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I live in a remote part of Tanzania with no mains electricity. We have had solar power for some time, but on purchasing a new inverter, battery and extra solar panels have run into problems.

The problem can be simply stated.

We have two N200 lead acid batteries both yielding 12-13V each. Connected in series we get 25v from the system.

The inverter (Shiv Solar Comercial UPS 2000-24V) requires that the batteries be connected in series. If they are connected in parallel the inverter quickly triggers a ‘low battery warning’.

But the batteries will not charge when connected in series.

Each will charge on its own. So there does not appear to be a problem with any of the connections.

Moreover both will charge together when connected in parallel.

Nor does it make any difference which way around we connect the batteries in series.

When I say that they do not charge I mean that the charging sign on the controller indicates that the batteries are not charging. Also the Amp level reads ‘0’ for both the solar panels and the batteries. The volts show a positive reading (batteries on 25, solar panels vary according to the sunlight).

We know that single batteries charge on their own because the charging sign on the charge controller indicates that they are and the ampage is positive both for the panels and the battery (varying according to the sunlight).

What would cause this problem?

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12V lead acid batteries typically require around 13.8 volts to charge. So in series your inverter needs to output at least 27.6 volts. Less than that and your batteries may not fully charge. I suspect your UPS may be intended for use with Lithium Ion batteries which have a lower charge voltage requirement. A google search for the UPS you specified did not show any results with specs.

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To charge a battery you need to put more voltage than it outputs on its terminals.

The charge controller will monitor the battery voltage and stop the charging when the voltage exceeds a preset value. When discharging the charge controller will also disconnect the battery when the voltage becomes to low. This means that you will need to adjust the settings of the change controller when changing the battery configuration from 13V to 25V.

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It sounds like your charge controller is designed to charge a 12 volt battery bank, so will not charge the 24 volt bank you have with the batteries connected in series to supply your inverter.

Some details of your charging source will be required to further diagnose the problem.

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