Voltage drop under load

SoCalEsben

We have a 2022 Tab400 and we have recently seen a significant voltage drop when putting some load on the standard agm battery. 

Dry camping and using lights, fans, (fridge on propane) and after a short shower running pump, we saw a sub 11V reading on an almost fully charged battery. See smart shunt image. Note that the solar is charging so the load might be around 100W. 

We have never had the battery below 60% and we generally see good battery life.

Any thoughts on this? It drop seems excessive and it also causes an Alde failure message.

pthomas745

If you are running any load, the voltage will always display a "false" low readout.  So, that phenomenon is well known, and it happens to any device that measures voltage.  The only way to determine a reasonably accurate state of charge is by turning the battery switch off, and stopping any charging coming into the trailer.

So....a lower voltage would be normal.

But...this does seem a bit excessive for only 19 watts in use.

SoCalEsben

Thanks @pthomas745! I did some general research on this drop, and it definitely seems to be way more than what would be expected. I also wondered if there was anything NuCamp specific info, but I couldn't find anything.

Just as a note, when removing the load the voltage went back to expected levels.

Grumpy_G

At merely 1.75A current draw the voltage should indeed be much higher, if the state of charge is correct. The fact that the Alde system complains means that the voltage reading is somewhat accurate. The no load voltage can be deceiving, I've seen batteries that recovered from single digit readings under load to over 12V. 

Either the state of charge estimate is way off, or the battery is already shot which would be unusual with such a new trailer. The best way to test it is to charge it off shore power for 24hr when you get home and then retest with the same load. See how long it takes until the voltage drops that low again. 

pthomas745

As I said...a "drop" in the voltage is normal.  But....this seems to be more than normal.  I have pictures of my battery displaying 12.1 with just the fan running, and 10 minutes later with the battery switch off the voltage settled to 12.46.

I would run this test again...running these items and watching the current and power numbers.  Your battery might be trying to tell you something, but I'm not exactly sure what that would be.  At most they might be three years old.

klenger

There are two places where the voltage drop can be created, either the battery itself, along the battery wiring, or a combination of those.  If you have a digital meter, you can check the voltage at the battery and along the wiring to see where most of the drop is.  You can do this my measuring the voltage from ground to the points along the run, or measure the drop along each segment of the wire by putting a probe tip on each end.  Voltage drops along the wire runs should be fractions of a volt. This test will help identify if the drop is normal, or if it is caused by battery sag, or wire loss. 

What kind of battery(s) do you have.  I assume AGM or Lithium.  Lithium is much less prone to sagging under load.  I assume that you are using some kind of heavy load on the inverter to be using 100 amps.

pthomas745

@klenger, thanks, I didn't notice the "inverter" comment.  @SoCalEsben can you show us your Solar controller display also?

Denny16

I wonder if the Inverter is switched on, even in standby with no AC load, it cand drain a battery quickly.  

Cheers 

SoCalEsben

Thanks all for all the input - it looks like I have some more testing ahead. Also - time to get that meter. Been putting that off for too long.

I'm already back home and this issue was a couple of days ago - unfortunately I didn't do a screen shot of the solar at the time. I estimate that it was pulling in about 80W from solar and that's why I'm guess that I was pulling around 100W total (Note: not 100 amps). It's two 6V AGM batteries - I'll do a closer inspection tomorrow. 

I'm not sure how the app calculates the percentage, because I would think it should be lower according to the Volt reading. But after most of the load was removed, it came back up to a Volt level corresponding with the 91%.

SoCalEsben

I checked the inverter when the issue occurred and it was not on. 

SoCalEsben

A few weeks ago - before this issue - I did accidentally leave the inverter on for almost a full day. The solar was keeping up but it was hot outside and I could definitely hear fans running. Wonder if the heat could have caused any issues?

MarkAl

I assume from seeing the temperature, you also are using the voltage/temp wire probe to the shunt. If so, the Battery voltage shown is at the battery. So you are not seeing wiring losses. I wonder if the battery is old or bad so the "91%" is based on what you set the capacity but not the actual capacity of the battery?

Sharon_is_SAM

There was another recent thread with this same issue.  I read that this kind of disparity can be an indication of a battery on its way out.  When you say “an almost fully charged battery” were you looking at SOC or voltage?

The other issue that can impact the accuracy of the BMV is the use of multiple charging sources.  Apparently, going from TV, to Solar to shore power can alter how the BMV reads.  From time to time, I charge using a separate battery charger and resync the BMV to avoid aberrant readings.

Dutch061

It is imperative as @Sharon_is_SAM mentioned that the Battery capacity is set properly, should be 224 Ah for the Harris 6-volt batteries.

The SOC is synched to 100% on the BMV-712 based on the following settings.

Charged voltage
Tail current
Charged detection time

If these are at the "default" which are auto configured based on detected battery voltage, they are all incorrect.

When I was using the Harris AGM, my settings were as follows and it worked well.

Charged voltage - 14.2 
Tail current - .5%
Charged detection time - 5 minutes

SOC is reset to 100% when the voltage is over 14.2 with less than 1.12 Amps for a minimum of 5 minutes.

What I believe may be happening is that your SOC is resetting to 100% when they are not fully charged. I had this exact issue early on and discovered that it was due to not properly configuring the BMV-712 when I installed it. I would venture to say that even if someone installed it for you that it may not be set up properly.

Brad

SoCalEsben

Thanks again for the input!
We are definitely moving a lot between power sources so that might be it. The “almost fully charged” was based on a SOC reading. On the first screen shot it doesn’t really seem to match the voltage but after removing the load, the voltage went back up to around 12.5. 

Good point about the settings too. I believe I got the initial settings somewhere in this forum. See current battery settings below.

if the battery is actually on its way out - what’s the best way of testing that? Trailer is under warranty but I’m guessing dealerships are not the best way?

Dutch061

With the settings you have it is too easy to obtain the needed results and the reset is likely happening long before they are actually charged. That is what was happening to me, as mentioned previously.

Now, the batteries may be on their way out also, that would require some load testing, or the common test today uses an internal resistance measuring device.

I would also suggest changing your Charge efficiency factor to 80%, as this is what Harris Battery told me when I inquired.

Brad

SoCalEsben

Finally had some time to do more testing. I tried a couple of rounds with continuous load but wasn't able to recreate the issue. I guess I'll just keep an eye out and see if it ever happens again.

On another note, my new updated battery settings for the smart shunt, it seems like Victron never get's to synchronize and it just hangs at 95-96%. Should I try and tweak the tail current - or synchronize manually?

Thanks again for all your input!

MuttonChops

SoCalEsben said:
. . . it seems like Victron never get's to synchronize and it just hangs at 95-96%. Should I try and tweak the tail current - or synchronize manually?

Disclaimer:  I have zero knowledge experience with the Victron.
                           While as a Production Hardware Engineer I spent a career trying to resolve
                           so called 'tested software' on many production lines.

So my advice;  Synchronize Manually, at least that way you know the data tried to update.

Sharon_is_SAM

@SoCalEsben - Charge up completely via a smart charger with the battery switch turned off and solar disabled, then synchronize manually.