# question

## Solar Output Current compared to Battery Charging Current

New to solar and charging....I have 600W of solar panels on the roof of van. I have two 12V AGM batteries connected to the panels via MMPT charge controller 150/85.

The battery is for example at 95% charge. The charge controller is saying that it has 15 amps of current to send to the battery, but when I view my battery monitor, it is showing that only 8 amps of charge.

Is this due to the state of the solar controller? (its in the absorption state currently) I dont quite understand the various states as I have not looked up what they all mean. I assume becuase its almost full that it slows down the charge rate? What state is where it sends full charge controller current to the battery?

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Charging can only hit maximum available current in bulk. As soon as the charger hits full absorption voltage, current steadily drops off (the battery controls that through basic non-smart physics of electricity). Once current tails off sufficiently, the charger switches to float (optionally after an equalisation period).

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If there is a difference between the output of the charge controller & what the battery monitor is measuring going into the battery it's normally because you are also running some sort of load or equipment at the same time (possiby even equipment in standby mode).

The load will be using the unaccounted amount DIRECTLY from the MPPT, so it's not measured by the battery monitor.

If your still having trouble & want to run a simple 'test', then temporarily disconnect ALL loads from the battery monitor shunt & ONLY leave the MPPT negative cable conected. This will physically ensure that no loads can draw a load directly & in this state the 2 readings should roughly match.

As Murph said, full 'available' charge current is normally only provided during the 1st charge phase - bulk. Then during the absorption phase charge current will initially start off at/near full 'available' current and slowly decrease as the battery 'absorbs' more & more charge.

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OK That makes sense then. Thanks for clarifying BULK-->ABSORPTION-->FLOAT...simple answers to simple questions as I am still learning this battery stuff. The user manual for the charger didnt seem to clarify that as simply.

There were no loads on the battery at the time, but yes I understand if there was, good point.

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If there were 'truly' no loads then in my opinion there is something wrong with either of the MPPT or the BMV current reading accuracy.

The BMV will always be more accurate & considered as the master (if it actually does receive the identical current), but there should not be this amount of measurement delta.

To be 100% sure the only way is to PHYSICALLY disconnect all other cables & run the simple test I recommended - there could still be something that you don't know about connected somewhere or even an error/fault drawing a load...

Also if you are confident that the MPPT output current will be <10A during a given part of the day (morning or evening), then you could run the current through a standard multi-meter that measures up to 10A to see what it reads. This will provide you with further confidence about what is going on & what device to investigate further.

If you run some tests with a multi-meter I would also recommend to connect it both at the MPPT output, as well as between the negative battery terminal & the BMV current shunt to see if the reading is really identical/repeatable at both points in the wiring. If you happen to have 2 multi-meters then it will be best to test both in parallel - that way the instantaneous solar output between tests will not effect the results.

Another option would be to use a multi-meter with a current clamp attached. This would be a little less accurate but much quicker to conduct the tests (no need to break the circuit to insert the multi-meter), as well as easier to move between different cables.

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