Battery condition

[candapack]

Our van has a control/display panel which shows the state of charge of the vehicle and leisure batteries. We also have a solar panel with a display showing the same things.

Except, the solar one always shows a higher reading for both batteries, by anything up to 0.8v for each.

Why is that, and which reading is likely to be the more accurate?

[Derek Uzzell]

It’s possible that both displays are accurate, both are inaccurate, or one is accurate and the other is not.

 

It might be that the solar-panel display is showing the direct output of the panel, whereas the control-panel display shows the batteries’ charge state with the voltage being affected by a load being placed on the batteries.

 

That’s just crystal-ball gazing though and, although it might be possible to make a better guess if you said what the comparative voltage readings were, as this might point to one display’s readout likely to be ‘righter’ than the other, the best way would be for you to use a multimeter to cross-check the voltage readouts.

 

I’d better add that some care needs to be taken and some understanding of electricity be needed when using a multimeter. There’a also the matter of accuracy - for example, if it were suspected that a battery-charger was not performing properly, a multimeter that was over-reading or under-reading by, say, as little as 0.2V might confuse the issue. But, unless a multimeter’s readouts were wildly inaccurate, the device should still be a handy tool for a motorcaravanner to own.

 

This MHFun discussion should be useful

 

http://www.motorhomefun.co.uk/forum/threads/multimeter-advice.108250/

 

but there’s plenty more information on-line

 

https://www.google.co.uk/?gws_rd=ssl#q=motorhome+multimeter

 

 

[candapack]

Thanks Derek. We're in France just now and all systems are go, it's just something that puzzled me when I was checking that all was OK.

Multimeter probably a good idea, might get one for Christmas if I leave enough hints.

Cheers,

[Cliffy]

 

We have the same, I have not checked which is correct but the guy I bought the van off, who was pretty clued up said the solar panel controller readings appeared to be nearer the truth when checked with a Multi Meter.

 

 

[coach2000]

You still will not know the correct voltage unless you have a multi-meter that has been calibrated and know its reading is accurate.

 

Clive.

[Brian Kirby]

candapack - 2015-08-21 2:32 PM

 

Thanks Derek. We're in France just now and all systems are go, it's just something that puzzled me when I was checking that all was OK.

Multimeter probably a good idea, might get one for Christmas if I leave enough hints.

Cheers,

You haven't said under what conditions the battery voltages are being read. With a solar panel, if read in daylight, it is probable that you are reading solar output and not battery state. If on EHU, you will be reading charger output and not battery state. Disconnect the EHU (if relevant) and wait for it to get dark so that there is no solar input, and then switch off all sources of 12V consumption (including the fridge), wait 30 minutes or so, and then see what the main panel says. That should give a fair indication of what the actual battery voltage is when it is resting. This is the only meaningful voltage reading, as load and charging sources otherwise continually change the recorded voltage.

[Derek Uzzell]

coach2000 - 2015-08-21 11:22 PM

 

You still will not know the correct voltage unless you have a multi-meter that has been calibrated and know its reading is accurate.

 

Clive.

 

That’s a given, but even if a multimeter were producing readouts that were not 100% accurate, as long as the instrument provided ’stable’ readouts it should be possible to identify whether Chris’s motorhome’s displays could be believed. Let’s say that the multimeter over-reads or under-reads by a full 1 volt. If the multimeter readout of the solar-panel voltage - even though it will be 1 volt out - shows it to be up to 0.8v higher than the readouts taken at the batteries, it’s reasonable to assume that the motorhome’s displays are not faulty. The displays might well not be providing accurate voltage figures but, comparatively, It would be reasonable to assume that some confidence could be placed in them.

 

I have one of these

 

http://www.amazon.co.uk/Digital-Multimeter-Tester-Voltmeter-Ammeter/dp/B00IBD3F1E

 

I bought it in April 2015 when I had begun to suspect that my good-few-years-old digital multimeter was under-reading by enough to concern me when I wanted accurate voltage readouts. As I haven’t had the VC97 multimeter calibrated I don’t KNOW that it’s accurate, but its readouts are higher than the old multimeter’s and are what I expect them to be. The Amazon reviews highlight the VC97’s negative points - it’s good enough for me, but obviously may not be good enough for professionals who will need a multimeter known to be accurate and almost certainly a lot more expensive.

 

Tyre-pressure gauges are similar in that the gauge may need to be very accurate for, say, a racing motorcycle’s tyres, whereas for a motorhome’s tyres inflated to around 80psi a 1% or 2% +/- inaccuracy won’t be that important. I have 4 tyre-pressure gauges - none of them ‘calibrated' - and I’m aware that one of them over-reads at higher pressures. For the motorhome I use this gauge

 

http://www.ebay.co.uk/itm/ACCU-GAGE-ITC-PROFESSIONAL-TOOLS-PRECISION-TYRE-PRESSURE-GAUGE-0-160-PSI-RDF160X-/380900205560

 

It seems to be accurate (ie. its readings cross-check with two of my other gauges) and it’s easy to use with the motorhome’s valves. I think (but don’t KNOW) that it’s accurate and it probably is.