Starter Cable Sizing, voltage drop

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Jan 21, 2008
Vessel Name
Vessel Make
'05 Mainship 40T
I have twin 4-236 perkins.* My batteries are mounted on the Stb side of my boat about a ten foot (one way) run from the starting battery to my port starter.

I was having a few issues with slow starting on my port engine so I replaced the positive starter cable and upsized from a 1/0 to a 3/0 cable.**Saw some improvment, the engine always starts in a second or two,*but still turns over a little slower than the stb one does.

I had a marine electrician on board doing some bonding work and he said he felt that I need to upsize my ground wire to the port*engine as well, it is still 1/0.*

He also felt that I should go up to 3/0 on the starboard engine.* When I asked him why (it turns over really fast and fires right up) he said it was "just better."

Two questions:

1. Should upsizing the ground cable to the port engine lessen the voltage drop?

2. Is there a good reason to spend $150 on cable on my Stb. engine?

I think 2/0 would be sufficient in size. I would make sure that you maintain 9.5 volts at the switch wire on your starter during cranking. I like disable an engine for this test. Battery- no less than 10.5 drop, switch wire, 9.5 volts. If you want, get in touch and I will tell you how to check the existing cables for too much resistance.
Upping the ground to the same size is a good plan.* You'll get voltage drop in each direction (the electrons don't really know that they're on the "ground" path

Steve's on the right track.* Because of the ground issue, be certain to measure the starter voltage relative to the engine, not the battery negative.* You're interested in the actual voltage that the starter sees, and a drop in the ground return will make the voltage appear higher.
A simple trick is to take the voltage for the V meter off the starter terminal.

That way you can see if the V is enough for a good start ,about 9+V is required by most starters when under load.

BOTH the supply and return should be as large as your walet will stand.

On newbuilds we used 04 and forgot about V problems.

After a long start , gently touch each connection between the Batt. and Starter and back.

the one that brings tears to your eyes by burning your hand is the one to clean.

Beware of OLD combiner switches , their contacts get burned over the years and can melt the plastic case .
So I check the voltage at the main lug on the starter before I crank the engine, then have someone crank the engine while checking the voltage at the same spot?

The reading while the engine*is being cranked but before it starts*will tell me how much voltage drop I'm getting right?

Do you guys also feel that I should up the size of my ground wire that ties the two engines together?
Thats right. Most starting problems are "not at all". In your case, you are looking for "works- but not quite right" which easies to troubleshoot with the system intact. I remember replacing a defective starter for a customer several years ago, only to find that I had a defective negative battery cable. Having to explain to the customer that a $10 cable was his real problem helped me to remember what I learned. I over E = R. Ohm's law. Voltage, amperage and resistance values change in an electric circuit when any of the 3 values change. Voltage drops, amperage goes up, etc.. I suggest you start with a known good battery (you already have this- one engine starts fine). Next I would disable the engine and have someone spin the engine over with the starter while you have your test leads from your VOM on the battery- at the center of the post. This eliminates loss of voltage between your battery and clamp. You should not drop lower than 10.5 volts at this point while spinning over for 5-6 seconds.
Next , keep your negative lead in the center of the negative post and locate the wire that gets voltage when the button is pushed. Put the red test lead here. You should not drop lower than 9.5 volts while spinning for 5-6 seconds.
If these two tests are within spec, you are ready to check for resistance or voltage drop in your battery cables. Again, keep your negative test lead in the center of the battery post and put your red lead at the other end of the negative cable. Sounds crazy I know, but spin the engine over again for 5-6 seconds. If the cable has excessive resistance- it will show up under a load. When you spin the engine your VOM will measure voltage lost between the 2 points. You should read .2-.4 volts while spinning. I personally think that .5 volts is acceptable. Above this indicates too much loss and the cable needs to be replaced. Run the same test on the battery positive cable next to prove/ disprove it. Sorry for so much typing. What size is your ground wire?
I'll give it a try.* My ground wire, I'm pretty sure is 1/0.

"Do you guys also feel that I should up the size of my ground wire that ties the two engines together?"

Why not , but first check to see that there are NO load carrying SS or steel nuts or washers in the system.

The SS ability to pass current is horrible , use COPPER ONLY as washers , not even brass has the ampacity of pure copper.

Also any starting battery that simply has a copper lug on a post on top of the batter (usually with a SS wing nut) MUST be replaced with a terminal with muvh more area.

Every NAPA will sell you boxes of "military style" terminals that have 10X the contact area , about $5.00 each in P or N to excatallt fit.

This is a great upgrade for any cruising boat , cheap and effective .

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