New batteries needed

Lithium batteries are not more expensive than AGM:
400Ah of Victron lithium cost me a bit under 4000USD. So let's call it 10USD per amp hour. My 'old' AGMs had a nominal capacity of 540Ah so the useable capacity was 270Ah. They cost me 2000USD, so about 7USD per (useable) amp hour. The AGMs lasted 18 months, so every month the lithiums last over 2 years, makes them cheaper than AGM. They are expected to last 10 years.
My AGMs have lasted more than 11 years so I’m not sure your calculation is quite so clear cut.
 
My AGMs have lasted more than 11 years so I’m not sure your calculation is quite so clear cut.
I agree, and my last AGMs had lasted the 6 years I'd had the boat plus however long the previous owner had had them. I either got unlucky, or the AGMs were quite happy when being used occasionally, but couldn't handle us moving on board permanently.

I think a number of factors hastened their demise:
1. After a little over a year living aboard I finally got down the list to the alternator, which had been mis-installed in numerous ways. As a result I think it always worked at 40% regardless of what the regulator told it to do. This probably resulted in some overcharging particularly the 4 days of solid motoring at the end of our Atlantic crossing.

2. The boat was left on the hard in Curacao over hurricane season. When we returned it had been so hot inside that some cans had exploded, I doubt the AGMs liked that much.

3. And finally, living aboard and relying on solar and wind to provide all the power meant that the batteries rarely got fully charged and I think they really disliked this - for us this is the biggest advantage of LiFePO4 - they don't care about partial charging, so are ideally suited to liveaboard boat use with renewables.
 
I agree, and my last AGMs had lasted the 6 years I'd had the boat plus however long the previous owner had had them. I either got unlucky, or the AGMs were quite happy when being used occasionally, but couldn't handle us moving on board permanently.

I think a number of factors hastened their demise:
1. After a little over a year living aboard I finally got down the list to the alternator, which had been mis-installed in numerous ways. As a result I think it always worked at 40% regardless of what the regulator told it to do. This probably resulted in some overcharging particularly the 4 days of solid motoring at the end of our Atlantic crossing.

2. The boat was left on the hard in Curacao over hurricane season. When we returned it had been so hot inside that some cans had exploded, I doubt the AGMs liked that much.

3. And finally, living aboard and relying on solar and wind to provide all the power meant that the batteries rarely got fully charged and I think they really disliked this - for us this is the biggest advantage of LiFePO4 - they don't care about partial charging, so are ideally suited to liveaboard boat use with renewables.
That sounds about right, enough abuse for significant detriment. I’ve always looked after our lifeline AGMs - mainly because they’re so damned expensive!
 
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