Battery size for 90hp Outboard

[slimtim]

So I'm looking into getting a 90hp Tohatsu TLDI engine. Tohatsu have told me I need a 120Ah, 700CCA (850MCA) battery for it (which weighs about 28-30kg). Now as far as I was aware, these engines ran on petrol, not electricity and I'm no battery expert but it appears that this battery is huuuuuge. The helpful guy at Tohatsu told me this is the battery I need and I could use something of lower capacity but it could result in errors being logged on the engine management system and eventually a a shut down into limp mode.

Given that the Tohatsu guy seems to know his stuff, I will get this battery but I'm just curious as why a 90hp engine needs a battery big enough to power a small town. Is anyone running one of these engines, or an optimax equivalent, with a smaller battery than this without any problems?

Thanks in advance.

Tim

[jyasaki]

Need to be able to crank the motor, which is why it's huge.

Trust the manufacturer; they've been there before.

jky

[Ian M]

Mmm, I'm not a battery expert, but I seem to remember reading that the CCA rating (cold cranking amps) is the measure of a batteries ability to turn an engine over, and it is not the capacity (amp hours), or in basic terms, the amount of electricity, that the battery is able to store? This is why you have starter batteries, able to supply large currents quickly (thin plates) and deep discharge batteries, that can hold a large charge and are able to deliver a reasonable current over a long period of time (thick plates!) without distorting. Or have I been at the mushrooms again

[slimtim]

Originally Posted by jyasaki

Need to be able to crank the motor, which is why it's huge.

Trust the manufacturer; they've been there before.

jky

Yes of course but a FAR smaller battery will crank the motor. Its not a big motor, 90hp, 3 cyclinder, 1.3 litres. Much smaller batteries turn bigger car engines. As I said, I will get the battery he recommends because this guy really knows his stuff and is extremely helpful but I'm just curious as to why and wondered if anyone else has had any experience of running smaller batteries.

I read somewhere that the compressor for the fuel injection requires a bigger battery but shouldn't the engine's alternator supply all the current required when the motor is running? If it didn't, the battery would just drain away as you used it.

Ian M, its been a while, how are you? Yes I think you're right there which is even more puzzling to me because the Ah are a measure of supplying a certain current over a long period (i.e. to power electronics and lights etc. in a boat/motorhome when at rest) which doesn't really apply to an engine battery as the alternator should be able to do this while its running. For the record, I have minimal electronics on my boat, some gauges a fish finder and a bilge pump.

Cheers,

Tim

[ashbypower]

Tim, you have hit the nail o the head.
The compressor needs to turn quite fast to supply the air pressure required.
Now this is a load on the battery in addition to the starter motor load.
When High current is taken,voltage drops, due mainly to the internal resistance of the battery. Now if this voltage drops below a certain threshold the ECU will detect this and this will cause a fault code et al..
Our Optimax engines all have high CCA batts and we never have a problem, you must remember however that an opti will never start first turn of the motor it WILL take 2 perhaps 3 turns to build up the air pressure.

[Ian M]

Originally Posted by slimtim

Ian M, its been a while, how are you?

Fine thanks Tim, still bumbling around the Solent, but I think you have been on your travels?

[slimtim]

I live in Switzerland now which means boating has been exchanged for mountain biking and skiing. I still wish I could bumble around the solent more regularly though :-(