Amps rating for starter motor vs size of cable amps rating

[Portsmouthsailorbloke]

Hi Jollies,

I have a 12v 1kw starter motor on my Yanmar YSB8 diesel.

I've added a extra battery and rewired the positive run from batteries to starter but used 60amp rated cables as a TEMPORARY measure. I understand that the starter sucks up about 83amps at 12v at the turn of the starter key. The original and still fitted negative cable is about 15mm thick.

My batteries always start at 12.8v. I've felt the battery cable after firing up the engine and, even being 23amp under spec, they never feel warm, ie OK.

Question: With reduction in the amps run, due to cable sizing, will this have any adverse affect on the armature windings?

I read that when the volts drop, the consuming cause draws more amps to compensate. Eg: 12v 12amp = 144W, 11v 13amps = 144W.

Cheers
Peter

 

[savageseadog]

I don't know what you mean by "60A rated cables" You should aim to lose no more than a volt to the starter through the cables, switches connecter etc., bear in mind the battery voltage itself will probably drop to 9-10V or so on start. In practice, if the starter is 3m or so from the battery, 25mmsq cable will suffice.

 

[Portsmouthsailorbloke]

Starter to batteries is under 2m. I think the 60 amp is was the current added positive cable is rated at handling on a 12v circuit - I think it's copper core is about 6mm diam. Just looks so thin against the thick origina car-like battery lead.

Your advice is appreciated.
Peter

 

[neil_s]

Current rise due to a volts drop into an inductive load (your starter motor) would only be a momentary effect in a DC system. The real, longer term, result would be less power available from the motor.

 

[savageseadog]

Starter to batteries is under 2m. I think the 60 amp is was the current added positive cable is rated at handling on a 12v circuit - I think it's copper core is about 6mm diam. Just looks so thin against the thick origina car-like battery lead.

Your advice is appreciated.
Peter

6mm diameter is approx 28mmsq, it will be fine. The other cable is massive.

 

[VicS]

Starter to batteries is under 2m. I think the 60 amp is was the current added positive cable is rated at handling on a 12v circuit - I think it's copper core is about 6mm diam. Just looks so thin against the thick origina car-like battery lead.

Your advice is appreciated.
Peter

To put the numbers in perspective

If you use 3 AWG cable ( 26.7mm² )

Assume cables, positive and negative, are each 2m long making a total of 4m.

It is possible to calculate that the volts drop at your figure of 83 amps will be approx 0.21 volt

Single core 3 AWG cable has a safeworking current rating of approx 150 amps

 

[Portsmouthsailorbloke]

Thanks all for your advice.
Peter.