One charger fits all (homemade charger hack)

[Fallingwater]

With all the LiIon cells of various shapes I have kicking around (RCR123, 14500, 18500 and a gaggle of different prismatic ones) I obviously couldn't have a separate charger for every size.

I was faced with a choice: either get an expensive computerized charger meant for RC use or come up with some sort of homemade hack.

Since I got a RCR123 charger along with the RCR123 cells, I decided to go for the hack.

LiIon cells all need the same charging technique, and (in contrast to NiMH) there can be no dumb charger for LiIon.
So any LiIon charger can charge any LiIon cell; all the difference between, say, a charger for RCR123 and one for 18650 is in the size of the battery receptacles. Well, ok, a 18650 charger probably charges at a higher current, but if waiting isn't a problem any small charger will do.

So I opened the RCR123 charger, soldered two wires to the connections for the battery contacts, and this is what came out:

That's a 18500 cell, with wires held in position by hard drive magnets, being charged by the RCR123 charger.
It takes a while to charge larger cells, but it works just fine.

 

[koala]

Easy and cheap to make, I think they will go well with your charger with croc-clips. First two are modified C sized cell holders.

1) CR123A
2) 17670 / 18650 (white pads in the middle lifts the cell so it makes full contact)
3) 14500 / AA

 

[Fallingwater]

For CR123 I already have the charger's own slots.
For the others: yes, I could build the appropriate holders, but I find it's quicker to just stick the magnets in place (after making sure the outer casing is properly isolated on the front).
Thanks for the idea, though.

 

[knot]

The magnetic field has no effect on the battery or charging?

 

[Fallingwater]

No effect at all. Or at least none I can see.
It makes sense though; magnetic fields don't have anything to do with the chemical reactions inside a rechargeable cell (well, they do on an atomic level, but that's a whole different story), so unless you apply such a strong external field that it deforms the cell's casing it won't affect anything.

 

[mdocod]

just try not pinch the cell with those magnets, them rare earth magnets aren't always forgiving, lol...

I have a stack of those HD magnets out in a bin somewhere.. they WERE going to be part of a wind generator project, but as with most of my endeavors, ran out of money and lost inspiration after getting half-way through, lol.

 

[koala]

Wow they sounds really strong.

 

[Fallingwater]

Rare earth magnets are freakishly strong.
Anyone who's used to black ferrite magnets will be amazed at how hard these buggers hold.
Click one on a solid ferrous surface and you're going to have difficulty getting it off. Click two together and you're gonna need pliers.

They aren't powerful enough to deform the casings of the cells, but they sure can pinch fingers.

Less powerful magnets would be better as wire-holders; I have a few smaller drive magnets, but who knows where I put them... until I find them I'm gonna use the big ones in the picture and be careful.

 

[Fallingwater]

Updatin' time.

Maybe because it had already charged two half-full 18650s, maybe because the third one was almost completely flat and its hunger for power overtaxed it, maybe because of both things, yesterday the charger overheated.

The charging light was blinking much faster than usual, and after a while I started feeling the characteristic hot-electronics smell.

I disconnected everything and let the charger cool, then I tested it by filling up a RCR123. It works ok, so thankfully nothing was damaged.

I immediately did what every responsible person would do. What? No, I didn't go buy a proper universal charger! Honestly, the very idea!
I put a fan on it

I originally wanted to put a small fan right on top of it, but the result would have been a hodgepodge mess of metal braces and melt glue, too flimsy and easy to damage.
So I got a project box (actually a plastic can that originally contained pickled shrimps) and got to work.

I soon realized the small fan would have been insufficient for the size of the box, so I upgraded the project to a 80mm fan I got off the PSU of an old 386.

Step one was to take the charger out of its casing, hot-glue it to the bottom of the box and drill the exhaust and wire holes:



The holes were drilled very roughly using my dremel. I don't care about the aesthetics of the whole thing, so I didn't bother with the alternatives (cutting many thin vertical stripes, or well-proportioned square holes).

Step two was to cut the angles off the fan with the dremel (it wouldn't fit otherwise), insert it in the box and hot-glue it into place:



Step three was to cut a hole in the lid of the box to precisely fit the size of the fan and put it on top of it:



I use a regulated variable voltage wall-wart transformer to power the fan. This way I can keep it at full power while charging large cells for long, and run it at lower power when charging 123s or other small cells. It's rather noisy when running at full blast, so it's good that it only has to work hard when the charger actually needs all the airflow. At full power a nice breeze comes out of the exhaust holes on the bottom of the can.

Mind you, 80mm fans are meant to get heat off CPU heatsinks and the internals of 400-watt power supplies, so using one for a small charger is really overkill.
But all that airflow should make sure the charger doesn't overheat no matter what I throw at it.
Here it is at work:



It's been charging 18650s for a few hours now with no weird smells or abnormal flashing, so I consider the project a success.

 

[65535]

Thats a pretty good idea, but about lithium chargers being smart chargers they are actually rather stupid. All they do is terminate the charge at 4.2 volts. And require some 3.0 volts in order to start the charge.

 

[Fallingwater]

What I meant is that they don't just put charge in the cell at a slow enough pace that nothing's likely to burst without ever stopping, like many cheap NiCD/NiMH chargers do.

 

[barkingmad]

I am sure I read that 0.3C (or higher) is recommended for charging most lithium cells - that charger is probably rated far lower for larger cells like 18670's.

Also instead of the magnets I would probably recommend a battery clamp (someone is selling a good one in the marketplace).

 

[LEDependent]

Thats a pretty good idea, but about lithium chargers being smart chargers they are actually rather stupid. All they do is terminate the charge at 4.2 volts. And require some 3.0 volts in order to start the charge.

Eh, not quite. Li-ion chargers are equivalent to a 4.2V supply limited to a max current (usually 0.5C). A discharged cell will allow way more than 0.5C to flow through it at 4.2V, so the current limiter will cause the voltage to be reduced to the correct level to allow the max current to pass. When a drained cell is below 3.0V, a very small current has to be used to charge the cell ABOVE 3V before the usual (max) current can be used.

As the cell is charged, its resistance increases, so it will allow less and less current to pass through. Chargers will signal 'done' when the cell allows only 0.02 to 0.05C of current to pass.

EDIT:
This is a great li-ion charger that you can make yourself using the good ol' LM317. I swapped R1 for a 5W 0.5 ohm resistor for a 1.3A max charging current. Make sure the LM317 is well-heatsinked (heatsunk?)!

Use a 6V source and adjust R4 to get 4.2V at the (unconnected) output. This PDF was originally for two cells in series (hence 8.4V).

 

[Fallingwater]

I am sure I read that 0.3C (or higher) is recommended for charging most lithium cells - that charger is probably rated far lower for larger cells like 18670's.

That may well be, but it's charged a few old but functional 18650s just fine. That's good enough for me.

Also instead of the magnets I would probably recommend a battery clamp (someone is selling a good one in the marketplace).

Eh, I dunno. Why spend when I already have what I need?

 

[DM51]

Fw, probably an unnecessary question, but when this thing heated up enough to make you think about a cooling fan, are you satisfied it was the charger itself heating up, not the cells? Did the cells themselves get hot to touch at any stage?

 

[Fallingwater]

Fw, probably an unnecessary question, but when this thing heated up enough to make you think about a cooling fan, are you satisfied it was the charger itself heating up, not the cells?

Yes, and my finger agrees with me.
The little transformer on the circuit board was too hot to touch and the yellow tape covering it was starting to become brown. Other components were pretty hot too. The cell was at ambient temperature.

I confirm the success of the project, by the way.
The charger's been charging 18650s since yesterday, and when I unplugged it the aforementioned little transformer was barely warm.

 

[Bimmerboy]

Cool hack, FW! One point of concern though...

Now, I don't know much about chargers, but since your charger was meant for much lower capacity cells than an 18650, is there a possibility it could miss termination and explode the cells? Something about detecting deltaV, or along those lines.

Do you know what current that charger puts out?

 

[LEDependent]

Li-ion chargers don't need to detect termination. The cell itself will accept less and less current as it is filled. You can leave a li-ion cell charging at 4.2V for a month - it won't draw any current, 'cause it's already full. Chargers will indicate they're done when only a fraction of the original current is flowing, but that's just to let you know that it's full.

delta V (change in voltage) is used in NiMH and NiCD. The cells heat up slightly when they are full, causing their voltage to drop a bit. When the charger sees the voltage sag, it MUST stop feeding current to the cell or the cell will overheat.

 

[Bimmerboy]

Ahh... ok. Delta V not for Li-ion. Ya' learn something new every day. Or rather, ten new things every day, and forget nine of them... at least in my case. :sigh:

Related question: So therefore, the only danger of leaving a Li-ion on a charger is if said charger is not properly calibrated to no higher than 4.2V?

Likely could've phrased the question better, but you'll probably know what I mean.

[/hijack]

 

[mudman cj]

I have tried using a DSD charger to charge large Li-ion cells with both the stock power supply and also higher current power supplies.

I have found that higher current power supplies tend to cause premature charge termination, with cell voltages around 4.10 V. The power supply I used is an LG phone charger, 850mA @5.2V.

Using the stock power supply allows cells up to C size to charge to 4.20 V, albeit at a slow rate. Using the stock power supply with D size Li-ion cells causes overcharging. I caught one at 4.22 V and took it off immediately to discharge it. It may have kept on charging. Either way, overcharging is not a good thing for Li-ion batteries, so I had to find another way.

I am now looking into other options including a Graupner LiPO charger and an adjustable power supply. The Graupner would be nice if I can get it to work; so far I can only get it to charge at 200mA but it should be capable of 3A. I need to test it again using a better 12V power supply to feed the charger along with reduced resistance connections to the battery and a way to measure current without adding resistance. I should know soon and will report my findings.