What is the point of bottom balancing batteries?

[Luft]

I was watching a pod cast from EV TV and they were talking about how important it is to bottom balance a battery pack. I guess I don't understand the importance of lowering all of the cells to the same value before charging. Do these cells have some kind of memory? I mean if you lower them to the same value then charge them all to the same value what possible difference would it make. How would "they know" if you had not bottom balanced them to begin with?

 

[DaveinOlyWA]

i would "think" the only reason is to prevent polarity reversing which can happen if the voltage drop across a depleted cell goes too far. voltage drops on batteries wont go to zero. they will drop to a point and then flip flop polarity. that is why balancing is key. a weak cell will have greater voltage drop and can reverse its polarity in a pack situation which lowers over all pack voltage.

 

[DaveEV]

There is some debate as to whether top or bottom balancing is best. Both methods will give you maximum usable capacity on the pack. As long as you do one or the other, IMO, that's all you need to do.

Top balancing seems to be more common than bottom balancing.

Either way, you need to have low/high voltage detection on each cell since even bottom balancing won't prevent a cell from dropping in voltage quicker than the rest.

 

[garygid]

If one rarely ever uses enough of the battery's energy
to get near the bottom, then bottom balancing would
rarely happen, maybe not for months or years.

More often, typical use of a battery will approach the
top end of the charge condition, so it would seem that
top balancing is considerably more practical.

 

[GerryAZ]

The Leaf's BMS will balance the cells any time that it deems necessary during charging. I have charging records that show periodic stopping of current draw from the wall a few minutes after the start of charging from Turtle or Shutdown as well as during the middle of charging and near full charge. I believe these are cell balancing events.

Gerry

 

[AndyH]

I was watching a pod cast from EV TV and they were talking about how important it is to bottom balance a battery pack. I guess I don't understand the importance of lowering all of the cells to the same value before charging. Do these cells have some kind of memory? I mean if you lower them to the same value then charge them all to the same value what possible difference would it make. How would "they know" if you had not bottom balanced them to begin with?

Bottom balancing is something invented by the EVTV gentleman and some other uninformed souls at DIY Electric Car. These are folks that do not use battery management at a pack- or cell-level, don't top-balance, and don't use the full potential of their battery.

The Leaf balances at or near the top. It has a 'real' battery management system that monitors and protects each individual 'cell' (as 2 cells in parallel act as a single cell), and keeps each cell from both overcharge and undercharge.

Any ideas to bottom balance are best forgotten unless one intends to power something from a 'naked' lithium battery and, like the host of EVTV, has the cash to maintain a DC3 at a local airport and import significant quantities of cells from Chinese manufacturers...

 

[GregH]

Bottom balancing would be a fine strategy if (as Gary points out) you were down there often enough to do so.. Most battery monitoring systems balance at the top because you're more often there and it's usually easy to see the highest cells pop out. I once worked with a battery guy who really wanted us to balance in the middle of charge! His idea was to minimize stress at the top and bottom end, although we were using phosphate cells back then and seeing variations in the middle SOC range was very difficult. I hope Gerry is right and that balancing happens whenever it's needed regardless of SOC. Looking at my new Leaf pack there is at least one cell group that is 45mV lower than the pack average and it's no more evident at "full" (which is only 4.1V) than it is down at VLBW (3.5-3.6V) or for that matter anywhere in between. Since most folks rarely see the turtle and the Leaf never goes to true 100% (4.2V) it seems reasonable that balancing would happen whenever/wherever needed. I'd like to think my 45mV low outlier evens out with time. We'll see..

Edit: I do NOT have a 45mV imbalance, I was reading the data wrong.

 

[mwalsh]

I could probably balance a battery on my bottom. If I couldn't though, I do know a few girls with sufficient "shelf" space.

 

[ksnogas2112]

I could probably balance a battery on my bottom. If I couldn't though, I do know a few girls with sufficient "shelf" space.

Wow, somebody went there... made me snort instead of laugh (siol?) :lol:

 

[Herm]

Bottom balancing is something invented by the EVTV gentleman and some other uninformed souls at DIY Electric Car. These are folks that do not use battery management at a pack- or cell-level, don't top-balance, and don't use the full potential of their battery.

^^^^^^ this

+10

 

[TomT]

I have an 8D that should present something of a challenge... :lol:

I could probably balance a battery on my bottom. If I couldn't though, I do know a few girls with sufficient "shelf" space.