Supercapacitor Battery Replacement

[redpoint5]

I tested the configuration of my supercapacitor / LiFePO4 in the TSX and recorded various parameters when starting the car.

The super capacitors are connected to the car with 4 awg cable and ring terminals. The bank of six 350 farad capacitors in series has a total effective capacitance of 58F at full charge (16v). However, the LiFePO4 battery only maintains a voltage of 13.3v, so the total capacitance is further reduced.

To protect the 4.2Ah LiFePO4 battery which only has a 2C charge rating, I have a 0.2 ohm 50W resistor connecting the negative lead of the battery to the super capacitor. The battery is in parallel with the supercap.

In line with the battery and super cap, I have two watt meters that measure peak and cumulative amps for both charge and discharge. They draw 14mA each, which is significant considering the TSX has a parasitic draw of 42mA. Without the meters, the car could sit for about 4 days before the battery was drained. With the meters connected, I could only sit for 2.5 days.

First, I did 5 engine starts with the capacitors only and recorded peak amps, and voltage sag (minimum voltage reached while cranking). Each time I began cranking at around 12.5 volts.

Peak Cranking Amps - Voltage Sag
128A - - - - - - - - - - - 7.0v
212A - - - - - - - - - - - 7.2v
182A - - - - - - - - - - - 6.2v (failed to start)
184A - - - - - - - - - - - 6.6v (barely started)
192A - - - - - - - - - - - 7.7v
180A average - - - - - - 6.9v average

Next, I connected the LiFePO4 battery to the supercaps to determine the peak amps during start and charge for the battery only. The 0.2 ohm resistor limited the current of the battery, causing the supercap to do most of the work.

Note that these figures are for the battery only. The supercap is still drawing ~200A at start. Voltage sag was again measured at the capacitor and not the battery. Battery sag was minimal given the limited discharge. All starts performed near 13 volts.


Battery Cranking Amps - Battery Charging Amps - Capacitor Voltage Sag
-15.5A - - - - - - - - - - - +3.5A - - - - - - - - - - - - 8.4v
-16.2A - - - - - - - - - - - +3.5A - - - - - - - - - - - - 8.3v
-15.8A - - - - - - - - - - - +3.4A - - - - - - - - - - - - 8.4v
-14.3A - - - - - - - - - - - +2.9A - - - - - - - - - - - - 9.0v
-15.5A average - - - - - - +3.3A average - - - - - - - 8.5v average

https://www.youtube.com/watch?v=LX_o3hl2Dv4

[Enki]

Have you considered doubling up on the capacitors to reduce the voltage drop at start?

[redpoint5]

Originally Posted by Enki

Have you considered doubling up on the capacitors to reduce the voltage drop at start?

If I were to make this a permanent solution, I would likely run 1500F capacitors instead of the 350s. The kit on Ebay is $209.

I am also considering reducing or eliminating the current limiting resistor so that the LiFePO4 can contribute more to starting the vehicle.

My third idea is to connect the battery to the capacitor with 2 parallel connections. 1 connection would have a current limiting resistor to reduce the rate of charge. The other connection would have a power diode that would allow the battery to supply cranking amps (at a 1v reduction), but not allow charging through that connection.

[2000mc]

looking at $209 for just the capacitors, what are the advantages against just running a big lifepo4 battery? such as Amazon.com: Shorai Lithium Battery LFX36L3-BS12 Lightweight Powerful: Automotive for $283, "claiming" a 540cca equivalent

[serialk11r]

I think your current limiting resistor has too much resistance. 2C means it can take about 8A right? Drop it down to 0.1ohm and you'll get charge protection plus a more effective battery. I bet you can get away with even less than that.

[redpoint5]

Originally Posted by serialk11r

I think your current limiting resistor has too much resistance. 2C means it can take about 8A right? Drop it down to 0.1ohm and you'll get charge protection plus a more effective battery. I bet you can get away with even less than that.

I think you're right. I got the 0.2 ohm with worst case scenario in mind, where the voltage has dropped to 10v before starting, and then the alternator puts out 14.5v; a 4.5v difference. When the battery voltage is closer to alternator voltage, a smaller resistor can be used, and I bet 0.1 is ideal. Perhaps allowing a higher current when the battery has been more deeply discharged is still safe.

I do have 2x of the resistors, so I could wire them in parallel and that would give me the 0.1 ohm resistance you suggest. The results of which we already know. Double the charge and discharge current.

I'm almost tempted to try a 0.05 or even a 0.01 ohm resistor. Heck, Lasersaber didn't even use a limiter... then again he blew his battery jump starting his lawn mower.

[Enki]

Keep in mind resistors don't have as tight of a variance as diodes do, so two of them may not have the output you'd expect (but it should be pretty close).

[ECONORAM]

Originally Posted by Enki

Have you considered doubling up on the capacitors to reduce the voltage drop at start?

That was the same thought I had. Running caps in a series/parallel config.

Originally Posted by redpoint5

If I were to make this a permanent solution, I would likely run 1500F capacitors instead of the 350s. The kit on Ebay is $209.

I am also considering reducing or eliminating the current limiting resistor so that the LiFePO4 can contribute more to starting the vehicle.

My third idea is to connect the battery to the capacitor with 2 parallel connections. 1 connection would have a current limiting resistor to reduce the rate of charge. The other connection would have a power diode that would allow the battery to supply cranking amps (at a 1v reduction), but not allow charging through that connection.

I am reading this thread with great interest, but don't think I am following your thought process here with the battery/capacitor connections thing.

[redpoint5]

Originally Posted by ECONORAM

That was the same thought I had. Running caps in a series/parallel config.



I am reading this thread with great interest, but don't think I am following your thought process here with the battery/capacitor connections thing.

The LiFePO4 battery has a rated output of up to 168 A, but the charge rating is only 8.4 A. My idea was to provide separate charging and discharging paths so that the full output was available for starting the vehicle, but the charge would be limited.

I don't think I'll go with this idea.

[Cobb]

I notice this isnt in the unicorn section, but Im not seeing any data in regards to fuel economy. I had hope the savings would be 2 fold. 1 from weight reduction, 2 from less load on the electrical system to recharge the starter storage device (battery and or caps).