Engine Rating for Maximum Battery Capacity

[alloyboy]

Now what would you make of an outboard motor maker publishing a maximum capacity value for a starting battery? I am having a hard time wrapping my mind around a battery being too large.

[jimh]

I can't see any harm coming from the battery being able to deliver too much current to the outboard engine's cranking motor. The battery does not force current into the cranking motor; the cranking motor demands the current (it wants) from the battery. As long as the cranking motor gets all the current it wants, it should not have any complaint about where it came from. An analogy might be cruising along at 4-MPH on a small boat with a 300-HP engine. The engine can supply more power, but there is no harm (to the boat) in getting the lower power it needs to go 4-MPH from a 300-HP engine.

The only possible situation I can imagine in which connecting a very large-capacity battery to an outboard engine might cause harm to the engine is when the engine is not being started, but, rather, when the engine has already been started and it is trying to re-charge the cranking battery. The harm could occur if there were a situation like this:

--the engine is running, and its alternator is trying to develop current to charge the cranking battery

--the cranking battery is deeply discharged

--the cranking battery has enormous energy storage capacity, say something like 250-Ampere-hours

In this situation the engine alternator is going to be asked to deliver its maximum output voltage and current to re-charge that big battery for a very long time. This long-duration operation of the alternator-battery-charger circuit might cause it to overheat. The overheating might cause damage.

There is one problem in my proposed scenario: how did the engine get started if the cranking battery were completely discharged?

You should ask your question to the manufacturer of the outboard engine who published this specification. Who was that?

[jimh]

ASIDE: I have posted that same or similar advice on the diagnosis of battery-powered devices before. For example, I also said:

Another maxim I have developed after many years of experience:
If trouble develops in any battery operated electrical system, the first component to check is the battery.

That was posted 10-years ago in an earlier article. See
http://continuouswave.com/ubb/Forum6/HTML/001139.html

I also wrote an article about engine cranking problems and finding electrical problems. In that article I said:

Sources of Trouble
The Battery

Whenever there is trouble in the engine electrical starting circuit, the battery is the first element of the circuit to investigate.

That article appeared in 2008.

[Jefecinco]

In my experience batteries do not fail gradually. During the past forty years every battery I've had to replace was due to sudden failure in which no symptoms were previously exhibited. I've not yet had a marine battery fail but I replace them at about the seven year point.

The battery in my wife's Milan failed suddenly two weeks ago as we were leaving for a few days on the Gulf. It would not turn the engine. We had slightly less than eight years use from that battery.

I highly recommend dual starting batteries or a jumper battery in boats.

[alloyboy]

Engine maker is Yamaha. No one at the USA distributorship would be able to answer a question about the maximum battery size specification. Maybe not even anyone from Yamaha Japan. I suspect that Mitsubishi makes the electrical stuff for Yamaha. Would be hard to find Fujimoto at Mitsubishi who might could answer the question. He might not even speak English.

Yamaha outboard motors use permanent magnet generators. The kind that are producing all of the electrical power of which they are producing at any given engine RPM. I can't see a PMG stator or R/R being harmed by continuously recharging a battery. Current is either going to go to a battery or up in heat. It should be mox nix to the electrical generation system. Maybe.