DMM

[tominator425]

Guys, please dont flog me.

I am anxiously awaiting my TK11 R2 (any day now) and already have the 18650 batteries. Reading on here, I have gathered that I should have a DMM to test these batteries to make sure they are not over-charging?

So, I am looking for them not to go past 4.2v right?

I have looked at alot of DMM, and just have no clue as to what to get. I am not an electrical engineer, just want a simple DMM to test my batteries. So, do I just swing by Radio Shack or do I buy from somewhere on the 'net? I just want to check my batteries, as you guys recommended.

TIA as always!

 

[chewy78]

I use one i got from my local sears sku#82139. It runs at about 30 bucks. Its a step up from your cheap 5 dollar multimeters. But its no high dollar fluke or whatever. It runs off of 2aa's and has a nice feature in when you leave it on it shuts off in 10 minutes. From then last time you use it if you forget to turn it off to keep the batteries from running down.

 

[tominator425]

That is what I am looking for Chewy, a simple inexpensive, but useable DMM. I was hesitant to buy a dirt cheap one (the whole "you get what you pay for" thing), but figured that, for now, I dont need a $100+ one.

I will check that one out at Sears. Thx!

But, simply - do I just check to see that it does not go over 4.2 when fully charged? I need to read more on here I think.

 

[TITAN1833]

I have 2 MM one expensive ISO TECH, and a blackspur DT 100 being the cheaper one,FWIW both read the same when testing batteries :twothumbs

BTW the difference in price is many many,pounds the BS DT 100 I got of ebay for about £2.00 the other was IIRC well over £80.00,so if you just want to test batteries of the charger get a cheapy :thumbsup:

 

[tominator425]

if you just want to test batteries of the charger get a cheapy :thumbsup:

This is really all I want to do at this time, just be sure that my batteries are doing what htey are supposed to be. So, the little $7-$10 ones are good for this?

 

[TITAN1833]

This is really all I want to do at this time, just be sure that my batteries are doing what htey are supposed to be. So, the little $7-$10 ones are good for this?

Yes really the cheap ones do that job really well.:twothumbs


[edit]If it helps any I use my BS DT 100 all the time for testing batteries off the charger and current at the tail cap it reads no difference than my expensive one.

 

[chewy78]

they will work also

 

[chewy78]

i too have a cheaper one that the one i listed that works for simple stuff.

 

[tominator425]

so where is the best place to look for one of these cheaper ones? Just go to Sears, or Walmart??

What do they roughly cost?

 

[TakeTheActive]

Interpreting DMM Specifications - A Detailed Example

Guys, please dont flog me.

...already have the 18650 batteries. Reading on here, I have gathered that I should have a DMM to test these batteries to make sure they are not over-charging?

So, I am looking for them not to go past 4.2v right?

I have looked at alot of DMM, and just have no clue as to what to get. I am not an electrical engineer, just want a simple DMM to test my batteries...

I've learned quite a bit about DMM Specifications regarding 'percent' and 'digits' from CPF Member Justin Case in this recent thread:

Digital Multimeter Which One?​

While many of the inexpensive, NOT CHEAP, 3½-digit DMMs are perfectly suitable for NiCd and NiMH cells, (under 1.999VDC), they lose a whole level of accuracy when measuring Li-Ion:

Interpreting DMM Specifications - A Detailed Example​

 

[Mr Happy]

Following what TTA said, I was going to mention the same thing. Voltmeters are more accurate generally when the measurement is nearer the full scale point.

If you want to measure 4.2 V on most DMM's you will need to use the 20 V DC scale, and this puts 4.2 V at only 20% of the scale. In addition, with the 20 V scale reading up to 19.99, you will see 4.2 V reading as 4.20 with only two digits after the decimal point.

If you can find a DMM that has scales that go 600 mV, 6 V and 60 V rather than 200 mV, 2 V and 20 V, or if you can find a DMM with 5 display digits instead of 4, you will be better off.

Such DMM's may be hard to find and more expensive, and I don't immediately have one to recommend, but it would be something worth keeping an eye out for.

 

[MarNav1]

I use one i got from my local sears sku#82139. It runs at about 30 bucks. Its a step up from your cheap 5 dollar multimeters. But its no high dollar fluke or whatever. It runs off of 2aa's and has a nice feature in when you leave it on it shuts off in 10 minutes. From then last time you use it if you forget to turn it off to keep the batteries from running down.

I have this meter as well, got it on sale for $20. Works good AFAIK, I spent extra money for better leads (silicone), I dont use the stock leads. I also have a Fluke 88, but for a beater meter or something to loan out it has worked fine. :thumbsup:

 

[tominator425]

Was looking at this one ($20) at radioshack:

http://www.radioshack.com/product/index.jsp?productId=2103174&tab=summary

not much on it tho really, needs a "special" battery and reviews says that the case is attached and way too small to store the attached cables.

And still the Sears one mentioned above. I have a Sears in town, so could go see what they have.

I looked on Amazon and they have a bunch! varying prices, mastech, etc. Should I look at one on there, but sheesh...which one to go with!

I did read the above post and will look for one that has the listed specs.

 

[Light Sabre]

I have several of the Radio Shack Cat# 22-812 DMM's. PC Interface 46-Range Digital Multimeter - RadioShack.com I love them because they have an RS-232 port and you can record data for a very long period of time on your computer. They' don't have an AC adapter. I thought that that was strange until I found out that with an Energizer 9V battery they can last about 2 weeks straight. They are battery vampires and have seen them still working (low bat warning on of course) with the battery at 5.5V. They are $70 and that includes software on CD and RS-232 cable. The next closest DMM that I could find would have cost around $160 with software and RS-232 cable.

The RS meter graphs the data on the screen. Gathers data once every sec, 5 sec, 10 sec, 30 sec, and 60 sec. It saves the data as a text file and you can import it into Excel. You can zoom in in a major way and see individual data points even if you have collected data for a very long period of time. Have used a photocell to see how long a flashlight lasts. Photocell to see whether a a flashlight goes into moon mode or just quits without warning (a sin in my book). Have used the same set of rechargeable batteries (with adapters if needed) in different flashlights to get an idea how they compare. Have used a thermistor to check and see how fast things heat up. I really need to get one of those thermocouple adapters that do 1mv/degree outputs for true temperature.

The graph on your computer screen is in real time, but unfortunately you can't reload a saved file to view it later. So I just do a screen capture to save the graph or you can print it out. Cursor mode (after you have stopped gathering data) allows you to measure delta T or delta in whatever unit you were measuring. One of the best DMM purchases I have ever made.

 

[SilverFox]

Hello Tominator,

After you get your $5 volt meter, you can always spring for a $30 voltage reference to check it accuracy...

If you don't want to purchase a standard and maintain it, you could always send your voltmeter to someone that has one of these and have it checked out... This way you would only be out some extra postage.

Tom

 

[tominator425]

Ok, Ok... I get it.

No $5 meter, but surely just to simply test batteries, I dont need a million dollar one. The ones mentioned above, and a messload I have looked at ranging around $20-$30 should do the trick right?

(remember, not electrical engineer). :ironic:

 

[Justin Case]

Hello Tominator,

After you get your $5 volt meter, you can always spring for a $30 voltage reference to check it accuracy...

If you don't want to purchase a standard and maintain it, you could always send your voltmeter to someone that has one of these and have it checked out... This way you would only be out some extra postage.

Tom

This will work for checking voltages that are close in magnitude to 5V and which reside in the same measurement range as for the 5V measurement. For the application at hand, fortunately this is the case. The Li-ion terminating voltage should be 4.20V and the voltage reference standard is 5.00V. If your reference measurement reads 5.02V, for example, then you can reasonably assume that your Li-ion terminating voltage measurement will also be 0.02V high. However, at voltage measurements far away from 5V, I don't think that this assumption necessarily holds since error is a combination of offset and gain errors, and gain errors are a function of the input.

Another related difficulty with this approach arises when your measurement of interest is in a lower measurement range than that used for the 5V reference. Most of these inexpensive 3 and 3 1/2 digit meters have measurement ranges that don't match well with this 5V reference if you want to measure something less than 4V. For example, if your 3 1/2 digit DMM has ranges of 400mv, 4V, 40V, 400V, and 1000V, then you will have to use the 40V scale to measure the voltage from the 5V reference standard.

But to calibrate a meter, you ideally should make reference standard measurements for each measurement range. Unfortunately, you can't do that since 5V exceeds the 4V range max in the example given. Thus, you can't be entirely sure what the voltage divider circuitry in your DMM is doing on the 4V scale vs 40V scale. It's probably better than nothing to simply assume that whatever error you get on the 40V scale also applies on the 4V scale for voltages close to 5V.

Most likely, none of these issues will matter since you probably don't need even 2-digit accuracy when measuring things on the 4V scale. The measurement where you'd like at least 2 digits accuracy is the Li-ion terminating voltage accuracy, and that magnitude should match up with the 5V reference standard in terms of using the same range and being close enough that you can assume the same error magnitude.

 

[Justin Case]

Ok, Ok... I get it.

No $5 meter, but surely just to simply test batteries, I dont need a million dollar one. The ones mentioned above, and a messload I have looked at ranging around $20-$30 should do the trick right?

(remember, not electrical engineer). :ironic:

If you want to make sure your Li-ion cells aren't getting overcharged (i.e., voltage not above 4.20V), then you need the accuracy to be sure that your measurement is actually 4.20V max and not say 4.23V. IMO, you can't depend on the cheap meters to do this, unless you calibrate it somehow -- comparison to a highly accurate, known-good meter or to the 5V reference standard previously mentioned.

Note that the reference standard also drifts with 9V battery source voltage, temp, and aging. So the reference also needs periodic calibration.

 

[Yoda4561]

I've got the craftsman 82139 from sears as well. It's probably the most meter you would want for this sort of thing, I've used it at work and at home for a couple years (electrician and low voltage/firealarm installation). For lithium batteries you can set the range out to 3 decimal points, I normally don't bother with that since it seems just waving the leads around in the air induces enough voltage that it's innacurate that far out. Maybe I just need better quality leads or something(using the stock ones, 100% servicable but better would be nice). I paid about 50 for it, and for the purposes of this hobby I can't see needing much more, unless you get into some seriously heavy duty hotwire mods where you need to measure more than 10 amps of current.

 

[tominator425]

Maybe I am just not getting it, so please bear with me.:thinking:

If I have protected 18650s, and the battery charger I have has an over-charge protection, why do I need a DMM to begin with? I mean, if I feel/think the battery is not fully charged, I would not check it with a DMM, I would just put it on the charger?

Or, are the 18650s something that really need to be monitored? I have NEVER used or owned a DMM, never tested a rechargeable battery on anything I have ever had to this point, and am just missing something I guess.

I dont mind spending $30, $40, $50+ on a DMM, if I knew WHY I was doing so? I do get that a cheap one is just that, cheap....but please tell me why I need to spend $30 or more on a DMM to simply test a protected battery that is charged in a "safe" charger?

Thanks guys!