Rechargeable lithium question

[rwolff]

I had originally intended to make a self-propelled flaslight (bike with custom light /ubbthreads/images/graemlins/grin.gif) using a 12V SLA. Recently, however, I've come across a source of cheap LiIon cells (originally intended for cell phones) - and LiIon has a much higher energy density (weight-wise) than SLA. I haven't had a chance to do much looking for chargers, but what is a good one for a 4-cell series configuration? Also, would cell phone batteries be capable of sourcing the current I need (these are roughly 1 AH, for capacity reasons I'd want to go with at least 5 AH, so it would be a 4x5 series-parallel arrangement, headlight would be a triple Luxeon series rig at 1 amp or less, turn signals around 100 mA, and the horn would probably draw about 1 amp but only be used for short periods)?

Since these are flat rectangular packs, would they be LiPolymer, and if so what would be the handling differences compared to ordinary LiIon? The only 4-cell charger I've seen so far is one through BatteryStation - can it handle ordinary LiIon (non-polymer) batteries too, and what is the maximum amp-hour rating that it can handle?

 

[VidPro]

CROSS LINK

there are specs on the polymer version of the li-ion, they are relative to the rolled cells.
the HUGE problem you have here is not parelleling the li-ions, as there is little problem with doing that other than one bad pack will self discharge the lot, and heat up when charging, and possibly ruin more of the lot.
The problem is the seiresing of them. you then MUST have seperate cuircuitry for EACH Unit of same voltage cells. for both charge AND discharge.

so if you have cells with the Low and high cutoff cuircuit IN them or on the pack, then that is some minor need for each cell.

hmm i will try an explain, need psudo pictures.

+ ----|||||||battery 3.6v||||||||-------- -
+ ----|||||||battery 3.6v||||||||-------- -
+ ----|||||||battery 3.6v||||||||-------- -

Parrelell this set of three batteries , all you need is to STOP discharging it at about 2.4-2.7 basically its when they got no more juice left in them. go any lower and it hurts them.
so if your running white Leds, when they go below 3.0V they are pretty darn dim, and drawing little current.
You MUST stop it at that point, or have a curcuit in that does stop it.

when you go to charge the above pack, you will have to stop charging it when it reaches 4.25V (see charge discussion for more info).
OR the pack itself Must have the cuircuit in it to stop it from charging over that rate, or the cells will DIE, in the case of li-poly they will probably bubble internally plate seperate, and expand, and just generally become 100% worthless, the bag they are in might fill with gass, and all kindsa terrible things.

so the above pack needs a protection curcuit, which you can buy, or buy them WITH the curcuit already attached. it is actually a LAW in the states, meaning you couldnt sell a product to consumers that didnt , without getting sued the first time sombody got trouble with it.

then the charger (reguardless of the packs protection curcuit) should have a high cutoff and proper charging for li-ion cells.

the unit that the pack goes in should have a low voltage cutoff, although the packs LVC is likely to be effective.


ok so piece of cake so far.
run a parrelel pack, feed some LEDs with it, they are reletivly the same voltage as your pack. charge it right, and discharge it right, via constant attention (see ADHA) or with correct curcuits.

last note on this single parellel pack, LI-Poly doesnt do many cycles, and pulling out old cell battereis they would already be cycled enough times to not want to bother with them.

 

[VidPro]

now you wanna put them in SERIES TOO , and then you need ALL of the same above things for each SET put in parellel.

+ ----|||||||battery 3.6v||||||||-------- - | + ----|||||||battery 3.6v||||||||-------- -
+ ----|||||||battery 3.6v||||||||-------- - | + ----|||||||battery 3.6v||||||||-------- -
+ ----|||||||battery 3.6v||||||||-------- - | + ----|||||||battery 3.6v||||||||-------- -

or THIS will happen
+ ----|||||||battery 1v||||||||-------- - | + ----|||||||battery 4.8v||||||||-------- -
+ ----|||||||battery 1v||||||||-------- - | + ----|||||||battery 4.8v||||||||-------- -
+ ----|||||||battery 1v||||||||-------- - | + ----|||||||battery 4.8v||||||||-------- -
(destroyed parellel set 1) --------------------- (destroyed parellel set 2)

and DOES, this is what seems to be destroying packs, just like it did with Ni-Cd, and NI-MHY , its just round 3.
but its round 3 FAR worse, because the li-ion will toast, big time, when it goes over, goes under, or reverse charges.

so INBETWEEN this SET , you gotta have a wire that feeds the curcuits for all of the stuff you had for parellel.
and if you ask me, i think that the built in protections are NOT enough to re-balance the cells, i think each parellel unit, needs its OWN charge curcuit (or similar), not magic cut off.

there is no discharge this SET to 5v, because that isnt how its going to happen.
there is no charge it to 8.5 because that isnt how its going to happen.
the Imbalances that occured in other rechargable PACKS will damage (destroy) li-ion permanentally in the First high charge.
and you cannnot just cycle or overcharge slowly the PACK to get the cells balanced, if they go over , even SLOWLY , they are ruined.

Disclaimer: there is a lot of info about, some of it differers, and there are a lot of PACKS that are dying for no reason other than POOR quality cuircuits.
chemistry and protection curcuits are different in each of the things, and chemistry CHANGES over time, so some of what i learned might not apply tomorrow even.

 

[VidPro]

i have done a scooter pack, with 20x9 parellel series, and IF i had (just) made sure that each 9 parellel sets had charge controll it would have worked , instead 20x1 one of the parellels Died in the series, and every cell in that parellel died.

"stupid is as stupid does sir" /ubbthreads/images/graemlins/smile.gif

hey sombody had to do it, and it was a long time ago, and it went 27Mph 4 times /ubbthreads/images/graemlins/smile.gif

 

[andrewwynn]

paralleling little batteries doesn't make much sense.. those batteries don't have a lot of energy.. typically less than 2 1/2WH each.. i would get some 18500s and build a pack with the appropriate voltage using the appropriate safety circuit. get a nice driver like fatman or nFlex depending on what you end up using and make yourself one baddass bikelight.

there is 'li-poly' and 'li-prismatic'.. the prismatic are typically the type in a cell phone and do not have the power capability of the poly cells.. i have a poly cell that is the size of a cell phone bat cell and it rather than being 2W is capable of 20W. It's quite a sight to see a battery 1/4 inch thick driving two lux 3 emitters at 4W each..

If you already have the cells.. you can build the circut described above.. risking the chance of killing all the cells in a particular rung of parallel.. but i'm considering doing a 3x3 to put into a 3D light with a 9AA to 3D size adapter... exactly how it was described above.

good luck and post pictures

 

[VidPro]

there is some sence to it, if your unregulated, the voltage will be uneffected by the load, so it will act more like a controlling curcuit would.
also then you only need one charger curcuit or controll stuff.
course with these rechargables, you can have them hooked to a hotplate and they will hold thier voltage well.

 

[rwolff]

Thanks for the advice. A few points that I might not have made clear:

1. I don't have the cells yet. I had been considering a 12V7.5AH SLA, but a local source has new 3rd-party (i.e. non-OEM) cell batteries (3.6V about 1AH) at a good price. I didn't want to buy them unless I could be reasonably sure this would work (4x7 series-parallel would be a LOT lighter than the SLA).

2. I know about the need to monitor each section in charge - BatteryStation has controllers available for 2, 3, and 4 cell series hookups. I was planning on using one of those (4-cell model) unless it either couldn't handle the current of charging a (roughly) 7AH pack, or something better (possibly with multi-cell discharge monitoring as well) were available.

 

[andrewwynn]

Hey i've used SWAKs.. they aren't terrible /ubbthreads/images/graemlins/wink.gif those charge circuits should be fine.. when you do the series-parallel.. you risk one 'rung' messing up the cells in that row.. i.e. if one cell dies it tends to take out all the cells in that row.. which means 7 in the case you describe.. I haven't plotted this out but it probably makes more sense to break it into two banks.. like 4 x4 and 4 x4.. or if you are set on the sapce of 4x7.. use 4x3 and 4x4..that way for only $5 more or whatever the safety ckt is.. you protect a lot more cells.. worst case you lose 3 extra cells vs 6.

-awr

 

[rwolff]

It seems like going with lithium in this application is asking for trouble (I'd only considered Lithium because the local surplus emporium had the NOS cellphone packs on sale for $2 each). The SLA is a few pounds heavier, but it's "stone axe simple" technology (plus I'd be able to use the same driver circuit in another application where 12V SLA is pretty much mandatory - an upgraded version of my automotive trouble light, where SLA would let me tap into my car's electrical system for charging, with only a diode to keep the trouble light from draining into the car).

Another option (with possible weight and size savings) would be NiMH (again, a 12V pack). Chargers are fairly readily available (if I can track down a cordless tool that uses NiMH instead of NiCD, that's a locally-available source), replacement cells are extremely common, and I could go with a modular approach (multiple parallel-connected packs - install only as many as I think I'd need). With 2000 mAH being common for AA, a 12V8AH pack (40 cells wired 10x4 series/parallel) would be roughly half the weight of a 12V7.5AH SLA.

The bike project is at least a year in the future - when something that (initially) looked like a good deal turned up, I wanted to get it while it was available.

[ QUOTE ]

Hey i've used SWAKs.. they aren't terrible


[/ QUOTE ]

Compared to TWOH they are. If Luxeon flux bins went low enough (unfortunately they start at F), I'd have used a sig along the lines of "AWOL - a Luxeon bin bad enough that it's a court-martial offence".

 

[rwolff]

Bumping because I'm now back in the Li-Ion camp. It seems that another branch of the surplus emporium had (note the past tense) in stock a different model of cellphone battery (again, it was 3rd-party NOS for an obsolescent phone). This one is 7.2 volts (i.e. 2 Li-Ion cells per pack instead of 1) with 1350-1400 mAH capacity (instead of roughly 1000 mAH), and was being cleared out at an incredible 91 cents per unit (instead of $2).