Add Battery Test to a Multimeter

Picture of Add Battery Test to a Multimeter
The photo shows an inexpensive (often free) multimeter from Harbor Freight. I added a white dot to the indicator. Notice that the white dot points to a battery test function for 1.5 volt and 9 volt batteries. 

Batteries should normally be tested under a load because a no load voltage test could read normal, but drop considerably when the battery is under load. This multimeter's manual indicates that the battery test circuit adds 360 ohms of resistance as a load. According to the label on the meter, a 1.5 volt battery in good condition should show a current draw of 4 milliamps or more and a good 9 volt battery should show a current draw of 25 milliamps or more. There is nothing mysterious about this. It is a simple application of Ohm's Law, which says that voltage (E) in a direct current circuit is equal to resistance (R) multiplied by current (I), or E = IR. Factored, that is current (I) equals voltage (E) divided by resistance (R). 
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Step 1: No battery test function on your meter

This is not my better meter, but one I needed in a second location. It does not have a battery test function, but I decided it would be handy to add one. I can use the 200 m function. But, I will need to add about 360 ohms in resistance when in use. (My better meter also does not have a battery test function, and I made one of these resistor probes for it, too.)
imperiusPT20 days ago

This is great. Thanks for sharing.

Phil B (author)  imperiusPT19 days ago

I love practical, useful Instructables. Thank you for looking and commenting.

tutdude9810 months ago
I ve got some 330 ohm resistor and if i put it serial to battery my reading should be 1.5/330 = 4.545ma?
Phil B (author)  tutdude9810 months ago
That is correct, and 27 milliamperes at 9 volts.
Homepwner10 months ago
I really like this idea, I've read some of the comments and I think that with these types of batteries, absolute thresholds are not important. I just need to know if the battery is decent and this seems to be a good option.
Phil B (author)  Homepwner10 months ago
You make a very good point. In step 3 I mentioned, "It may well be that a battery considered depleted in one application would still function in a less demanding application." Over the years I usually tested a small home-use battery by taking a direct voltage reading with no load. There has usually been a fairly direct relationship between the remaining voltage and the depletion of the battery. In practice I quickly got to know at what voltage reading the battery would no longer do the job required for a specific application.

Perhaps we are all spoiled or deceived by the little battery testers with a red and a green zone on the indicator dial. We assume the battery is no longer useful if the needle falls slightly into the red part of the scale when it may still be useful for some applications. Somewhere I found a discussion that suggested testing a battery with a load resistance is truly useful only if the test resistance equals that of the actual circuit in which the battery will be used. Still, adding a resistance of 360 or 370 ohms is probably better than nothing, although I did actually test a 1.5 volt battery recently that showed the proper current flow with the test resistance, but a direct voltage reading showed a very depleted battery. I am not quite sure how that happened, but I have begun to check both the current flow under an artificial load and the voltage from a direct reading.

Thank you for looking and for commenting. 
dejure10 months ago
Thanks for the [two millionth] tip. Like you, I keep a few meters around (one in the shop, one in the van and one for the house). It would be nice to build probes for each that would stay with the meters.

I have some one inch square plastic tubes LED's came in and I might see if I can rig those to house the resistors or a small rheostat set to 360 and epoxied. The tubing would accommodate a label with values and a reminder of why it shouldn't be tossed ;)
Phil B (author)  dejure10 months ago
Thank you for looking and for your comment. I encased resistors in a plastic soda straw because I wanted something thin I could hold against the side of the meter while I wrapped the test leads around it and the meter. Your square tubing should work well for you.
rimar200010 months ago
Phil, I measure directly Amps when testing batteries. I do that fast, to avoid discharging even more the battery.

If the current is near the max specified for that battery, I assume it is good. When it is discharged, amperes are very low, almost zero.
Phil B (author)  rimar200010 months ago
Osvaldo, that is a good idea if you know the amps capacity of a battery.
rimar2000 Phil B10 months ago
The batteries I use are these little 1.2 V, generally rechargeable. They always says their amperage.
The only problem doing that I suggest is the procedure discharges even more the battery. But if you are conscious on that situation, and are fast do measure, the problem is less important.
Phil B (author)  rimar200010 months ago
That helps me better to understand what you are doing.
technovative10 months ago
This is a good mod.
Phil B (author)  technovative10 months ago
Thanks. I wish I had thought of every aspect of it. I simply borrowed somethings others did and gave it a broader, more portable application.
MrOddjob10 months ago
As usual Phil, a very useful instructable. I have an ex British Telecom meter with battery condition function built in, but it is a fairly large chunk of meter to carry around. Three resistors and an alligator clip is a vast improvement. Thanks for sharing Phil, I know what I'll be doing tomorrow!
Phil B (author)  MrOddjob10 months ago
Thanks. I believe you could use a single resistor, maybe a 330 ohm or a 470 ohm resistor; or, you could use a small potentiometer you dial in to the resistance you want. The thresh hold current you are looking for would simply have different numbers. So, according to Ohm's Law, with a 330 ohm resistor the current would be 4.54 milliamps for 1.5 volt batteries and 27.2 milliamps for 9 volt batteries. With a 470 ohm resistor you would look for 3.2 milliamps on a 1.5 volt battery or 19.1 milliamps on a 9 volt battery. Life would be simpler if you could just go down to Radio Shack and buy resistors in almost any value, but I believe they square the value, double it, and the square root of that is the value of the next resistor. So, the next standard resistor from a 330 ohm resistor is a 470 ohm resistor (The square root of (330 x 330 x 2) equals 466.

Anyway, I hope you find it useful.

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