Why won't lithium balance in series?

[GoneDiving]

Would someone please explain to me why lead technologies will self balance when cells are in series but lithium will not.

Thanks

 

[Wolf]

Lead batteries do not "self balance" they need an equalization charge every so often, especially if you have ( 4 )12v batteries in series for 48V.

During normal battery operation some lead sulfate (sulfation) will build up on the plates. This build-up will continue after each recharging cycle and gradually the battery will begin to lose capacity to store a full charge and eventually must be replaced. Lead sulfate build up is reduced if battery is given an Equalizing Charge once every 10 discharge cycles or at least once a month. An Equalizing Charge increases charging voltage to 14.4 volts or higher for a short period. This higher voltage causes gassing that equalizes (re-mixes) the electrolyte solution.
Wolf

 

[Korishan]

If he's referring to "voltage" equalization, it's really simple. Lead acid batteries can be overcharged w/o hurting them much. You can take a lead battery to 15 or 16V and it'll only complain with venting off some of the liquid electrolyte, which "usually" can be refilled (even sealed can be refilled if you know how to do it)
Whereas Lithium batteries do not like being overcharged. They have a very narrow voltage range to operate in and going out of that spec is putting the cell/battery at risk.

It's like the difference of inflating a balloon vs plastic bottle. The balloon can expand to quite a size to accommodate the extra air. The plastic bottle will just flat out explode without warning when it reaches critical pressure. (to be fair, a balloon will too, but you have a lot of warning you are approaching its limit before hand)

 

[GoneDiving]

If he's referring to "voltage" equalization, it's really simple. Lead acid batteries can be overcharged w/o hurting them much. You can take a lead battery to 15 or 16V and it'll only complain with venting off some of the liquid electrolyte, which "usually" can be refilled (even sealed can be refilled if you know how to do it)
Whereas Lithium batteries do not like being overcharged. They have a very narrow voltage range to operate in and going out of that spec is putting the cell/battery at risk.

It's like the difference of inflating a balloon vs plastic bottle. The balloon can expand to quite a size to accommodate the extra air. The plastic bottle will just flat out explode without warning when it reaches critical pressure. (to be fair, a balloon will too, but you have a lot of warning you are approaching its limit before hand)

Thanks. This is exactly what I was referring to and explains it well.

One more question: what about when there is no charge current? Lead tends to still even out voltages (I'm unclear if the cells self discharge to the same voltage or if higher voltage cells drain into and charge low voltage cells) whereas lithium will remain unbalanced. Why the difference?

 

[Wolf]

Thanks. This is exactly what I was referring to and explains it well.

One more question: what about when there is no charge current? Lead tends to still even out voltages (I'm unclear if the cells self discharge to the same voltage or if higher voltage cells drain into and charge low voltage cells) whereas lithium will remain unbalanced. Why the difference?

The chemistry of Pb Lead acid is such that the individual cells in a ,lets say 12V battery which has 6 cells, will come to a "natural" rest voltage of 2.1V per cell for a fully charged battery. Since you can "charge" a Pb battery with a higher voltage to break down the sulfation and remix the electrolyte, the cells after charging will then seek their equilibrium of 2.1V. The interaction between the sulfuric acid and the lead plates dictate this.

If you have ever charged a Pb battery, that has caps so you can see the electrolyte level, with a higher voltage you will see that some of the cells will start to "boil" sooner than the others which means the electrolyte is saturated and cannot take any more charge allowing the other cells to catch up.
There is no interaction between the cells of a Pb battery at rest. Neither is there any interaction between Li-ion cells in series at rest.
This is the reason for a good BMS as we charge our Li-ion battery pack to the proper voltage the cells that are "higher" in voltage will get a resistive load applied to them to "boil" the excess energy off. allowing the lower cells to catch up.

it'll only complain with venting off some of the liquid electrolyte, which "usually" can be refilled

Yes the excess energy of "overcharging" the Pb battery has to go somewhere and the result is Hydrogen and Oxygen (H2O) Water being "boiled out of the electrolyte. This can be refilled with distilled H2O.

Wolf

 

[GoneDiving]

Thanks

 

[Redpacket]

+1 to above, summary: Pb "balancing" is done by gassing which results in a similar effect to "bypassing" for Li-Ion cells.
Pb equalizing energy = H2 gas, for Li balancing energy = resistor heat.
Pb doesn't mind mild overcharging (as long as the acid is topped up) but Li chemistries are not tolerant to over charging.