Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Is this a bad idea? Supercapacitor bank to crank engine, fed by the main battery

Status
Not open for further replies.

Sam5253

New Member
I'm thinking of putting together a source of backup power that could crank the engine and start the car, even if the battery is discharged and/or cold. A bank of supercapacitors would provide the power to turn the engine. 6 x 2.7V/500F in series with balancing circuitry, to total 16.2V/83F max. The supercapacitors would be charged from the main car battery via a voltage boost circuit, so even when main battery voltage drops below 12V, the supercapacitors can still be charged to about 14V. A diode would prevent power returning to the battery, and the starter would be the only thing connected to the supercapacitors.

Here is the general order of the circuit I have in mind:

car battery > diode > voltage booster > supercapacitors > starter

Could this damage the car's battery or electronics? Am I overlooking anything here? Thanks for any input.

Sam
 
What is the effective series resistance of your 83F capacitor? The peak in-rush starter motor current (depending on engine displacement) to get the engine turning is like 500A, and then the cranking current until the engine starts is > 100A.
 
I would say yes, it is a bad idea.

Keeping MikeMI's post in mind, note the graph below:
upload_2017-5-9_10-54-57.png

The voltage, across ALL types of caps (an "Ultra" type in this case) when under a load, drops very quickly, vastly reducing the power (I times E) that can be delivered.

They simply aren't suitable for your application at a reasonable price.

Not that it can't be done, For instance: but the cost:
upload_2017-5-9_11-17-24.png

is remarkable. Note also that it's only rated at 100A.
 
Thanks for the replies. I hadn't considered the internal resistance of the capacitors. The ones I was looking at (on eBay) don't even list it, so not going to look further into that particular item.
https://www.ebay.ca/itm/1-6x-2-7V-5...hash=item2a7915507b:m:mnLfkxjV1hetPVW03Taprtg

I see Maxwell units, as suggested by cowboybob, have more complete specs. 1900A is overkill for a car starter, but a smaller model such as this one might be usable?
https://www.ebay.ca/itm/Maxwell-2-5...101735?hash=item25cbed1327:g:MEMAAOSwo4pYYgvM

At 15V and 19 mOhm, I calculate a peak of 789A. Seems this should be sufficient to get the starter going, as the starter current drops dramatically after the initial rush of current.
 
At 15V and 19 mOhm, I calculate a peak of 789A.
That would be the current into a dead short, not into a starter motor, if 19 mOhm is the cap resistance. Or is 19 mOhm the starter resistance?
 
BTW, Welcome to ETO, Sam5253!

I think the major downside of using caps (of whatever flavor) as a "battery", in particular as you envision, is that their full voltage level immediately starts to drop from its peak as soon as a load is applied. This has the effect of, in a like manner, of quickly reducing the power transferred to the load, as graphically represented in the graph in post #3.

This effect can be lessened, to a degree, by massively over building the circuit by increasing the total capacitance to, in my opinion, ridiculous and very expensive high values for automotive use.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top