# Battery Tester - How do these things work, and why should I buy a more expensive one?

#### Joe Stavitsky

Joined Apr 5, 2020
107
I'd like to see if I can build one of these myself.

This is this the first Google result I got for a circuit

Is it accurate? Meaning, is it the kind of thing I'm likely to find in a commercial model? If not, is a commercial model better in any respect? In what way is the $79 dollar unit "better" than the$11 dollar unit?

Thanks so much

Joe

ETA: My strategic concern is not specifically car batteries (although I may get one eventually) but large batteries in general, like UPS and jump starters.

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#### MrChips

Joined Oct 2, 2009
27,144
How does one test a battery?
What is the criteria for differentiating between a good and bad battery?

#### Joe Stavitsky

Joined Apr 5, 2020
107
How does one test a battery?
What is the criteria for differentiating between a good and bad battery?

A good battery holds charge for optimally infinite time and passes current with no internal resistance, and a bad battery... doesn't.

#### MrChips

Joined Oct 2, 2009
27,144
A good battery holds charge for optimally infinite time and passes current with no internal resistance, and a bad battery... doesn't.
Then that is what the instrument should measure.

#### strantor

Joined Oct 3, 2010
6,112
The devices you linked to are load testers. They place a high current draw on the battery and see how much current it can source, measure voltage sag, measure internal resistance of the battery, and some of them test other external factors such as alternator output, starter current, chassis ground resistance, etc. They are relative complex compared to the circuit you posted, which merely measures voltage level of a battery.

The $11 unit is probably the commercial equivalent of the circuit you posted and won't test any of the things I mentioned above. You can DIY this simple circuit with relative ease, but it will probably not cost you less than$11. You're better off buying the $11 cheap China tester if all you care about is testing battery voltage. If what you care about is diagnosing automotive battery charging and starting systems then your best bet is to buy one of the more expensive commercial units. If what you care about is learning electronics with a somewhat practical starting point, building the circuit you posted is a good starting point. If you're a masochist and what you care about is learning how to swim in electronics by diving off a cliff into shark infested rocky waters, rolling your automated load tester is a good place to start. If I have a good understanding of what you're after (a good load tester that provides useful data for a decent price) then I recommend letting go of all notions of DIY because you already found your answer and it costs >$50.

#### crutschow

Joined Mar 14, 2008
30,806
Just measuring its voltage doesn't necessarily tell you whether the battery is good or bad.

#### Joe Stavitsky

Joined Apr 5, 2020
107
The devices you linked to are load testers. They place a high current draw on the battery and see how much current it can source, measure voltage sag, measure internal resistance of the battery, and some of them test other external factors such as alternator output, starter current, chassis ground resistance, etc. They are relative complex compared to the circuit you posted, which merely measures voltage level of a battery.

The $11 unit is probably the commercial equivalent of the circuit you posted and won't test any of the things I mentioned above. You can DIY this simple circuit with relative ease, but it will probably not cost you less than$11. You're better off buying the $11 cheap China tester if all you care about is testing battery voltage. If what you care about is diagnosing automotive battery charging and starting systems then your best bet is to buy one of the more expensive commercial units. If what you care about is learning electronics with a somewhat practical starting point, building the circuit you posted is a good starting point. If you're a masochist and what you care about is learning how to swim in electronics by diving off a cliff into shark infested rocky waters, rolling your automated load tester is a good place to start. If I have a good understanding of what you're after (a good load tester that provides useful data for a decent price) then I recommend letting go of all notions of DIY because you already found your answer and it costs >$50.

See edit. Thanks for the suggestion, and I'll look into what you said. I have a retail location nearby that lends tools, I'll see if they have anything.

#### MrChips

Joined Oct 2, 2009
27,144
Simple.

I take a 12V automotive headlight bulb and socket and mount in on a metal box with two toggle switches for low and high beam. I put a shield over the lamp because the light output is intense.

You can leave it on as long as it takes to get an estimate of battery capacity.

#### Joe Stavitsky

Joined Apr 5, 2020
107
Simple.

I take a 12V automotive headlight bulb and socket and mount in on a metal box with two toggle switches for low and high beam. I put a shield over the lamp because the light output is intense.

You can leave it on as long as it takes to get an estimate of battery capacity.

This is also a solid plan.

#### dl324

Joined Mar 30, 2015
15,117
A good battery holds charge for optimally infinite time and passes current with no internal resistance, and a bad battery... doesn't.
You're describing an ideal battery that doesn't exist in the real world.

#### strantor

Joined Oct 3, 2010
6,112
Simple.

I take a 12V automotive headlight bulb and socket and mount in on a metal box with two toggle switches for low and high beam. I put a shield over the lamp because the light output is intense.

You can leave it on as long as it takes to get an estimate of battery capacity.
The testers he showed are automotive battery testers so I assume the application is an automotive battery. A car head lamp won't be near enough load.

#### debe

Joined Sep 21, 2010
1,320
This is what i use & seems to work for me. I test a new fully charged battery when i buy it & record the results. That is then my reference as the battery ages. ( most new batterys i have ever purchased are not fully charged when bought}

#### LowQCab

Joined Nov 6, 2012
2,506
If the device You are using does not provide at least
the "average" Load the Battery will experience in normal service,
then it leaves-out a lot of very valuable information.

A tester that digitally displays the "Cold-Cranking-Amps" of a Battery
using ~20-gauge Test-Leads, is an absolute joke.
It might give You some various useful bits of information, but don't believe the display.
.
.
.

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#### debe

Joined Sep 21, 2010
1,320
On those displays as the years go by the cold cranking amp display actualy drops, as does internal resistance. In the Ford dealership i used to work in you had to use one of these testers with a print out. This was used for Ford warranty claims, if the tester did not come up with battery fail, then no warranty claim.