Cell balancing? Any idea?

[SnowDrifter]

I've heard of using cell balancing for batteries (like the li-ion in your laptop) but I only just now read about using it for capacitor banks.

Google didn't turn up much info.

Is this something that is needed/beneficial? And if it is: does anyone have a link on how to build an equalization circuit?

[SnowDrifter]

Bump

[RobertR]

Used to balance cells back when I raced rc cars. helped a lot with battery life, faster recharge time and less heat, small but noticeable performance gain. Not really sure if cell balancing would apply itself in this situation but i would say its worth looking into.

http://www.mpoweruk.com/balancing.htm

[SnowDrifter]

Used to balance cells back when I raced rc cars. helped a lot with battery life, faster recharge time and less heat, small but noticeable performance gain. Not really sure if cell balancing would apply itself in this situation but i would say its worth looking into.

http://www.mpoweruk.com/balancing.htm

That's actually where I got the idea haha. I'm really not sure if I need one or not... If I do I don't want to risk the lifetime of my caps by not using one. But if I don't: well... Than I guess that's that.

They are used to ensure that all the cells have the same charge level so they don't get overcharged or over-drained, correct? Capacitors don't have that lifetime that typical li-ion cells have, so I'm questioning if there is a need for one. I can't seem to dig up much on Google either.

And looking into balancers a bit more: I'd really want one that I can just plug and play. I can find the circuit diagrams on Google but they are a bit over my head in terms of what I have the skill to build. All the pre-made ones I've searched for are for lithium ion/polymer batteries. So I'm really in a tough spot right now.

Tony? Ed? Want to come to the rescue here? Lol

[SnowDrifter]

I just stumbled across this: using balancing resistors.

The only thing I'd be concerned about is leakage current with the car off. Hmm....

http://www.coilgun.info/theorycapacitors/capacitors2.htm

[HHR Ed]

Cell balancing does help CAPS that are connected in series. It protects from over voltage and extends life.

Using a passive cell balancing (resistors) on a bank of caps when connecting caps to batteries can create a slow drain on the batts.

[SnowDrifter]

Cell balancing does help CAPS that are connected in series. It protects from over voltage and extends life.

Using a passive cell balancing (resistors) on a bank of caps when connecting caps to batteries can create a slow drain on the batts.

That makes sense

How big of resistors would I need? I've found this equation but I don't deal with this stuff often, so I'm unsure of what numbers to put in. Is it the capacitance of an individual capacitor or the entire bank?

For 2 capacitors in series: R = (2V_m - V_b) / (0.0015 C V_b) For N > 2 capacitors: R = (NV_m - V_b) / (0.0015 C V_b) where R = resistance in megohms

V_m = max voltage you'll permit on either capacitor

V_b = max voltage across the entire bank of two (or N) capacitors

N = number of caps in series

C = capacitance in microfarads

Since I have 7 paralleled pairs wired in series: Would it be this?

R=(7*2.6 - 16.6)/(0.0015*850,000,000*16.6) - capacitance of bank

R= .756 ohms

Or this?

R=(7*2.6 - 16.6)/(0.0015*6,000,000,000*16.6) - capacitance of a capacitor pair

R= .107 ohms

Or maybe this?

R=(7*2.6 - 16.6)/(0.0015*3,000,000,000*16.6) - capacitance of an individual capacitor

R=.214 ohms

[Wicks]

You'd want this version (the resulting capacitance in each series location):

R=(7*2.6 - 16.6)/(0.0015*6,000,000,000*16.6) - capacitance of a capacitor pair

R= .107 ohms

Although check your calculations, I get 0.0107ohms.

Regardless that resistive load is way to low for practical use, even with 7 in series.

I'm guessing the leakage current expression in that formula is off for these large caps.

Reversed-biased Zener diodes are commonly used as voltage clamps/regulators.

You should be able to connect seven ~2.4V Zeners in series and use that.

They'd also have a very small reverse current too so the parasitic draw would be small.

You'd have to check some calculations to see what it would be though.

I'll check more into it.

[Wicks]

So, forget my previous comment on the zener's.

http://www.instructables.com/id/Lets-learn-about-Super-Capacitors-A-Practical-G/step7/Balancing-Your-Series-Banks/

They're good with low power circuits, but wouldn't work when dealing with hundreds of amps of current.

The best person to comment on this would be Tony D as he used a ton of ultra-caps in the T15K.

Although his power supply probably takes the balancing in account and charges each level of caps separately.

Sounds like you either put up with a good amount of waste in balancing resistors or go with an expensive active solution.

[SnowDrifter]

Is there any way to calculate this based off the leakage current? Each cap is 5.2ma