What about using super capacitors in battery bank

AzJames

AzJames

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I've been thinking about making a portable battery bank that can go to jobsites to power a chop saw. I thought about using some super capacitors to take the brunt of the start up load when I turn on the saw. That way I can extend the life of my batteries and maybe even use a smaller power bank, but get the same results as a larger power bank.
Has anyone tried this?
 
there are a few. But I think the chop saw will kill the batteries fairly quickly, even with a bank of super caps. Not knowing the ratings of your chop saw, but I'm going to guess you'd need at least 2kW inverter with 4kWh surge. And if you want any life out of the cells, you'd need to go with 48V and probably around 80p.

Maybe you could extend their life a little by also having a solar panel or 2 to help charge while on the job site.

Now, if your chopsaw uses a universal motor, then you could potentially run it on DC and forego the inverter. But then you'd need to run at least 96VDC, but better to run 120VDC. Buuuut, this is getting into dangerous voltages with DC, so not advisable unless you have a LOT of safety in place to keep from something/one getting fried.
 
I think your only real hope to run powertools (inductive load) off an AC inverter (right? am I understanding your plan?) is to use a SLOW / SOFT START mechanism, on the AC side.

Its going to come down to how much surge can your inverter take. which largely depends on voltage of your DC supply (how many S are you planning? 24V or 48V? 12V for every thousand watts you want as AC ...)

Caps aint gonna do anything for you. Powertools are brutal. Think washing machine or a vacuum cleaner. That's my $0.02.
 
And I'd gather, using soft start on the job site is probably not a desired function. It would mean that your saw(s) take longer to spin up to full speed, which means it takes longer to make that cut. And depending on how many cuts you need to make, it could add 10's of minutes to the job per day, or even up to an hour just waiting on the saw to spin up.
 
Korishan said:
And I'd gather, using soft start on the job site is probably not a desired function. It would mean that your saw(s) take longer to spin up to full speed, which means it takes longer to make that cut. And depending on how many cuts you need to make, it could add 10's of minutes to the job per day, or even up to an hour just waiting on the saw to spin up.
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although not ideal, it would work, which is better than not. same principle for running a fridge etc ...

i'm not sure if slower startup for tools would matter to a wage earner. maybe to a contractor. ; ) we're talking a ramp up in half a second, not 20 seconds.

the main point is that this is better solved on the AC side, and its not an issue which caps (on the DC side) are going to address.
 
IF the inverter doesnt have enough caps then buy better inverter. Adding caps to a battery bank made of lithium is generally useless due to lithium being high Power. An inverter can easily take a shorter high current surge. And IF the battery fails its most likely a bigger issue from My experience :)
 
Yes, I tried it.....

image_lcqqsg.jpg


The electrical reason is why they are not that good in a conventional battery setup is that energy in capacitors is stored as a charge relative to the voltage and not as a chemical state that holds a relatively flat voltage during discharge so the economics and issues are slightly different.

Energy = 0.5 x Farads * Volts * Volts

If your battery pack drops 0.5V before it can deliver 3kW a supercap would only be able to discharge 0.5V of charge before it is at the same voltage as the battery and just sit there.

For a string of 5 supercaps (rated at 500 Farad at 2.7V) this would store 9113 Joules of energy (just over 2Wh) at a terminal voltage of 13.5V. Dropping the voltage to 13V would only discharge around 663 Joules of energy or 50A for 1 second or 663 Watts for 1 second.

It works BUT economically, supercaps are still way too expensive to use.... just add batteries.
 
completelycharged said:
Yes, I tried it.....

[...]

It works BUT economically, supercaps are still way too expensive to use.... just add batteries.
Click to expand...

Um, so we are agreed then, that NO it does NOT work. (assuming using an AC inverter..)

Surge rating on the AC inverter is the key.

If your inverter is rated ok, and your battery can supply it, then ... no caps needed. If your inverter cant handle the surge, and/or the battery cant provide enough SUSTAINED power to the inverter then super/caps wont help.

Good idea, just not applicable for your intended application.
 
Capacitors inside inverters are usually only intended for surge/noise balancing 20mS or less of the exported power (half or full wave).

The startup of a chopsaw typically takes just under a second (at least mine does with no soft start - 2200W) and the startup power is over 8kW in the first few cycles (kick) with a 300mm disc (more inertia slowing the startup) and trips a 16A Type B breaker on the odd occasion. The 220mm angle grinder is even worse..

The pictured capacitors (15 x 500F 2.7V) would only provide less than 20% of the startup load surge so would need to add a few more packs to decrease the spike on the battery pack to the sustained power draw level. Not economically viable today but an interesting experiment that proves it works at taking some of the spike away from the pack.
 
Or just buy some more batteries for the same cost that van take the surge and hold more capacity :)
 
If you had the solar panels on site you could leave the saw running at slow speed with a speed controller and just wind it up and down as needed.... maybe....
 
camthecam said:
If you had the solar panels on site you could leave the saw running at slow speed with a speed controller and just wind it up and down as needed.... maybe....
Click to expand...

OH&S!.... I'd want that blade stopped!
 
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