How to measure mah with multimeter?? - RC Groups
Dec 01, 2005, 07:31 PM
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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)????

Thanks,
John
 Dec 01, 2005, 07:50 PM Permanently Banned 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
 Dec 01, 2005, 07:52 PM (My website goes here) I hooked the 10amp directly to the battery and everything got real hot...I guess that's why you need the light huh....??????...
 Dec 01, 2005, 07:59 PM Permanently Banned That's a SHORT circuit in disguise on what U did
Dec 02, 2005, 12:51 AM
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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......

### Images

Last edited by johndreid; Dec 02, 2005 at 12:57 AM.
 Dec 02, 2005, 02:40 AM Registered User 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 Last edited by ebill3; Dec 02, 2005 at 03:07 AM.
 Dec 02, 2005, 03:48 AM Registered User 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
 Dec 02, 2005, 05:00 AM (My website goes here) thanks guys
 Dec 02, 2005, 05:29 AM Registered User 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
Dec 02, 2005, 09:32 AM
Registered User
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
 Dec 02, 2005, 10:15 AM Registered User 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
 Dec 02, 2005, 11:30 AM Permanently Banned 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
Dec 03, 2005, 11:32 PM
Registered User
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
 Dec 04, 2005, 05:01 PM (My website goes here) Thanks for all of the help guys....