Switched Capacitors

[Tiberius]

I remember a few discussions on here about using switched capacitors for cell balancing, but the topic seems to have gone quiet.

It struck me as quite a good system, with maybe other advantages too. Did some fundamental reason for not doing come up, or did it just get overtaken by other projects?

What was the conclusion about the balancing currents that could be achieved?

Nick

 

[OneEye]

The idea came up in the PIC based BMS thread, around page 20 or so.

Gary and Fetcher were hashing out the concept; Fetcher posted a schematic that may have been workable; randomly pointed out the current transferred between cells would be minimal based on the part used; I asked about another part number Fetcher had suggested to see if it would work; Fetcher hypothesized about creating a FET based switched capacitor design. Then the thread went quiet for a while and when it started up it took off in a different direction.

In a different thread I recall Gary showing off picture of a 3-cell switched capacitor experiment he was working on with the intent to expand it if successful. I don't think he ever posted back about results for that one though.

 

[fechter]

Gary tried it with some off-the-shelf LM2662 switched capacitor chips. The problem is the existing chips have too high of an on resistance to pass much current and tended to run hot. I'm not sure exactly how much current they were running at, but they were rated for 200ma max.

The same topology would work with lower resistance FETs, but you'd have to build drivers for the switches. At low power levels, the off-the-shelf units run with very high (98%) efficiency. In theory, this could be scaled up if the right parts were used.

 

[Tiberius]

OneEye, fechter,

Thanks for the replies. Yes, my quick calcs showed that the on resistance is the key thing. I reckoned 100 mA was all that was easily achievable with standard muxes.

What did occur to me though was that this was also a way to sample the individual cell voltages. Switch the cap between the cells and an ADC input on a micro, and all the cell voltages could be measured. It would only need one micro, and no problems with different voltages.

Nick