j4mes-17 avatar image
j4mes-17 asked

Smart Solar 75/10 will not charge batteries

Hi all.

FYI, I'm an IT guy and an electronics noob, so please be kind. ;)

I've had my Smart Solar 75/10 since 2nd March this year, and it has been great! However, a few days ago, the battery ran completely flat as the battery had not been charged. At the time I chalked this up to bad weather, but since then, the sun has come out and the charge current remains at 0 watts, 1 watt or 2 watts.

Here are the facts of my set-up:

  • Two 12v panels wired in parallel, one 10W, one 80W.
  • Always-on load of a Raspberry Pi, plus weather sensing instruments and WiFi antenna. Usually draws 0.2-0.3A from the controller's load output.
  • Two lead-acid batteries in parallel, 12v 12Ah, and 12v 2.3Ah.
  • The controller has not been disconnected or powered down since first installation . The RPi had an uptime of over a month when the battery stopped charging.
  • When the battery died and the load was shut-off automatically, I kept the load disconnected to allow the battery to recharge over several days. However, the controller only reported a yield of 3-10 watts all day. Compared to my average yield of 80-100 watts all day.

Here is what I've checked so far:

  • Both solar panels are working fine and gave a 20v output under moderate sunlight for the UK.
  • I connected the batteries directly to the solar panels with my multimeter in-line. It showed a draw of 3-4 Amps. Immediately I heard a hissing sound like gas escaping from the batteries, and I stopped. Am I an idiot for directly connecting the panels and batteries, or does this show the batteries are toasted and the controller is not at fault?
  • I both visually inspected and continuity tested the 20A fuse and it is fine.
  • I have reset the charge controller to default settings.

Observations of the charge controller when operating:

  • Despite only charging the battery with 1-2 watts, the status of the controller shifted from bulk, to absorbtion and then float within a few minutes.
  • When set to 'battery equalisation' mode, the charge current rose to what I would normally expect at that time of day, 20-30 watts.

Apologises if this doesn't really make sense. Please feel free to ask any questions.


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As a side note, I did not observe the LEDs displaying any error codes and none appeared in the Connect app.

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Typically a lead acid suffers damage iif discharged more than 50%. Well worth checking actual voltages at the terminals and compare it with the manufacturer spec. From my novice perspective, I do however fear the worst. Consider investing in a battery protect / bms to ensure this does not happen in the future.

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4 Answers
Stefanie avatar image
Stefanie answered ·

The fact that your MPPT is shifting charging states from bulk up to float in very short time is a good indicator that your batteries are dead and needing to be replaced.

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tilo avatar image
tilo answered ·

As Stefanie said, seems like your batteries are damaged. This is most likely due to a too high charge current. A battery has a maximum charge current recommended by the manufacturer. Usually between 20% and 30% of the battery "Ah", example: 100Ah should be charged with no more than 20A or 30A. Please check the datasheet of your battery. Sometimes it is written directly on the battery. In your case the maximum charge current for your 14,3Ah battery bank is about 3A to 4A. Your solar panels can charge more than 5A together, therefore I think that might have damaged your batteries.

You can limit the charge current in your MPPT to let´s say 3A.

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Thanks for your reply, tilo.

Looking at the charger's history, the maximum charge current ever reached in its lifetime was only about 46W. Over the past fortnight, it was around 26W. In any case, I've taken your advice and limited the maximum charge to 3A.

However, I do think you're right about the batteries being toasted. They were the cheapest batteries on Amazon with no datasheet, with the intended purpose of powering burglar alarms. Perhaps the repeated recharges were not suitable for this kind of battery. It just seems strange how they would both fail so suddenly.

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adev avatar image
adev answered ·

having two panels with such different characteristics is not ideal.

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j4mes-17 avatar image
j4mes-17 answered ·

Thanks for the replies everyone.

It seems that the general opinion is that my batteries are toasted. However, what I don't understand is that when I put the charger into 'equalise' mode, it charges the batteries with the normal current I would expect for the weather.

The panels are only attached to the roof of a garden shed the maximum I have ever seen is charging at 40w, which wouldn't appear to be over specification...?

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