Such a thing as too big a battery???

[halcyon]

As said an excessively large battery feeding a starter motor through suitably heavy cables may overheat and damage the starter motor.

Starter motor stall current is related to rated starter motor torque and mangetic field, not winding resistance. Voltage will effect current draw, but will, normally reduce the current drawn as voltage increases.

In the case of the Sealine it has one engine starting from a 95 ah battery, the other from 2 or 3 95 ah batteries, so you are saying one engine will burn out it's starter ?

Brian

 

[duncan]

interesting thread

firstly, is there any possiblility that the volvo spec relates the battery Ah to the standard alternator capability? I suspect not but it would make more logical sense to me!

my Yanmar specifies a minimium 600 CCA battery, but make no reference to Ah. Such a battery can be found with 90Ah - or 200Ah+. I have a 185Ah/1000CCA one in place but am always careful not to run the starter for any length of time as is normal practice anyway.

 

[William_H]

Starter motor stall current is related to rated starter motor torque and mangetic field, not winding resistance. Voltage will effect current draw, but will, normally reduce the current drawn as voltage increases.

In the case of the Sealine it has one engine starting from a 95 ah battery, the other from 2 or 3 95 ah batteries, so you are saying one engine will burn out it's starter ?

Brian

Sorry Brian I think you are completely wrong on stall current. Magnetic field once developed and unchanging will not affect coil current and I don't know how rated torque could affect stall current. Only so much as a bigger starter motor will have lower resistance windings.
Yes it is likely that the starter operating off 3 batteries could have a shorter life especially of commutator and brushes. Certainly under extended operating conditions there is more danger so be aware of the time limits (duty cycle) on operating the starter if engine is giving trouble. Prolonged operation will see windings overheat with insulation failure.
Please note that the calculations for power of a stalled starter ie at first powering up do not follow for power increase at 12v when the stater is actually turning. This because the starter spinning will reduce current so with more voltage you will get higher speed of cranking so current will be more limited.
What I am saying is that power increases with the square of the voltage but only on a resistive load (stalled starter) the back EMF of the starter is what reduces current and this is dependent on field current and rotation speed. olewill