Quote:
Originally Posted by 19bonestock88
How badly does the supercap bank self discharge? Take a charge more/ less efficiently than lead acid? No battery in parallel with it?
Where did you get the parts to build?
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These are the caps I ordered:
https://www.alibaba.com/product-deta...27fc3e5fZyJBmC
The caps would probably hold enough energy to start the car for a few weeks (just a guess) but the car has a little bit of parasitic draw on its own and leaving them connected to the car drains them pretty quickly. My DC-DC converter only charges to around 13.6v, so the voltage most cars would top the caps off at would probably last a little longer, but I estimate I have 48-72 hours of standby with no battery, if your car has no more parasitic drain than mine. Or in other words, you can *probably* start a car on a Monday if you park it on a Friday, if you don't disconnect the caps.
I have access to an unlimited supply of lightly used 7 amp hour sealed lead acid batteries at work and the most economical thing for me to do would be to just keep one of these in parallel with the caps to increase my standby time to greater than a week. I had to complicate things though, and added a 20 amp hour LiFePO4 battery, which weighs less than a 7Ah lead acid and has greater charging efficiency. This battery would likely run the starter motor safely on its own in good weather, but it hasn't been above 0F here in around 4 days and anything below freezing is extremely dangerous for lithium batteries. So, I added a 0.3 ohm resistor with heatsink between the battery and caps. The battery only serves to trickle-charge the caps, and, in a pinch - let's say I leave my lights on and run the caps down while I'm in a store - I can just shut them off and wait 20 minutes for the caps to fill back up.
After starting the car with the caps, I hear the fan for the DC-DC converter kick on for around 6-8 seconds, and then they're full again, and it goes back to a low current mode. I have no idea how much energy is wasted keeping a 12v lead acid battery around 14v when driving around, but it's effectively zero with the capacitors.
Another perk is that my car starts just as well at -10F as it does on a warm summer day. Those suckers have some cold cranking amps.
This is similar to the resistor I used:
https://www.mouser.com/ProductDetail...Ju4VZEcCWjA%3d
This is the LiFePO4 battery I ordered:
https://www.batteryspace.com/lifepo4...assed-dgr.aspx
You can get them cheaper without balancing circuits, but I felt it was a good idea.