Re: [EVDL] capacitor balancing
Lee Hart wrote:
>> You're designing it backwards; picking the parts, and then trying
>> to build something with them. Instead, start by specifying what it
>> needs to do. THEN design it, and pick the parts.
Jack Murray wrote:
> I thought I described what I want it to do?
There was a rough description, but not enough for me to be sure of what
you wanted. In this email, you provided more details. Let's see if it's
enough to do a design. Is this correct?
- "Stop regen recharge when any of 135 caps reaches 2.7v."
- "Shunt 330ma above 2.7v (copy Maxwell)"
- what should the current be from the supercapacitor at 1v? 2v? 3v?
This spec is important because it tells us how much current the circuit
can draw below the threshold. If it draws a lot, it will run the
capacitors down to 0v much faster than if the capacitors were just left
to self discharge. I think Maxwell used 50 microamps for this, which is
"negligible" compared to their capacitor's self-discharge rate. I think
you could probably allow it to be as high a 1ma at 2v before it begins
to noticeably affect the self-discharge rate.
The current at 3v (above your 2.7v threshold) is important because it
tells what happens if the cell goes overvoltage anyway (if whatever is
supposed to stop regen fails to stop it). A "perfect" 2.7v zener diode
would try to draw a huge current at 3v, and probably burn up. When it
fails, it will short! Now all the energy in that capacitor will get
dumped into the short! This could lead to a fire! So with a zener,
you'll need some kind of fuse or protector to prevent this.
> Here is a graphic picture http://nimblemotorsports.com/capcharge.jpg
I don't have the time to design it all for you for free. But here is a
taste of the process you need to go through.
If you use regular forward biased diodes for your shunt, the current in
them goes up about 60-120mv for each 10:1 increase in current (depends
on the type of diode). If you only want 330ma, a 1N4001 1 amp diode
might work. Reading from a data sheet, its forward voltage drop is:
0.3v at 0.001ma
0.39v at 0.01ma
0.49v at 0.1ma
0.6v at 1ma
0.71v at 10ma
0.83v at 100ma
0.95v at 1000ma
So at 330ma, we have about 0.89v drop. Three in series is about 2.67v;
pretty close to what you want.
They would burn up if the current gets much higher, as this part can
only dissipate about 0.5 watts. So, you need a fuse in series as well.
A real zener might also work. Let's see... a 1N5221 zener is spec'd at
2.4v at 20ma, and the graph shows the following current at other voltages.
1.0v at 0.1ma
1.5v at 1ma
2.2v at 10ma
2.4v +/-5% at 20ma (guaranteed)
2.8v at 100ma
So it performs worse than the 3 diodes at low currents, but better at
high currents. The problem is that the voltage at 330m is too high (and
this is the lowest zener voltage available in this series).
> The diodes get the current to flow when 2.6v occurs, which would
> send the opto output low, which the recharge controller would
With your circuit, the opto gradually turns on as the voltage gradually
increases. With all the optos in parallel, their output currents add.
You won't get a sharp threshold; instead, you'll get a cutback signal
that is some kind of average. For example, when...
1 capacitor hits 2.7v
2 capacitors hit 2.69v
10 capacitors hit 2.6v
100 capacitors hit 2.5v
This is why I suggested something like a reed relay; its output is all
on (0 ohms), or all off (infinite).
You could also do it with a "programmable zener" like the TL431. This is
an IC, but in a 3-pin package that behaves like an ideal zener. You set
its "zener" voltage with two resistors. An LED in series with its
cathode turns on sharply when you reach the set voltage.
But.. when you have a circuit with a sharp threshold, it can oscillate.
The cap goes overvoltage, the cutback circuit stops the charging, so the
cap voltage falls back below the threshold, which activates the cutback
again... You need extra parts to prevent this sort of instability.
>> - do you want it to be fail-safe, so if something breaks, it gives
>> a FAULT indication rather than a SAFE indication?
If you do, wire all your optos (or relay contacts) in series, so any one
of them turning *off* causes the alarm or cutback. Your circuit needs to
have the opto normally on, and turn off when you exceed the threshold.
There are parts intended as low battery warners that have this behavior
(low=on instead of low=off like the TL431).
Hope this helps!
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
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