8s charger from balance plug

[etriker]

Discharge the laptop cell hard at 5 amps and watch the voltage drop then drop the discharge to 1 amp and watch the voltage go up while discharging at 1 amp.

Of course it does.

Most cells behave like this when discharged beyond their rating for a time then have the discharge rate decreased. It's pretty fundamental to the way cells work, and is down to a couple of effects. The first is that higher than rated discharge current causes the cell to heat up from power loss (I²R loss) primarily, that is the resistive loss in the cell. Cell internal resistance (IR) is temperature dependent - make the cell hotter and its IR increases, which increases the I²R loss, which then makes the cell hotter still, etc, etc. Lower the discharge current and the cell starts to cool, as the I²R loss will have dropped (in the case of going from 5A to 1A the I²R loss will drop a great deal). As the cell cools, IR drops, reducing the resistive loss so terminal voltage increases.

I'm shocked that someone so obviously expert on the best cell charging methodology as yourself should find this surprising....................... :wink:

That may be the misunderstanding. I Have been in electronics repair for over 25 years and the more I do it the more I know that I am far from knowing it all. I am and will always be a newbee steep on the learning curve !

I do know enough to build battery packs and get down the road using cheap discarded cells. Don't have to hire HK or Ping or cellman to build my packs. Works for me !

Some people think the IR goes down when the cell gets hotter.

http://www.buchmann.ca/chap9-page3.asp

https://docs.google.com/viewer?a=v&q=cache:Nec_uFeqtnEJ:www.mdpi.com/1424-8220/10/6/5604/pdf+li+ion+internal+resistance+increases+with+heat&hl=en&gl=us&pid=bl&srcid=ADGEESjcvVWuyY7OntEYcctC7m8EBA4Y7Wo9I3-YW-VTRlEByqRLVI69BG8-vedyy3Nt1HRKbjdJVZOP6xj2WJfkpxxIJrBFgQEg6PHw3djcduA3J96HyCxbQzp3eQyORCUg0baRYJUL&sig=AHIEtbSDP0OEljz9fPZG89u0ElWRDpUemQ

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090023862_2009023573.pdf

I am thinking the 5 amp discharge makes the cell get hot, lowers the IR then when the current discharge is lowered to 1 amp the cell is hot, has a lower IR and the voltage goes up because of the lower IR caused by heating up the cell ?

 

[Jeremy Harris]

Cell heating isn't straightforward with lithium polymer cells, unfortunately. Certainly IR does decrease as the cell is warmed up externally, but there are other effects at higher internal temperatures created by high discharge current that then drive IR up, rather than down. It depends a lot on the way the cell is constructed, as some of the effect is down to the mechanics of the cell and the way it expands internally as it heats up and pushes the electrodes apart, reducing the effectiveness of ion transfer and increasing the apparent IR. Generally, when a cell gets hot from discharge, rather than charge, it is a pretty good indicator that it's already damaged anyway; this shows more clearly with RC type packs, as they'll puff as they expand.

 

[etriker]

Cell heating isn't straightforward with lithium polymer cells, unfortunately. Certainly IR does decrease as the cell is warmed up externally, but there are other effects at higher internal temperatures created by high discharge current that then drive IR up, rather than down. It depends a lot on the way the cell is constructed, as some of the effect is down to the mechanics of the cell and the way it expands internally as it heats up and pushes the electrodes apart, reducing the effectiveness of ion transfer and increasing the apparent IR. Generally, when a cell gets hot from discharge, rather than charge, it is a pretty good indicator that it's already damaged anyway; this shows more clearly with RC type packs, as they'll puff as they expand.

It is getting hot because it is being over discharged. That is what happens when you overdischarge one.

Do it just long enough to get the cell hot then drop the current and watch the voltage go up even while still discharging.

I got to go now but again,

Thanks for the across the pond battery talk

When we do get a kick butt ebike charger it may well be from the UK.

 

[dumbass]

Cell heating isn't straightforward with lithium polymer cells, unfortunately. Certainly IR does decrease as the cell is warmed up externally, but there are other effects at higher internal temperatures created by high discharge current that then drive IR up, rather than down. It depends a lot on the way the cell is constructed, as some of the effect is down to the mechanics of the cell and the way it expands internally as it heats up and pushes the electrodes apart, reducing the effectiveness of ion transfer and increasing the apparent IR. Generally, when a cell gets hot from discharge, rather than charge, it is a pretty good indicator that it's already damaged anyway; this shows more clearly with RC type packs, as they'll puff as they expand.

It is getting hot because it is being over discharged. That is what happens when you overdischarge one.

Do it just long enough to get the cell hot then drop the current and watch the voltage go up even while still discharging.

I got to go now but again,

Thanks for the across the pond battery talk

When we do get a kick butt ebike charger it may well be from the UK.

I would think it does this because the cell/cells are recovering from the higher discharge rate. That's totally normal. Most cell will have some voltage reduction and recovery voltage. Even if you are still discharing them but at a lesser discharge.

Bob