Musings on starting engine by combining batteries...

[JDCAVE]

So my son has a project boat (as in a REAL project boat) and has been upgrading the electrical. He has a 175 amp terminal post fuse on his house bank, sized for the cabling.

He tried to start his vessel and discovered his starter battery was flat. So he tried to jump it and it blew the 175 amp fuse in the process.

I don't know what the demands are for his starter, but thinking about the situation, it would seem to me that by combining the house and start batteries, the house would be dumping amps into the starter battery and this would be continuing when he tried to start the boat, so the the combined load into the start battery and the load into the starter exceeded the rating on the fuse.

Just thoughts on this.

Jim

 

[rslifkin]

That's likely the case. And depending on the engine in question, the starter alone may draw enough power for long enough to blow the 175 amp fuse (or it may not).

For my pair of starting batteries, each engine and the generator has a switch to select between "start battery 1", "start battery 2", or "both start batteries" (no provision to start from the house bank in my case).

I like the switches better than an emergency combining solenoid, as even though I have to make a trip to the engine room to flip the switches, it gives me the choice of whether to combine the 2 start batteries or to switch the non-starting engine entirely to the other battery and isolate the problematic battery (if one or both start batteries were low for some reason but otherwise healthy, I might want to combine, but if one lost a cell or had some other failure, it might make more sense to switch to the other and leave the bad one isolated).

 

[Xlantic]

I would think the moment the starter begins drawing power, the voltage would drop low enough that the dead starting battery would not be taking any amps.

Just my non-expert opinion

 

[rslifkin]

I would think the moment the starter begins drawing power, the voltage would drop low enough that the dead starting battery would not be taking any amps.

Just my non-expert opinion

Good point, as long as the starting battery doesn't have a shorted cell or other significant failure.

 

[Bacchus]

Starting batty (or circuit) is the one ABYC exception to fusing as close as possible to the source/batty. It does require cable sizing to handle the hi A load.
It shouldn't matter whether a switch or solenoid is used to combine battys for starting the key is adequate cable size for the worst case load (one batty dead or near and only one handling the starting A).
A fuse sized for relatively low house A won't be capable of handling starting for many engines. Your engine maual should spec starting MCA.

 

[PM]

My guess is that he either has a dead starter or a stuck engine. A good starter on a free engine would spin so instantly that blowing a high amp fuse would be unlikely.

pete

 

[KnotYet]

My guess is that he either has a dead starter or a stuck engine. A good starter on a free engine would spin so instantly that blowing a high amp fuse would be unlikely.

pete

JDCAVE doesn't say the size of his son's boat's engine but almost any diesel
engine's starter will appear to be a short circuit to a 175 amp fuse. It did it's job.
It may not be a diesel engine either but old dying batteries combined with old
corroded connections will increase the amp draw and time required to spin it up, too.

 

[twistedtree]

Jim, did the fuse blow as soon as he jumped the batteries together, or did it blow when he tried to crank the engine?

 

[Flatswing]

As a point of reference, my JD 6068’s draw almost 500A on constant cranking of the starter. This is why ABYC has the fusing exception for start batteries. (I use a 600A JLN fuse for my start batteries). Most clamp dc meters only go to 400A, but if he has a smaller engine, a good place to start as to causality.
Perhaps not pertinent, but ABYC also recommends using a JLN fuse for larger house banks (otherwise large current flows with [for instance] a dead short can blow the fuse but arc across it anyway).

 

[Bacchus]

As a point of reference, my JD 6068’s draw almost 500A on constant cranking of the starter. This is why ABYC has the fusing exception for start batteries. (I use a 600A JLN fuse for my start batteries). Most clamp dc meters only go to 400A, but if he has a smaller engine, a good place to start as to causality.

Perhaps not pertinent, but ABYC also recommends using a JLN fuse for larger house banks (otherwise large current flows with [for instance] a dead short can blow the fuse but arc across it anyway).

Good point but to clarify for others. ABYC calls for Type T fuses for Hi AH bank protection.
Littlefuse uses the JLLN Designator for their Type T fuses
BleuSeas uses a different designator BEP- FT for their Type T

 

[JDCAVE]

Ok so here’s the update. He put a new fuse in and disconnected the starter battery and successfully started his engine! It started immediately. So it supports the hypothesis that the combination of a “load dump” into the battery together with the load into the starter was sufficient to blow the fuse.

WRT the fuse. The fuse on the house bank was sized for cables, which were determined based on expected maximum loads. He probably won’t have an inverter (when he gets one) that exceeds 1000 amps.

It’s a very old engine, of British manufacture.



He has an old concrete hull boat that he got for free together with moorage. He has done a lot of work. Now completely rewired, according to “Caulder” and pretty much ABYC, over current protection etc, Blue Sea, and so forth.

I told him “Don’t do this!” But he did it anyways!

Eric’s pretty capable. He repairs small engines, bob cats and excavators etc.

 

[KnotYet]

I would like to see/hear more about that engine.
A quick web scan reveals it to be a 5.7L 6 cyl diesel.
Beefy!

 

[JDCAVE]

Eric has corrected me. He used a “Charger Booster” to start his engine, not the house bank.

 

[JDCAVE]

I would like to see/hear more about that engine.
A quick web scan reveals it to be a 5.7L 6 cyl diesel.
Beefy!

Hmmm. I think it’s a 4 cylinder. They come in a 4 and a 6.



Jim

 

[KnotYet]

Hmmm. I think it’s a 4 cylinder. They come in a 4 and a 6.

View attachment 136215

Jim

Obviously if you or he has eyes on it, you will know. I was going by the ID tag's '57'.

 

[BruceK]

BMC stands for British Motor Corporation (long before it was a racing bicycle team brand).


BMC cars

• Austin.
• MG.
• Morris.
• Riley.
• Wolseley.

Odd fact: A Sydney charter ferry engine was started by the equivalent of a clutch start. Towed in neutral, reaching necessary speed, into fwd, engine started.

 

[CharlieJ]

@Bacchus #10:

Good point but to clarify for others. ABYC calls for Type T fuses for Hi AH bank protection.

A fine point; ABYC does not specifically call for "Type T fuses...". E11.10.1.2.3 calls for an overprotection device with a minimum AIC of 20kA @ 125VDC or greater. Fuses that meet these requirements are Class T or HN. There may be others. This requirement does not rule out qualified circuit breakers.

 

[CharlieJ]

@JDCAVE: An ABYC compliant system for emergency starting from the house bank would be a properly sized conductor w/o a OCPD routed from the house bank B+ to the line side of a ON-OFF battery switch marked "Emergency Starting Cross-connect". The load side of this switch would be connected to the load side of the isolation switch between the starting battery B+ and the engine starter.
If the starting battery is weak/fails, remove it from the circuit by opening the engine battery isolation switch and closing the cross-connect switch. The house battery provides power directly to the starter.

 

[KnotYet]

Hmmm. I think it’s a 4 cylinder. They come in a 4 and a 6.

View attachment 136215

Jim

Looking at that photo anew, there are 4 exhaust runners but maybe 6 injectors?
It's not unheard of to siamese exhaust or intake manifold runners on old designs.
I think that's the 5.7L.

 

[SteveK]

Remove the valve cover and count the injectors.

BTW, I think it is a 4 cylinder as I can only count four spark plugs (hehe).

 

[KnotYet]

Those look like injector tops. I see no plug wires but do see fuel pipes
and think the engine in that photo is a 6 cylinder diesel.

JD, tell your son we need better photos!

 

[SteveK]

Those look like injector tops. I see no plug wires but do see fuel pipes
and think the engine in that photo is a 6 cylinder diesel.

JD, tell your son we need better photos!

I find myself agreeing with KY.
It is a 6 cylinder, 2&3 share one port and 4&5 share another. The timing must allow this sharing.

 

[Nautica]

That picture is clearly of a 6 cylinder.

 

[JDCAVE]

Sorry for the late reply. Yes it is a 6 cylinder. My mistake. I was looking at the manifold. Thanks for everyone's contribution.

Jim