Charging up a 18650 lithium battery

You might try posing your question over at http://candlepowerforums.com/

–MV

I'm not registered at the CPF.

–B.G.–

battery university is usually pretty good:

http://devices.dslshop.co.uk/product_details.aspx?idProduct=29

Whilst that article is really about automated charging, the basic principles should stand. In particular, the charge rate suggested for your cell is 0.8C

'C' is an interesting concept, based on the current to discharge the battery in one hour. So the C rate for a 2Ahr battery would be 2A. You have to charge for longer than an hour when charging at the C rate, due to the inefficiency of the electrochemical charging process.

Charging at lower than C rate is recommended, as it involves lower charge powers, so less chance of overheating if you overcharge. If you charge at 4.2V for a nominal battery voltage of 3.7V, then for every amp of charge current, you're dumping about 0.5W into the battery, which will be dissipated as heat. Depending on the size of the battery, this may or may not cause dangerous overheating…

But if you use a regulated power supply with voltage and current limits, you can make a constant current/constant voltage charger, which is essentially what a LiIon battery charger is, barring the end point detection. You'll have to do end point detection manually, based on time, or estimating the initial charge state form open circuit voltage (tricky, sine LiIon batteries have a prett flat discharge curve), or stop when it gets hot.

[goes off to look up 18650 battery] 2600mAh capacity gives C as 2.6A, so your suggested 100mA would seem very conservative compared to the 2A 0.8C suggested charge rate.

Since this is not a real charger (it's only a bunch of power sources and wires and meters and alligator clips), I won't push the limit. I don't have any automatic monitoring.

I may just leave it at 100mA or so for a time until the voltage comes up. That will get me by until the real charger arrives.

Meanwhile, these damned 18650 batteries seem to store a lot of energy!

–B.G.–

Meanwhile, these damned 18650 batteries seem to store a lot of energy!

They sure do. One way to see that is to take a look at the Zebralights that use them — look at the light output and run time figures.

–MV

"One way to see that is to take a look at the Zebralights that use them — look at the light output and run time figures."

Tell me about it.

I just got a new H600, dropped a fully-charged 18650 battery into it, and switched it on. As soon as my vision returns to normal, I can tell you how bright it was.

–B.G.–

Understood.

I have an H502W (single AA battery, for those who do not know). I just went out to the kitchen, full dark, and turned it on — it lit up the whole kitchen with a nice even light. It's hard to imagine what your light must be like :)

EDIT: I just noticed a review for the H600 saying that the brightest level is brighter than his car headlights!

–MV

My first Zebralight was getting me in the neighborhood of 100 lumens, and I thought that was plenty bright. This newest one is in the neighborhood of 1000 lumens, and that is why there is this big temporary spot in my vision.

–B.G.–

Hi Bob

The 'standard' charge rate for a battery with a capacity of C ampere-hours is C amps. Yours are 3.4 A-hr, so the standard recharge rate is 3.4 A. But that is a bit too simplistic.

Most significant rechargable lithium batteries contain a safety circuit. This circuit does two things: it protects the battery in the case of an external short circuit, and it protects the battery from being over-charged. It does this very simply, by having a MOSFET transistor inside the case which can open circuit between the +ve terminal of the real cell inside and the external + terminal. (Or ws it the -ve terminals? I forget.)

This means that when you are charging the battery with an external supply, the battery itself will turn off when it is full. This, crudely, is how all the rechargers work. However, in more detail: they stick about 4.2 V across the battery but limit the current input. If you are buying a charger *designed for these batteries*, it may well be able to pour 3.4 A into the battery. When the current suddenly cuts off, the charger knows the battery is full.

Real chargers actually have a more complex algorithm that I have explained here, to do with profiling the V/I charging curve. Shall we skip that for now?

However, when you pour that much current into ANY battery, it can get hot. The extra terminals you find on camera and phone batteries give access to an internal temperature sensor (a thermistor), which the proper chargers monitor. (The cheap chargers may not.) If the temperature inside the battery goes too high, the charger shuts down for a while. Provided the charger monitors the temperature, it can recharge at the full 1C rate.

It is possible to recharge at a lower rate, in which case a different and simpler charging profile is possible. 100 mA might be a bit low for your batteries in fact: a full charge could take 34 hours! Anyhow, if you want to use a slow charge, you generally don't need to worry about the voltage limit so much. Yes, this will fully charge your batteries just as well – but who wants to wait 34 hr?.

I sometimes use a 12 v solar cell with a home-brew controller to recharge my 3.7 V and 7.4 V lithiums. Power input is then limited by solar intensity. When the battery is full and shuts off the solar cell voltage rises to >12 V and a little white LED lights up.

Cheers

That 34 hours is a little slow, but I won't have to worry about the 18650 battery that way. I simply had a 100ma source laying around, so it works. Fast-charging a lithium can result in fewer total charge cycles usable for the battery. All that will work fine until the real charger is delivered.

–B.G.–

As long as your cells are protected then the cells cannot be over charged. For most cells overcharge cut off is around no more than 4.5 volts. but of course charging over the 4.25 max recommended voltage is going to reduce the cell life. But that only matters if your Vin is greater than 4.5 volts, any less and charging will effectively stop when the cell reaches the same voltage as Vin (result of the cells internal resistance)

Regarding charge rate. Easiest option is to take a look at the technical specs of the charger you have on order. You should be able to see how many amps out the charger produces. Simply set your DIY charger to deliver no more than the same current as the dedicated charger.

BG-

How hot does your ZL get when on High for a few minutes? Just warm or too hot to hold? I've got a few flashlights that run the XM-L2 in the 900 lumen range and they get fairly warm even with all their heat sinks. I'm wondering how ZL gets around this(or if they do). Thinking they may step back the power after a few mins…

PS – All my 18650 chargers are in the 1A-1.5A output range.

Ryan

Sort of scary to rig up a charger for an 18650 battery. I personally use the Nitecore Intellicharger to charge mine up (I also own a Lucfest Charger).

I suggest finding a local "Vape" shop…you can get the Nitcore 4Bay Intellicharger for $20 now.

nm

"Sort of scary to rig up a charger"

Not at all. I've been smoke-testing commercial electronics for decades.

"Easiest option is to take a look at the technical specs of the charger you have on order."

Nothing!

"How hot?"

I haven't used it for very long yet. I expect it to get warm, but not uncomfortably warm to the touch. I mean, it is supposed to be worn on the forehead.

If it gets too warm, I can always step down the intensity.

–B.G.–

> Fast-charging a lithium can result in fewer total charge cycles usable for the battery.
That may be true for NiCds and NiMH, but afaik it is not true for good lithiums. To be sure, the net is full of all sorts of rubbish, but I have yet to find a genuine technical reference which states that.

Cheers

"I've got a few flashlights that run the XM-L2 in the 900 lumen range and they get fairly warm even with all their heat sinks. I'm wondering how ZL gets around this(or if they do). "
Zebralights get hot, too, hence the stepdown on the high of the 600 series.
They're supposed to get hot, not in spite of the heatsinking, but as a result of proper heatsinking transmitting the heat to the body.
If they *didn't* get hot, now that'd be a problem.

"How hot?"

This evening, I used the new LED light for about 90 minutes. The air temperature was 62*F, and I could never feel any heat around the LED head. There seems to be a decent heat sink there.

–B.G.–