SethB wrote: ↑Fri Apr 20, 2018 11:49 am
CapnLoki wrote: ↑Wed Apr 11, 2018 3:43 pm
...I try to replace them when they're down to about 80% capacity...
CapnLoki, how are you evaluating the capacity of batteries/battery banks as they age?
There are a number of ways to determine the capacity and state of charge (SOG) of a battery. My primary tool is a Amp-Hour meter which measures AH in and AH out continuously. Since I live on board during the summer, I'm glancing at that at least a dozen times a day. I also have a AH meter on the primary load, the refrigerator, and I record that number daily so I can track the fridge health over time.
I also look a the voltage frequently, but since there is always a fluctuating load its difficult to interpret. Although many folk make claims like "12.6 V means a fully charged battery" its actually the least reliable measure.
Health of a bank is often measured with a load tester, where you measure the voltage decline when a fully charged battery is given a large load. I don't have such a device, but I can turn on the microwave (through a 2kW inverter) which pulls a 90 Amp load and note the drop. Nowadays you can get a conductance meter battery tester for under $100 - there may be one of these in my future. There is also the "full capacity test" where you fully drain under controlled conditions, but this is rather extreme.
When I'm considering replacement, I start testing the specific gravity with a hydrometer (remember I have flooded cells!) so I can determine the SOG when the AH meter says 200 AH have been used. This is as close as I can come to a real health test. I often do this at the end of a season to know if I'm getting new batteries in the spring.
Normally I can "just feel" when my batteries are getting old. And when they are old they are more susceptible to dying from minor abuse - tired batteries are more likely to die when neglected in a harsh winter. So my solution is to replace after about 6 years at the first sign of significant decline. If I were only using the boat occasionally, I could probably push it a year or two longer, but since I live aboard, and problems usually happen in hard to get to places, I replace early. My net cost per year is about $100, while the cost of an untimely problem could easily be 10 times that.
hubby rigged a 12 volt thing in our motorhome. Works great.