Bill avatar image
Bill asked

Wondering why I see negitive amperage on my battery monitor when the sun is shining bright

I am wondering if there is something that can make the "smartsolar" controller be a bit smart? Let it put out more amperage than it thinks the battery can accept so solar panels can help overcome the loads in the RV (or boat)? It pains me to see the negitive numbers on my trimetric when I know there is plenty of sunshine to keep charging the battery but the smartsolar controller is not aware we really need more power. From what I can tell the BMV's won't talk to the controller to help with that.

In case I am not asking that correctly.

Example, Its just past midday and my 640w of panels could be producing 350-400 watts then. But the controller is thinking it should only be producing 5 amps as the battery is getting closer to 95-100% SOC. In actuality, for my RV its pretty normal for the inverter to be pulling 8-10 amps from the battery all day long due to network devices, DVR, Fridge, radio, weather radio, TV, phone chargers, cameras and 12v "phantom draws" of which there are many also.

The solar irradiance at that time of the day could easily produce 300-400w @ 80v but the charge controller appears to be limiting it to 25-75 watts at 80v. as it thinks it wants to charge the batteries at 5 amps due what I assume is an algolrythm and maybe the batteries chemical reactions pushing back.

Is there something (maybe a BMV) that can let the charge controller know the battery is not getting charged and up the amperage till its back at +5amps again or whatever the CC thinks it should be pushing right then.


BMV Battery Monitorsmart solar set-up help
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2 Answers
Mark avatar image
Mark answered ·

What phase is the controller in when this is noticed? All?

Does the battery voltage drop below the set-point for the particular phase?

How are you measuring/establishing the current that the load is drawing?

The MPPT should be providing whatever the system NEEDS to maintain the voltage set-point for the particular phase - obviously within the limitations of available PV.

If you are at the end of the absorption phase the battery will inherently need less & less current to maintain the voltage set point.

If you have a load on at the same time then part of the MPPT output will go direct to the load & the remainder will go to the battery (as measured through the battery monitor/BMV)

If the battery monitor is indicating a '-ve' current flow then to me that indicates that you actually don't have enough PV to cover BOTH the loads & the current required to maintain the battery voltage set-point (for the particular phase it's in).

Also remember that the current flowing from the MPPT directly to the load will NOT be measured by the battery monitor/BMV. So you will need to COMPARE the MPPT output current to the battery monitor/BMV to understand this - unless you have a CCGX or similar that can monitor the lot at once.

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Bill avatar image Bill commented ·

Mark, Mostly I see it occur in Float.

What I think you are saying is that the controller "should be" upping the amperage to compensate for the extra load because the battery voltage should drop due to that load?

My Trimetric shows the amperage going into the battery is normally about 8 amps less than what the CC says its putting out concurrently, If i shutoff the PV and run with just the battery my Trimetric will consistently read around -8 amps with my normal daytime loads.

I am just getting started with the Raspberry Pi GX but I do not have a BMV yet. Just my trusty Trimetric. But the GX info makes me want a BMV to get that data into the system too.


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Mark avatar image Mark ♦♦ Bill commented ·

Hi Bill,

Yes that's exactly what I'm saying that the behaviour should be.

So if the situation is;

- MPPT on & in float phase

- Battery is already at float set-point voltage

- There is plenty of sun available

- External loads are small


- The battery will only take/require a very small current (somewhere near 0 & upto a few A) to maintain the float voltage setpoint

- The rest of the MPPT output will go directly to covering the load

- If the load changes then yes it will effect battery voltage & the MPPT will need to slowly react by adjusting it's output current to compensate

- During the few seconds following the load increasing it will be normal for a a '-ve' current draw from the battery (until the MPPT compensates)

So, you should only be concerned if the battery is dropping under its voltage setpoint & staying there while there is plenty of sun available for the MPPT to increase its output (and bring the voltage back up).

If the batt monitor is reading '-ve' in the scenario I discribed above then I expect it to be only momentarily or a very small '-ve' reading could also be posdible due to measurement error / lack of calibration.

Lastly a '-ve' reading could also be possible if the battery is somehow charged to a point ABOVE the set-point voltage - during the period of voltage drop to the correct setpoint there may be '-ve' current flow. This scenario 'could' be possible due to temperature compensation & a rapid temperature rise (which would lower the voltage set-point). But normally I think this will happen too slow to notice.

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Bill avatar image Bill Mark ♦♦ commented ·

I am currently only running a Trimetric so I am not seeing a -ve.. I assume that would be on a BMV? I will probably be buying one after we find out what the tax man wants to extract from me next month. I will need to keep a close eye on the voltage. Currently, the negitive numbers stay up there a long time.

From what I have read the BMV doesn't really communcate with the controllers, except maybe voltage and temp. Right now, I have an SBS talking to the controller "sometimes" -Bill

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Mark avatar image Mark ♦♦ Bill commented ·

Hi bigbillsd,

Your question is asking about 'why I see negative amperage on my battery monitor' which is the Trimetric.

But your reply above starts off by stating that you are 'NOT seeing a -ve' on your Trimetric...

Then you state that 'the negative numbers stay up there a long time'...

So I am confused!

To clarify, just in case this is the source of the confusion - when I say '-ve' I mean 'negative' or the '-' symbol.

I have never used a Trimetric battery monitor myself, but I would expect negative current flow when the battery is being drained & positive current flow when the battery is being charged - just like a BMV. Otherwise there may be a charge/discharge indicator light or similar to know.

Yes I can confirm that you can pair the BMV to the MPPT to share voltage & temperature - but it will not control the function on the MPPT.

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JohnC avatar image
JohnC answered ·

Hi Bill. If your Smartsolar is running on default settings and not subject to external controls, then it should be doing what you expect, and just targetting (mostly*) a V profile. It will just attempt the V, and if you add loads, then it will compensate by adding W. The SS doesn't know where the power is going, it'll just chase V.

Let's say you have Pb's. Once V reaches Abs, the W will taper quite sharply, turning the expected bellcurve to a sharkfin shape. Then when it drops to Float, another W stepdown.

Your target V's may be getting slightly modified by temp compensation, but if they're in the ballpark the the SS is doing all it can.

Of course if you're not achieving V and have other kit involved, then please tell us what it is..

* Little things like Battery Safe profile may impinge, but not worth worrying about.

Composed before I saw Mark's answer, so forgive me if I seem to repeat anything. :)

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Bill avatar image Bill commented ·

John, only kit is a GX, SBS and Trimetric currently. I am using custom battery settings per my Bat Manufacturer. 15.5 -E, 14.5 -A and 13.5 -Float.

I will pay closer attention to what the CC is seeing for voltage from what I am reading between the two answers. -Bill

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