck, and heard it beeping like mad. It was the low voltage alert going off.  It had reached the cut-off voltage of 11.2 volts, and everything got automatically disconnected to save the batteries from discharging too far.

To check out how much charge was going into the battery from the panels, I installed a shunt and a digital meter which let me measure the current going in. On a nice cool sunny day it was only putting in a maximum of 4.6 Amps, which is very low (even allowing for the sun being low in the sky in May) for 600W of theoretical input.

The people who supplied the solar panels and the MPPT charger have been very helpful. They asked me to do some tests to try and work out if it is the batteries or the panels which are the issue.  They don't think it is the MPPT charger.

The basic components involved are:

  • 6 x 100W ultra thin solar panels, connected in pairs, so feeding in as 3 separate inputs to the MPPT charger at around 34V
  • A 30A GSL MPPT charger
  • 3 x Optima D31M Blue Top batteries connected in parallel. They are 75AH / 900CCA.

The first thing to do was to test the 3 batteries separately.  So they got isolated from each other, and fully charged using a 240v chargers. It took 2 days to get them fully charged!

Over the 2 days of charging, I 'rotated' the chargers over the batteries so each battery had the same amount of time with each of the 3 chargers (because the chargers were all different capacities).

Next I made up 3 identical loads, which were 3 LED spotlights I happened to have laying around. So I wired up some eyelets to screw onto the battery terminals.

Once the charging was finished, I let the batteries 'rest' for 4 hours, and then I connected the spotlights up to each battery and started monitoring the voltage.

I ran this setup for nearly a whole day. These are the results:

Pretty much a linear degradation over the 20 hours of testing.

I'll have to pass these results on to the 12V people and see what they think this means, but it seems to me like the batteries are OK.  Which leaves the Solar Panels as the next thing to check out.


Perhaps 0.8A was not enough to properly test the batteries?  A couple of people have suggested I hit it with a much heavier load, like about 10% of the AH rating of the battery. Which would make it 7.5A.


So I've made up a new test rig to load the batteries up and I'm going to re-measure things.   This is the new plan: 2 x 50W halogen globes, which in theory should draw about 8.2A when hooked up in parallel.

And indeed they pretty much draw what I was expecting:

8.0 Amps going out for the twin globes, and the battery charger throwing in 7.2A trying to keep up (it is only a 7A charger).

Looks like this will be a good next test, so I'm off to fully charge the batteries before giving it another try.

Another day, another test!  Today I did the following:

  • Charged everything up again, but this time using one charger across all 3 batteries
  • Separated the batteries
  • Attached a 4.3A load across all of the batteries (as compared to a 0.8A load last time)
  • Recorded the results in a new graph for 8 hours (compared to 20 hours last time)

Again, the voltages across all 3 batteries are very consistent.

I'll be contacting the solar store tomorrow to see what they think of this new information.

So.... I went out and bought a battery analyser to see if that would throw any light on the situation. And I've got something interesting for sure!

The Optima batteries have 4 terminals per battery: 2 with threaded wingnuts and 2 with the traditional round automotive type.

For some reason, when I was taking measurements for the voltage graph, I swapped from using the threaded terminal to the round ones, and to my surprise they were different. So I got to wondering how 2 terminals that in theory should be identical voltages could be different.

The readings from my Battery Analyser are vastly different, depending upon if I use the threaded post or the round post. Here are the three batteries with their threaded readings on top and the round post readings underneath:


What a HUGE difference!  I am going to get in touch with Optima to see if they can explain this.

Total: 1 Comment(s)
Andrew Kee
  Be interested in the outcome ,
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