Solar controller amps vs. what goes in battery

[SKMT]

Is it normal for the actual amps going into the batteries to be 30% less than the amps displayed on a solar controller?

I have a battery monitor, so i can turn off all loads and see how many amps are going into the bank. I always notice a big difference between the amps displayed on the controllers vs the battery monitor. For ex: yesterday my co trollers were showing 28+28+28+21=105a. Meanwhile my monitor was showing +75a (with no loads on).

My wires are sized for 1% loss. I understand there is conversion loss in the controllers. Is the amps displayed on the controller pre or post conversion? I can't imagine another reason for such a disparity...

[Sailmonkey]

What make are your solar controllers?

[Captain Bill]

You didn't tell us the state of charge, the bank or the size of or the battery type/chemistry. If the voltage is at absorption and you are using some form of lead acid, it would not be unusual at all to be seeing this. Even then you would probably have to have a pretty big battery bank to absorb 75 amps having reached absorption. If you have a LiFePo4 bank of a decent size it would be an issue unless the bank was nearly fully charged. We really need the additional information to address your question.

[donradcliffe]

The discrepancy bears looking into. The nice thing about current is that amps are amps, and they don't change with state of charge.

After first checking AGAIN that there are NO other loads or other batteries which don't go through be battery monitor shunt (like a starting battery), I would take my clamp-on DC ammeter and verify each current against the displayed value.

[flee27]

All asked/stated as a question... Amps are amps but the acceptance rate of the batteries does change with charge. Like donradcliffe is asking. If the solar is producing the stated 105 amps and the lead acid bank is small or past the bulk stage of charging the internal resistance of the batteries will limit the current they will accept. So in the case you would see less amps going into the batteries the the solar has the potential to deliver.

The OP could also start turning on known loads and as long as the load(s) don't exceed the 105 amps in the above scenario he should see a reduction in charge current. The net add of current should still be the 75 amps with 20 amps of additional draw.?

Foster

[sailcrazy]

Following. Our solar panels are old, and trying to know what decrease is due to panel age vs battery SOC vs ??? has long been a concern. FWIW, we're using a Victron solar controller with 4-80watt panels, and a windgen, and the battery charger when we run our genset(nearly daily) to keep out 6xTrogen T-105s up.

[SKMT]

Battery bank is new lead acid 1075 ah. I have a 10 year old 500w array with a xantrex/trace c40. And then i just installed 1200w of new panels spread across 3 40amp mppt rich solar controllers.


I see suggestions it may just be because the batteries are past bulk charge. That's possible, except I saw the same 30% difference back when i only had the old 500w array. Also, when my batteries are more discharged, the controllers show more amperage (able to accept more), so that still doesn't really explain why there is always a 30% difference between the controllers and the monitor.

[Dsanduril]

Originally Posted by donradcliffe

The discrepancy bears looking into. The nice thing about current is that amps are amps, and they don't change with state of charge.

After first checking AGAIN that there are NO other loads or other batteries which don't go through be battery monitor shunt (like a starting battery), I would take my clamp-on DC ammeter and verify each current against the displayed value.

+1. Amps are amps, battery SoC is irrelevant. If the solar controllers are putting out 105A then 105A has to be going into batteries+loads. If loads = 0...

Most controllers are >95% efficient in conversion, so if they are reporting current on the panel side then most I would expect to see "lost" would be ~5A. And most controllers in 12V systems are step-down, so the high side current (if that is what is reported) is lower than the low-side current going to the batteries+loads.

I'm with Don, check the currents with an alternate instrument. Also check the BM configuration, does the shunt size in the configuration match the physical shunt installation?

[Edit] as you posted while I was typing, if the 30% happens across a large range of currents then I'd start by comparing the shunt size in the BM with the physical shunt size. And maybe the shunt connections and checking the shunt voltage directly.[/Edit]

[Nord Sal]

I'd check the configuration of the battery monitor. Assuming those amperage readings from the controllers and the BM are accurate, that current has to go somewhere.


Do you have a battery combiner that is sensing a charge voltage level and then sending current to an unmonitored battery bank or other places that are not measured by the BM?

[SKMT]

Yes it does make me suspicious of the monitor.

Well guess i finally have a reason to buy an amp clamp. Recommendations on the brand/model with the right price/capability?

[jamiehc]

If your monitor has a shunt to allow amp measurement you may have wired part of the PV array to the non-shunted side.

[ikanode]

It’s better to think in terms of watts than amps: watts = voltage times amps.

Amps are confusing because 10 amps @120 volts or 10 amps @ 18 volts is not the same as 10 amps @ 12 volts.

10A x 120v = 1200 watts
10A x 18v = 180 watts
10A x 12v = 120 watts

[AndyEss]

Battery charge monitors can have zero offset problems (reading charge or load) but 30% seems like a really error.
Is the correct sized shunt being used? Is there oxidation at the connections to the shunt?
(Oxidized connections would increase the voltage drop across the shunt, fooling the monitor to read higher amps than actually flowing across the shunt).

[sailingchiro]

A good clamp on amp meter with voltage and ohm meter is one of the best tools on a boat.

[donradcliffe]

My goto clamp-on these days is the uni-t UT210D for $50 at Amazon. Not as big a clamp and only maximum 200 amps compared to 400 for the Fluke 325 at $280, but mine has lasted for years.