Author Topic: Balancing a lithium string  (Read 3271 times)

0 Members and 1 Guest are viewing this topic.

dnix71

  • SuperHero Member
  • ******
  • Posts: 2513
Balancing a lithium string
« on: December 07, 2014, 09:32:40 PM »
Seems nearly impossible without a fancy custom built circuit. I have a Black and Decker GrassHog weed whacker that uses an 18v nominal slide in pack. The original packs were high capacity NiCads and the charger was transformer based [rated 21.75 @ 0.85 amps].
 
15 NiCads at 1.45v = 21.75 so in theory I could leave the pack plugged in for a long time without damage. The minimum charge time was supposed to be 8 or 9 hours.

Both packs failed at about 25 cycles. In the meantime I had bought 4 more chargers and 3 more packs [standard capacity, not hi-cap]. All of those standard packs still work.

The price of a pair of hi-cap packs is 3/4's the price of the whole original machine + 2 packs, so I looked into building nicad or NiMH packs from single cells and found it was cheaper to just buy a new pack than to rebuild. Last week I saw protected 18650 lithium cells for cheap on eBay and bought 20. These can't be tab welded, as the bottom of the cell is a plated disk soldered to the BPC at the base.

I soldered a pack of 6 and charged the pack with one of the nicad chargers I had. All the remaining chargers are switching supply types. Being B&D there is not much quality control. The Voc of the transformer supply was 21.7. The switching supplies were 24.6, 28.0, 28.8 and 29.6. Only the 24.6v supply will charge a string of six 3.7v lithium cells without going over, but one cell stays at 3.85 while the others go to about 4.05.

The other chargers pushed at least one cell waaaaaay over 4.2v and would have burned up the pack if left alone long enough. The battery protector circuits don't seem to work very well at all.

I went to eBay and bought plastic holders and twin-pack wall chargers for the 18650's and intend to charge them loose and insert and remove loose cells when I need to use the weed whacker, even though it isn't exactly convenient to do so.

The lithium cells do not like the high rate of discharge the weed whacker demands, but at 3 times the voltage of a nicad they are still cheaper even if they don't cycle any more than the nicads did.

The only way to leave lithium cells in a pack and get some balance to the string seems to be a circuit that charges each one separately, but I don't know how to do that in a simple manner.

Bruce S

  • Administrator
  • Super Hero Member Plus
  • *****
  • Posts: 5370
  • Country: us
  • USA
Re: Balancing a lithium string
« Reply #1 on: December 08, 2014, 10:36:24 AM »
Not sure I can help at all :(. I'm still working with them individually too.
I'm thinking now would be a good time to bribe GHURD into seeing if his now famous diversion controller can be modified down to a 3.2Vdc level , I say 3.8Vdc so that even the levels need for the LiPo units can be utilized.

Best of luck
A kind word often goes unsaid BUT never goes unheard

mab

  • Sr. Member
  • ****
  • Posts: 428
  • Country: wales
Re: Balancing a lithium string
« Reply #2 on: December 08, 2014, 03:23:40 PM »
The only simple way I've considered for fully charging a 'pack' is to buy lots of 4.1v wallwart type supplies and daisy chain their outputs to match the pack. But I've not actually tried it.

I have put 7 18650s (from an old laptop battery) in series and then paralleled them with my 24v lead acid AGM battery just because they were sitting there unused; the only balancing I included was a daisy chain of 2v zenners (to give 4v across each cell of course). But then this 18650 battery is constrained by the AGM charge regulators and never gets fully charged or discharged, and in practice, these 2v zenners don't work as cleanly as higher voltage zenners seem to (so they still leak a little current at <2v and don't 'open up' a huge amount above 2v). Nevertheless, after a year+ in parallel with the AGM and with no other maintenance, the cells are still within 20mV of each other.

Simen

  • Sr. Member
  • ****
  • Posts: 479
  • Country: no
  • Grimstad, Norway
Re: Balancing a lithium string
« Reply #3 on: December 08, 2014, 05:22:43 PM »
You really need something like this:
http://www.batteryspace.com/PCM-with-Balancing-Function-for-22.2V-pack.aspx

Don't know if 20A would be enough for your weed whacker, though... :)
I will accept the rules that you feel necessary to your freedom. I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do. - (R. A. Heinlein)

dnix71

  • SuperHero Member
  • ******
  • Posts: 2513
Re: Balancing a lithium string
« Reply #4 on: December 08, 2014, 09:02:51 PM »
Simen It might work, but at $40 plus shipping I could buy 2 cheap packs. The battery holders came in today and are a lot neater than soldering. I have 5 double 18650 chargers on order. It seems that not charging a lithium fully does it no harm it just lessens the runtime. The motor bushing on the weed whacker are getting a bit worn. A B&D rebuilt with one battery and charger is $28 shipped.

Simen

  • Sr. Member
  • ****
  • Posts: 479
  • Country: no
  • Grimstad, Norway
Re: Balancing a lithium string
« Reply #5 on: December 09, 2014, 12:57:16 AM »
A charger much used by rc-hobbyist would also do nicely for you; something like this:

http://hobbyking.com/hobbyking/store/__18066__Turnigy_Accucel_6_50W_6A_Balancer_Charger_w_Accessories_US_Warehouse_.html

You're right; not charging a Lithium cell fully doesn't harm it - on the contrary; it prolongs it's life. :) (And, as you say, lessens its runtime (but not permanently).)
Over-charge (above 4.25V) and over-discharge (below 3.00V) does harm it, :) so keep the discharge voltage in mind when using the whacker...
I will accept the rules that you feel necessary to your freedom. I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do. - (R. A. Heinlein)