new AGM or new starting battery??

[Sailorman Ed]

I am helping out a recently widowed friend who spends considerable time living in the marina on her trawler. I went to start the SB 4 cyl Mercedes diesel but the single 825CCA Diehard AGM battery was weak. Her husband had previously told her that he planned to replaced one battery ( the other bank is 2 AGM in parallel) She checked today and found the battery was hot ( is on a continuous charger) so I had her disconnect it and said I would bring down a new battery. This boat moves once or twice a year but is normally started once a month. The fridge, lights and all is powered by the double AGM bank. I normally would only use a starting battery for starting but since it is used so infrequently, would an AGM be better in this case?

[tkeithlu]

Let me make sure that I understand here.

Correct that there is a two battery AGM house bank, plus a separate starter battery?

If they are on the same charger or the same alternator then making them all similar is indicated. A charger or a voltage regulator (at least in the sophisticated versions) needs to be programmed for AGM charging. Being AGM structure does not eliminate being a starter battery. Starter batteries are designed to supply a whopping current briefly while house batteries are designed to supply lower current for a long time.

But, and it's a substantial but, this sounds like a simple system that is really not tuned as such, and probably does not rely on 100% efficiency from the battery set up. Battery purists may kill me in my sleep, but my recommendation is that you find a battery, AGM or not, that is similar to the decision made by the late skipper, and move on. That solution apparently worked OK in the past.

[Sailorman Ed]

plus a separate starter battery?

No separate starting bat. The charger looks to be a 2 zone charging sys. I am not sure it was set up for AGM or not.



I know it is best to have all batts the same age so I don't think adding a new AGM to a couple older AGM in the same bank is the best way.
I am thinking that a regular starting batt with a small trickle charger might be a better way to go.


Trying to get info on the charger there now.

[smac999]

Originally Posted by Sailorman Ed

I normally would only use a starting battery for starting but since it is used so infrequently, would an AGM be better in this case?

the use frequency would not matter as I'm assuming it's always on a charger anyways. so either would be fine.

I would go with an agm since the house already is.

[Scubaseas]

Assuming the charger is automatic and set up for AGM, stay with AGM. AGMs have a much lower self discharge rate than FLA but if it's plugged in all the time it won't matter.

Lots of starting batteries are AGMs nowadays. I use a H8 on my Yanmar 54hp diesel. The price difference between AGM and FLA starting batteries isn't much these days.