Battery cable size - confused by calculators

[johnphilip]

Best to be sure

I'm getting confused I'm trying to decide on the correct battery cable size for replacing my aged and corroded cables. The existing cable has no markings on it and the conductor thickness is only 7mm (9mm including insulation) which seems a little on the small side.

The bigger the better, the Forum way! Taking delivery of the new cable for the binnacle compass light

 

[noelex]

4) keeping the starter circuit as short as possible, including the return helps more than fatter cable.

If the voltage drop is the same the result will be the same. A short relatively thin cable will give the same result a longer thicker cable with the same voltage drop.
That is why calculations are done to ensure the voltage drop stays within acceptable parameters.

 

[oldsaltoz]

I just trot down to the scrap yard and pick up some almost new welding welding cable, they often chop off small lengths after the insulation gets damaged leaving some very hany length and for the price a scrap.
This cable has very high capacity and durable insulation and can be tinned as well.

Good luck and fair winds.

 

[maej]

On removal of the old cables and subsequent close inspection they were poorly made, the terminals were for much thicker cable than used and a well of solder was then needed to fill the void, except where screw clamps were used. The cable was not the flexible sort as it was made up of fewer large strands, and near the soldered joints was very stiff. In several places strands were broken and where it connects to the terminal on the 2nd battery most the strands had broken restricting power from the engine starter and 2nd domestic . Much of the cable was exposed and had a lot of surface corrosion and the cable was quite brittle.

So I replaced all the battery cables this weekend with 35mm2 tinned flexible cable. I attached 8mm eye terminals to them which I crimped on with a hydraulic crimper tool from ebay then sealed each one with adhesive heat shrink. I replaced all the battery terminals with those that have a short vertical 8mm stud that can take the eyes I crimped on to the cables.

The difference is astounding , the starter motor spins up so quickly now and the engine turns over twice as quick as before which is helping it start, and no more smoke from the starter when starting the engine. Plus now that the cable runs are balanced for the 2 domestics they should start working as a proper pair rather than draining one more than the other.

All in all a very worth while exercise and absolutely worth the cost and effort in my case

When it comes to a starter motor, there are a few things to consider:
1) The initial inrush current to a virtually stalled motor is a lot more than the nominal current.
2) Once the inrush is over, the difference between 0.2 and 0.4V wiring drop is probably not what decides if your engine starts well or not. It pales into insignificance, compared to the effect of how well charged or healthy the battery is.
3) During the inrush, or while trying to start with a low battery, cranking at low RPM, you are giving the starter motor a hard life.
4) keeping the starter circuit as short as possible, including the return helps more than fatter cable.
If you are routinely needing to boost the engine battery with the house batteries, I suspect the charging is not right, or the battery is not up to it.

The starter battery I replaced about 6 weeks ago with a Varta I4 because I assumed the old battery was worn out, while the new one would crank for longer it still could not crank any quicker, even from a fresh full charge, that is untill now
I believe the alternator is working, the guage shows current coming out and the domestics are certainly getting charged.

Thanks everyone for the help

 

[pvb]

Great news - well done!

 

[Westerly Jetstream]

Batter cables are only critical for starting and for that bigger the better, however the most critical cable on the boat is the alternator cable to the split charge unit (if fitted) or master switch. VD is critical here as even 0.4v can make a big difference especially if you have an advanced alternator control. In one Yacht I rewired recently with battery banks fwd at foot of mast I needed to run a 25mm alt cable to prevent the Stirling going into an over voltage trip.
Remember to regularly remove clean and refit all connections, the marine environment is brutal to electrics.