The first thing is to perform a test on the battery, you won't be able to do this, your best off taking it to a specialist as they have the equipment for testing it (when it's fully charged!), it may be the battery has had it.

The main thing with a battery is the CCA, with a bigger engine and higher compression this increases but in doing so does the AH figure as well.

This AH figure is how many amperes it can supply over an hour, theres far more to it than this, but as a rough guide if you had a 10A load, it would last four hours in your case.

Remember it's the charging system, i.e. alternator that provides the power, in addition to charging the battery.

If your battery is a little on the small side for turning over that engine as so this is killing it.

Martin