Charging Li Batteries w/o A Real Charger

We just got setup for night time bike riding with a Romisen RC-N3 II R4, a Fenix LD20+, some Eneloops, and a C9000. It all works great except that my night vision turns out to be worse than we knew. So I'm now trying to find a brighter light and I'm finding that anything brighter runs off of a 18650. Grrrrrr. The cost of something like an MG P Rocket, two 18650 cells, and a real 18650 charge kinda breaks the bank right now. However, the P Rocket and a single 18650 is sorta doable. But I need to charge that cell...

I have a 1A 0-120VDC CC/CV bench power supply. If I set the CV limit to the max charging voltage (4.2VDC, yes?), the CC limit to 0.5C, and then shut it off when the current drops to 0.05C, that's a safe and reasonable charger right there, isn't it?

I did that while I was waiting for my "real" charger to arrive. Worked fine. Ya best not leave it unattended so you can watch for heat buildup and bad behavior from the power supply. Probably a good Idea to use a reasonably accurate meter to set the voltage.

Along with olephart's excellent suggestions, it'll work. I've done it quite a few times, particularly with LiFe cells, before I had a proper charger. And yes, I would use use an accurate meter along side, rather than trusting the PS's readout.

It's sorta a PITA. but it'll work.

Dave

williaty said:

I have a 1A 0-120VDC CC/CV bench power supply. If I set the CV limit to the max charging voltage (4.2VDC, yes?), the CC limit to 0.5C, and then shut it off when the current drops to 0.05C, that's a safe and reasonable charger right there, isn't it?

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I charge lithium-ion cells with CC/CV power supplies all the time. I recommend plugging it into a timer so it will shut off after a few hours if you forget about it. Also, please be sure to double check the voltage output of the power supply with a good voltmeter (or two).

Also, remember that you can charge multiple lithium-ion cells simultaneously in parallel, so you could go for the light that takes multiple 18650s. You just need to buy or make a holder.

Cheers,
BG

How do you charge multiple cells in parallel? If the cells aren't perfectly identical, don't you end up with problems?

williaty said:

How do you charge multiple cells in parallel? If the cells aren't perfectly identical, don't you end up with problems?

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With Li-Ion cells this is not a problem, provided the cells are EDIT: within 0.5 Volts of each other, of course the closer the better, as this reduces the strain on the higher voltage potential cells, as they balance. It is also possible to charge Li-Ion cells of different mAh rating in parallel, as long as the charge rate is compatible with the lowest mAh cell. I've never done this, as it just seems wrong, but it is possible.

With nickel chemistry cells, due to their nature, parallel charging is not possible. These cells are incapable of balancing when parallel charged. This is why nickel based cell packs are balanced in series.

Dave

Its not ideal, I have done it before for a few minutes with a small 3.6V / 500mah power supply... just to "poke" the protection circuit and get it to reset.

It will work, but you can not be lazy about it. The minute you let your guard down you increase your odds of a flame/vent incident.

I think your biggest concern will be with tired / dead / expired / defective cells. I have had a hand full of these that simply would not voltage-peak higher than ~3.95V. These are extremely dangerous, as the charger will continually try and pump current into them, never reaching the 4.2 cutoff. These cells get blazing HOT if left unattended in a runaway charge scenario (don't ask me how I know). IMHO if you are buying your 18650 cells from china, ebay, DX (etc) there is a higher chance of this occurrence than buying 18650 cells from respected retailers like AW, BJ or LH.

So you have to be vigilant with your safety practices.

45/70 said:

With Li-Ion cells this is not a problem, provided the cells are within +/- 0.5 Volts of each other, of course the closer the better, as this reduces the strain on the higher voltage potential cells, as they balance. It is also possible to charge Li-Ion cells of different mAh rating in parallel, as long as the charge rate is compatible with the lowest mAh cell. I've never done this, as it just seems wrong, but it is possible.

Click to expand...

I am pretty much in full agreement. I would argue that there is absolutely nothing wrong about charging lithium-ion cells (assuming they have the same chemistry) of different sizes in parallel. The only concern is if you connect two cells in parallel that are at very different states-of-charge. For example, connecting two similar cells in parallel that are at 100% and 0% SOC will result in a very large initial "surge" current. In fact, if you connect a 100% SOC small cell to a 0% SOC large cell, you will effectively short circuit the small cell. Therefore, I think that the +/- 0.5V range is actually a bit too large, as one could interpret this as a full 1V spread, which would essentially be 0-100% SOC for a lithium-ion cell. I would say that if the cells are within 0.1V, they can be connected in parallel without concern. Once the cells are within that voltage range, they should be able to be charged/discharged in parallel without concern.

45/70 said:

With nickel chemistry cells, due to their nature, parallel charging is not possible. These cells are incapable of balancing when parallel charged. This is why nickel based cell packs are balanced in series.

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Quite true for NiCd and NiMH cells. The PowerGenix NiZn AA cells can/should actually be charged in parallel. But the PowerGenix cells are probably a topic best for a separate thread.

Cheers,
BG

Battery Guy said:

Therefore, I think that the +/- 0.5V range is actually a bit too large, as one could interpret this as a full 1V spread

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You are absolutely correct, BG. What I meant to say is "within 0.5 Volts of each other" and have edited my previous post.

I remember SilverFox and some others discussing this in another thread, but have no idea where it is now. The conclusion was that the current surge between cells of 0.5 Volts or less difference in voltage, was acceptable.

Thanks for bringing this to my attention, BG. :thumbsup:

Dave