Victron Telecom AGM battery - horizontal installation and cell endurance

DIY
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1

I just bought Victron Telecom AGM 165Ah batteries to replace my battery bank. I had the same Victron batteries, which lasted 4 years in marine environment, which seems not too long.

I have to put batteries in horizontal position, due to catamaran manufacturer space limitations and I can not use other form factors.

Question 1: Horizontal installation is possible, but not recommended. What is not good in horizontal installation?
Question 2: Can AGM Telecom battery develop a short cell after few years (12V become 10V)?. I read opinions that AGM construction protects from shorts very well, and it is hard to have short cell on AGM

2

Longevity usually comes down to cycling and proper charge care and practices.

Agm tends to sulphate dry out before anything else, so that will likely be what happens before plate warping.
I have never side mounted. Did look into it once. what you actually need to do since you have purchased your replacement is check if the manufacturer has specific requirements for the side mounting, (if they actually allow it) there may be additional things to consider for venting and other reasons.