I used a marine starting battery for my solar power system, but it is pretty much worn out. I had it for about three years, and it was about three years old when I got it.
The old battery has almost 100 milliohms at 4 amps. A new one should have less than 20. To test this, I ran a 40 watt load on an inverter for about one minute and measured the voltage, then turned it off for a minute and measured again. Following Ohm's Law, R=V/I. I took the difference between the two measurements and divided by 4.2.
The capacity has also dropped significantly. It should take over a week to charge (with no usage) once the under-voltage protection activates. However, it only took a few days with heavy usage.
The final straw was not having enough power to charge a phone overnight or open my motorized roller blind after a couple of cloudy days.
I decided to replace it with a used car battery from a junkyard. I found a few places that sold batteries for around $40. Some had no warranty. Other places had extra fees. None of the junkyards were close to where I live or along my travel routes. I decided that it made sense to buy a brand new battery for about twice the price. I chose a type of battery that is designed to be charged and discharged frequently. This one has terminals that are easy to connect to, and it is possible to check the fluid levels. It didn't come with a data sheet, but I was able to look up most of the information online. Making sure that the fluid level is correct will allow the battery to last a long time.
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