AGM battery drop diesel heater

[Robert2891]

I've posted about my battery and solar problems in a different thread - the conclusion seems to be that I've got a dodgy controller and or solar panels. However, while the solar panels aren't outputting anywhere near where they should do, they have kept my batteries topped up to 13.8V while the van is not in use.
I have an eberspacher diesel heater and was testing it and within less that a minute, my 13.8v twin parallel 100ah batteries dipped from 13.8 to 12.9, and continued to drop 0.1 every 10 seconds - I turned the heater off. It's evening and overcast so no charge going in from solar, but is this normal? The readings were taken from the batteries with a multimeter. Batteries were brand new less than a week ago. TIA

 

[Robert2891]

I should add, once tye heater was turned off, voltage reading returned to 13.2V

 

[pappajohn]

13.8v is the charge voltage.
After an hour or so off charge the reading should be around 12.6v with no load.
Sounds like you are still getting something from solar.

 

[Stealaway]

I would recommend starting your engine before starting the Eberspaecher - they take a large load at start up.
I seems the heater is taking more power out than you are able to put in to the hab battery.

 

[tonka]

Diesel heaters will usually take about 10amp as they start up. Once running on tickover will drop down to a couple of amps. So something to be aware of.

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[autorouter]

I think you are worrying unnecessarily. A fully charged battery has a 'resting voltage' (no charging, no loads) of about 12.7V. If you disconnected the solar and left it a few hours, it would gradually settle down to that voltage.

If you apply a load to a battery, its voltage drops immediately, due to the internal resistance, it's normal. So I would expect the battery to start off at about 12.7V, drop to maybe 12.4V during the startup phase, then go up to maybe 12.5v when the demand drops back a bit. If you switch the heater off, it will go back up to 12.6V or maybe even 12.7V. The point is, battery voltage is a bad indicator of battery charge state when it's actually under load or being charged.