# Capacitor to help acceleration?

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moveon70
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Capacitor to help acceleration?

The LiFePO4 pack that I bought some times cuts out when pulling from a stop.
The bike never did that with the lead acid batteries.
I am assuming that the BMS is current limiting.

Has any one tried to add a super capacitor in parallel with the battery to help at take off?

I am wondering if the cap could hurt the controller some how by allowing current transients.

Calculations:
Kinetic energy in Jules=(mV^2)/2 (m in kg, V in m/s)
Energy stored in a capacitor in Jules=(CV^2)/2 (C in Farads, V in Volts)
It would take 5kJ to bring the mass of the bike and myself up to 9m/s (20MPH).
No-load voltage is 48V (lets just say 50) and to keep a good strong acceleration, we dont want the voltage to drop below 40V. would store 5kJ in a 10F capacitor.

So, if I add a 10F capacitor, I should have enough energy stored in that capacitor to get me up to full speed. Once the load reduces, and the battery voltage increases, the batt would recharge the cap while I ride.

Any one try this?
The Caps are cheap these days. \$50 for 100V, 10F

-Mark

My blog

moveon70
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Re: Capacitor to help acceleration?

Sorry all for my error here.
I was off by three orders of magnitude.
A 10 or 20F capacitor is not cheap, nor easy to get.
What are available are 0.02F capacitors. I bought one and tried it, and it did not help with acceleration. The capacitance is too small to store any appreciable amount of power.
If high voltage, high power capaictors ever became so cheap, the EV world would change.

marylandbob
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Re: Capacitor to help acceleration?

Another consideration is the present SIZE and weight of a 20 Farad 100 volt capacitor!--Try paralell connection of a high "C" rate lithium polymer battery, or simply use TWO identical batteries like the present one, in parallel.(Charge separately BEFORE connecting!)-Bob

Robert M. Curry

moveon70
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Last seen: 10 years 4 months ago
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Re: Capacitor to help acceleration?

Thanks for the good advice Bob,
I have seen those LiPo batteries in action, an they can dump an incredible amount of current compared to Li-Ion or LiFePO4. So impressive! I saw a radio control airplane use one, and the little pack the size of a cell phone can dump over 30A!!

After some searching, I did find some interesting capacitors out there, but they are not cheap.
Maxwell makes some ultra capacitors limited to 2.7V each. They could be run in series, but need controls to limit the voltage.....same as a BMS for a Li battery. Here is a pack that they all ready put together with controllers internal.
http://www.maxwell.com/ultracapacitors/products/modules/bmod0083-48-6v.asp

Also EEstore in Texas has worked to developed an ultra capacitor to be used in Zenn EV and possibly Raytheon EV for defense.
http://www.eestor.biz/

I was originally interested in adding the capacitor in parallel to help accelerate from a stop, but they guys are working on capacitors to replace the entire battery.
The advange would be very fast recharge times (limted only by the charging circuit) and no appreciable amount of degradation over time.

They still weigh more than Li-Ion for a given amount of energy though.
http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

Still, if the recharge time was only a couple of minutes, I would not mind having reduced range and thus a smaller capacitor bank.

If I can find a deal, I think I will try this out on a little stand up scooter for experimentation.
At current cost, a capacitor to power a little stand scooter for 15min up would cost over \$2k
Its exciting to watch the technology progress.

For the capacitors I all ready bought....I may be able to make a Capacitive Disscharge spot welder to make my own Li-Ion battery packs.
http://www.ledhacks.com/power/battery_tab_welder.htm

-mark

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