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Old Dec 01, 2005, 06:31 PM
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Purvis, Mississippi
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How to measure mah with multimeter??

Just got a DT-830B Multimeter ...I can measure the voltage...but how the heck do I measure how much energy the pack has stored (lipo, MAH)????

I think i ruined a battery trying to do so..the battery got hot and so did the leads on the tester.....PLEASE HELP

Thanks,
John
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Old Dec 01, 2005, 06:50 PM
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discharge thru a auto headlite in series with the 10 amp input jacks of multimeter if there is one to check discharge current . Then time it using a lcd timer set around 1 hour

A whattmeter or AF 109 will show AH discharge automatically
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Old Dec 01, 2005, 06:52 PM
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I hooked the 10amp directly to the battery and everything got real hot...I guess that's why you need the light huh....??????...
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Old Dec 01, 2005, 06:59 PM
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That's a SHORT circuit in disguise on what U did
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Old Dec 01, 2005, 11:51 PM
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Purvis, Mississippi
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I can't make it work......maybe it won't do it...I've never used a multimeter before...you can prob. tell

By looking at the picture of it....will it measure amp draw and/or capacity?
If so, what settings....Dang I feel stupid......
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Old Dec 02, 2005, 12:26 AM
CamLight Systems
New York City, USA
Joined Oct 2003
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Yup, you essentially short circuited your battery with your meter. You need a load of some type to keep the current down to the level you want to use for measuring the capacity of the battery.

You can't use a headlight or any other fixed-resistance load to measure the capacity of a battery cell or pack. The current level will drop as the voltage level drops so you won't be able to multiply the current level (amps) by the discharge time (hours) to arrive at the battery's capacity (ampere-hours). The current level is constantly changing and you need to integrate the value over time to figure out the capacity. This is the way chargers/dischargers work. But, no one shold have to do calculus by hand to determine the capacity of a cell/pack....there's a better way.

You need a "constant-current" load. This type of load compensates for the battery's voltage drop as it discharges to keep the discharge current level steady. Since the current level never changes and you know how long it took to discharge the battery down to its cutoff voltage, you only have to multiple the current (amps) by the time (hours) to arrive at the AH capacity for that battery at that current level.

There are tons of schematics available to make a constant-current load if you want, only takes a few inexpensive parts to make a 1-amp constant-current load for lower-voltage packs.

Regarding your meter and measuring current...
Dial to the 10A DCA setting but also make sure that the positive (probably red) lead is plugged into the 10ADC jack and that the negative lead (probably black) is plugged into the COM jack.

IMPORTANT: Be sure to move the positive lead back to the V-omega-mA jack when done measuring the current and before you use the meter for anything else!!

Otherwise, if measuring voltage or anything with a voltage present, the meter will still think it's trying to measure current. And without a load to keep the current to a certain level, there will be short circuit (like what has already happened to you). In fact, you may have blown the fuse inside your meter. Check that before trying to measure current again.

I recommend the purchase of a battery analyzer or wattmeter to check voltages. capacity, and current levels. A multimeter is handy, but one of these units is handier.
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Old Dec 02, 2005, 01:40 AM
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johndreid,

First, an ammeter is always put in series with a source and a load. A voltmeter is in parallel with the source and or the load.

To measure capacity, you need two values current and time. To read the current, you need a load which can be a bulb, a motor, a suitable resistor, etc. Select a load that will cause a current close to the capacity of the battery the C. Of course, JohnMuchow in post #6 is correct - as the voltage decreases, so will the current, so your results will be skewed. But you can get a pretty good indication of capacity with a simple load.

The battery, ammeter, and the load are connected in series. You really need to have another meter so that you can monitor the battery and stop the discharge when the voltage drops to the desired minimum. For LiPos that is 3.0 volts per cell; for Nickels, that is 0.9 volts per cell. A wattmeter is very useful here, in that it displays current, voltage, and even computes the mah for you.

As JohnMuchow stated, there are devices that keep the current constant. In addition to DIY circuits, there are off the shelf products available. A couple of such devices are here and here.

Bill
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Old Dec 02, 2005, 02:48 AM
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To answer your original question....there is no way that you can measure how much charge is currently contained in a battery.

The guys are telling you all sorts of useful ways to take all the energy out and measure how much there USED TO BE in there. But if you just want to know how much is in there at the moment it's not possible to measure it directly. You can get a reasonable estimate for a Lipo by measuring the voltage, which you already know how to do.

Steve
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Old Dec 02, 2005, 04:00 AM
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Purvis, Mississippi
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thanks guys
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Old Dec 02, 2005, 04:29 AM
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San Carlos, California, United States
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Let's say you have a water tank, and you don't know the capacity, and you want to calculate the capacity.

You have a meter which measures the gallons per second of water flow.

If you connect this meter to the water tank, it won't tell you "Oh, this tank holds 400 gallons" or something. It only tells you how much water is coming out of the tank.

If you have 1.5 gallons coming out of the tank for 100 seconds, you can reasonably deduce that the tank holds 150 gallons, but there is no "150 gallons" indication on the meter.

Toshi
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Old Dec 02, 2005, 08:32 AM
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Quote:
Originally Posted by TMorita
If you have 1.5 gallons coming out of the tank for 100 seconds, you can reasonably deduce that the tank holds 150 gallons, but there is no "150 gallons" indication on the meter.

Toshi
If you have 1.5 gallons per second coming out of the tank for 100 seconds you can deduce that the tank can hold at least 150 gallons but it's now empty.

Unless you are completely certain that it was absolutely full when you started it could hold more than 150 gallons. And if you put some more water in, then use some, then put a bit more in, you have no idea how much is in there now .

It's the difference between measuring the maximum capacity of a battery and measuring how much is in it now. The first you don't usually need to do because it's written on the label and the second you can't do in any practical way.

Steve
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Old Dec 02, 2005, 09:15 AM
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You can get a rough idea of how much capacity remains in the pack by checking the open voltage.

Approximate numbers as I remember them;

4.2 volts/cell-100% remaining
4.1 volts/cell-80%
4.0 volts/cell-60%
3.9 volts/cell-40%
3.8 volts/cell-20%
3.7 volts/cell-discharged

I generally fly a pack if it's over 4 volts per cell, charge it if it's under 4 volts per cell.

Dan
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Old Dec 02, 2005, 10:30 AM
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Dan

That's true for a normal pack, but what if the pack has one bad cell at 50% capacity.

The only true test is a thorough DISCHARGE CAPACITY test after a normal charge cycle
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Old Dec 03, 2005, 10:32 PM
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Quote:
Originally Posted by slipstick
If you have 1.5 gallons per second coming out of the tank for 100 seconds you can deduce that the tank can hold at least 150 gallons but it's now empty.

...
Steve
Yah, I was assuming that was obvious, but thanks for pointing it out.

Toshi
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Old Dec 04, 2005, 04:01 PM
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johndreid's Avatar
Purvis, Mississippi
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Thanks for all of the help guys....
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