Supercapacitor Battery Replacement

[redpoint5]

Originally Posted by ECONORAM

I was looking at All Electronics today, and found they have 2.7Vdc Maxwell 1500 Farad capacitors for $25 each. All Electronics | Electronic and Electro-Mechanical Parts and Supplies at Discount Prices

This is a phenomenal price since Mouser is selling the same thing for $47 without a free shipping option. It amounts to 60 farads per $1. This is an even better price per farad than the BCAP3400 capacitors at 48 farads per $1. Mouser's current best price per farad is the weldable BCAP3000 at $58 each, or 52 farads per $1.

Here's my conundrum; I need 3000 farads for my truck, and to achieve that with the 1500 farad capacitors, I need to buy 12 of them (6s*2p) at $25 each, for $300 total. If I went with the 3400 farad capacitors, I would only need to buy 5 (5s*1p) at $68 each, for $340 total.

The 3000 farad "battery" consisting of 2 parallel banks of 6 series cells would have a total series capacitance of 500 farads. The 3400 farad battery with the higher 2.85v rating would only need 5 cells in series, for a total of 680 farads. So buying the "cheaper" 1500 farad capacitors would give me 1.66 farads per $1, while buying the "expensive" 3400 capacitors would give me 2.0 farads per $1.

I'd still be better off getting the BCAP3400 for my application, even though the price on the BCAP1500 is better in most other applications. I still might get the BCAP1500 though, because the price can't be beat.

[Enki]

It may also be significantly easier to fit the 12 pack, not to mention the additional balancing that happens if you connect them up a certain way (6 parallel banks of 2 caps connected in series). Charging them all up at the same time should tell you which ones are slowest charging vs fastest, and combining a slow with a fast should have a decent balance per bank of 2, if that makes sense.

[Speedtoy]

I'm new to this thread, and I am following it with great interest. I'm enjoying the technical content much.

My curiosity, is to use a 6x3000 bank for starting, with a lightweight 12v racing battery in parallel to keep the cap bank charged and the .05a drain that the car wants, happy.

At the same time, upgrading the starter cable from the rear battery tray, to 0/1ga as well.

[Enki]

Depends on how often you will be starting the car to recharge everything. My car has a drain of half what you're seeing so I have to start it at least once a week (usually twice in the winter) on a Braille 23 AH reserve lead acid. Even then, I have a spare that I swap out after de-sulfation every six months or so. Any idea what your total amp draw is when cold starting?

Couldn't hurt to try.

[Speedtoy]

Originally Posted by Enki

Depends on how often you will be starting the car to recharge everything. My car has a drain of half what you're seeing so I have to start it at least once a week (usually twice in the winter) on a Braille 23 AH reserve lead acid. Even then, I have a spare that I swap out after de-sulfation every six months or so. Any idea what your total amp draw is when cold starting?

Couldn't hurt to try.

It's either a daily driver, or on a smart charger. (Porsche 928)

I have asked this, and been told "in excess of 200A" but since the motor swap isn't done yet, I cannot measure this on my own yet.

[redpoint5]

I've been distracted lately by a Prius purchase, but I'm looking at buying caps soon. My main concern at the moment is engineering the best balance circuit possible. I'll be buying 5x of the BCAP3400 caps to maximize the farad rating and keep costs down.

I created this very rough spreadsheet that auto-calculates things like how many days a vehicle can sit and still be able to start. It must be opened to input your own values. Look for the Cap Calculator tab and enter values in the top right box to see the calculated values.

[Enki]

Here's what mine looks like.

Edit: Was going to attach directly but the size limits here are ridiculous.

Click for big.

[Speedtoy]

What would be the gauge of cable you would use to jump from one 3400f cap to another?

Maxwell shows nice little buss bars with their integration kit, but I wont install these in a row of 6, but in a 3x2 configuration. So at some point I will have to use a cable.

I will use the screw version of the cap, I dont believe I can weld to the terminals by hand fast enough to prevent overheating the cap.

[Enki]

I just used pieces of aluminum and steel nuts.

[Speedtoy]

Photobucket wont bring it up...