Simple way to measure coach battery capacity

[Gerald55]

I'm looking for a reasonably simple way to measure the current capacity of the single coach battery in my newly purchased (used) RV.

I have a basic voltmeter which I don't think will be much help, but I do also have a kill-a-watt - so I figure maybe I just charge up the battery, then turn everything off except say the TV which I plug in through the kill-a-watt and measure it that way? Any downfalls to that approach?

[MEXICOWANDERER]

A significantly capacity impaired battery will fall flat on it's face when a load test is conducted. Thirty seconds rather than the BCI 15-seconds. Sample electrolyte clarity and density and call it a day. Ten minutes rather than many hours. Doing a spectroscopic analysis of the debris in your radiator bug screen would be on par with PT'ing something nearing terminal failure. If the effort is for self-education my comments here will summarily do an abrupt 180 degree reversal. But exploratory testing is best performed on other than a mortuary bound corpse

[BFL13]

Well true, but let's change that to a 96 year old RVer trying to swap out his four T-1275s!

That is why I said you don't run the full 10 hours. If the battery is not up to rating, then going 10 hours will put you below 50% somewhere. Maybe well and fatally below!

So the test is to see how long it takes to get to 50%. If it is only 5 hours, then that should tell you something, without killing them dead right then. 🙂

[MEXICOWANDERER]

Very politically correct. Stephen Hawking is one of my personal heroes.

But attempting to measure how little remains in a 7+ year old battery is like watching a 96-year old man deal with 300-lb barbells as a test to see if he would qualify as a bouncer.

[BFL13]

MEXICOWANDERER wrote:
Isn't this somewhat similar to strapping a pedometer to a quadrapalegic?

Just to alleviate any suggestion of "political incorrectness" clarification is sought---Is this before or after we use him as First Base?

[MEXICOWANDERER]

Isn't this somewhat similar to strapping a pedometer to a quadrapalegic?

[BFL13]

You don't run it for 10 hours and then see where you are at. You run it down at the 20hr rate until you are at 50% by SG, confirmed by bounce back voltage when you stop. Now you go by the time it took to get to 50% wrt 10 hrs.

EG, if you got to 50% in 9 hrs you are at 90% of rated capacity.

[westend]

Other than inaccuracies in how close one was to applying exactly 5 amps, how close one was in timing for exactly 10 hours, and how close one was in measuring the ending battery voltage .... why isn't this a "perfect method"?

One should also allow for the Peukert effect to achieve close to perfection.

[CA_Traveler]

pnichols wrote:

landyacht318 wrote:
Draw it down 50% of the original nominal capacity at the rate at which it earned that capacity and wait for voltage to rebound.

IE if it were rated 100AH at the 20 hour rate, apply a 5 amp load for 10 hours, remove the load, and wait a few hours for voltage to rebound.

If it rebounds to 12.2 or so, then the battery is still healthy.

By no means a perfect capacity test, but on a single battery of unknown age and condition it will give you a general idea.

Other than inaccuracies in how close one was to applying exactly 5 amps, how close one was in timing for exactly 10 hours, and how close one was in measuring the ending battery voltage .... why isn't this a "perfect method"?

It sure looks brilliantly eloquent in it's simplicity to me. The only reason I can see as to why battery shops couldn't/wouldn't use it is the time it takes to perform it while the customer is waiting!

If someone can explain what I'm missing here ... please do so.

It's a rare battery shop that knows how and actually will preform a proper load test. It can take a lot of hours and of course they have to know what to check, etc.

[pnichols]

landyacht318 wrote:
Draw it down 50% of the original nominal capacity at the rate at which it earned that capacity and wait for voltage to rebound.

IE if it were rated 100AH at the 20 hour rate, apply a 5 amp load for 10 hours, remove the load, and wait a few hours for voltage to rebound.

If it rebounds to 12.2 or so, then the battery is still healthy.

By no means a perfect capacity test, but on a single battery of unknown age and condition it will give you a general idea.

Other than inaccuracies in how close one was to applying exactly 5 amps, how close one was in timing for exactly 10 hours, and how close one was in measuring the ending battery voltage .... why isn't this a "perfect method"?

It sure looks brilliantly eloquent in it's simplicity to me. The only reason I can see as to why battery shops couldn't/wouldn't use it is the time it takes to perform it while the customer is waiting!

If someone can explain what I'm missing here ... please do so.

[landyacht318]

Draw it down 50% of the original nominal capacity at the rate at which it earned that capacity and wait for voltage to rebound.

IE if it were rated 100AH at the 20 hour rate, apply a 5 amp load for 10 hours, remove the load, and wait a few hours for voltage to rebound.

If it rebounds to 12.2 or so, then the battery is still healthy.

By no means a perfect capacity test, but on a single battery of unknown age and condition it will give you a general idea.

[Gerald55]

I'm not sure how long it has been in there since I don't know the history. I guess I should say that it's likely between 3 and 8 years old.

[kellertx5er]

Gerald55 wrote:
Thanks for the suggestions! Yes I was definitely asking about testing the actual capacity of my battery, not the nominal capacity which is easy to look up.

I've just bought a new rig with a battery that has probably been in there for ~8 years, and possibly abused, so I wanted to know how much extra capacity a replacement could give me.

I wouldn't even spend time analyzing an 8 yr old battery.

[Gerald55]

Thanks for the suggestions! Yes I was definitely asking about testing the actual capacity of my battery, not the nominal capacity which is easy to look up.

I've just bought a new rig with a battery that has probably been in there for ~8 years, and possibly abused, so I wanted to know how much extra capacity a replacement could give me.

[wa8yxm]

Group 24 73 amp hours
Gropu 27 95
Group 29 105
Group 31 130
Two GC-2 in series 220 give or take 10
4D 230
8D is I think 220 but not sure

Or is your battery not on this list.

This applies to batteries about 1 week to 1 month old

IF your battery is getting on in years and you suspect loss of capacity take it to a genuine battery store.. NOT an auto store but someplace that sells batteries and only batteries.. Many of them can test it for you.