# 12v auto battery capacity testers accuracy?

#### BobaMosfet

Joined Jul 1, 2009
1,996
Neither one do a load test. The top one positively not. The bottom one, they only draw 1mA so can't. But they may do more in evaluation based on battery behavior during cranking- when the start itself draws the current- in which case they can evaluate the voltage cycle.

Of the two, if I had n o other options, I'd go with the bottom one.

Joined Jan 15, 2015
6,698
Is this simply measuring voltage with no load and mapping that to a percentage?
That's about it. That's all they do. There is no load applied.

Ron

#### dl324

Joined Mar 30, 2015
14,683
Is this simply measuring voltage with no load and mapping that to a percentage?
That's what it appears to be doing. An open circuit voltage test is pretty useless for measuring battery health/capacity.

#### crutschow

Joined Mar 14, 2008
30,100
No load voltage can tell you the approximate battery state of charge, but it tells nothing about the battery capacity or health.

#### MrAl

Joined Jun 17, 2014
9,155
Hi,

Even with a load test it's a little hard to tell but just knowing the open circuit voltage only tells you if the battery was charged up enough.

With a load tester it's hard to tell too, but you can do a comparative analysis. Measure the voltage and ambient temperature when the battery is new with the load applied for 10 seconds. Some time down the read after the battery has aged a little, do the exact same test then compare results. When you see the voltage dropping lower and lower you know the battery is getting old.
Harbor Freight makes the cheapest load tester i know of at the moment, and i think it applies a 100 amp load.

If you dont want to buy a load tester than use the cranking period to do the measurement. Get an analog meter (not digital) and monitor the voltage while cranking the engine. You'll see the voltage drop as it cranks before the engine starts. As the battery ages that voltage drop will increase. It might be 11 volts at first when new, but after a whjile it will drop lower and lower. This can give you an idea how bad the battery is getting. You'll also hear the engine turn over slower. This may be the best test really but you need an analog meter with fast response so you cant use a digital meter.
What i do is i have one installed permanently so i can check it every single time i start the car. So i get a check every time i start it.

Now of course in the cold months it is worse. If you have temperatures in your area that drop to near freezing then you really have to test it then. However, during the summer it may still work good enough so you can wait till autumn to buy a new battery as that are expensive these days (mine was $220 USD plus they charge you$45 to install around here).

Also remember that the OBDII system requires constant 12v so you need to supply an external 12v somewhere to keep the computer powered up. If you dont do that, the monitors will go unready and you have to do a drive cycle or two or just try driving the car around for a while or back and forth to work. This part can be very easy or very hard depending on what kind of car you have. My car is the worst it takes weeks to get the monitors to go to the ready state again. You cant pass inspection if the monitors do not go ready (for most cars).

Good luck with it.

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Joined Jan 15, 2015
6,698
Surprised to find they still make these battery testers. You want to test a car/truck battery you give it a load. Note the link provides about a 100 amp load and you only load the battery for 10 ~ 15 seconds maximum. Had testers like this when I was a kid working the local Exon gas stations. I am sitting here in NE Ohio where winters are cold, very cold. I still ride a '92 Harley Davidson and during winters it stays garaged with a battery tender so maybe once a week I can run it for several min. Keeps my battery ready to go in sub freezing temperatures.

Ron

#### bassbindevil

Joined Jan 23, 2014
457
You can sometimes pick up old but functional load testers at vintage car swap meets for a few bucks.
But this thing from Ali looks like it might work:
https://www.aliexpress.com/item/1005003477765500.html
And if your car doesn't have a real voltmeter, consider adding a digital voltmeter.