Fechter's Capacitor Coupled Cell Balancer

Batteries, Chargers, and Battery Management Systems.
Knuckles   10 kW

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Re: Fechter's Capacitor Coupled Cell Balancer

Post by Knuckles » Jun 02 2013 6:14pm

Maybe something like this ...

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Knuckles   10 kW

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Re: Fechter's Capacitor Coupled Cell Balancer

Post by Knuckles » Jun 03 2013 3:20am

I got the idea from this source ...
http://www.celnav.de/hv/hv9.htm
(scroll thru)

I expect a three phase 'generator' at 20-50 kHz would be good. At higher frequencies the system is more efficient and smaller caps will work for a given power input. Individual battery cells act as capacitors so the caps adjacent to the bats probably are not needed (but can't hurt). The bats must remain connected at all times while the phase generator is 'running'. Cheap (general purpose) 5-10 volt electrolytic capacitors should be fine.

A high kv outrunner (or inrunner) would be a great 'mechanical' three phase generator.
There might be a potential for LVC and HVC control here also.
And a 'chainless' drive system and an end to 'fake pedaling'.
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fechter   100 GW

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Re: Fechter's Capacitor Coupled Cell Balancer

Post by fechter » Jun 03 2013 8:19am

Look who's back...

The problem with that arrangement is the cell furthest from the AC source has to pass through all the series capacitors and will have lower current than the ones closer. If the capacitors in series were big enough, the total ESR might be low enough for it to work, but I think you'd still have uneven current distribution in a long string.

I'm not sure if using a 3 phase setup will be worth the extra components, but you could probably push quite a bit more current. You could use the same series arrangement single phase. If there was a 'smart' element between the rectifier and the cell to regulate the voltage, it could compensate for the uneven supply (probably needed anyway).
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Re: Fechter's Capacitor Coupled Cell Balancer

Post by docnjoj » Jun 03 2013 9:23am

Welcome back, stranger!
otherDoc
E-bike stable at our house

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Re: Fechter's Capacitor Coupled Cell Balancer

Post by Arlo1 » Jun 03 2013 10:27am

I have been thinking about this. I need to re-read this thread. But I was thinking how one might use isolated dc-dc supplies to pass current from a Hi cell to a low one. I would like to think of a bms that can be turned on when ever you ride the bike and when one cell (or string of cells in parallel) get higher then the average it starts taking power from them to the others. If you can prevent one cell from dropping before the rest on the go it would help, well it would help me with my crazy experiments.
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fechter   100 GW

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Re: Fechter's Capacitor Coupled Cell Balancer

Post by fechter » Jun 04 2013 8:05am

Arlo1 wrote:I have been thinking about this. I need to re-read this thread. But I was thinking how one might use isolated dc-dc supplies to pass current from a Hi cell to a low one.
I've seen some setups like that and it does have some merit. Parts count becomes sort of a killer again.
I worked on a simplified version of this a while back. Each cell has a little transformer that takes excess charge and boosts it to pack voltage and feeds back into the main pack terminals during charge. The transformer takes the place of the shunt resistor in a typical setup. This allows very high shunt current without generating a lot of heat. I found some nice transformers that were about $2ea in quantity. These could easily handle about 3A shunt current.

The TI PowerPump arrangement is similar but shuttles charge to the adjacent cells only. In more complex arrangements, there is a 'share bus' that high cells dump into and low cells draw off of. For sure this requires microprocessor control. This would allow for full time balancing and help out weaker cells during discharge. Very elegant, but way too complex for me.
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Re: Fechter's Capacitor Coupled Cell Balancer

Post by Arlo1 » Jun 04 2013 8:27am

I just thought of another SIMPLE B.M.S. I think you could use mosfets hooked to each Cell that when turned on would connect the cell to a cap bank.
So it would be this simple. You pulse the mosfets on at different times. All feeding or pulling from the same cap bank. If all fets turned on at once You would blow it all because it would connect all cells in parallel. But if pulsed at different times it would work.
Does your project need a high performance motor drive, battery charger or other power electronics developed? Let's talk!
www.powerdesigns.ca
Leaf motor controller build. http://endless-sphere.com/forums/viewto ... 27#p963227
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Re: Fechter's Capacitor Coupled Cell Balancer

Post by amberwolf » Jun 04 2013 12:48pm

You'd want fuses on each cell or cell group, though, because FETs tend to fail shorted, so if one did fail for any reason then you switch to another one, you'd then be shorting *at least* two groups together. :(

The fuse would prevent this from being a serious problem, especially if your system has a monitor to detect that a group is no longer connected and stops the process, and alerts you.

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Re: Fechter's Capacitor Coupled Cell Balancer

Post by fechter » Jun 04 2013 1:28pm

Arlo1 wrote:I just thought of another SIMPLE B.M.S. I think you could use mosfets hooked to each Cell that when turned on would connect the cell to a cap bank.
So it would be this simple. You pulse the mosfets on at different times. All feeding or pulling from the same cap bank.
The FETs would need to be controlled by some kind of smart software to shuttle the charge around. I think the FETs would all need to be rated for full pack voltage since one side of them would always be connected to a cell and you would need two per cell, maybe 4 due to the body diodes. It gets complicated to drive the FETs from a common microprocessor. For pulse only operation, you might get away with coupling capacitors for the gates.
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