# Add Battery Test to a Multimeter

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## Introduction: 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).

## 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.)

## Step 2: Solder Resistors

It would be ideal if I could have gone to Radio Shack to buy a pack of 360 ohm resistors, but they are not available. I rummaged through my junk box and found a 270 ohm resistor and a 100 ohm resistor. Soldering these two end-to-end (series) gave me one resistance of about 370 ohms. (Resistors are usually not exactly their stated value, but vary just a little. Check the actual resistance with the Ohms function on your multimeter. My chain of resistors is actually 367 ohms. Also, resistors use a color coding system to indicate their value. The colors shown on the resistors in the illustration are accurate for the values indicated.)

See the second photo. I soldered a small alligator clip to one end of the resistors. I slipped a soda straw over them and added some hot glue at each end to provide some physical strength. I scraped the resistor lead opposite the alligator clip to remove any hot glue that might act as insulation.

I could have brought the 367 ohms down to exactly 360 ohms by soldering another resistor of 19000 ohms in parallel to the two resistors. Resistances in parallel function differently than resistances in series. (Adding 19,000 ohms, if such a resistor were to be available, is more of a hypothetical example than a serious suggestion.) Here is an on-line calculator for parallel resistances

To use, just attach the alligator clip to one of the test leads. Set the multimeter to the 200 milliamp scale. Use the bare resistor lead as the probe. Touch the probes to the battery under test.

## Step 3: Make a Label

It is easy to forget what the thresh hold figures are for testing batteries. I made a label for the back of the meter. My figures are not quite accurate, but are close. It may well be that a battery considered depleted in one application would still function in a less demanding application. See the second photo. The battery that read 3.9 milliamps in the photo for the Introduction with 360 ohms added reads 3.8 milliamps with my 367 ohm resistance added to the circuit.

You can easily make your own resistance load from resistors in your junk box for checking common batteries under load with a meter you already own, even though it may not have a battery test function.

## Step 4: One Caution

This meter contains a fuse. It is easy to blow the fuse if the selector was set to the wrong range for a test you performed recently. If the fuse is blown, the milliamp scales will read zero (0). At first you may wonder what is wrong. You simply need to install a new fuse. The fuses used by this particular meter are not easy to find locally. I ordered a package of five from Amazon.

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I have three different multimeters with 3 different dedicated battery load tests. Their spec sheets read as follows:

---Tacklife DM10:
---Another Tacklife DM01
---Bside ZT-M1 - All volt batteries with 10mA load current
* The ohm load for Bside is based on 10mA load current for all batteries.

Why does each meter choose a different load and manner of designating load? Which loads are preferred? I would think that the higher the voltage of the battery the higher the ohm load should be, like in the Bside; not like the Tacklife DM10.

Use a voltage reading instead of a current reading. You can put your load (resistors) across the battery in parallel (instead of on one end in serial) and then take your voltage reading across the battery as well. If you get 1.5 or near to 1.5 then the battery is holding up under load and is good. If the voltage drops significantly lower than that then the battery is dead.

An advantage to this method is that you won't have to worry about blowing fuses on your meter.

This is a great instructable. I only had one side comment about resistor values. Many resistors have an advertised tolerance of +/-20%, so you may want to double check the resistances of your specific resistors using the multimeter and do the specific calculation for the current you should expect.

Thank you. You make a good point. I do usually check. My nominal 370 Ohm resistance came in at 367 Ohms.

There is nothing magic about 4mA for 1.5V and 25mA for 9V. It is only saying that is what it draws.

In fact, a 1.5V cell should actually be tested with -more- current than a 9V battery. I'd flip those numbers around.

They are just being cheap and putting one range on there to measure both. It'd be better if you were to add separate inputs to your adapter, maybe load the 9V cell to 10mA and the 1.5V cell (for AAA and up) to about 30mA.

Don't test 1.5V buttons cells at that current, though.

I have a Lead Acid rechargeable battery which is used in emergency lamp.

The voltage range mentioned in the battery is about 6-7.5v

Max charging current mentioned in the battery is about 1.5A

When I measure the voltage during charging it shows 6.5V

When I measure the voltage using multimeter (without charging) it shows only 4.4V

I assume that the battery life is over and not working.

To test this can I use the same method which you explained in this article ?

If I take lets say voltage is 6.5V and use 360 ohm resistance to measure current.

V=IR. Hence the current should be 18mA.

Note : The emergency lamp is not working and I suspect the battery.

Not sure if I can replace this battery with four 1.5V battery in series to check if the lamp works.Please comment on this as well.

You can use this same method to test the battery. However, when a battery will not reach a full charge, the battery is defective. You should be able to test the lamp with four 1.5 volt batteries in series.

What does the 48 volt battery power? Can you measure the current flow in the circuit when its normal load is connected? Then you would use Ohm's Law to compare what the battery produces with what it should produce.

Otherwise, a standard battery test uses a carbon pile to handles.l of the current. Carbon pile testers are very expensive. Most you would find are likely to be for 12 volts. A tester for a 48 volt battery would be even heavier duty. I do not know where you would find one.

This is the best answer I know. I see you posted this question eleven times various places. Perhaps you could delete the other ten. Thank you.

Hi Phil,

Thanks for this tutorial.It helps me understand the concept very easily.

Could you please explain why do you ask the battery power ? From where I can get to know this ? Without knowing the power if I blindly connect 360ohm what would be the consequence ? And lastly, why you chose 360 ohm ?why not lets say 10 ohm or 1ohm ?

The ideal would be to connect the meter to a resistance that is the exact resistance of the load in the device the battery will power. 360 Ohms probably represents an average approximate load for many battery devices. Sometimes the voltage of a battery not under load appears to be good, but the battery is too far depleted to provide the current needed to power the device. Adding a load of about 360 Ohms to the battery gives a better indication of whether the battery is still good. Even then, one application or device may be able to work on a very low current draw with a weak battery while another device needs a fresher battery because that device draws more current.

Hi Phil,

"Could you please explain why do you ask the battery power ? From where I can get to know this ? Without knowing the power if I blindly connect 360ohm what would be the consequence ?"

I suspect you mean current. Power usually means Watts. Do you know Ohm's Law? Voltage equals amperes (current) multiplied by resistance (Ohms). Watts equal volts multiplied by current in amperes. You are simply employing Ohm's Law to test a battery by adding a resistance. If a 1.5 volt battery is fresh, a load of 360 Ohms should show a current draw of 4 milliamperes. Anything significantly less than 4 milliamperes indicates the battery is somewhat depleted.

Hi Phil,

This was totally clear for me.What I wanted to ask was about power rating.Like how much heat the resistor dissipate and will it withstand the heat if we connect 360 ohm to a 48V battery.

Thank you for this useful tool. I've been using it mostly to test button cells. Those batteries are really pesky. I have some that were testing at 3v with no load. That's exactly what they are supposed to be, but they still wouldn't power my equipment. Replacing them with new ones (also testing at 3v) solved the problem. That was somewhat of a mystery to me. Using your method, I've now discovered that my 3v button cells, which should be testing at 8.3 milliamps with a 360 Ohm load, are somewhere between 1 and 5 milliamps. Finally, an explanation as to why they didn't work.

I am glad this is helpful to you. Button batteries are a challenge. They are small and convenient, but do not last nearly long enough. Often the voltage output seems sufficient, but the ability of the battery to produce sufficient current is exhausted. If the application allows it, some people add a battery holder for AAA or AA batteries.

It wasn't mentioned but it's just as easy IMO to measure the voltage of the battery when loaded with a resistor, a dying AA battery might measure almost 1.5V with no load, but when loaded with a 150 ohm resistor for 10mA draw, the voltage may begin to collapse if the battery is weak. I prefer voltage simply because we already know what the nominal voltage for a 1.5V AA battery should be. Additionally one can choose the right resistor based on application, for example my digital camera that requires two AA batteries is labeled 0.6A at 3V, so I know each battery needs to be able to supply 0.3A, so a 5 ohm resistor loaded on the battery will quickly tell me if the battery can support this camera.

Thank you for your comment and additional information. A voltage reading often tells all I need to know and works quite well.