# Capacitor battery pack

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#### Corrie

##### New Member
New project, i want: A 'battery' pack that will charge from mains, and give me one good crank every 10 minutes, initially for motorbikes, with optional added capacitance for cars.

So, for a bike i need: 170 cold crank amps for 5 seconds; 170(amp hours)/3600(seconds in an hour) =0.04722222222222*5 =0.24ah =864(coulombs)/12 =72Farads.

I can get 500F 2.5V capacitors. I'd need to string 5 together in series to get 12.5V and that would leave me 100F (1/(1/500*5) ).

Can get a 12v 5a psu, so that would take (.24ah*20=4.8)=(60/20)=3 minutes to charge (triple needed for car, so 9 minutes, which would be fine).

Think a tptt switch and relays to simultaneously make the connectors live and isolate the PSU, and a simple volt +amp meter to show when charged and how much juice the bike is taking when i try to start it.

Its just in the planing phase right now so any advice/ criticisms welcome, also I'm new at this kind of thing and really not sure about some of this math..

#### Nigel Goodwin

##### Super Moderator
Capacitors are a very poor substitute for a battery, every few weeks someone asks about using 'super caps' in place of batteries, and it's a poor idea

#### Corrie

##### New Member
I'm not using them in place of batteries outside my workshop. i work on bikes through cold winters, batteries are not good left for months in -15c neither are they happy with cranking a bike over every ten minutes for hours while i look and test for a problem. Its also not ideal buying a battery for a bike i plan to park for 6 months. A capacitor bank hooked to a mains PSU would seem to solve all these problems.

#### ronsimpson

##### Well-Known Member
i need: 170 cold crank amps
Check if you "capacitor" can do 170 amps. What is the part number?

Capacitor verses Battery: A battery at 10% charge has a voltage of about 12. A capacitor at 10% has a voltage of about 1.2 volts.

#### Nigel Goodwin

##### Super Moderator
I'm not using them in place of batteries outside my workshop. i work on bikes through cold winters, batteries are not good left for months in -15c neither are they happy with cranking a bike over every ten minutes for hours while i look and test for a problem. Its also not ideal buying a battery for a bike i plan to park for 6 months. A capacitor bank hooked to a mains PSU would seem to solve all these problems.
A capacitor bank wouldn't, but a battery hooked to a sensible mains charger would.

Why would you imagine a capacitor bank would be more capable of "cranking a bike every ten minutes" than a much more powerful battery?.

#### Corrie

##### New Member
> A battery at 10% charge has a voltage of about 12

That's just flat out wrong, a battery at 10% would be down to 4 or 5 volts and would likely never charge again unless it was a deep cycle in which case i couldn't use it to start a bike/car anyway.

>battery hooked to a sensible mains charger would

thought about this, it would be huge though, maybe put it in a trolley kind of set up, relay switching system that would isolate the charger and connect bike simultaneously, problem is that the charger would need 40+ minutes between cranks. Plus if i left it for a month the thing would need 4 hours before it would even give me one crank

>Why would you imagine a capacitor bank would be more capable of "cranking a bike every ten minutes" than a much more powerful battery?

This seems like a silly question in context, maybe go back and read my second reply to you; I highlighted all my requirements and where the battery had shortcomings.

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#### Pommie

##### Well-Known Member
You've obviously made up your mind so why did you bother asking the question here and then ignore (argue with) all the answers?

Mike.
BTW, try doing your maths in Joules.

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#### rjenkinsgb

##### Well-Known Member
Your "170A for 5 seconds" can only discharge the capacitor bank from 15 to 11V (or possibly 10V), to stay within the normal working range of a 12V battery.

170A at 1V per second needs 170F
200 - 250F would be better to allow for internal resistance etc.

You need at least six 2.5V caps in series to give the initial 15V
I'd say a minimum of three sets in parallel to give adequate capacity to do what you need.

A 10A charger would bring it back up to full voltage in under two minutes, or a smaller charger in proportionately longer time.

#### Corrie

##### New Member
rjenkinsgb, thanks, that's a hell of a lot more caps than i had planed, but it gives me something to work with.

#### Les Jones

##### Well-Known Member
A capacitor bank will not give a more or less constant voltage the way a battery does. When half the stored energy of a capacitor charged to 12 volts has been used it's voltage is down to about 8.5 volts. 12/root 2 (12/1.414) If 25% of it's energy has been used it is down to about 10.4 volts.

Les.

#### ronsimpson

##### Well-Known Member
> A battery at 10% charge has a voltage of about 12

That's just flat out wrong, a battery at 10% would be down to 4 or 5 volts

The Lead Acid battery is mostly flat from "0 discharged" to "100 discharged". It works on a chemical reaction that produces voltage.
A capacitor doing the same job would start out at (some) voltage and ramp down to 0V. A discharged capacitor has 0V on it.

#### Nigel Goodwin

##### Super Moderator
This seems like a silly question in context, maybe go back and read my second reply to you; I highlighted all my requirements and where the battery had shortcomings.
That was my point, the capacitors have lot's of shortcomings as well - probably more so - a battery would generally be more effective, and cost much less for the same amount of stored power. Keeping the battery on trickle charge when not in use solves all the 'shortcomings' anyway.

#### dknguyen

##### Well-Known Member
Isn't storing batteries cold preferable as long as you don't crank them cold? You don't warm up the garage when you're working in it?

#### Pommie

##### Well-Known Member
If you're charging and discharging continuously then they will never get a chance to get cold.

Mike.

#### picbits

##### Well-Known Member
I've recently finished a project which used a couple of Maxell 500F 16V capacitors in parallel to start a car engine every minute. In between starts they were charged up by a current limited switch mode PSU set to 40A @ 14V. Tens of thousand starts later it was all performing as it was when first built. Quite fascinating really

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