Overcharging/overdischarging li-ion battery

[kitchenman]

My previous experience of batteries is NI-MH and I was warned of the danger of OVERCHARGING(leaving the charger on after the green light comes on) and OVERDISCHARGING (leaving the battery connected to the charger once charged and the charger has been switched off or leaving the battery connected on bike)
Do I have to worry about these things with a LI-ION battery? Or is this more a question of functions/limitations of my equipment?

 

[piotrmacheta]

You do have to worry (a lot) about overcharging and over-discharging LiPo batteries but you rely on the speed controller to cut out at 3.2V per cell and the charger to stop at 4.2V per cell. I have been using RC model batteries for about a year now and there is no BMS so I have a low voltage alarm to stop discharge too far and the charger has a ballance mode where it keeps all the cells ballanced during charging. If you discharge below 3V per cell it ruins the battery - the battery retains its capacity but doesn't want to give out the current and the voltage sags. I have found that the battery pouch swells and you risk it rupture during charge. Overcharge is defo not good as they catch fire (a bit like NiMH's).
As for leaving the battery connected to bike or charger I suspect that it makes no difference as there should be no current drain when off.

 

[NRG]

Alan, if the battery is fitted with a BMS then you have no worries, charge, fit and forget. However, you should try and remove the battery from the charger soon after the green light comes on even with a BMS. Li-Ion doesn't like to be 'held' at a high potential for extended periods after a full charge...

 

[kitchenman]

Thanks piotrmacheta and NRG, Should be receiving a manual shortly. WIll have a good read in case there is anything special to consider with my kit ...

 

[Scottyf]

As others have said as long as there's a decent BMS all in the battery then it really is as simple as plug and play. Or charge and leave.

Unless leaving for a long period of time...

Much better than Nicad or Nihm though!

 

[kitchenman]

As others have said as long as there's a decent BMS all in the battery then it really is as simple as plug and play. Or charge and leave.

Unless leaving for a long period of time...

Much better than Nicad or Nihm though!

I dont know yet as I haven't got the user guide. But having read in another thread about the 12 hour conditioning process it seems to me that the charger will continue to charge the battery after the green light has come on. If it didn't I would not have been able to condition the battery assuming that is what I have done! .... If I put the battery on to charge now and go to bed then in the morning the light will be green and the battery would have be overcharged by about 4 hours? or is 4 hours ok? ... whats a "long period of time" ? Life would be much easier if the charger stopped charging when the green light comes on and had a conditioning button for those times that conditioning is required? ...

 

[NRG]

You cant 'overcharge' the BMS sees to that. The conditioning talked about is merely to reset the BMS 'fuel gauge', one complete discharge and then full re-charge will do that. Leaving the charger connected overnight is probably not wise...as stated before holding the battery at a high potential for long periods (yes overnight) does them no good. I suggest if this is likely that you time how long a full charge takes and buy a 24hr mains timer and set it to cover the charge period with a little bit extra at the end....

 

[kitchenman]

You cant 'overcharge' the BMS sees to that. The conditioning talked about is merely to reset the BMS 'fuel gauge', one complete discharge and then full re-charge will do that. Leaving the charger connected overnight is probably not wise...as stated before holding the battery at a high potential for long periods (yes overnight) does them no good. I suggest if this is likely that you time how long a full charge takes and buy a 24hr mains timer and set it to cover the charge period with a little bit extra at the end....

Great! >.. I've done 3 cycles of 12 hour conditioning! ... being careful not to completely discharge as I thought that was bad! ... I discharged to 20% ... so ... what do you think is the status of my BMS 'fuel gauge'? ...

 

[NRG]

Unknown! run it down....all the way until you can ride no further with power, charge it fully, disconnect charger. Ride bike and forget about charging and leaving 20% capacity! Use it all, thats what its there for...

 

[flecc]

That's good advice from NRG. Not using the top and bottom 20% of a battery's charge can extend life if in an environment designed specifically for that kind of usage, currently mostly in hybrid cars or satellites.

E-bikes are designed with cutouts that operate during usage and charging, these ensuring that only the safe charge region is used, freeing you of that responsibility.

 

[kitchenman]

That's good advice from NRG. Not using the top and bottom 20% of a battery's charge can extend life if in an environment designed specifically for that kind of usage, currently mostly in hybrid cars or satellites.

E-bikes are designed with cutouts that operate during usage and charging, these ensuring that only the safe charge region is used, freeing you of that responsibility.

freeing me of responsibilty ... I like the sound of that ... "feel the wind blowing through my hair" ...I'm in a flake advert where I belong! ...

 

[flecc]

freeing me of responsibilty ... I like the sound of that ... "feel the wind blowing through my hair" ...I'm in a flake advert where I belong! ...

That's it exactly, the marketing paradise, just hand over a large lump of cash and we'll make your life perfect in every way!

 

[Haku]

Life would be much easier if the charger stopped charging when the green light comes on and had a conditioning button for those times that conditioning is required? ...

In the early days I used to unplug the battery when the charger light went green, then I gave up and just left it connected all the time, perhaps this has contributed to the normal dwindling battery capacity I don't know, but the first two li-ion packs I've used on my UM36 have each lasted around 2 years by which time the capacity has in my estimate halved.


I've been pondering the 'disconnect after charged up' thing and after a little testing I'm certain that I can whip up a small circuit to automatically do that. Which I intend to do in the next couple of days.

The circuit, mainly consisting of a microcontroller and MOSFET, would alter the charging process slightly; plugging in the charger to the battery would not automatically start charging it, you'd have to press a button to start the charging, but once the microcontroller senses the red LED on the charger has gone off it then uses the MOSFET to electronically disconnect the charger from the battery.

 

[piotrmacheta]

Would it not simpler to do what NRG suggested and just use a mains timer to unplug the charger after a few hours?

 

[Scottyf]

Does sound much simpler and would only cost a fiver ;-)

 

[Haku]

The parts to mod the charger come to under £3, and you wouldn't have to set a timer each time, just press a button

 

[Synthman]

On my charger it recommends that it is left plugged in for an hour after the green light comes on. I've used a power monitor and it does indeed still draw a small amount of current when the green light is on, gradually dropping to the same level as with no battery connected, about 4.5 watts.

 

[Scottyf]

Usually its probably balancing a little bit while replacing a little bit of charge while doing so..

 

[Haku]

The manual that came with my recent bike says I should unplug the battery after charging, so that's what my circuit will do

This evening I soldered a MOSFET and load of test wires into the charger, and through testing have worked out that I can make the resulting circuit completely transparent to the user - no need to press a button to start the charging, the Picaxe microcontroller will easily be able to sense when a battery is plugged into the charger then start it charging and 'unplug' it after it's charged up.
I'll either replace the existing LED with one controlled by the Picaxe or add another one or two.