# Is there an instrument that can measure the capacity of lead acid batteries

Background

I have an electrical scooter with 6 x 12Volt lead acid batteries (72 Volts total), which are now in such a condition that they can barely get me to my work and back. I got a bunch of used UPS batteries from a friend, and want to check their capacity and compare them with the batteries that are currently in my scooter.

I roughly think I know how to measure their capacity, after loading them I should discharge them at a certain rate and see how long it takes before the voltage drops below a certain threshold. I'm not sure about the exact numbers though, and I'm also not quite sure how I should discharge them at the same rate as my scooter would typically do (power a heater or something like that?).

Question

Is there a device/machine/instrument that can do all of that for me? Load the battery, discharge it at a constant rate and log the results.

If not, I guess a UPS hooked up to a computer could be programmed to do the job? Any ideas?

• Why not just hook them up to the scooter and do laps around the block? Jun 20, 2013 at 18:29
• I want to measure individual batteries, throw the worst ones out and keep the best. It would take quite some charges, replacements and drives to test all combinations. I have 12 used batteries to test, plus the ones currently in my scooter, so 18 in total. Jun 20, 2013 at 18:35
• Mixing batteries in any case would give you the capacity of the worst one, so do not expect optimal results unless the replacement pack had been used always together.
– FarO
Jun 3, 2016 at 11:35
• You can test them using multiple LM317 in paralle, each one in "constant current" configuration (look it up), where you set the total current to the rated one (usually it's the value of Ah: 7Ah => 7A discharge) and check how long each one lasts until the voltage drops under a specified threshold.
– FarO
Jun 3, 2016 at 11:37

Yes, there are lead acid battery testers that will tell you the condition of each battery. They are often used by UPS service technicians during preventative maintenance checks to check on the health of each battery in a large series string.

Here are a couple examples:

The good ones probably cost more than a set of replacement batteries for your device, but if you can find a UPS dealer or service tech, they may be able to test your batteries for a small fee.

• Thanks for those links, a search with the keywords "Battery Capacity Analyzer" did bring up some interesting results but nothing economically worthwhile. The most interesting thing I found was a ProfiPower accutester, but I guess it's unlikely that I can do good enough comparisons with this gadget. Jun 22, 2013 at 20:56
• You could always do the load testing you mentioned in your question by creating your own custom battery tester - find out what kind of current draw your scooter has, then check eBay for some high wattage resistors that would give a similar drain, then hook them up across each battery and measure voltage over time. I see a pair of 25W 6 ohm resistors on eBay now - one of those across the battery would give 2 amps of power draw (24W). Put 2 in series for 12ohms/1amp/12W of power draw. Be careful, 25W will make the resistor quite hot, and it might need a heatsink. Jun 23, 2013 at 16:22

Take them to your local NAPA/O'Reilly auto parts store. They offer free battery testing on their machines for lead-acid wet cells. Other auto parts stores usually offer this service for free too just call around and make sure they can test your type of battery (6V vs 12V etc).

They punch in the battery parameters (capacity, cca, etc.) from the manufacturer and the machine will run it through its paces in about 5 minutes or less.

But for UPS batteries these UPS and other telecom type stationary batteries are designed for standby emergency service and not daily deep discharges. They basically only have 150 to 300 cycle life or 5 to 10 years whichever comes first. I doubt you'll be happy with their performance in your scooter.

• It's probably true what you say about the UPS batteries, they have the same specs in terms of voltage and capacity (12V 17AH) and even the dimensions were identical. I was told that they were replaced as 'preventive' maintenance, so I had good hopes. I built them in today, the charger gave a green led "charged" after only 5 minutes, but they couldn't even bring me around the block! I'm still planning to "cherry pick" the other batteries though. Is there any other unit or rating to look out for that will reflect the deep-cycle discharging characteristics of a battery? Jun 22, 2013 at 20:47
• @LouisSomers if it doesn't say "deep cycle" then it probably isn't. Deep cycle batteries have larger plate surface areas and as a result are heavier. If a manufacturer makes something deep cycle they brag about it on the package. Deep cycle and non-deep cycle can have the same basic electrical parameters, the only difference you'll notice is the life time under heavy discharge/charge cycles. Jun 23, 2013 at 18:10

A garage should have a battery tester, I've only seen one hooked up once a long time ago. I beleive all it did was measure the voltage then discharge through a fixed load for a minute? and then gave a go / bad indication. I'm guessing that you can do the same from fully charged and then see which is the worst and dispose of it.