engine start battery voltage drops below 10v when starting engine

Does the engine start battery actually start the engine? There will be a large voltage drop when a very high current is drawn although this will be less noticeable in the domestic bank as it's usually a much higher capacity.

Richard
 
webcraft said:
Domestic battery bank starts engine no bother. Engine start battery runs domestics inc. Webasto no problem.
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Perfectly normal behaviour. Partly it's the effect of a large current flowing through the small internal resistance of the battery and partly it's a chemical effect. If you see a bigger drop in voltage when teh start batter is starting then when the domestic battery is starting then I guess your domestic battery capacity is bigger.

It's why cars with ballast resistor coils generally bypass the resistor when starting, getting a strong spark when it's needed most.
 
No, turns it over maybe half a revolution. Voltage drops enough for VHF, GPS to go off. But it will start the Webasto and run all the domestic electrics. Charges at 14.1 v with the engine running.

Domestics show the same voltage, but virtually no voltage drop when starting the engine.

Domestic bank is 220 amp hours, engine battery is 70 amp Hours.

- W
 
The start current imposed on a small start battery is huge. Typically this is the failure mode of small batteries used to start engines. (like your car). If it still starts your engine then hang in there with it. You have a dual battery system to give you the back up when the engine start battery dies. As said it may be normal ie still a long way from dead. olewill
 
its probably had it, but double check the terminal connections first. they might be good for a lower power item (though a heater usually draws plenty for the first few mins) but not good enough for the start load.
 
I have seen a good voltage from batteries but hardly any stored energy .
Sounds like a new battery required but before paying out you might test with another known good battery.
 
webcraft said:
No, turns it over maybe half a revolution. Voltage drops enough for VHF, GPS to go off. But it will start the Webasto and run all the domestic electrics. Charges at 14.1 v with the engine running.
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Turn off all the electrical kit before starting the engine then you will have nothing to worry about.
 
Where's the 10v measured? Is it across the battery terminals? If not might be worth measuring there (and at the starter) with a min hold meter to double check it's not dropping because of some wiring problem further down the line before chucking the battery .
 
JumbleDuck said:
The tittle is a little misleading: "engine start battery voltage drops below 10v when starting engine" would have been better put as "engine start battery voltage drops below 10v when failing to start engine"
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Agreed.

The two key facts, that seem to have got a little lost, are that the engine battery does not start the engine, but the domestics do. In which case i would have to concur with post #5 :encouragement:
 
I'd expect the battery voltage to drop much lower than 10V, if it's completely failing to crank the engine.
 
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