Can a multimeter be used for accurate battery testing?
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lightmad said:
Sure. I use my DMM for all my battery charge evaluations...before, during, and after.
I think what he is getting at is what you take as a reading will drop when you turn on your light. Some people try to measure the voltage at the bipins of the bulb while it is turned on.
I think he is trying to tell you to find something that would simulate the load the battery would be under if used. That's getting into the details that the experts pay attention to....rather than us noobs who just want to know if it's soup yet. LOL!
But you can get reliable information just measuring a battery by itself similar to measuring it with the kitchen drawer battery tester. It just gives you a more accurate number with a DMM.
I think he is trying to tell you to find something that would simulate the load the battery would be under if used. That's getting into the details that the experts pay attention to....rather than us noobs who just want to know if it's soup yet. LOL!
But you can get reliable information just measuring a battery by itself similar to measuring it with the kitchen drawer battery tester. It just gives you a more accurate number with a DMM.
Handlobraesing
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You can even test the mAh capacity if you have a data logging DMM with sufficient resolution (every minute or so) with enough memory space.
bfg9000
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Assuming you are interested in how the cells perform in regular use, the "gold standard" would obviously be to use the mentioned data logging to measure the battery's performance during an entire runtime test, using the actual device you intend to power.
Fortunately just one voltage reading while the cell is under load is usually enough to tell us if the cell is "bad." It doesn't tell us anything about runtime but it quickly tells us if the cell can run the load at all, even if only for a moment.
Why do we want to use the same load the battery is going to see in real use? Imagine a charged car battery that reads 12.5v with your DMM--is it any good? The way to check is to have someone crank the car while you are checking the voltage. If it drops below 9v while the car is cranking, then that may not be enough to run the car's computer or injectors or spark plugs and the car won't start. You can stop troubleshooting right there because the battery is obviously bad.
So a battery that reads 12.5v no-load can still be bad (if it drops below a usable voltage under use). Do you care about the 12.5v at all? Well it does tell you the battery can probably power a miniscule load with no problem, but it doesn't tell us anything about its ability to power a big load (and here at CPF we usually are interested in pushing batteries pretty hard).
That's the reason the auto parts store sells those load testers for car batteries (usually in a big metal box because it must have enough resistors to dissipate some 100A of current), and why those battery testers from Radio Shack (which do have a small value resistor load in them) will only work if you intend to use the cells with a similarly light load. And that's why the best practice is to test with the actual light you will be using the cells in.
Fortunately just one voltage reading while the cell is under load is usually enough to tell us if the cell is "bad." It doesn't tell us anything about runtime but it quickly tells us if the cell can run the load at all, even if only for a moment.
Why do we want to use the same load the battery is going to see in real use? Imagine a charged car battery that reads 12.5v with your DMM--is it any good? The way to check is to have someone crank the car while you are checking the voltage. If it drops below 9v while the car is cranking, then that may not be enough to run the car's computer or injectors or spark plugs and the car won't start. You can stop troubleshooting right there because the battery is obviously bad.
So a battery that reads 12.5v no-load can still be bad (if it drops below a usable voltage under use). Do you care about the 12.5v at all? Well it does tell you the battery can probably power a miniscule load with no problem, but it doesn't tell us anything about its ability to power a big load (and here at CPF we usually are interested in pushing batteries pretty hard).
That's the reason the auto parts store sells those load testers for car batteries (usually in a big metal box because it must have enough resistors to dissipate some 100A of current), and why those battery testers from Radio Shack (which do have a small value resistor load in them) will only work if you intend to use the cells with a similarly light load. And that's why the best practice is to test with the actual light you will be using the cells in.
Right, sorry for my terse message above. The open (unloaded) voltage of a battery (as measured by just hooking up the voltmeter leads to a cell out of a circuit) is almost meaningless. Even completely dead NiMH cells will often read close to 1.2 volts when out of a circuit, which is about the same as a cell with a great deal of capacity left in it.
Almost any kind of load is better than testing with no load. What kind of batteries do you want to test? Assuming 1.2V AA rechargables, try about a 15 (or so) ohm 1/8 or (better) 1/4 watt resistor across the leads while you test. That will draw about 80ma from the cell as you test, which will start to distinguish fresh cells from dead cells (but which won't draw too much from them or get too warm). Now get a baseline by measuring the voltages of an known exhasuted cell, a medium charged cell, and a fresh cell.
Almost any kind of load is better than testing with no load. What kind of batteries do you want to test? Assuming 1.2V AA rechargables, try about a 15 (or so) ohm 1/8 or (better) 1/4 watt resistor across the leads while you test. That will draw about 80ma from the cell as you test, which will start to distinguish fresh cells from dead cells (but which won't draw too much from them or get too warm). Now get a baseline by measuring the voltages of an known exhasuted cell, a medium charged cell, and a fresh cell.
HCaul said:
HC, I figured that was the idea...what you and BFG said. However, if lightmad is in the relative noob category like me, trying to figure out what in the world you mean by "15 ohm 1/8 or better 1/4 watt resistor across the leads" is like 1,000 times over our heads in techno-speak.
Just to give you an idea of the level of many flashlight users...I have never used a resistor (consciously) in my life for anything, and am guessing that ohm must be a term used for resistance, and think about watts in terms of 75 or 100 watt light bulbs.
I know everyone means well when they give advice, but I really believe there are a lot of CPF'ers who are starting out from ZERO knowledge about electronics, batteries, charging when they join here.
I NEVER had any understanding about the many differences betwen NiCad, NiMH, Lithium, Lead Acid batteries...or how to properly care for them, life spans, dangers, etc. etc. A battery was a battery. When the D cells in my old flashlights wore out, you throw them away and get new ones. At some point alkaline batteries came out....but all that meant to me was they lasted longer than non-alkaline.
If something like my Laptop or Cell phone came with a battery charger...well I pretty much always kept it plugged into the charger when not taking it on the move. I had no idea that constant/overcharging frequency of many batteries damaged it....nor that Lithium rechargeables lasted about 3 years and then started crapping out no matter what you did.
It was not until I bought my Hyperion charger because Mad Maxabeam told me to get it for charging the Larry14K 28.8V spotlight that I began to pay proper attention to finding out the details. It honestly started when I was trying to figure out what this "LBA10 balance charger accessory" was all about, and if I should be using it while charing the NiMH in the Larry14K. Recently, I just graduated to making some custom tap leads for a FiveMega Li-Ion pack.....I was so proud of my progress.
I say this just so you know that is how "stupid"....or....ummm....rather...."uneducated" many of us are about all these topics....just so you remember back to when you were starting out. LOL!
Oh, and back to your resistor deal....who/what/when/where/how does that get setup with a DMM? I assume you have to get a whole set of various resistors for all the various battery types & sizes....that seems a bit overwhelming to consider....let alone trying to figure out which should go with which battery.
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wptski
Flashlight Enthusiast
Load testers for car starter batteries are still sold these days but they have testers now that are of the non-destructive type. They are like the AC 1kHz impedance testers talked about in this forum.bfg9000 said:
LuxLuthor said:
Lex,
Sorry again for the terseness of the reply.
First, the easiest solution is to get a dedicated battery tester scuh as the ZTS. Another alternative is a multimeter with a battery test function (which basically has the simple resistive load scheme I'm describing here built in to it).
The basic problem we're trying to solve is that for many many kinds of batteries (including just about everything in the AAA/AA/C/D form factor), the open circuit voltage doesn't tell you very much about how much energy is left in the cell. By "open circuit", I mean the voltage read when the battery isn't actually in use, e.g., when you take the cell out and just measure the voltage at the + and - terminals. All but the very deadest NiMH cells will read about 1.25 - 1.35 volts, regardless of how charged it actually is. An alkaline cell will similarly read about 1.5 volts under these conditions.
In other words, you can't distinguish between a good NiMH battery and an almost-dead battery just by measuring the voltage across its terminals.
But if you measure the voltage WHILE THE BATTERY IS ACTUALLY BEING USED, it will be lower than that, and the more exhausted the battery gets the more the voltage will drop compared with when you measure it when it isn't in use. For alkaline cells, this voltage drop may be considerable; for NiMH, it will be smaller but still quite measureable.
So you learn much more about the state of a battery by measuring it's voltage while "under load". You can use a resistor (one of the simplest of electronic components) as a load for this purpose.
Now, we'll need to get slightly technical for a few paragraphs. If you understand what follows, you'll go a long way toward understanding basic DC electricity theory and practice, so please bear with me...
You already know about voltage, more properly called "potential" in electronics-speak. It's measured in, well, Volts. A car battery supplies about 12.8 Volts. A rechargable NiMH or NiCad AA battery supplies about 1.2 Volts. Standard residential house wiring (in North America) supplies about 120 Volts (although there are also higher voltages used for certain fixed appliances like heaters and washing machines). Batteries supply "Direct Current" (DC); commercial power is "Alternating Current" (AC), which reverses polarity 60 times per second (in the US). AC is a bit more complicated to analyze and measure, but the basics here still apply. voltages over about 48V are considered a shock hazard and require special precautions. Lower voltages can still be a fire or spark danger if there's a short. So always be careful when working around electricity of any voltage, especially from higher energy sources (like car batteries or commercial power).
Resistance is measured in Ohms. The more resistance something has, the less current flows through it at a given voltage. A short piece of copper wire has a resistance of close to 0 ohms; a dry piece of plastic has a resistance of close to infinite ohms. "Resistors" are the electronic components that provide the values in between...
Any electronic circuit looks to the battery like a resistor of some value. (It's not actually that simple, because many circuits involving certain kinds of electronic components, including light bulbs, have a resistance that depends on the voltage, but for our purposes, any circuit can be thought of as a fixed-value resistor). For most of the circuits you'd use with AA batteries, the overall circuit resistance is between about 2 and about 2000 ohms or more. A flashlight will be closer to 2 ohms; a pocket calculator will be closer to 2000.
The amount of current flowing through a circuit depends on the voltage being supplied to it and the resistance of the circuit itself. Current is the measured in Ampres (or Amps). The number of Amps in a circuit can be calculated by dividing the voltage (in Volts) by the resistance of the circuit (in ohms). So a 48 ohm circuit with a 12V battery will have a current flowing through it of 0.25 Amps, (usually written as 250 milliAmps, or 250mA).
Now you're equipped to understand how battery capacity is measured: in mA/h, or milliAmp-hours. A 2000 mAh battery can theoretically supply 1 milliAmp for 2000 hours or 2000 milliAmps (2 Amps) for 1 hour or 500mA for 2 hours, before it goes dead and has to be recharged. (In reality, it's not quite as simple: there are some non-linearities and inefficiencies at the extremes, but for the purposes of this discussion, that's the idea).
So you'll want to measure the voltage of your cell with a resistor across its + and - terminals. A load of about 15 Ohms will draw about 80mA from a single 1.2V AA NiMH cell. (How long would you expect to be able to do this from a fully-charged 2400mAh cell?)
So, assuming you want to measure single AA NiMH cells: Go to Radio Shack (or whatever your local electronics chain is called) and get yourself a roughly 15 Ohm resistor. The exact value doesn't matter; anything more than 10 Ohms and less than 30 or 40 Ohms will do. (The larger the resistor, the SMALLER the load it will be placing on your battery when you test). You'll need one rated for at least 1/8 Watts (voltage*amps), but that's not a problem because 1/8 Watt resistors are the smalest you're likely to find at any electronics store.
Now you'll need to find a way to have the resistor be across the + and - terminals of you battery at the same time as your meter's leads are across the + and - terminals (that is, you want to measure the resistor in PARALLEL with the battery). The easiest way to do this might be to get a battery holder and solder the resistor across the terminals, or if you're highly dexterous you may be able to just hold everything together with your hands.
Finally, you need to test some batteries to get an idea for what values to expect with your resistor. Test at least three: one right off the charger, one that's been used for a little while, and one that's completely dead, and note the values. That's all there is to it!.
This won't be as accurate as a proper tester, but it will be sufficient for many purposes, and certainly better than measuring without any load at all.
H. Caul
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WOW....that was just at the perfect level for my understanding, and I REALLY REALLY appreciate that entire post. I understood everything after reading it a few times, but had a couple questions if you don't mind.
But going back to the plastic with infininte ohms...why wouldn't something that had large resistance (higher ohm value) put more of a load (I think "drain") on a battery? I'm thinking it would take more energy from the battery to make a circuit through the high resistor (& giving more heat)...and thereby use up the battery faster.
I guess I don't have a good handle on what the voltage readings mean with & without a resistor and with a good vs. bad battery.
Again, thanks for all your help and patience with these questions.
I'm confused why a pocket calculator would have more resistance than our flashlights....my logic is based upon there is so much heat you can feel from most of our lights, vs. practically no heat in a pocket electronic device. I would have thought all that heat in the filaments was from high resistance....not like a piece of copper wire which doesn't typically heat up at all with nearly zero ohms.HCaul said:
Did you mean 0.4 Amps? 12 V/30 ohm = 0.4HCaul said:
300 Hours?HCaul said:
Sounds easy enough to setup with a battery holder...and must be similar to testing flashlight voltage by measuring with leads attached to both bi-pins while bulb is turned on. A resistor is just a replacement for the flashlight components and bulb filament.HCaul said:
But going back to the plastic with infininte ohms...why wouldn't something that had large resistance (higher ohm value) put more of a load (I think "drain") on a battery? I'm thinking it would take more energy from the battery to make a circuit through the high resistor (& giving more heat)...and thereby use up the battery faster.
I'm confused by how you would know what it means when you test a battery right off the charger, after it has been used in the light a while, and one that is exhausted. I'm assuming they would show progressively lower voltage respectively...but that would be the case if they were tested without a resistor.HCaul said:
I guess I don't have a good handle on what the voltage readings mean with & without a resistor and with a good vs. bad battery.
Again, thanks for all your help and patience with these questions.
(responses inline below)
Glad to help...
Well, remember that there's an INVERSE relationship between current and resistance at a given voltage. So the higher the number of Ohms, the smaller the current if the voltage is maintained at the same level. A flashlight uses a lot more power than a calculator (but uses the roughly the same voltage batteries), because the flashlight has lower resistance (and allows more current to flow).
Heat -- which we didn't discuss -- has a funny relationship to all this. In any series circuit of a givent total resistance, the most heat will be given off by the parts of the circuit with the highest resistance. But as resistance increases, current (and power) decreases. So your house wires don't heat up when you turn on a light, because the wires contribute very little resistance (especially relative to the light bulb). But because they don't actually have zero resistance, they are heating up a little.
Yes, sorry, ment to type 48 ohms. I have no idea where the 30 came from. Old age.
30 Hours. Careful of those order-of-magnitude errors...
Remember the inverse relationship between Current and resistance. You can control how much current goes through the circuit by varying its resistance.
The problem is that the differences between good and bad are very, very small without the load. Measuring under load will spread the values out more, and you'll get more more accurate picture of the battery's charge state.
Hope this helps.
H. Caul
LuxLuthor said:
Glad to help...
Well, remember that there's an INVERSE relationship between current and resistance at a given voltage. So the higher the number of Ohms, the smaller the current if the voltage is maintained at the same level. A flashlight uses a lot more power than a calculator (but uses the roughly the same voltage batteries), because the flashlight has lower resistance (and allows more current to flow).
Heat -- which we didn't discuss -- has a funny relationship to all this. In any series circuit of a givent total resistance, the most heat will be given off by the parts of the circuit with the highest resistance. But as resistance increases, current (and power) decreases. So your house wires don't heat up when you turn on a light, because the wires contribute very little resistance (especially relative to the light bulb). But because they don't actually have zero resistance, they are heating up a little.
Yes, sorry, ment to type 48 ohms. I have no idea where the 30 came from. Old age.
30 Hours. Careful of those order-of-magnitude errors...
Remember the inverse relationship between Current and resistance. You can control how much current goes through the circuit by varying its resistance.
The problem is that the differences between good and bad are very, very small without the load. Measuring under load will spread the values out more, and you'll get more more accurate picture of the battery's charge state.
Hope this helps.
H. Caul
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I think I understand your answers. I'll look at it again tomorrow.
Maybe what would be helpful is a couple examples of types of readings with a resistor (which I will buy some this week at Rat Shack) that would illustrate good vs. bad cells.
LOL! I wonder what happened to Lightmad who started this thread?
Maybe what would be helpful is a couple examples of types of readings with a resistor (which I will buy some this week at Rat Shack) that would illustrate good vs. bad cells.
LOL! I wonder what happened to Lightmad who started this thread?
@lightmad:
-turn Your meter to Amperes DC (the 10 A reading) and put the wires into the right openings
-load Your batts with the charger and place them into the flashlight,
-leave tail cap open (best if the end of the batts still stand out, if not You need some kind of spacer isolated from the flashlight body).
-Put a (strong) rubber wire around the light
-hook one cable under the rubber, connecting to the battery,
-hook other cable to body of flashlight
--> light should work now and the meter should show a reading (around 800 mA for a good part of lights)
Check readings at some point (say every 1/2 hour) and remember them.
When the light gets lower in output stop the whole thing.
Calculate together Your readings and You have the mA Your batts give
(say: 1/2 hour 800, 1/2 hour 790, next 1 h 780 and 1/2 hour 770 = 1960 mAh)
-turn Your meter to Amperes DC (the 10 A reading) and put the wires into the right openings
-load Your batts with the charger and place them into the flashlight,
-leave tail cap open (best if the end of the batts still stand out, if not You need some kind of spacer isolated from the flashlight body).
-Put a (strong) rubber wire around the light
-hook one cable under the rubber, connecting to the battery,
-hook other cable to body of flashlight
--> light should work now and the meter should show a reading (around 800 mA for a good part of lights)
Check readings at some point (say every 1/2 hour) and remember them.
When the light gets lower in output stop the whole thing.
Calculate together Your readings and You have the mA Your batts give
(say: 1/2 hour 800, 1/2 hour 790, next 1 h 780 and 1/2 hour 770 = 1960 mAh)
abvidledUK
Flashlight Enthusiast
See...
http://www.candlepowerforums.com/vb/showthread.php?t=162640
Easy under load testing which gives comparative results.
http://www.candlepowerforums.com/vb/showthread.php?t=162640
Easy under load testing which gives comparative results.
ynggrsshppr
Newly Enlightened
I think voltage from a DMM gives a good approximation for the remaining charge on a lithium ion battery. I charge them when they fall down to 3.6V. Why would you need to take a reading under load? I thought lithium ion was different from NiMH in this regard.
I'm still trying to digest some of this stuff which I vaguely remember from Physics class 30 years ago. It makes sense but gotta read it slowly a few times. Anyway will pick up some resistors to play with. I recall having a neighbour who used to fix up TV sets(pre-PCB) in 1970s as a hobby and he would have his multimeter and test each component. Took hours/days but hey! such is our madness.Lux is spot on regarding noobs.
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