DMM accuracy and charging voltage.

This probably looks like a recipe for disaster: A ***fire cell in a ***fire charger on a combustible surface. Not to worry though, this is not how I normally charge my cells, and I use AW 14500 and 16340 in my EDC lights. I didn't want any metal near this setup, that's why I used the chair.

Now, on to my questions. As you can see, the DMM reads a higher than optimal voltage (I'm charging at the time) but it is possible that the actual voltage is about 4.25 V.

The specifications for the DMM say "basic accuracy: ± 0.5% of rdg ± 2 digit" and the way I interpret that is that the lowest possible voltage is (0.995*4.29) -2 digits ~4.25. Obviously there is a worst case scenario as well, (1.005*4.29)+2 digits ~4.33.

What I'm wondering is, since most of my cells (all are protected) measure 4.25 V when I take them off the charger (I charge until the light turns green) and assuming the charge terminated because the protection tripped, does this suggest that the DMM shows too high a voltage rather than too low?

I know it's not recommended that you rely on the protection circuit for charge termination, but at the moment this is the only charger I have. I don't mind if using this charger shorten my cells' service life a little, my main concern is safety. So my second question is: do you think I'm pushing my luck, or am I well within safe limits as long as I make sure I never leave the charger unattended, and that the cells' voltage when fully charged doesn't exceed ~4.25V?

Also, has anyone else who uses the Ultrafire WF-139 charger measured its charging voltage, and if so, what results did you get?


Thanks in advance for your input.

You might try to find someone with a different (more expensive, more accurate) meter to compare your meter against. With the various cheaper meters I have purchased it would not be unusual for the reading to be off by about 0.03 V around 4.00 V.

As the the risk of fire, I think it's pretty low. To make a lithium ion battery fail you have to do bad things to it like short it out bypassing the protection circuit, or charging it way up to 5 V or more without reducing the current. To just charge a cell up to 4.25 V when it is spec'ed for 4.20 V is only likely to shorten the life of the cell and little risk of anything worse.

From what I've read here on CPF a lot of cheaper chargers charge at higher than 4.2V so that the cell reads 4.2V the end of charge.

How many times have you read that the Pila charger is no good because it doesn't charge every cell to 4.2V, the CV stage of the charge is 4.2V and therefore the cell will not reach 4.2V, the resting V will be dependent on the condition of the cell.

Cheaper chargers try to force the cell to 4.2V by charging above 4.2V. As stated above part of the error will also be meter inaccuracy.

I'm sure someone is going to come along and explain it much more eloquently than I ever could.

Norm

there have been many many reviews of UF chargers from way back when and foreward, almost all of them indicate that UF does not follow CC/CV charging methods.
http://www.candlepowerforums.com/vb...sting-of-Ultrafire-WF-138-and-WF-139-chargers (just one of them)
Most UF chargers are screwey Cheap get by kinda charger that will work, other than that i dont know of any reason to use any of them. even if there is one out there that does follow all the rules, they have negative credibility.

join the club there is a rare person who has done all the testing, that thought the UF chargers were a Good thing.
it would be a bigger problem on batteries that are bad or going bad, or if you mess around and keep putting it back on charge.
it can be faster to get to a green light (assuming you get to a green light) because they are faster at the end of charge, which is not proper.
They are another good reason for protected cells, although it doesnt solution the end rate thing (not cv) seen on most of them.

If you only observe the final charge on the cell, it would look ok, because usually they stop before any serious overcharges. Once you start trying to view everything, that is where you see that it doent use an actual CV, and instead some kinda reduced rate , that is not the same at all.

What your showing could very well be 100% real, and not an anomoly of inaccurate metering. Because charge is still applied, and it is not CV. So the voltage of both the battery and charger can indeed be over the final termination voltages the battery reaches. Yes, it Could also pound the protection, with the protection kicking in and out , over and over again, untill the cell is sitting at the protection end, something that is way less likly to occur when using proper CV.

Thanks for your replies. If I try to sum up the advice and suggestions I got and distil a take-home message, I think it would sound something like this: "Go ahead and use your cheap charger for now - you're not likely to set your house on fire - but the second you get yourself a better one, put it away somewhere and never look back!"

Update: Today I charged two AW cells, one 14500 and one 16340, both discharged to 60-70%. I took them off the charger before the indicator lights turned green, and was a little surprised to find that the voltage on the 16340 was 4.23 V and the 14500 was 4.25 V.

I don't think my cheap meter is very accurate, so I downloaded an android app that shows the voltage of a mobile phone battery. It was 3.96 V, but when I turned off the phone and measured the voltage with my unreliable DMM, it said 4.03V! Quite a discrepancy. I don't know how accurately phones measure voltage, but I'm going to repeat the test with my other phone (which has different battery) and see if I get a similar result.

Edit: Second phone battery
Phone app: 3.81 V
DMM: 3.89 V


I have reached one conclusion already: I can't recommend the Velleman DVM 831 Digital Multimeter :thumbsdow

Swede74 said:

I don't think my cheap meter is very accurate, so I downloaded an android app that shows the voltage of a mobile phone battery. It was 3.96 V, but when I turned off the phone and measured the voltage with my unreliable DMM, it said 4.03V! Quite a discrepancy. I don't know how accurately phones measure voltage, but I'm going to repeat the test with my other phone (which has different battery) and see if I get a similar result.

Edit: Second phone battery
Phone app: 3.81 V
DMM: 3.89 V


I have reached one conclusion already: I can't recommend the Velleman DVM 831 Digital Multimeter :thumbsdow

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I wouldn't say that. The app on the phone measures the voltage with a load on it. It's is the same as, say leaving the light on. The DMM is measuring the no load voltage which will almost always be higher. That's the same as measuring the battery with the light off.

I think your DMM is accurate enough.

It is possible to build (or buy) a voltage reference to compare a meter against, both for checking its accuracy and for keeping tabs on whether the accuracy changes over time.

uk_caver said:

It is possible to build (or buy) a voltage reference to compare a meter against, both for checking its accuracy and for keeping tabs on whether the accuracy changes over time.

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voltage reference kit

I didn't look too far but this couldn't be simpler, I can't comment about quality or accuracy.

Norm

If I used Li-Ion cells, I'd probably build a reference around one of the 4.096V reference chips to give me a good idea of likely accuracy ~4.2V.

uk_caver said:

If I used Li-Ion cells, I'd probably build a reference around one of the 4.096V reference chips to give me a good idea of likely accuracy ~4.2V.

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How about a 5 volt reference that would be the closest standard reference.

Norm

I've had good results with a simple voltage standard.

http://www.voltagestandard.com/Home_Page_JO2U.html

itguy07 said:

I wouldn't say that. The app on the phone measures the voltage with a load on it. It's is the same as, say leaving the light on. The DMM is measuring the no load voltage which will almost always be higher. That's the same as measuring the battery with the light off.

I think your DMM is accurate enough.

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I didn't know that. You learn something new every day, even at my advanced age (almost 40).

I must admit I also didn't know that such a thing as a voltage reference existed. It makes me wonder, what does one use to determine the accuracy of a voltage reference?:thinking:

Anyway, although they seem fairly cheap, I would only use it once if I bought one, so I think comparing my meter against a different one will have to do.

I'm going to spend my money on a new charger instead, either the Nitecore Intellicharger i2 or the new on from Xtar that has a LCD display.

:thanks: again.

Swede74 said:

I didn't know that. You learn something new every day, even at my advanced age (almost 40).

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I do not believe that you will ever stop learning stuff, how much you are learning does, of course, depend on yourself.

Swede74 said:

I must admit I also didn't know that such a thing as a voltage reference existed. It makes me wonder, what does one use to determine the accuracy of a voltage reference?

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Another reference, all the way back to the definition of the volt.

Swede74 said:

Anyway, although they seem fairly cheap, I would only use it once if I bought one, so I think comparing my meter against a different one will have to do.

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And when you DMM is running out of battery and starting to show wrong values, are you going to panic or just check the DMM on your reference .

Charging my 3++ year old AW 14500. Surprisingly on the UltraFire 139 charger, it stopped at 4.04V. Took the battery out and put it back on the Xtar SP2 and let it charge for another 20-30 minutes before it stopped at 4.18V. My old UF139 has been very conservative with its charging voltage so there's no overcharging here. Good eh?

Brera said:

Charging my 3++ year old AW 14500. Surprisingly on the UltraFire 139 charger, it stopped at 4.04V. Took the battery out and put it back on the Xtar SP2 and let it charge for another 20-30 minutes before it stopped at 4.18V. My old UF139 has been very conservative with its charging voltage so there's no overcharging here. Good eh?

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It looks like you won the Ultrafire lottery :twothumbs