single capacitor battery balancing

[Rager]

I would really like to build a single capacitor battery balancing system. Like the following:

The capacitor transfers the charge from a full battery to a more empty battery.
The problem comes with the switches...
I need a MOSFET switch and it can be done for example like that:
But there is a Voltage problem with the gates of the MOSFETs... I want it to be able to have a chained system voltage of up to 48V and the solution above does not work for such a high voltage..
The "better" option could be the TLP250, but it's old and quite expensive (also only working from 10V-36V and not down to the 3v of a Li-ion battery...). I would like to do it with discrete MOSFETs and cannot find a good solution for it.
The Switching frequency will be in the <100kHz region.

Has someone a good idea/ solution for me?
Thank you in advance!
Florian

[Siwastaja]

Don't do it, it makes absolutely zero sense!

Redistributive balancing is not needed at all in 99.9% of cases, the only thing it does is to offer energy savings which pay back in approx. 1000 years or more, if ever. Any quiescent or "managing" power it requires may tip it to negative.

If you still want to do it, capacitor distribution is out of question, as it is inefficient (25% theoretical maximum efficiency AFAIK), and the whole point to build redistributive balancing was the efficiency. Same goes to any architecture that only moves charge between two adjacent cells.

Capacitor distribution is only presented in academic papers as a filler in the preface section. It seldom is actually realized, because it makes no sense. True redistributive balancing is IMO done with bidirectional flyback converters so that any cell can charge or discharge to any other cell with maximum of two conversion steps. That's what I prototyped before I came to my senses and ditched redistributive balancing altogether.

[dmwahl]

Like siwastaja said, it doesn't make sense in the majority of (if not all) cases. Consider the total energy dissipated by any resistive balancing scheme, and unless your cells are horribly unmatched the overall charge efficiency will be affected by less than 1%. You're likely to waste more power in the charger, or in the wiring, than you do by resistively balancing. Look at any commercial BMS and they use passive/resistive balancing. Not to mention the cost of a flyback converter per cell gets high quickly.

[Rager]

Okay,
Thanks guys!
I will forget about it.

[splin]

Don't do it, it makes absolutely zero sense!

I'm sure it can make sense in some cases - eg. when the cells aren't well matched, but if they are it's unlikely to be worthwhile compared to a simple resistive balancing scheme (if even that can be justified).

If you still want to do it, capacitor distribution is out of question, as it is inefficient (25% theoretical maximum efficiency AFAIK), and the whole point to build redistributive balancing was the efficiency. Same goes to any architecture that only moves charge between two adjacent cells.

That's not true - capacitive switching convertors can easily reach 95% or more efficiency under the right conditions. See pages 4.11 to 4.15 of:

http://www.analog.com/media/en/training-seminars/design-handbooks/power-thermal-mgmt-sect4.pdf

[madalf71]

Rager.

Great idea, depending on battery chemistry and how many you have. Battery balancing is a very common practice.
Not too sure on the capacitive side of things, most smaller chargers are switchmode or pwm, would that cause an issue eg responeses, or would the battery be dampening it all out?