#1 12-04-2017 14:32:59

From: basecamp
Registered: 29-04-2016
Posts: 44

Why the voltmeter is a waste of time ;-)

An inbuilt voltmeter looked like a real plus to me when I was researching what CT to buy.  However ...

We found that it could read 12.8 v at the end of a day's driving but still plummet overnight to 12.2v after a modest draw, and leave us scratching our heads.  We also found that it consistently read 0.2 volts more than it should, which is quite a lot.

Over two years of struggling to get the two batteries in the Argyle SE to properly charge off the alternator, which has been the standard rig for this, we learned a lot of things, and one of those is not to rely on that meter.

The voltmeter is attempting to show State of Charge and this is not the same as load capacity, which is what we're really concerned about.

A voltmeter is unreliable anyway unless the batteries are prepared for measurement.  Just the surface charge is being measured.  The batteries need to be disconnected, with nothing in or out, for around 3 hours before the voltage can be measured.  The only other reliable way to is to measure the specific gravity of the cells' electrolyte.

Well, it's not complete waste of time as it can be handy to have an indication that some level of charging is taking place.

The other key learnings have been that we didn't understand batteries and recharging them, and that an alternator won't properly charge a deep cycle battery let alone two - but that's a story for another post.

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