question about charging li-ion vs. lipo

[bryguy17]

Hey flashaholics.

I'm working out the battery I'm going to use with my bike light, but im a little stuck. I was originally going to use a single 3s (11.1V) 2170mAh lipo pack for each head (2x cree XR-E, probably R2). However, I was digging around and found a dell laptop pack that I'm pretty sure is still alive. As far as I can tell, it is a 4s3p li-ion pack with 2150mAh cells.

My question: Will I be able to charge these li-ions with my lipo charger? I have one that will balance charge through a balancing tap, and one that will charge through the power leads.

If i solder on balancing leads to the cells, can i charge and balance like a lipo? or will i need a li-ion specific charger?

a little away from that, would there be any problem in resoldering the cells into a pair of 3s2p packs instead of the single 4s3p? just wanna be safe about it

btw, I'm familiar with the precautions necessary for using lipos, just wondering if they transfer over the standard li-ion cells.

thanks in advance,
-Bryan

 

[DM51]

Moving this to 'Batteries Included'.

 

[SilverFox]

Hello Bryan,

As far as charging goes, most Li-Ion cells charge to the same voltage and use the same CC/CV algorithm as Li-Po cells do. I would recommend figuring out a way to balance the cells in your pack, then charge them as a Li-Po pack. Keep in mind that Li-Ion cells are limited to a 1C charge rate, and prefer a 0.7C rate.

Tom

 

[bryguy17]

thanks for that silverfox.

I have a balance charger that is lipo specific, and has adjustable charge current in .25A increments. For these packs, i would be stuck with a .7C charge rate since the charger caps out at 3A and the packs will be 4300mAh.

Also, as far as discharging goes, should I apply the same 80% of rated capacity or 3V/cell cutoff point?

 

[SilverFox]

Hello Bryan,

Quality Li-Ion cells seem to be able to withstand a lower low voltage cut off. If you are going for maximum cycle life, the 3 volt cut off works great. However, I have taken some Sanyo cells repeatedly down to 2 volts, at 1C, and they just keep going. I should add that the resting voltage always is rebounding back up to over 3.4 volts after these deep excursions.

The various specifications for low voltage cut off range from 2.5 - 2.8 volts, but they don't reference a current draw.

Tom

 

[bryguy17]

okay cool. I'll play around a little with the cutoff once i get that far.

I'm currently getting ready to solder together a pack to try stuff out, so wish me luck.

 

[bryguy17]

So I wired up everything last night, and tried to charge the pack this morning.

Seemed to hook up well to my charger (FMA cellpro 4s) and all of the voltages showed up for the cells as they were supposed to. Only thing, once I got to roughly 4V/cell, everything started heating up significantly. The pack itself only got warm, but the charger got a lot warmer than it does when charging my lipos.

any ideas as to what went wrong? I'm sure theres no shorts in the pack, and that the balance tap im charging through is wired correctly. I just dont want to blow up my pack or my charger (or both :shakehead)

-Bryan

 

[mdocod]

soldering directly on li-ions can cause internal cell damage.. did you do it pretty fast? I'm not very good at soldering and wouldn't trust myself to do a big job like that at all.

If the pack is "old" from the laptop, then it's very possible the cells are simply worn out. Heating up towards the end of a charge is a tell-tale sign of aged cells. After it finished charging did it hold ~4.20V per cell? Or did it drop below that within a few minutes resting?

 

[bryguy17]

I was fairly quick when i was soldering onto the cells, plus it wasn't directly onto the cell, but onto tabs that were welded on there (doubt it makes a difference)

With regards to cell age, I'm pretty sure they weren't heavily used at all. The laptop didn't last longer than a year (friggin dell), and even then there wasn't much use.

I didn't complete the charging because I wasn't sure if Li-ions get warm during charging (lipo dont, and thats all im used to)

I'll check the voltage on the cells really quick to see if they dropped significantly overnight

 

[bryguy17]

just finished up the charge to the 4.2V/cell and left it for 20 minutes. After the 20 minutes, the voltage had dropped to ~4.16/cell according to the charger (has individual cell voltages). I take it the cells are getting ready to go the way of the dodo then? or should i be able to get at least a few more charges out of them?

 

[SilverFox]

Hello Bryan,

When your cells drop to below 4.0 volts/cell, then it is time to retire them. 4.16 is not all that bad.

Tom

 

[mdocod]

should be fine, some warmth is semi-normal depending on charging rate. you just don't want it to be hot.

 

[bryguy17]

The pack has held voltage quite well at 4.15V/cell, so I'm pretty sure its fine.

Thanks for the advice. I'll keep the charging rate down to around .5C, since the pack didnt get very hot when i finished up the charge earlier.

Thanks for all the help/advice guys!