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How Do I test amps on cell phone battery using a multimeter?

How Do I test amps on cell phone battery using a multimeter?

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  #1  
Old 01-12-13, 08:53 AM
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How Do I test amps on cell phone battery using a multimeter?

Hi. I have a Greenlee DM-20 multimeter. I want to test the voltage and amperage on two cell phone batteries. I was able to test the voltage. I am having a problem with the amperage. The batteries are 3.7V and 950mAh. I can't find the rights selector on the multimeter to test the amperage. The choices on the multimeter are V (squiggly line) 300 and 200; A (straight line on top and dash line on bottom) with 2000u, 20m, 200m; battery symbol with 1.5V and 9V; An omega sign with 200, 2000, 20k, 200k, 2000k; and a V with straight line on top and dashed line on bottom with 300, 200, 20, 2000m, 200m.

The multimeter is working. The batteries are not dead. If I get any read out on the multimeter when I test for amperage, it looks like I am getting an overload indicator.

Which of these selectors do I use to test for the 950mAh on the cell phone battery? Thank you for your help.
 
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Old 01-12-13, 09:11 AM
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You can't test the amperage of any DC battery until a load is applied. You connect your multimeter between the load and the battery to determine it. I know it's a weird question, but why do you want to measure the amperage?
 
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Old 01-12-13, 09:16 AM
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Hi:

I am trying to see if the battery is holding its charge properly after recharging. As I understand, if the voltage reading is high but the battery is still not working well, I may need to check its amperage.

I thought I could set the multimeter to read amperes and connect the positive and negative contacts as for testing voltage. And if the reading is much lower than the mAh noted on the battery, then the battery's storage capacity might have declined, and it is time to replace it. Will this type of testing not work?
 
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Old 01-12-13, 09:24 AM
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Nope, gotta have a load attached between the battery and the tester. About the only test you can run is a timed test in the phone itself. Also, if it is a smart phone, check the applications and automatic updates running at any given time. They can drain a battery fast. Live desktops, high screen resolution, all reduce battery life.

Here's a primer on YouTube I found that is self expalatory. Read Amps using a multimeter - YouTube
 
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Old 01-12-13, 09:30 AM
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No, it will not work. The load must be in series with the meter to test. Of course a cell phone is a variable load depending on if the back light is on, if it is transmuting etc. so it is not necessarily a good way to test. You need a fixed resistance. What that resistance is I couldn't tell you.

When I think a battery is going bad I just replace it and save the old one as a spare. I put it in to the phone or an older no longer used phone a couple of times a month to check the charge and top off the charge if needed. I carry the spare with me just in case my phone battery dies just when I need it.
 

Last edited by ray2047; 01-12-13 at 10:20 AM.
  #6  
Old 01-12-13, 09:36 AM
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Thank you for the advice.
 
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Old 01-12-13, 05:00 PM
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I thought I could set the multimeter to read amperes and connect the positive and negative contacts as for testing voltage.
Absolutely not, as the others have said.

In addition: The others have explained how to measure CURRENT by placing the leads of the multimeter in series with a load. But based on your question, you're not interested in how much current is flowing - you're interested in mAh - milliamp HOURS, which is the total charge the battery holds. That can only be determined by measuring current thru a load over a substantial period of time.

Finally, it's a real mistake to place a multimeter in the current (amps, milliamps) position directly across a battery. Unless the battery is very weak, that will cause a very large current to flow (since there is no series load resistance), triggering whatever protective mechanism the meter has. You could blow a fuse in the meter or damage the meter.

Hope this is helpful.
 
  #8  
Old 01-12-13, 11:26 PM
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You can't test the battery with only a multimeter but it's not that hard to test the battery if you have a properly sized resistor. For your 3.7 V battery, a 20 ohm resistor rated at 1 watt would let you discharge the battery at about 185 milliamps. You would fully drain the battery in just over 5 hours.

To test the battery, charge it fully and then remove it from the phone. Then, measure the battery voltage and record this number. Switch the multimeter to the 200m setting and make sure the positive lead is connected to the A port Connect the battery's positive terminal to the resistor, then connect the resistor to the positive lead of the multimeter. Finally connect the negative multimeter lead to the negative battery terminal.

Your battery will start discharging and you should notice the resistor is getting warm. If it starts smoking, you probably used an undersized resistor.

Record the current reading every 15 minutes for about 1 hour then disconnect the battery. If you average the current readings, it will give you a good estimate of the mA hours that you drained from the battery. Give the battery a couple of minutes to stabilize nd then measure the new battery voltage. Don't forget the switch the DMM back to voltage mode and plug the positive lead back into the V port first.

Depending on the type of battery you have (NiMH, Li-ion, etc) you can find different state of charge curves (also called depth of discharge) to look up the percent charge in the battery at the beginning and end of the test, based on the voltage measurements you took.

Lets say you discharged 185 mAhrs (19% of total battery capacity) during the test and your battery started at 98% full. You would expect that the voltage at the end of the test would indicate 79% charge remaining. If your results show a much lower charge remaining, your battery is failing.

Note that for lithium ion batteries, you may have to discharge the battery for 60 to 80% of its total capacity because the cell voltage changes very little over much of the discharge range, making it hard to estimate the actual state of charge.
 
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Old 01-13-13, 05:31 AM
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If you're dealing with a newer phone, it likely has a Li-Ion battery. You have to be careful about tinkering with lithium batteries because of the chance of fire. If they are charged or discharged too quickly, they have a tendency to overheat and can catch fire.

When used in their normal manner, they are quite safe. But just be careful if you're starting to connect resistors and such. NEVER leave a battery like that charging or discharging without you around.
 
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