DMM

[Yoda4561]

The real problem here is poor quality chargers, which is most of them. The other problem is what lithium ion batteries tend to do when they are overcharged (yay housefires!). So it's important to make sure they're being charged properly. Use of a top quality hobby charger or Pila IBC will mitigate that to some extent, but even the hobby chargers have missed end of charge or popped defective lithium polymer batteries resulting in more than one house fire incident. All it takes is .2 volts over the 4.2 volt recommendation to start causing severe battery degradation, keep that up and it's possible the battery will vent with flame while charging. Also if the battery is somehow discharged flat, below 2 volts. If that happens just dispose off the battery, don't even bother trying to put it on a charger since it's certainly been damaged, and charging it will just damage it more, at best you will have a barely useable battery, at worse it will catch fire while you aren't looking at it. The really cheap meters that are accurate are just fine, it's just that unless you have a reference to ensure it's reading accurately to at least .1 volt it's basically useless for this job.

 

[SilverFox]

Hello Justin Case,

I may point out that if you have a voltmeter with ranges of 400 mv, 4 V, 40 V, and 1000 V, and are measuring 4.20 volts, you will be on the 40 V range.

I may also point out that there are also reference standards of different voltages available from the same source.

I should also point out that this particular standard will drift as battery voltage changes, but the difference between a battery with 9 volts and one with 6 volts is only 0.000015 volts. It seems that this would be stable enough to check a 3 1/2 digit voltmeter.

With this particular reference, temperature effects are greater than voltage effects, but if you can keep the temperature within 14 F of 65 F, you should be within the 0.01% spec.

The standard is aged 200 hours prior to delivery, and has an aging rate of about 10 ppm/1000 hours.

All in all, I think it is reasonably stable, but calibration services are available if greater precision is needed.

Tom

 

[tominator425]

So a Pila IBC should definitely be my next purchase? And the craftsman mentioned above is a good enough one to buy?

Man, you guys got me to thinking I should only charge the batteries when and where I can watch them, and pull them off charger as soon as they reach a full charge! :huh:

 

[SilverFox]

Hello Tominator,

Having a volt meter gives you an edge on safety.

If Li-Ion cells were safe, you could run down to the local store and pick some up. However, since there are dangers associated with them, and liability concerns, the local store won't carry them.

Since the malfunction of the protection circuit and/or charger could lead to starting a fire, we think a little extra care in using Li-Ion cells is worth the expense of purchasing and learning how to use a volt meter.

Tom

 

[SilverFox]

Hello again Tominator,

It is always recommended that you attend to charging, and with Li-Ion cells it is always a good idea to pull them when they are fully charged.

Tom

 

[Mr Happy]

Man, you guys got me to thinking I should only charge the batteries when and where I can watch them, and pull them off charger as soon as they reach a full charge! :huh:

This is always true of loose lithium ion cells, especially the larger sizes like 18650.

You need to discard them if their voltage ever gets much above 4.3 V.
You need to discard them if they spend any length of time below 2 V or so.
You need to charge them very slowly at first if their voltage starts out below 3.3 V, before commencing the normal charge rate (good chargers should do that).
You should charge them in a safe place where they cannot cause a fire if they go up in flames while charging.

These cells have significant dangers associated with them. Care and safe handling is needed.

 

[tominator425]

Thanks guys! That helps alot!

This is the charger I have on order:

http://www.dealextreme.com/details.dx/sku.5837

Is this one sufficient, or should I really look into the IBC charger?

And is the Craftsman above a good one?

 

[Justin Case]

Hello Justin Case,

I may point out that if you have a voltmeter with ranges of 400 mv, 4 V, 40 V, and 1000 V, and are measuring 4.20 volts, you will be on the 40 V range.

I may also point out that there are also reference standards of different voltages available from the same source.

I should also point out that this particular standard will drift as battery voltage changes, but the difference between a battery with 9 volts and one with 6 volts is only 0.000015 volts. It seems that this would be stable enough to check a 3 1/2 digit voltmeter.

With this particular reference, temperature effects are greater than voltage effects, but if you can keep the temperature within 14 F of 65 F, you should be within the 0.01% spec.

The standard is aged 200 hours prior to delivery, and has an aging rate of about 10 ppm/1000 hours.

All in all, I think it is reasonably stable, but calibration services are available if greater precision is needed.

Tom

Yes, you are correct that 4.2V would be measured on the 40V scale for my hypothetical DMM. I stated as such in my original post:

"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."

I asked Doug Malone and he stated that he sells only a 5V reference. The TI chip he uses does come in different voltage levels. But that's a different matter.

You can buy both aged and non-aged (which are also non-trimmed) references from Malone.

Malone's web site has some aging data and he estimates roughly 200 uV change over his 200 hr aging process.

There is also the temp stability of his trim resistors, in addition to the temp stability of the TI chip itself. Malone states that the trim resistors add another 10 ppm.

I grant that even adding up all of these other drift sources, you probably are going to affect at most the 3rd decimal place. Since most 3 or 3 1/2 digit DMMs will display only 2 decimal places at that voltage magnitude, drift issues can be moot for that case. Nevertheless, IMO it is still wise to get the standard calibrated yearly. Otherwise, what's the point of having a voltage standard if you don't really know what its output is? Maybe you got a bum chip and it drifts more than you expect from the TI docs.