Problem with Battery Balancing Circuit

[arthurartmaster]

hi all so basically the voltage is monitored with a seperate circuit that feeds to a microcontroller, the microcontroller then sends out signals to the opto to enable balancing

 

[MrAl]

Hi,

Ok well that can get a little complicated. How long is the code for your microcontroller?

 

[ronsimpson]

Why!
6 batteries in series.
If all the batteries are purchased at the same time......
If all batteries are charged with the same current.....
If all the batteries have the same load....
Do we need a battery balancing circuit?

 

[MrAl]

Why!
6 batteries in series.
If all the batteries are purchased at the same time......
If all batteries are charged with the same current.....
If all the batteries have the same load....
Do we need a battery balancing circuit?

Hi there,

Well for one, they might age differently or even be different anyway when they are new.
As min we'd have to monitor the voltage of each cell and turn all cells off when the first one reached it's limit.

 

[ronsimpson]

they might age differently

A car battery has 6 cells in it.

Battery theory; I do not know how they work!!!
I think at 50% charge a battery stores a large(er) percentage of the charging current.
I think at 95% charge a battery stores a small(er) percentage of the charging current.
(could be wrong, it happened once last year)
If my theory is true, then some battery balancing will happen automatically.

 

[MrAl]

A car battery has 6 cells in it.

Battery theory; I do not know how they work!!!
I think at 50% charge a battery stores a large(er) percentage of the charging current.
I think at 95% charge a battery stores a small(er) percentage of the charging current.
(could be wrong, it happened once last year)
If my theory is true, then some battery balancing will happen automatically.

H there,

The mechanism behind "automatic balancing", where we charge the cells in series with one current and do not add any additional circuitry for sensing battery state, is the overcharge behavior of the cell and is considered abuse of the cell.
However, some chemistries take this abuse better than others. Lead acid seems to have no problem with this, and NiCd's are pretty good too although they have been overstated in the past i think. To the contrary, NiMH and Li-ion are more sensitive to this kind of overcharging abuse and at best will be damaged and at worst will cause a bad fire or explosion. With NiMH we can get still get away with it as far as i know because if the current is not too high the cell will just get damaged, but with Li-ion the risk of fire or explosion sets in and that's something that nobody wants.
So the problems that could come up are based on the chemistry type, where one type will survive for a longer time.

The reason this happens in the first place is because of the individual cell ability to take a charge. The voltage rises for all cells in the series string, and if one cell is older than the other (for example) then it charges up faster and thus reaches the maximum state of charge (and maximum allowed voltage in some cases) faster than the others. After that point it is being overcharged and that's when the overcharging behavior sets in. It then depends on how the cell behaves as it is being overcharged and each chemistry has it's own characteristic.

So while lead acid can take it pretty well, NiMH will get damaged faster than NiCd, and Li-ion poses a risk of fire or explosion. This is why forced cell balancing is recommended for some cells like NiMH and especially Li-ion.

One of the examples is when one cell is older, but this can happen if one of the cells is already charged a little, more than the others, and over time it could take a few charge cycles to show up. If we charge all the brand new cells fully and then use less than the full charge, if there is a 5 percent difference in cell state of charge then when the cells are put back on the charger we could have one cell that is already 5 percent charged and thus it's voltage will reach maximum before the others. So it gets over charged and damaged, so next time it charges up even faster and gets more damaged, etc.

I dont like charging Li-ion in series myself but then i dont have any devices that need cells in series. Those Li-ion portable drills use series Li-ions but i never got one because i dont like the price tag.

Yes with different voltages levels for Li-ion we have different state of charge. I accept a little less in order to help the cells age better.

I just reread my post and when i said "NiCd's" have been overstated in the past that was meant to say that they were said to be able to take an indefinite over charge period, but that's not true from my years of working with them.
In a device that constantly charges them at the normal charge rate (not too high but not too low either) the cells may last one year. That may not be considered too bad. But in a device that charges them at the normal rate for only the time needed to replenish the self discharge on a daily basis, the cells can last five years. That's a big difference to me.
The reason i bring this up is because long ago it was believed that we could leave our power device plugged into the wall for indefinite time periods, like 24 hours per day, 7 days per week. So that is what a lot of people did, and after about a year the cells would get very weak. Maybe some do consider 1 year not a bad time frame, but for my portable drill i could not see paying 40 dollars per year when i only have to use the drill once in a while, like maybe once a month. After two battery packs and two replacement sets of cells, i finally switched to lead acid for the portable drill. I've had it for many years now.