18650 li-ion pack building newbie question

instead of buying the 7.2v battery pack with pcb like this?

http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=507

can one just solder two 18650 2600mah 3.7volt protected cell together to get the need ed 7.2 volt battery like one below?

http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=1600

and use the same charger like this

http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=478

set to 7.2volts.

Hi eddielee70. Welcome to CPF. :wave:

You could but you would have to be both highly skilled and knowledgeable at assembling that type of pack. Soldering lithium-ion rechargeables together is something I'd never do and it's not something I'd suggest you do either as you would end up with a unprotected pack. Cells are welded together with metal tabs instead of soldered. Soldering could easily damage the cells by introducing too much heat to some critical areas during the process.

Protected packs are the way to go. It may cost a few dollars more but it's money well spent.

If you wanted a pack that had the cells end to end instead of side by side then you could just order it made that way. Ask them for a B Format pack. They make them. You should also enquire about this slim protection pcb with the end to end pack if your application will allow a bit of room for it on the side of the cells.

Soldering to LiIon cells is possible, but it's highly advisable to do so with a soldering station.
It's much easier when your iron can get to 450 °C...

ok, soldering is not a problem. I solder them together and they tested out to the voltage I want. 2 protected cell at 8.4 volts 100% charged. 3 protected cell at 12.5 volts 100% charged. I soldered trail tech connectors to 2 cell and 3 18650 cels and test run my bike home build LED lights. the 2500mah printed protected cut off power at 2.7volt per cell and protect it from overcharge at 4.2volt. my question is that if I use a 7.2volt charger that charges at 8.4volt at 1amp. I don't have equipment to match the cell or balance the cells. will it still charge the 2 protected cells solder together? voltage is split b/w 2 cell. will the one amp current blow the cells. I'll give it a try in the garage in the metal can and give you a feedback, if I don't hear back. worse case is that I'll have blown cell in the metal open top can. good case is cell read zero bc circuit are blown. These one cell protected circuit do stand up to reverse or short to protect the cell and reset themselves, but I do get a hot burn hand when I accidentally short them out. They seem to charge up fine up to 4.2volts.

If the charger is explicitly meant for LiIon/LiPo cells it'll charge them just fine. One amp is actually less than half of the full 1C charge current for most 18650 cells, so you'll have no trouble with overcurrent.

You might have some trouble with balancing in the long run, but it's easy enough to balance a 2-cell pack if you have a wire for every terminal and if your charger can charge single cells as well.

All you have to do is take some extra time every few charging cycles to fully charge the single cells.

long story short. 2 cell 7.2v setup drain well. but neither 2 cell 18650 and 4 cell 7.2v 18650 worked. It need a balance charger. or PCB board on the cells. the charger won't charge 2 cell. so I desolder the 2 cell and they seem to still work. The 4 cell 18650 7.2 volt drain 2 of the cell to zero voltage and almost blew up from heat. and think they are protected cells. didn't even think about charging the 4 cell. :mecry:

I disagree with Fallingwater saying "possible," and you then assuming that soldering is not a problem. No one should consider that soldering Lithium Cobalt ion cells is adviseable.

I don't regard the new "safe" Lithium (Saphion) type chemistry cells in the same category regarding being soldered.

I've just soldered to an unprotected LiIon RCR123 with no problems whatsoever.
Two seconds (three, tops) with the iron at full power are enough to get the solder to stick. The cell becomes a bit hot, but MUCH less than if you heat it for 8-10 seconds with a cheapie iron, which is what you need to do if you don't have a soldering station.
I've held it in my hand with no discomfort after two or three seconds from the soldering, and I've touched the soldered terminal after maybe four or five and only found it mildly warm.

As for the "not a problem" part, I should probably rephrase. You need a soldering station, capable of getting to 450 degrees Celsius (that's the max temperature of mine and I solder to cells on that setting). You should not solder to LiIon cells if all you have is a cheapie iron.

DISCLAIMER DISCLAIMER DISCLAIMER DISCLAIMER DISCLAIMER!

I assume no responsibility if you follow my suggestions, solder to LiIon cells and manage to catch them on fire, explode them or open dimensional portals to Hell through which nameless abominations pour out to enslave humankind.

yup you need protection.
i solder 18650s all the time, but i wont make error, the action either goes FAST and occurs with little heat going internal or i stop. you should not in any way be heating the cell item itself.

here are some battery soldering tips, its not like welding is without heat.

Sand the tops and bottoms to make a very clean tinnable surface, you dont want to be stuffed waiting for "flux" to clean the metal have it clean already. use 320-1000 grit aluminum oxide type sandpaper , or a green scouring pad will do the same basic thing. (no not washing, just cleaning off the oxides)

dont use a 15W or a 25W soldering gun, its to slow, a 25 will work, but i find the 40W setting or a temperature adjustable soldering is faster, you want a one shot quick speed occurance, if your not getting it fast, then stop, wait till it cools. cleaning the metal makes it much faster.

tin the tops and bottoms of the cell (just a small point), it should be taking 2-3 SECONDS to get a quick connected flow on a clean metal surface, dont put the wire or tab on then, wait till everything cools back down again. if your taking longer than 2-3 seconds , practice somewhere else or fix something so it does.
clean the tab or wire your going to use to connect, just like the battery, pulling the copper wire through hand gripped sandpaper 3 times will wisk off all the oxides and make it very fast to accept solder, you can tin the wire or tabs too, as needed.
then go through placing the wire or tab on the solder, heating the wire into the connected blob, another 2-3 seconds Max.

if your going to interconnect cells like tabs, use some copper wire , get a quick slap on of the wire, then fold the cell and wala you have a stacked cell, without a welder or a tab, tabs arent usually copper anyways :-(. i would not attempt to solder a cells top to another cells bottom, just put a tab or wire there, and fold it over.

the safety issue is heating the internals of the cell or the can or the PCT on the top or the anode connect, speed is very important to insuring that the heat doesnt go (much) past the solder point location. solder point locations are best in the CENTER of the cells connection, which puts the heat furthest from reaching critical contents. if you damage the critical components at the top of the cell, you might ruin the cell, even if it seems to work at first.

with protected cells you run a good risk of desoldering things off the protection, if i was going to be making the pack, i would probably want pack protection soldered on after making the pack, use some nice copper wire to add in a 2 cell protection, also i could use LG raw cells, and high power protection.

Disclaimer, See fallingwater disclaimer above, beware firey demons from hell, they attack SLOW solderers

VidPro said:

Sand the tops and bottoms to make a very clean tinnable surface, you dont want to be stuffed waiting for "flux" to clean the metal have it clean already. use 320-1000 grit aluminum oxide type sandpaper , or a green scouring pad will do the same basic thing. (no not washing, just cleaning off the oxides)

Click to expand...

I've found a Dremel cutting wheel works fine for sanding cell terminals, and has the advantage of being able to sand the very center. Just apply it slightly enough that it doesn't cut but abrades the metal.

you can tin the wire or tabs too, as needed.

Click to expand...

I think tinning the wire before you solder it to the cell is essential. It makes it more willing to stick to the solder blob and keeps it in one piece, so you don't get wire "hairs" separating from the main bundle.

i would not attempt to solder a cells top to another cells bottom, just put a tab or wire there, and fold it over.

Click to expand...

I saw a tutorial on youtube some time ago of a guy using a hammerhead soldering iron to heat and join NiMH cells end-to-end in a plastic tube, to form a pack for a RC helicopter (I think).
I don't think that's a good idea. Solder joints are too weak to hold together big objects such as batteries. Any side impact would probably break a number of connections in such a pack.

Everyone please keep in mind that the last two posters should be considered as very experienced (? expert ?) members, and I still say the casual member should not solder Lithium cells. I repeat that I am unaware of ANY manufacturer that solders lithium cobalt secondary, or lithium primary cells. They all use welded nickel strip battery tabs, which says a lot about what should be done.

The only thing I would add having soldered well over 100 NiMH cells, mostly using the 60W iron with hammerhead tip and this magnetic jig...and following the aforementioned video here....is to use flux on surfaces after cleaning. I have never had one fail or pull apart, as long as I followed the guidelines outlined in the video.

Ok i must be crazy i built 3x18650 in series and all connections are soldered with 60w solder no more than 3 seconds contact. Preperations was dremel, flux, tinning on both contacts. Seems to do the jobs. Oh i also make 3S balance connector for charging safely. OH to those who asking the black thingy it is a pvc tape i know it is ugly but i'll get shrink wrap latter

here are the pictures:






Thanks

LuxLuthor said:

I disagree with Fallingwater saying "possible," and you then assuming that soldering is not a problem. No one should consider that soldering Lithium Cobalt ion cells is adviseable.

I don't regard the new "safe" Lithium (Saphion) type chemistry cells in the same category regarding being soldered.

Click to expand...

I'm usually not as big a safety advocate that Lux is, but I'm with him on this one. Experience is the big one here. It's possible to jugle chainsaws too, that doesn't mean it's for the novice or that it's entirely safe.

Regarding two in series... I think the Sony NP-FM50 is two 18650s in series, with plenty of circuitry including a battery capacity monitor, but I do not think it has a balancing tap.

Is it typical that a 2 cell pack of that size would stay in balance enough to not cause problems?

radellaf said:

Regarding two in series... I think the Sony NP-FM50 is two 18650s in series, with plenty of circuitry including a battery capacity monitor, but I do not think it has a balancing tap.

Is it typical that a 2 cell pack of that size would stay in balance enough to not cause problems?

Click to expand...

2 in series is generally OK. I wouldn't exceed 2 in series without balancing. I'd have to look at the specs, but it's possible to have a balancer built into the pack. The charger isn't the only way to balance a pack.

The biggest problem with putting a PCB in Maglite mods is the space. It is easy to take cells out and balance charge them (at least periodically). Poor man's way of balance charging them is just to fully charge (in a proper Lithium charger) each cell. Keeping an eye on the DMM voltage readings is always wise when dealing with Lithium secondary cells.

JimmyM said:

2 in series is generally OK. I wouldn't exceed 2 in series without balancing. I'd have to look at the specs, but it's possible to have a balancer built into the pack. The charger isn't the only way to balance a pack.

Click to expand...

I have seen some people making balanced chagers, but they are not so small, not even as small as 2x18650 ...
Do You have some ideas, schematics, distributors how to make or buy balancing - protection circuit for 2 serial Li-On cells?

P.S. I am looking to make 7,2v pack from 8x 18650 (4p 2s) ...

MatajumotorS said:

I have seen some people making balanced chagers, but they are not so small, not even as small as 2x18650 ...
Do You have some ideas, schematics, distributors how to make or buy balancing - protection circuit for 2 serial Li-On cells?

P.S. I am looking to make 7,2v pack from 8x 18650 (4p 2s) ...

Click to expand...

Oh, no. Not a built in balancer AND charger. But the Astro Flight "Blinky balancer" is pretty small. It can be left connected to the pack while charging or balanced afterwards.
http://www.astroflight.com/store/st...m=products:af-106&sid=0001AHaH4RxVFZoUN84A2K6

It does up to 6 cells for $25USD. I'd love to get my hands on a schematic. Perhaps I'll take a look around the net.

Edit: I just found a schematic for a cell balancer. It can "balance" a single cell. It switches on a bypass resistor(s) when voltage exceeds 4.15V (adjustable). You would just combine more of these for multiple cells. It's SUPER SIMPLE.