Lars, i think that by researching only the numbers, you may be suffering from tunnel vision. There are more considerations than you are looking at, such as what repeated use will do to battery longevity.
Yes, I have used a 17AH AGM battery to start a Volvo
MD7A, which is similar to your engine size. It fired right up and turned right over, but I would never use that battery for that purpose regularly. The current
being pulled by the starter would eventually damage the plates in the battery. As an emergency
start or to get the last pound out of a racer
, sure. But for a 1kW satarter motor
, in the US we would probably specifcy a "Group24" SLI battery for that engine. Same battery that is used for compact cars, which typically have a 1kW or 1.5kW starter motor
The larger battery takes less damage from each starter use, and provides a more reasonable lifetime.
You CAN use the house deep cycle batteries for starting as well, once their combined capacity sifficiently exceeds the draw from the starter, it makes no difference that they are deep cycle. But all in all, if you want a dedicated starter battery (what we call an SLI Starter-Lights-Igntion battery) your best bet is just to go to an auto supply store and buy whatever they sell for a sub-sompact or compact car. The load is about the same, the non-marine price
will be significantly lower.