battery

Get yourself a battery load tester. $30-$40 at any auto place or Amazon. Handy gadget to have around. It has a coil in it like a heater which puts a large load on the battery. You look to see how fast/if the battery recovers voltage after the load is taken off it.

I have a group of batteries and one was hot with shore power plugged in and the rest normal to the touch. Pulled the battery and tested it and it was bad. Do a simple 'feel' test.

When you think it is fully charged... disconnect the battery for a couple days and see if it holds 12.6+ volts. Then check with a hydrometer.

Or just unplug the RV 24 hours and see how it performs. If it works for you it is good enough.

The old fashion way. Disconnect the battery and clean it. Then put it on a "smart" charger for at least 24 hours or until the charger has completed its cycle. Let the battery rest for about an hour and then check the voltage. It should be between 12.0V and 13.0V, higher is better.

Buy a cheap battery load tester from HF. Connect it to the battery but only for about 10-30 seconds. The meter on the tester will give you an approximate idea of your battery health, but you should also have your volt meter hooked up. If it drops below 11.0V in that short period, your battery is definitely near the end of its life. When the load tester is removed, the battery should return to about the same voltage before the test.

You can check a few things to see how it's charging, but the bottom line is when the battery no longer has enough reserve to meet your needs is when it is bad.

The most common "failure" mode of a deep-cycle battery is that it doesn't last long enough for what you need it to do. After the first few charge/discharge cycles, the battery gets smaller every cycle, but my battery needs and yours may be so different that there's no test or number to tell you whether a battery is good or not. Someone who frequently boondocks may call a battery bad 2 or 3 years before someone who camps with full hookups every night would replace the same battery.

Just about everything has been mentioned;
1. Measure voltage at rest. Meaning; disconnect battery after it's charged, let it sit 24 hours and measure the voltage. Should be 12.7 volts.

2. Check the electrolyte specific gravity. This assumes your battery is wet cell and not a sealed battery. Also assumes you have a quality tester and know how to safely measure and what is good or bad readings.

3. Load test the battery by putting a load on and checking the battery's ability to sustain the load without a disproportionate loss of voltage.

4. Replace the battery every 5 years as preemptive maintenance.

While I do perform steps 1, 2 and 3, I still also perform step 4 and just replace the thing. Seems a battery will always fail at the most inopportune time. Buying a battery on your time schedule allows you to shop for a sale, use a coupon, research what would be best for your application, etc.

I usually buy my batteries from Costco or Sams. Their return policy can't be beat; bring it in, get a new one. No hassle.

grahampconrad wrote:
How do know when your battery is on the way out? I check it for water and check the light meter to make sure its charging when the trailer is plugged in .

Other than performing a true load test, pulling the specified 20A for the amount of time it takes to meet battery AH specifications, the other best indication is measuring the Specific Gravity of each cell.

If you can unplug from power a day or two and your slides still work well afterward then the battery is probably still good. But a seemingly good battery that is nearing end of life can suddenly fail. Ever had a vehicle that always started fine until one day it didn’t? Batteries have a warranty period depending on the quality of the battery. Good idea to replace that battery before that time is up. The manufacturer knows about how long they are designed to last and then they set the warranty time accordingly.

The light meter? You mean the 3 or 4 LED meter labeled "full" "1/2", etc?
That meter is worthless for determining much of anything.
When a battery is going bad it uses more water. If you've been noticing a pattern of water use and it increases that's a bad sign. Also, it will lose capacity and not last as long as it used too. There really isn't any way to know "This battery is 80% gone and needs to be replaced in the next 50 days". It's just not that exact.

You won't know really if you are plugged in most of the time. The battery acts as a capacitor/storage in that case. If you are unplugged and it fails to run the slides properly, it is on the way out. Others may have differing opinions, but that has been my experience.