Start battery spec capacity vs. MCA

[roaringgirl]

Hi, the engine (Yanmar 4JH2-TE) says it wants a start battery of at least 100Ah. The only thing the engine battery runs is the starter motor, engine instruments and the fuel pump. The engine battery has a dedicated alternator. In the case of engine battery or alternator failure, I have an emergency parallel switch to power the engine from the 400Ah house bank (which has its own alternator, and other charging systems).

I don't really understand why a start battery would need a specific capacity, if the MCA (marine cranking amps) is sufficient, why should I care about the capacity?

 

[[2574]]

I suppose the only reason might be battery endurance in case the engine doesn’t start immediately. I recently fitted a thruster bank and primarily concerned myself with MCA and CCA, AH capacity wasn’t of great significance other than, again, endurance.

 

[Tranona]

Hi, the engine (Yanmar 4JH2-TE) says it wants a start battery of at least 100Ah. The only thing the engine battery runs is the starter motor, engine instruments and the fuel pump. The engine battery has a dedicated alternator. In the case of engine battery or alternator failure, I have an emergency parallel switch to power the engine from the 400Ah house bank (which has its own alternator, and other charging systems).

I don't really understand why a start battery would need a specific capacity, if the MCA (marine cranking amps) is sufficient, why should I care about the capacity?

That is correct - but with "ordinary" batteries the CCA rises with capacity. So if you want a smaller capacity start battery with high enough CCA you need to go for a dedicated AGM start like a Red Flash or an Odyssey. I have had both at 28Ah, the current Odyssey starting a 30hp Beta. Red Flash do a 42Ah which would probably be suitable for your engine tayna.co.uk/car-batteries/red-flash/1200/?gclid=EAIaIQobChMI25HN-vnP_QIVQu7tCh3mygxMEAQYASABEgIFfvD_BwE

As you see not cheap at twice the price of say, a 95Ah Exide stop start AGM as used on Jaguar and Land Rover diesels which I had on my last boat as engine start, house bank and bow thruster and have on my current boat as house with the Odyssey as start. Big advantage for me is that I could get all three in the existing battery box, but if I did not have a space issue I would have used a basic 60Ah car battery for start.

I am a great fan of Red Flash/Odyssey types - the 26Ah one in My Morgan lasted 19 years from new!

Photo shows current boat installation - not fully wired at this point.

 

[noelex]

That is correct - but with "ordinary" batteries the CCA rises with capacity. So if you want a smaller capacity start battery with high enough CCA you need to go for a dedicated AGM start like a Red Flash or an Odyssey.

Agreed, I think Yanmar are using AHs just because it is more easily understood and it avoids the confusion of different CCA standards.

We have a newer version of Roringirl’s engine a Yanmar 4jh4-te. The start battery is only 50AHs, but is a spiral wound AGM so it has plently of cranking amps, which is what is important.

 

[LittleSister]

Agreed, I think Yanmar are using AHs just because it is more easily understood and it avoids the confusion of different CCA standards.

That's what I've always assumed. Other manufacturers such as Bukh do the same. (It's only since the internet, I suspect, that consumers have concerned themselves about CCA.)