How to evaluate used batteries

[Lee72]

Hello all,

I am thinking about buying used batteries. Someone offered me 2 AGM batteries which are a year old but where never used only charged.

Is there a way to figure if they are good beyond reading the volts and see if they are 12.8 or more?

Any suggestion?

Markus

[Mike90250]

Testing used batteries is a bit of a science project. Get a voltage/State Of Charge chart for your prospective batteries (from the battery mfg) Charge them fully, with the proper charge rate/voltage. AGM are tricky, too much charge and you vent / loose elecrolyte that cannot be replaced.

After they are charged, attach a known load to them, and remove about 70% of the rated capacity. Remove the load, and wait another couple hours before you measure the voltage. If it corosponds to the chart, they are pretty good.

Likely, they have not been on a maintainance charge, and have lost capacity. Even if they test good, I'd not pay more than what new flooded batteries of same capacity cost.

[Sunking]

You are between a rock and a hard place. Only way to tell is with a load test. It is time consuming, and unless you have th equipment is a bit expensive.

As Mike mentioned you have to first fully charge the batteries. If you do not have a charger to do that, well then it gets expensive.

Once charged you have to apply a known load of the proper size, and then monitor the voltage until you reach a given set point, let them set and rest for several hours and them measure the voltage again. Last step is then to do some calculations to get you in the ball park.

My advice is to pass and buy new batteries.

However here is the process. Problem is I do not know the size of the batteries, so I cannot be precise with the times and loads. For this discussion I will assume you have a 12 volt 100 AH battery.

You want to put a load on the battery that will discharge at the 4 hour rate To find that rate is C/4 where C = the 20 hour Amp Hour capacity of the battery. In this example C = 100 AH, so 100 AH / 4 H = 25 amps. Next problem is how do you find a load that can handle 25 amps. Well the Poor Man James Bond Method is car headlights so you would need 25 amps x 12 volts = 300 watts worth. That is about 3 to 4 car head lights worth. Getting any ideas? Yep put it in you rcar and turn on the head lights.

You then stay there and monitor the voltage, and when it hits 10.5 volts terminate the test and note the time it took. So let's say it took 3 hours. The capacity is 3 hour x 25 amps = 75 amp hours of capacity.

Next let them rest for several hours and remeasure the voltage and then see what what the state of charge is. Lets say it measure 10%. 10% of 100 AH is 10 AH, plus the 75 you removed = 85 AH capacity. They are good batteries.

Now reread my advice and see what you think.

Second option is buy them and take your chances. Pay no more than 50% of new.

[Lee72]

Good guess, they are 100Ah! The test sounds doable but not when I am at the person selling the batts.
They are of type 8A31DT and he wants $360. Is that good?

[Sunking]

They are of type 8A31DT and he wants $360. Is that good?

Well you can buy a new Deka 8A31DT all day long every day for $210 to $220. So for about $60 more you can buy new.

Here are some tips.

• Ask if the batteries have been on a charger since purchased. If not walk away
• Inspect the batteries real closely. He claims they have never been used right? If true they will show no signs of wear, and look new. If you see signs of wear, he lied, walk away
• Inspect the date code to see when they were made to validate his claim they are only a year old. If his claim is true, then the manufacture date should be no more than say 18 months or less. If they indicate more than 18 months he likely lied and you need to walk away.

[Lee72]

I have batteries from the same manufacturer (MK, type 8A22NF) - where do I look to find the manufacturing date?

[Sunking]

Try the battery terminal post.

[Lee72]

I wrote MK about the manufacturing date an thats the answer:

There should be a round black sticker with the letter and a number that would indicate the month and the year. If not on the top label inside of a black box near the part number a letter and number would indicate the date code.
Either way a-2 would indicate the battery was delivered or distributed in January of 2012. B-2 would indicate February 2012 and so on and so forth.


really cryptic encoding...

[Sunking]

really cryptic encoding...

Just about all use encryption coding to confuse the Chi-Coms

[Lee72]

So the batteries are from March 2011 and have been charged 3-4 times just sitting in the shelf. Does not sound to bad, right?
I will take a look at them on sunday.

[Sunking]

So far so good anyway.

[straitace]

I also just got a used DEKA 8G4D GEL battery how do I test if its 183AH rate at c/20 as its spec suggests.
do i plug a 9.15AH load for 20 hours or a 20 Ah load for 9.15 hours which one would be more accurate and for one test will the battery get damage if I recharge it right after the test