Replacing Batteries… When?

[Howler]

We have 7 AGM batteries: 4 house, 1 starter and 2 bow thruster. They have performed well for 6 season. Next year will be the 7th season. I’ve heard different philosophies:

• Change them before they go bad when you really need them (5-7 years)
• Wait until they show signs of going bad
• Test them one at a time and change those that fail test

What are your best practices on this question? Thanks!

[sailorboy1]

I don't replace them till they start showing signs of going bad. Even then most of the time you can wait a little if needed till more convenient to do.

But I learned last year that it isn't a good ideal to take off for a months long anchoring out trip without testing the batteries. I ended finding mine were pretty much useless right before crossing to Bahamas, which would have been an issue. So, turn off charging and run them down to see how they response before hand.

[rslifkin]

I replace batteries when they're no longer performing adequately. I don't worry too much about if a starting battery fails. If it does, I'll flip a switch to start that engine (or generator) from another power source and obtain a replacement battery.

For house batteries, I pay attention to voltage vs estimated SoC when we've drawn them down a bit. If the voltage (especially under a typical load) at 60% SoC is significantly lower than it used to be, the batteries are probably getting tired and either further testing or replacement is in order.

In terms of battery age, my AGM house batteries are on their 3rd season (so ~2.5 years old) and showing no signs of degradation.

Start batteries (also AGM) are doing just fine as well (both engines and the generator crank nicely, even if I turn off the charger for the start batteries and let them just sit for a few days). Those were installed spring of 2018, so this is their 6th season (and a bit more than 5 years old). I expect I'll get another couple of years out of the start batteries before it's time to worry about them.

[cr180]

Depends on your cruising style. If you can tolerate a failure, get towed back to marina for example, run to failure. But if you cruise long distances to remote areas, then replace at 5 years.

I’m of the replace at 5 year interval school. Cruise northern and central Bahamas for months at a time. Have never had a battery failure. That assumes proper charging regimen; for lead acid batteries, don’t let house bank go below 12V before recharging.

[Senojev]

When the annual capacity check of the batteries at the start of the season shows less than 85% of their original capacity.

I am in a situation when I could get replacements in a week if they totally fail but with 8 batteries, I could probably cut out the worst and make do.

[smac999]

Do a c20 test on the house bank. Google it. If it’s ok the others should be. If the house is under 80%. I would replace them all.