Multimeters and lithium battery safety

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1000yardstare
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Multimeters and lithium battery safety

Due to reading about numerous exploding lithium battery stories on CPF, I've now convinced myself I need to buy a multimeter. So, I did some looking around on ebay/dx... but all the choices have confused me further - so here I am seeking advice!

 

All I want to do basically is be able to measure battery voltage (make sure my cells aren't being overcharged/discharged) and do basic tailcap measurements on how much current a torch's LED is pulling from a battery to see if an LED is driven as specified, or over/under driven.

 

What's the difference between this (http://cgi.ebay.com.au/Digital-LCD-Multimeter-Voltmeter-Ohmmeter-Ammeter...) and this (http://cgi.ebay.com.au/New-LCD-Digital-Voltmeter-Ammeter-Ohm-Multimeter-...)? Or can anyone reccomend me a budget multimeter than works well?

 

Thanks Big Smile

Lights I own/have bought for others: Aurora AK-P7-3 (2x18650), Trustfire TR-1200, Solarforce Skyline 1, Uniquefire HS-802, Ultrafire 501b MC-E, Ultrafire 501b P7, Kai-Domain NF-500 Flood-to-throw, Generic Ebay 1AA Zooming light, Auror

brted
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I have a different version of the black one, but it is also labelled DT-830B and seems the same except mine is missing the blue circle thing. Generally I've been happy with it until lately when I was using it to measure tailcap currents and realized the leads were so lightweight they were distorting the amount of current the battery was supplying to the LED and giving me low readings (1.6A when it should have been 2.8A). For measuring voltage the lightweight leads won't hurt you. And the yellow one seems to use the same exact leads. One thing the 830B has is a LED setting. You can connect the leads to the + and - of a LED and it will light it up (just barely, won't hurt your eyes). That has come in handy before. I couldn't tell if the yellow one had that or not.

So now I am looking for some heavier duty leads which may cost more than the meter itself.

Don
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You don't really need anything fancy - the transistor/diode test socket on them is not likely ever to get used. Essentially you gets what you pays for, but you don't need all that much. Either of those will do, personally I'd go for the first one as it is likely to be a better meter. If you are near a Jaycar store (I'm assuming you are in Australia from the ebay.com.au links) it might be worth looking in there to see if they have any special offers. Don't place much faith in the last digit, or if it has a lot of digits (>4), don't put much trust in the last 2. The ability to tell the difference between 4.1 and 4.3 volts consistently is what matters. Apparently you should be able to expect 5% errors in the cheapies, maybe less.

 

Current draw measurements are often iffy on meters as the leads have more effect on the current than most people realise. Some 6mm2 solid copper mains cabling bodged into 4mm plugs will sort that one out.

 

Or the best way is to buy a cheap shunt and measure millivolts instead. There's links to a lot of useful stuff about electricity and meters in the last section of this article. I enjoy Dan's writing and recommend http://www.dansdata.com to everyone. 

http://www.dansdata.com/io107.htm

 

The resolution and accuracy figures on both of those are what are known as "lies". They always are. You need to pay serious money to get properly calibrated meters for it to be otherwise. Take a look at what calibrated Fluke meters cost. I can get a dozen meters for the price of a secondary calibration of a Fluke meter - get it calibrated against primary standards and you could buy a used car instead. That said, I paid the equivalent of US$50 on an old and battered Fluke some years ago and it has been worth every penny for what I want it to do. At least till the smaller of the two fuses in it died (Got blown up by the idiot owner) - I could buy two of both of the meters you link to for the price of that fuse...

 

The numbers from my light tests are always to be found here.

https://spreadsheets.google.com/ccc?key=0ApkFM37n_QnRdDU5MDNzOURjYllmZHI...

Don
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brted wrote:

So now I am looking for some heavier duty leads which may cost more than the meter itself.

 

Just get a couple of feet of solid core copper wire as used in house wiring. Force it into some 4mm plugs. You should be able to get a short length of scrap cable from any electrical contractor who happens to be passing. Insulation is a matter of personal preference - I use heatshrink or PVC tape when I bother. As it is solid core and copper is not infinitely ductile, keep some more around for when it breaks. I use the stuff left over from when my house was rewired in the 80's and am not about to run out.

 

The better bet is to get a shunt and measure millivolts instead. Put the shunt in series with the circuit (In practice attach it with some nice thick copper wire to the negative end of the battery and some more nice thick copper wire to the light body and measure the millivolts across the shunt. Then apply Ohm's Law. So if we have an 0.1 ohm resistor and the voltage across it is 100mV, from V=IR we get V/R=I so 0.1V/0.1Ohms=1A

 

The numbers from my light tests are always to be found here.

https://spreadsheets.google.com/ccc?key=0ApkFM37n_QnRdDU5MDNzOURjYllmZHI...

brted
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I found a good set of probes for only $3 on DX. I wasn't sure what gauge the wires were, but a customer posted a picture and you can read on the wire that is 18AWG. That should be good enough. I think the other probes I have worked fine for lower currents because I would measure very close to 1.05A or 1.4A for drivers that had 3x7135 or 4x7135 chips on them respectively.

 

http://www.dealextreme.com/details.dx/sku.33451

Don
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brted wrote:

http://www.dealextreme.com/details.dx/sku.33451

 

I bought a couple of sets of these to replace ones that were falling apart. I've had no trouble from them.

 

The numbers from my light tests are always to be found here.

https://spreadsheets.google.com/ccc?key=0ApkFM37n_QnRdDU5MDNzOURjYllmZHI...

FlexakaMal
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Thats where I got this one, the extra lead from DX. Got sick of changing the red led aroundLaughing  $15  

multimeter

1000yardstare
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Thanks Brted + Don. Just bought the DT 830-B for less than 10 AUD. Not sure if I have any 4mm plugs lying around... but I do have about 3 metres of 'tinned' copper wire?

Lights I own/have bought for others: Aurora AK-P7-3 (2x18650), Trustfire TR-1200, Solarforce Skyline 1, Uniquefire HS-802, Ultrafire 501b MC-E, Ultrafire 501b P7, Kai-Domain NF-500 Flood-to-throw, Generic Ebay 1AA Zooming light, Auror

Don
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1000yardstare wrote:

Thanks Brted + Don. Just bought the DT 830-B for less than 10 AUD. Not sure if I have any 4mm plugs lying around... but I do have about 3 metres of 'tinned' copper wire?

 

Radio Shack, Jaycar or equivalent will sell you them - they are very common items. The one in the picture is a cheap and nasty one but it gets the job done.

4mm lead

 

The numbers from my light tests are always to be found here.

https://spreadsheets.google.com/ccc?key=0ApkFM37n_QnRdDU5MDNzOURjYllmZHI...