Yanmar Operation Manual states it should be 70AH. There a lots or previous posts on batteries, including one in the last few days. I would buy A leisure battery from a caravan shop.
I've found most auto shops can order leisure batteries if you ask them and most are cheaper than chandlers or mail order ,if they look at you like you have two heads when asking for a "leisure battery" get them to check their suppliers catalogues.
It depends on what else runs from the battery. My old 2 1/2 ton Hillyard had just a couple of interior lights and a masthead light and a regular heavy duty car battery was fine - about £20.
Deep cycle batteries, as the name suggests, are designed to be be continually run down to about 50% charge before re-charging is necessary. Thus a 70AmpHour battery would provide 1A for 35 hours before needing to be recharged.
Remember also that the alternator on the 1GM isn't that big. So the above example would probably need a few hours to recharge completely.
Alex Rogers
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I have a 1GM10 and have used a 70 amp gel battery for six years, but was concerned about the ability to cope using a small cool box recently purchased, it takes 4amps. Last year I installed a second battery, with a split diode a dual battery switch and a 600amp bus bar. The bus bar is probably slightly overrated, but the next size down was too small, the cold cranking of a 1GM10 is 460amps as stated by jonathan.hay@barrus.co.uk. Now on measuring the voltage output that the battery sees with this set up, if I remember correctly was below 11 volts with the engine at idle, and I believe you really need 11.7v. I am not sure if the voltage would increase any with more revs nor did I test this thought. This year I am going to add one of those smart charge regulators which claim to give you more from your battery. Bus bar obtained from Power-Store also see www.yachbits.fsnet.co.uk where I got the battery switch and diode splitter, they also had a diagram of the a installation. If you wish, I can sketch what I did and fax it to you- fax number required and an email address and I could email some photos.