Hello Jorge, (George?), For a fine selection of inexpensive digital multimeters, try MCM Electronics. www.mcminone.com. I recommend getting one with detachable test leads so you can replace them if they get chewed up or wear out. An AGM sealed battery is the best one for the boat, higher current power and capacity and completely sealed. To the best of my knowledge, the lowest compression post lined up with the mid post and mast on my M17, but I will need to look next time I'm aboard. Later, Bones