Leisure batteries question

Devaprem · Nov 14, 2020

Hello. I'm a little concerned about the leisure batteries in our T6 Ocean. I recently decided to test the passenger area heater and noticed the indicated battery capacity on the centre control unit drops from 100% to 70% in about 5-10 mins of it being turned on, with a current draw of about 10 amps average. The voltage level drops from about 12.6v to 12.3v over that period. Hot air takes about 5 -10 mins to start flowing. After this, the battery stays at this charge level and the voltage seems to stay around the 12.3v level when running for about an hour. My concern is that significant drop in percentage capacity over such a short period, even though I do realise that the displayed figure is very approximate. I understand too that the heater demands a lot of power at initial start-up before reaching a steady state, but should I be seeing such a drop in charge level? Even if this is an approximate value, the relative drop seems still to be high for such a short period. If anyone has any advice, I'd be very grateful.

TripleBee · Nov 14, 2020

AGM batteries have a voltage of 12,8V when not in use.
They have a working voltage of about 12,3V, and they will keep that voltage until the current has become too much and then they will start dropping their voltage.
Nothing wrong here, it seems.

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Absorbent Glass Mat (AGM) Battery Information - Battery University

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Devaprem · Nov 14, 2020

Thanks, TripleBee. Interesting article too.

MattBW · Nov 15, 2020

The indicator isn’t 100% reliable. As a rule the voltage is a better indication. Unless it’s been on hookup for a day it’s unlikely to actually be 100% just driving around it won’t generally be above 80% but may read as such after a journey.

The reading is most accurate with no load 1-2 hours after the last use.

The heater will take a big draw on startup but drop right down once it’s upto temp. It will cause a voltage drop when in use.

That said there is a fuse between the rear and front batteries which can on rare occasion blow and will half your capacity. Easy way to tell is use a volt meter on each battery to check they match.