technical question about 6VDC batteries

[jdarwin]

I have a technical question about voltage and capacity in batteries. I had a dealer tell me my batteries (2 6VDC golf cart batteries connected in series) needed replacing. I checked them with an analog load tester. They checked OK. The repair tech used their digital tester which showed both batteries had 6VDC but one had 78% capacity and the other had 75% capacity. I then took a hydrometer and checked each cell of each battery. One battery showed each cell as good. The other had 2 cells at the mark between good and marginal (the line between the green and white marks). Since both batteries were identical (235AH) how does a battery show fully charged 6VDC but only show 175AH (235*.75)?

[AZ Traveler]

John,

It sounds like your hydrometer is showing you have an issue with one of your batteries. The load tester is best to check starting batteries instead of deep cycle batteries.

Are you seeing shorter times your batteries will run items in your rig? If so, you are likely getting near the point where replacement is recommended.

[Cat320]

How old are the batteries?

[Creeker]

What does each battery measure with a digital Volt meter?
A fully charged 6v golf cart battery will read around 6.35v to 6.37v (at rest) depending on manufacturer.

If your battery or batteries are only 6.00v at rest, they are actually less than 50% charged.

Several "State of charge" charts for 6v batteries available on line.

[jdarwin]

Thanks Zack. I guess I'm trying to understand how capacity is measured. For example, a battery rated at 12.6 VDC and 100AH how can it measure 12.6 VDC and measure 75% capacity (75AH)?



When we tested both 6 VDC batteries they tested at 6.2 VDC. I googled a chart showing 6.2 VDC is equivalent to 75-100% "state of charge". The dealer was trying to tell me the batteries needed replacing. My question is why does a battery showing 75% capacity at 6.2VDC need replacing. Using the hydrometer, one of the batteries will need some maintenance or possibly replacing in the future. I just didn't see the need to replace both batteries.


What am I not getting?

[AZ Traveler]

John,

The voltage give you an idea of the state/level of charge not the capacity. Please let us know how old the batteries are and if you have had any trouble with them.

Lead acid batteries are relatively inexpensive, I tend to place them based on age unless they give me problems. Decent golf cart batteries should last 6-7 years with regular watering, a good charging system and not letting them discharge below 50%.

[jimcol]

I think the important question is Zack's, "Are you seeing shorter times your batteries will run items in your rig? " Batteries start losing capacity the minute they are put in service, albeit ever so slowly. If using these batteries is not affecting your RV'ing use/style, I'd keep on using them.
Remember, the RV dealers number one goal is to sell stuff.

[JABURKHOLDER]

You said you possibly need to replace one battery but don’t see why you need to replace both. Any time you run pairs of batteries, they both degrade over time. Replacing only one will cause the old one to drag down the new one. Always replace in pairs with exact same ratings, same brand and same manufacturer dates. You will avoid replacing one battery at a time every three or so years.

[jdarwin]

The batteries are "US Battery US2000XC2 GC 2". 6 VDC/220AH. I bought them in March of 2021, so they're almost 2 years old. I've had to do little maintenance on them. Everytime I check the levels they are full. I've only put distilled water in them once. Since we live in the rig FT the batteries have been in constant use since March 2021. We're on campground power or a generator 95% of the time (except when traveling). I haven't noticed a reduction in run times.