Starter Alternator

[ditchcrawler]

14 hours ago, robtheplod said:

................................................just had a thought, its not true nothing has changed.. the boat did have a smartbank with relay (VSR) between engine/leisure batteries.... i'm now wondering if this may have been masking the problem? I have no idea when this was energised........ just a thought!

It could well have as the domestic alternator would have probably charged everything when the VSR closed

[Tacet]

7 hours ago, Tony Brooks said:

 

If it has worked satisfactorily for years then why worry. It is not too good to overthink theses things. At present you seem to have one large battery bank so any starting or bow thruster load will discharge both batteries, but until a clown leaves the ignition turned on when you leave the boat for a period or a battery fails I don't think it will be a problem. That is what I would use  VSR for, it would not help the volt drop along the cable.

 

I suspect (but do not know for sure) that with batteries in good condition and the correct use of the thruster burning the cable will not be a problem because as the current rises so will the volt drop and that will limit the current flow (happy to have this disagreed with/discussed).  The "bad" thing with this layout is that it will tend to lengthen the time needed to fully charge the bow thruster battery. As the thruster battery charges it will start to limit the charging current so the volt drop falls and the charging voltage at the thruster battery rises, but the voltage  will be lower for longer than at the engine battery, although they are both fed from the same source.

 

Best practice is for a fuse at either end of the cable so if a short to the hull developed along the run the fuse or fuses would blow and prevent further damage.

As the current drawn by the bow thruster battery reduces (due to its increased voltage) so does the losses as a square.  And the lower the temperature of the conductors, the lower their resistivity.  A 2.5 sqmm copper has a resistivity of about 7 ohms per km at 20 degrees C.

 

Clearly you need a sensible size cable to ensure the thruster battery is recharged following use and the lag does not mean the starter battery is much discharged overnight or whatever by the equalisation of voltages. But at least some of the factors work in your favour.

 

Maybe this is one occasion where the cable can be sized more by reference to the current drawn by the thruster rather than voltage drop?

[robtheplod]

Thanks for the replies. The cable going from stern to bow to charge the BT battery is either 6mm or more likely 10mm (not sure which). I'm just getting a shopping list together so i can fuse this at both ends...