Soldering 18650s is Entirely Possible

[CraigHB]

So, I'm working on a new mod I dubbed the Powerblok 6000 since it will use two 3000mAh cells in parallel.  The cells are the LG 18650 D1 cells that I talked about in another post here, a really cool cell with a 4.35V terminal charging voltage and relatively low internal impedance.

Now, I want to use a pack and I want to solder it in for a few reasons.  For one, it eliminates the need for battery contacts which reduces the length of the box some amount.  Since it's one of my usual high power boxes, I need at least a 12 Amp output from the battery.  I need two of these LG cells in parallel to achieve that.  I need to minimize power supply impedance and soldering in the cells does that.

So, I've never soldered a round cell before.  These are a new gen chemistry that has similar volatility to a LiCo cell.  Of course, getting one of these cells too hot can cause thermal runaway which is just the technical term for saying it could catch on fire.  So I was pretty leary about attempting this.

First I buffed the area to be soldered with ScotchBrite then applied flux liberally.  I made a work space on the garage floor, set an extinguisher close by, put on my goggles and welding gloves, cranked up the soldering iron to max temp (850F) and and went at it.    Much to my surprise the solder instantly flowed onto the metal with only a couple seconds contact.  After completion, I touched the cells with my bare fingers and they were still cold.

So, it was a piece of cake, much easier that I thought it was going to be.  Soldering wires onto battery packs made with round cells is no big deal.  In fact, it's rather easy.

Here's a photo of the ground strap.  It needs to be at the top due to the circuit board I'm using, same board I use for the LiPo cells.

[fsors]

wow another 1st Craig you are a great modder and I like your safety precognitions!

[CraigHB]

Pretty happy with this, box is gonna happen.  Battery is always the first thing to sort out. 

Here's the finished product, ready for a PCB to sit on top.

Oh, testing now, really impressive internal impedance, only 33 mOhms for the pack!!!

[CraigHB]

Just finished a drain down test on this pack.  At 6 Amps, I got an hour run time.  That's 6 amp-hours.  That's really good because ratings are done at 1/5C typcially and this pack made the rating of 3000mAh per cell at 1C. 

[Breaktru]

Thanks for the info on these batteries. Good to know 

[iusedtoanalog]

Holy Mackerel Batman, That will make one heck of a cell for a booster mod. Cant wait to see what kind of runtime you end up with in your new Powerblock. Side note, and Question, Is that 14ga silver solid core wire?....

[CraigHB]

The leads are nothing that fancy, just solid 18 AWG nickel plated copper  Silver is somewhat more conductive than copper, but it's not a huge amount.  Though if I wanted to reduce power supply impedance absolutely, silver would be a good idea.  Or, I can just use a thicker copper wire.  The leads I'm using introduce less than an mOhm so it's nothing to get terribly concerned over.

These cells run at a higher voltage than a LiCo cell so they store more energy.  They don't need to put out as much current for a given power output.  That 6Ah will go about 10% further.  The discharge curve is tall for these cells (image below).  Though the bench data in the chart shows 2.83Ah at 1C, I got 3Ah at 1C.  Don't know why that is.  Maybe the tester's cells were old or he had some inaccuracy in his test rig.  He measured 80 mOhms for a cell, but I got 66 mOhms for a cell (that's incredibly low for a round cell, IMR territory).  Whatever the case, I should get amazing run times with this pack.

I can actually discharge a battery pretty fast with my chain vaping at the power levels I use (regularly as much as 20W).  I'll probably get a full day of run time out of it, but for an average vaper at lower power levels, it would be several days run time.

[utak3r]

The only problem now is... to properly and safely charge it. Those cells should be balanced during charging...

[CraigHB]

When cells are hard wired in parallel, they become like one cell so balancing is not an issue.

These cells are volatile like a LiCo cell, but they have a PTC.  I actually tested it with a hard short and it works, just got a little spark then the PTC kicked in.  Wanted to be sure the PTC actually works before I made an e-cig with these.  If I do that to a LiPo, it makes a loud crack and puts a melted spot in the tabs.  I've not done that on purpose, but I accidentally dropped some desoldering braid on one and that's what happened, pretty much vaporized the braid.  These cells are actually safer than the LiPos I've been using.

My electronics have the normal Li-Ion safety features built-in, overload, short circuit, and over-discharge protection.  They also monitor booster temperature and shut down input power with a warning if it gets too hot.

My charger controller is a really good one, Microchip MCP73833, has a full set of safety features including temperature monitoring and a timer.

I also have a TVS on the board to protect the charger controller from any USB power supply spikes or ESD from handling the connector.

I don't think it's possible to build a safer mod than this one. It's as safe as I can possibly make it at least.  Nothing with a Li-Ion cell is ever absolutely 100% safe.

[utak3r]

When cells are hard wired in parallel, they become like one cell so balancing is not an issue.

Hm, I don't think it is so.
Observe they are two phisically separate cells (soldering them doesn't change anything here really). Now, if one of them will get more "tired" than the other one, charging gets bad: either one of them will be under-charged, or the other one overcharged. That's why there're special charging curcuits for charging such packets.

[CraigHB]

If you look up the definition of "battery" in Webster's dictionary, you get this;

b : a group of two or more cells connected together to furnish electric current; also : a single cell that furnishes electric current <a flashlight battery>

Several battery types consist of plates run in parallel to make up a cell then a series of cells that make up a battery.  The plates in each cell are connected electrically in parallel.  Lead-Acid batteries and the large industrial Lithium batteries are constructed in this manner.

Wiring these two cells together as I have is basically the same idea.  There are no electrical issues that arise from this other than the cells must match in voltage level prior to connecting them otherwise large currents could flow as the cells equalize in charge.

What happens when parallel cells become mismatched in wear is the cell with less wear will carry a bigger portion of the load, but voltage between them always remains the same.  The internal impedance goes up in the one with more wear so less current flows out of it for a given load.  As such, the cell with less wear will acquire more wear since it's carrying more current and the cells will balance themselves out.

This is not the case for series cells.  Balancing issues can be bigger problem since unlike parallel cells, it's possible for one cell to become dishcarged before the other.  Care must be taken for batteries comprised of series cells that they are not discharged low enough to damage one of the cells. 

Since Li-Ion batteries are sensitive to terminal charging voltage, it's also difficult to avoid overcharging one of the cells when charging series packs.  That's why it's best to use a "balancing charger" with those.

[Breaktru]

I use two Li-Po's in parallel and charge them in parallel. If the batteries are the same type and were bought at the same time (same age and same state of charge) and used equally there should be no problem. If one does fail the PTC would protect against a catastrophe.
Aren't some battery packs batteries in parallel?

[CraigHB]

If you're thinking of the kind that use round cells, they're generally series packs.  The Darwin uses two individual LiPo cells in parallel so at least they don't think there's any problem with it. 

Laptop computer battery packs are generally a 2x4 configuration of 18650 cells.  That's where two cells are in parallel for each set of cells in series.

[Breaktru]

If you're thinking of the kind that use round cells, they're generally series packs.  The Darwin uses two individual LiPo cells in parallel so at least they don't think there's any problem with it. 

Laptop computer battery packs are generally a 2x4 configuration of 18650 cells.  That's where two cells are in parallel for each set of cells in series.

I just looked for 3.7v Li-Po packs and couldn't find any. But found plenty of 3.7v Li-ion packs.
My Series/Parallel configuration is still holding up and as yet I haven't blown anything up.
USB Charging

[utak3r]

I just looked for 3.7v Li-Po packs and couldn't find any.

Maybe try looking in RC shops, they're using Li-Po often.