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Old 09-28-2021, 06:22 PM   #1
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Discrepancy in voltage, actual versus WeRV App

I was going to spring for a cheap battery monitor but I noticed my WeRV app indicates battery charge - but it's been dropping steadily from 12.8 to 12.5 while the camper has been parked in the driveway for just two days. So I measured the batteries with a voltmeter, and they're fully charged at 12.8. Why the discrepancy?



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Old 09-28-2021, 06:27 PM   #2
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Where is the sensor for your WeRV app? You have to take into account cable loss due to the length of cable from the battery to the sensor and the load that is on that cable. If your sensor for the WeRV is at the power center instead of directly at the battery, that could cause the difference.
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Old 09-28-2021, 06:32 PM   #3
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I don't know where it is, I suppose at the power center makes the most sense. I haven't found any documentation on it.
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Old 09-28-2021, 06:41 PM   #4
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I don't know where it is, I suppose at the power center makes the most sense. I haven't found any documentation on it.
That's probably the source of your discrepancy then. You would need to take your own voltage reading at the same location the sensor is located at.
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Old 09-28-2021, 06:52 PM   #5
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I don't know where it is, I suppose at the power center makes the most sense. I haven't found any documentation on it.
The WeRV app works off the LCI One Control system/app.

The discrepancy could bell be in the translation between apps themselves and/or interface.


When I first installed my Victron Smart Solar Controller along with my Victron BMV 712 monitor I found a discrepancy in voltages as reported on the app and a voltmeter. Victron then announced the issue was due to the fuse that fed power to the shunt being too small. They shipped with a .1 amp fuse and changing to a 1 amp fuse eliminated the small voltage drop that caused the discrepancy.

Not saying that's the problem, just showing how things like that can happen for the strangest of reasons.
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Old 09-28-2021, 06:55 PM   #6
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In addition to cable loss there are also some small losses associated with connectors. Each reputable manufacturer of connectors publishes or make available the tech specs for a given connector. For example, a relatively small connector may have a spec rating of dropping 4mVolt (0.004) per amp passed. Thus, this connector passing 10 amps would would account for a voltage drop of 0.04 volts.

In the case of the WeRv app you would have to know how many connectors stand between the battery and where the app samples the voltage and each of those connectors tech specs in addition to knowing the length of the cables and their associated losses.

Another factor is the quality of the connection relative to corrosion and tightness. Fun fact: loose & corroded connectors = increased resistance = increased current load = increased heat = possibly of burnt wires, tripped breakers, fire, etc.
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Old 09-28-2021, 06:58 PM   #7
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So in practice, can I add .3 volts to the WeRV number and consider it close enough for occasional dry camping? I have a hybrid so getting up under the bed to measure the batteries directly is kind of a pain.
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Old 09-28-2021, 07:06 PM   #8
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So in practice, can I add .3 volts to the WeRV number and consider it close enough for occasional dry camping? I have a hybrid so getting up under the bed to measure the batteries directly is kind of a pain.
Well, that depends. If your using the voltage reading to determine your percent of charge you should know that in order to get an accurate reading there should be no current draw and the battery should have been in a resting state for some period of time. A more accurate way to determine that would be the use of an accurate shunt thatís attached to a coulomb counter style of meter.
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Old 09-28-2021, 07:20 PM   #9
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So in practice, can I add .3 volts to the WeRV number and consider it close enough for occasional dry camping? I have a hybrid so getting up under the bed to measure the batteries directly is kind of a pain.
Probably. Plug it in to get the charger going and compare the readings then.
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Old 09-29-2021, 10:34 PM   #10
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"So in practice, can I add .3 volts to the WeRV number and consider it close enough for occasional dry camping?"
For VERY occasional dry camping sure...but for anything else....
NO you cant and you should consider ANY volt readings WHILE camping unreliable. READ the tables that show you STATE of CHARGE. They ALL state for STATIC disconnected batteries and it takes 24 hours to settle out after charging or use. You need a REAL battery monitor with a shunt instead of relying on a 12.8V surface charge to think your battery is done charging. Those batteries disconnected after 24 hours are FULL at less that your measured voltage as they are wet cell Deka's. So are they undercharged or overcharged? Only a real monitor will tell you. Sure you can get by without one occasionally...but you'll be buying batteries sooner than you would hope 4 if dry camping is often a choice.
It's really nice to know WHEN you can turn off the generator too...you can't do that with a volt meter. Get a Victron or Trimetric.
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Old 09-29-2021, 11:00 PM   #11
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Wait. Am I not understanding, or is the OP observing a DROP in voltage over time on the WeRV that he is not seeing at the batteries. It appears from the post that there is a decay in voltage over time observed by the WeRV that is not occurring at the battery. Am I correct? If so, nothing discussed so far addresses the decay.

OP, are you seeing 12.8 on the WeRV and the voltmeter on the batteries, but later 12.5 on the WeRV and still 12.8V at the batteries?
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Old 09-30-2021, 03:55 AM   #12
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Wait. Am I not understanding, or is the OP observing a DROP in voltage over time on the WeRV that he is not seeing at the batteries.
That was my thought. Static differences could be precision or voltage drop with some significant current flowing but decay over time is a totally different animal.
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Old 09-30-2021, 05:42 AM   #13
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Wait. Am I not understanding, or is the OP observing a DROP in voltage over time on the WeRV that he is not seeing at the batteries. It appears from the post that there is a decay in voltage over time observed by the WeRV that is not occurring at the battery. Am I correct? If so, nothing discussed so far addresses the decay.

OP, are you seeing 12.8 on the WeRV and the voltmeter on the batteries, but later 12.5 on the WeRV and still 12.8V at the batteries?
Yes, that's what I was seeing. We camped two nights with hookups, and then parked it in the driveway without plugging it in (I normally leave it plugged into the house but since we're starting to think about dry camping I wanted to leave it unplugged for a while to see how the batteries do). It was an hour or so after that I noticed the voltage on the app, which read 12.8v. Over the next 24 hours, that number dropped to 12.5 gradually while the batteries didn't.

I should probably plug it in again and see if this is repeatable.
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Old 09-30-2021, 06:15 AM   #14
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State of charge for the OP's batteries should be checked with a battery hydrometer during and after charging. That's the only way to verify the state of charge on flooded batteries. Not gel or AGM batteries, only flooded batteries.
My batteries, located at the front of the trailer, are 20 feet away (8 awg wire length) from the converter. The OneControl is another 15 feet (8awg wire length) away from the converter, making a round trip of 70 feet of conductors from the batteries to the OneControl. So I'm not surprised that there is a difference in voltage between my batteries and the OneControl. The WE app voltage reading is correct as I've verified the voltage at the OneControl's terminals. Mine is behind the microwave.
Personally, I think my trailer's 8awg wiring between the batteries, the converter and the OneControl is undersized, considering the distance and loads (12-volt Magic Chef refrigerator and intermittent loads like slideout, awning, tank heaters, etc). My "to-do" list includes running another pair of 8awg wires from front to back.
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