How to check secondhand batteries are ok?

[shadiya]

Found some secondhand batteries for sale, can someone walk me through what would need to be done to check they are as good as seller describes? Thanks very much.

[oliver90owner]

Load test, voltage checks, electrolyte levels and specific gravity (FLAs), any date codes.

Likely not as good as the vendor suggests.  Just a case of buyer beware, particularly if only purchasing a specific number, so without any possibility of selection for matched sets.  You pays your money and hope you get what you paid for.

[shadiya]

These are gel batteries, so I'm guessing that electrolyte and specific gravity isn't relevant. What would I be looking for with load testing and the voltage check? I know nothing.....

[clockmanFR]

Hi,
Gel second hand, should read no lower than 6.5volts for a 6volt battery, and 13 for a 12volter.

Batteries should be the same make for each string, ie coupled together for say 48v.or what voltage you require.

Do a first reading test of each battery, and use a felt tip pen to mark each, go away for a cup of tea say 1/2 hour, return and re_test each battery and they should be the same.

Some battery sellers will charge your batteries just before you arrive, the above method will show thier trickery. 

[Scruff]

Dunno about Gel much if they were new OLA I'd discharge them one at a time from full full (not a typo) charge at rated C20 curve for 10 hours and see they hold 2.025V per cell after a 24hours rest.

An older set takes longer to get an estimated C20 because I'd take them off discharge periodically and note the resting voltage to avoid over-discharging reduced capacity batteries. Either way 2.025V per cell is my cut-off; time x discharge rate x 2 {temperature compensated} = new rated capacity.

Then leave them disconnected for a week fully full charged and see how much they've self-discharged over that time.

Build the bank from similar capacity that hold their volts or hold the same volts.
Then discharge the bank to at   C20 for 10 hours to verify bank capacity.

[oliver90owner]

Scruff is right for selection for a bank after purchasing.  Perhaps substitute C/20 for discharge rate.  Also he means 'at' and not 'to' in the last para ( ie discharging to 50% DoD of original capacity).

Battery capacity at C/20 discharge rate is not necessarily the same as the original spec rating, but sensible capacity is more likely found this way than discharging at multiples of C.

Of course, return the batteries to full charge ASAP after the test.  Vented flooded lead acid cells are more easily improved with an equalisation charge - something unavailable to the sealed batteries (liquid or gel electrolyte).  Terminal voltage for a sealed battery is an important criterion and any poor single cell should be spotted with voltage checks after resting at full charge.

I would recommend a DVM (with a fresh battery installed) to check voltages.  Small deviations from expected results are important.

[clockmanFR]

Oly is correct.....

Use a DVM a Digital Volt Meter, I use one that shows 06.51 volt, say for the 6 volt battery, and say 13.02 for the 12 volt batteries.

Trust this helps?