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18650 lithium battery pack charging

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haxor5354

Sat Jul 13 2013, 04:56PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

I currently working 100W LED flash light project right now, its almost done.
what I basically have is a DC-DC step up module to drive my LED array in CC mode

Im currently using five 18650 lithium-ion cells in series to power up the whole setup, which works quite nicely.
Is it safe to charge five lithium-ion cells connected in series? or am i better off charging each cell individually?

I usually charge my 18650 cells by using my CC CV power supply, first I set the current limit to 1A and set the output voltage to 4.2v then charge the cell until its pulling less than 100mA from my power supply. Can I do the same to charge 5 cells in series if I change the output voltage to 21v?

Patrick

Sat Jul 13 2013, 05:56PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

charging in parallel and discharging in series is always best, fires and explosions are a possibilty with batteries. (though the explosions arent as bad as RDX, torpex or ammonium nitrate, there still scary and dangerous. just believe me)

the use of a balance connector is best IMO, even when it isnt absolutely required. (on my batteries its a must), on your system id recommend it. then your charging/monitoring cells in parallel but discharging in series. for a prototype system, as many of us 4hv'ers build it shouldnt be an overwhelming burden, though it could be easier to charge them one at a time by means of diassasembly. And watch for smells, bulging, heat and unexplained sudden differences from previous performance.

i would strongly advise making sure the cells are balanced in terms of SofC one way or another, simple series charging definately doesnt do that, even with CC and or CV.

Boeing is seeing the consequences of a poorly implemented battery system as we speak. (yet again!)

ConKbot of Doom

Sat Jul 13 2013, 09:48PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

Either parallel charge, or have a tap-per-cell hooked up to a hobby charger.

If you have nice matched cells, you might be able to get away with just a protection circuit that monitors each cell voltage and no balancing. My packs built with legit sanyo cells pulled from a laptop pack all stay very well balanced, always at the same voltage, to 0.01v after discharge could get away without balancing (like laptops do) However the previous pack I build using deal extreme cells always took an a lot of balancing to keep them even. (Ebay cells dont count as nice cells either as there is no guarantee of cell age, or matching)

Patrick

Sun Jul 14 2013, 05:31AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ConKbot of Doom wrote ...

If you have nice matched cells, you might be able to get away with just a protection circuit that monitors each cell voltage and no balancing. My packs built with legit sanyo cells pulled from a laptop pack all stay very well balanced, always at the same voltage, to 0.01v after discharge could get away without balancing (like laptops do)

I thought about making a similar explanation, but the whole "matched cell" thing still scares me when applied to Lithium chemistry.

Steve Conner

Sun Jul 14 2013, 10:11AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Most Li-ion battery packs for consumer electronics don't have balancing. The protection circuit just monitors the voltages of the series cells and disables the pack if they get out of balance. I save the protection circuit boards from dead packs and reuse them with new cells.

The batteries used with cordless tools and RC models are an exception. The cells are charged in series, but with active balancing through the tap points.

Patrick

Sun Jul 14 2013, 02:54PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Steve Conner wrote ...

Most Li-ion battery packs for consumer electronics don't have balancing. The protection circuit just monitors the voltages of the series cells and disables the pack if they get out of balance. I save the protection circuit boards from dead packs and reuse them with new cells.

The batteries used with cordless tools and RC models are an exception. The cells are charged in series, but with active balancing through the tap points.

my instruments indicate that my (RC drone charger) the series charge is rapid up to 70-80% then the taps are bringing the SOC to 100%.

brandon3055

Sun Jul 21 2013, 10:58PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Quick question while on the topic of li-ion i'm currently building a charger for 18650 cells using Arduino and I have a fairly good system for charging single cells but multi-cell charging is next on my list. Dos anyone have any idea how I might achieve active balancing?

Erlend^SE

Mon Jul 22 2013, 05:31PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Destroyer of mosfets:
The usual way is to have a resistor that can be connected over the cell for each cell. (via a FET)
You can do coils and active energy moving, but do you plan to have midpoint outputs that discharge parts of the pack? If not.. it's probably no big loss by just resistors.

Still, it doesn't cover how to protect against faults.

How do you plan to do the safety? if you don't have a fail-safe way, go for a pre-made SPI/other controlled solution with protection built in.

brandon3055

Mon Jul 22 2013, 10:43PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

I was thinking something along those lines thanks. I have a lot of safety built in to the charger program to prevent any problems when charging including temp monitoring but I don't really have any protection for discharging although I am considering low voltage cut off to prevent draining them to low.

I will mostly be using it to charge my DIY high power tourch (cree MC-E led) which runs on 4 cells but once I have a good multi purpose charger I will be using li-ion in a lot more of my projects.

Edit: so just to make shore I understand you correctly your suggesting a resistor across each cell that will apply a load if that cells voltage rises above the rest?
I was thinking a resistor that would apply extra current two any calls that fall behind but that would over charge all of the cells before it.

Patrick

Tue Jul 23 2013, 08:26AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Destroyer of mosfets wrote ...

I was thinking a resistor that would apply extra current two any calls that fall behind but that would over charge all of the cells before it.

typically there are isolated sources for each cell and level shifted monitoring. but thats hard to engineer in a garage for us individuals....

i was a fool for not spending the $60 more for the version with the embedded SMPS.... now i have to lug around the linear PS.

a

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