Inrush limiter for supercapacitor bank

Thread Starter

PavloRdK

Joined Jun 27, 2020
4
Hi all, this is my first thread on this forum. I've been popping in here quite a few times now, a lot of useful info can be found here.
There's something I couldn't find any info about in a form that I could understand, haha I'll need some help with this...

Ok, so I'm building a motorcycle. Small 125cc project. I've decided to use a supercapacitor bank to enable the bike to start from an electric starter motor. (it has kickstarter too, so not a problem if the pack goes flat). I went this route mostly because normal battery would ruin the look of my project, supercaps are nicely hidden away.

So, I've got a single row of 500F 2.7V capacitors (x6), each one has overcharging protection PCB on top. This is sufficient to start the bike, no problem.
I need a solution to slow down the current going into the capacitors in case if they go flat. The bike has a kickstand but if the pack is flat it will draw all the power and the bike won't start. I've been reading about inrush current limiters etc but electronic components are a bit like black magic for me.

Also I was thinking about diodes... Do I need them on the circuit? My understanding of electronics is very basic so every help would be appreciated. If there is a ready made solution on the market that would be great as my soldering skills are even worse! :D

The bike: Yamaha YBR125
Capacitors: row of 500F 2.7V capacitors (x6 so ~83F ~16V)
Max amps on the bike would be 15A (according to the main fuse on the bike)


I'm attaching bike's original electrical diagram. My supercaps would be replacing the battery.


Thanks!
 

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Thread Starter

PavloRdK

Joined Jun 27, 2020
4
Hi jpanhalt
That was my first thought. I was going to install circuit breaker (20A) with test button to isolate the pack. When isolated the bike will start from the kickstarter and it will run without batterry BUT when the pack is re-introduced to the circuit it will kill the engine as supercaps would be drawing too much power. The circuit will need some kind of charge limiter to re-charge the pack slowly, without killing the engine.
Hope this makes sense :)
 

jpanhalt

Joined Jan 18, 2008
11,088
How much current can your alternator supply before it kills the engine?

A resistor is often used as a simple current limiter, but it can get very hot as with fully discharged caps as it would "see" a dead short to ground. So, lets say you had a 6 Ω resistor. That would limit to 2 A a 12 V supply (your alternator is probably higher). The power dissipated formula is ( I^2 x R). Thus, 2 A @ 6 Ω = 24 W. A 50 W resistor will get quite warm. A 12 Ω resistor will limit to 1 A and consume 12 W, and so forth. Of course, the higher the resistance the slower the charge too.
 

Thread Starter

PavloRdK

Joined Jun 27, 2020
4
Thanks, that's really helpful
My generator output varies depending on revs of the engine (if I understand the concept right). The output gets higher with revs than when the engine is just ticking over on idle. If I understand it right it gives out roughly 160W with regulated volts - 13.7~14.7 (depending on the engine's RPM).

Now the problem that I can't get my head around is how to limit the charge current to the supercap pack without limiting the output power as all this juice will be needed to crank the engine (discharging the caps).
So let's say that the pack is fully charged, the bike's engine will start from both the electric starter as well as manual kick start. And generator will recharge all this drawn power without issues as usually it would only discharge the pack by 1-2V for cranking the engine.
When the pack is flat the bike won't start at all as the power drawn to the supercap pack is too high, it kills the engine (or actually prevents it from starting).

So I'm looking for some kind of a board which will:
* Limit charging current of the circuit so it will charge much, much slower (5-10 mins to recharge 6x 500F capacitors is fine, so minimal current is needed)
* Won't limit the output current of the capacitor pack as it needs all the juice for cranking the engine

So basically a charging limiter that only works one way. Do resistors work in both directions ?
 

jpanhalt

Joined Jan 18, 2008
11,088
I would use a switch with center off. One position would connect for starting, the other would connect for charging with a resistor in series, and the center would be disconnected so whatever minimal drain there is from the bike will not flatten the capacitor pack when it is parked. With a flat pack, put the switch in the center position for kick starting. Lots of switches like that. Something with a rotary motion (like typical automotive starter switches or large battery select cut-off switch) might be less susceptible to being accidentally put in the wrong position than a simple toggle switch.
 

Thread Starter

PavloRdK

Joined Jun 27, 2020
4
I would use a switch with center off. One position would connect for starting, the other would connect for charging with a resistor in series, and the center would be disconnected so whatever minimal drain there is from the bike will not flatten the capacitor pack when it is parked. With a flat pack, put the switch in the center position for kick starting. Lots of switches like that. Something with a rotary motion (like typical automotive starter switches or large battery select cut-off switch) might be less susceptible to being accidentally put in the wrong position than a simple toggle switch.
Wow, simple solutions work best, this is actually really good! Thank you.
After doing some research I've come up with this: (photo attached)

What do you think ? Will it work? I'm absolutely rubbish when it comes ti electronics like this so any help with the values of the components would be great. Or should I just stay with the switched solution ? Thanks!
 

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jpanhalt

Joined Jan 18, 2008
11,088
That is simple and worth a try. Diodes can be viewed as switches. If the limited current is a problem for starting, just pop the fuse in series with the resistor when the caps are flat.
 
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