Approximate cold cranking amps of starter on Universal M25 and similar size diesels

[peaman]

I am in the process of revising my electrical system to designate dedicated start and house battery banks. A Blue Sea Systems BatteryLink charger will be installed, with its integral ACR (Automatic Charge Relay), along with a Blue Sea "Dual Circuit Plus" battery switch. In such a system, the start battery is dedicated to starting duty, but in event of failure of that source, the house bank may be called on, with the battery switch set to combine both banks.

ABYC calls for a fuse to be installed within a few inches of any connection to a battery, except for a conductor connected to the engine starter motor. So all leads from the house bank should likewise be fused to protect against a short circuit condition.

And that brings me to my question. I have a 1/0 cable from the house bank to the battery switch, and generally, that conductor serves nothing more than the house panel, and so should be fused. But in event of a failure of the starter battery (?), moving the battery switch to "combine" or "both" will allow the house bank to serve the demand of the engine starter motor (as it does presently). So what fuse will be adequate to carry that load? Nigel Calder, in his Boat Owner's Mechanical and Electrical Manual suggests that a pair of jumper cables could serve in this case in lieu of the battery combine/both switch. But I'd rather not need to pull out the jumpers if a suitable fuse at the house bank will let me simply switch to combine/both, since that's what my reflexive response would be.

In summary (whew!) with failure of the start battery, the house battery can be switched to supply power to the starter. ABYC would like to see a fuse in that house conductor, so what size fuse would likely work to power the start motor?

 

[RCsailfast]

I mounted dual fuse Blue sea MRBF fuse holders, on the batteries, with 350A fuses on both the 1/0 House and 1/0 Starter battery. 75A fuses on each battery for the #4 cables linking the blue sea SI-ACR and a new #4 cable from the alternator to the house battery.
The fuses satisfy the ABYC rules and allow for protection in the combined mode on the battery switch.
The dedicated cable from the alternator to the battery along with the battery combiner protects the alternator from frying diodes if someone flips the battery switch off when the engine is running by accident ( that has never happened ).
The new battery switch also means no one should be touching it for charging purposes either

I don’t recall what the actual amp draw of starting the engine, however it was well under the rating of the 1/0 cable or the fusing to protect the batteries and cables. I installed the Blue Sea system 2 years ago and has never had a problem

 

[Kenneth K]

I was told by an old head on this site years ago that the typical Universal starter was rated at 1000W, but I never verified further. That would equal about 80-85A depending on batt voltage. Everything on the engine panel (including gauges, blower, glow plugs, and starter solenoid) typically draws less than 30A (and is protected by its own 30A in-line fuse--in the red wire in the engine harness). If you leave a few lights and the VHF turned on while cranking, maybe you're at a max load of 135A.

I put 150A MRBFs in both battery banks and have never tripped one, nor had a failed start due to insufficient cranking. Sure, 1/0 cable will carry more than twice that current, but my boat should never be drawing a load that high. I do carry a 200A MRBF in case one of the 150s trips, but it hasn't happened yet (3+years since install).

I suppose, if you have a windlass installed, you could be forced to run both the starter and the windlass in an emergency, so I'd probably add that to the fuse amperage too, but I don't have a windlass.

I crank the engine using 1, 2, or All indiscriminately on the batt selector. Starter doesn't seem to care what the source is--they're all 12V batteries. The "start" batt is a non-deep cycle tho.

 

[peaman]

Great responses, both of you. Thanks very much.

 

[Jaybee]

I measured my cranking amps with an ammeter on an M18 and it peaked at 60ish amps with steady draw of 40ish