DIY power tool battery powered compressor?

[Greg_E]

This converter is "rated" at 40amps and the compressor is tagged as being 26 amps@12 volts from the factory. I should have enough current overhead to handle the job. The batteries should have only been a 3c discharge (1.5c each), way less than any circular saw would put on them, so again should have been enough.

But this is a cheap DC-DC converter from unknown origin, the "rated" values may be way off. I'm also guessing it sags to 11.5v or slightly less under load because it's cheap. The no load voltage was 12.2v. I'll do more testing in the next day or two, getting too late to be making that kind of noise. I need to juice up my pair of 9ah packs and see what happens, the current draw on each should be under 1c.

What does worry me is that if the compressor is running at 11v, it will be using more current to perform the same work (power=volts*amps) and it could overheat the electric motor. Something I may need to check on.

And since this is a "simple" buck converter, I may need to find a nice switching converter, didn't think of that until now. Efficiency would go up a bit with a switcher, and I might be able to get the output up to the 13.8 that a running car would produce. The instructions say that you should run the car when filling, so that voltage should be fine.

[Rob M]

all makes sense.. The sag seems normal also .. I am shocked that converter is rated for 40 amps , but yea that would cover anything the compressor could draw..I wonder if those battery packs have the BMs module on board , or if it resides in the charger.

[Greg_E]

The Ryobi batteries have some form of BM in the pack, but it does not have low voltage cut off! I can ruin them if left in one of the radios for too long because that radio keeps pulling power even when off, probably the USB charging circuit. I've wrecked one or two of the old cheap 1.5ah packs on that radio.

All the other tools seem to have some form of LV cut in the tool, I've run them all pretty low to where the tool stopped spinning. Exceptions might be the lights, never run one down that low (yet). Mostly I use the 9ah in the lights, they will run for more than an entire day, but they do give a big boost to my circular saw, string trimmer, and 1/2 inch impact. The staple gun should never be used with anything bigger than 1.2 ah slim batteries. You get no additional performance, a lot of weight, and run time on the little ones exceeds anything I've used it for, which is about 200-250 staples in one day.

And yes, I was wondering about the DC-DC device I bought, it's probably junk and I'll have to measure things to find out. At least it will be easy to swap, everything is on XT60 connectors, so glad they used a common connector on the compressor!!!

Weekend plans have changed, need to rip wet insulation out of attic, and replace with new insulation and spray to stop mold/mildew... All on a 1.25 year old roof. Nope not happy, yup roofer is blaming other things, not looking like warranty covered. Warranties are only as good as the provider, lifetime might be the lifetime it takes to get the money transferred from one account to the other. Should have forced them to build the roof the way I wanted, but I'm not a roofing expert so I let them do it the way they do all their roofs. Needed metal drip edge about 2 feet up, and needed rubber ice and snow shield all the way to the peak... Would have been happy to pay the small increase at the time. Probably going to slam a metal roof on this summer so I don't have to deal with this again. This not worthy because Saturday is supposed to be new airgun day (according to UPS) and I wanted to see how this battery mod would work up to 4500psi.

[Greg_E]

Long time coming, but finally a follow up! New gun turned into a pumpkin, waiting for the refund check in the mail.

Just gave my compressor a run on my Pitbull extended and regulated Bulldog from 3000 up to 4500psi.

On AC it did this in right around 5 minutes.

On dual 9ah batteries it runs more slowly, and did this in 7-8 minutes. The batteries had been sitting for a month, so not completely full, but close. Used 1 bar out of 4 on the little power meter. I did not measure the voltage, too hard to get the probes in there, but betting it is slightly under 12 volts.

I'm guessing that I could put the batteries direct and the voltage sag would be enough to be safe. But that's a guess and I do not take responsibility if you fry your compressor!!!

Ultimately I think this 24 to 12 volt converter is safe to use, and I'll probably figure out how to mount it and the batteries to the compressor. But I'll keep an eye out for a 40 amp 24 to 13.8 volt converter, so a limited success for now!

[Greg_E]

Did another test, batteries were off the charger for about 30 minutes.

While pumping somewhere between 3500 and 4000 psi, I measured the expected 12.03 volts at the compressor, and 19 volts at the battery Y joint. Not bad for pulling about 12 amps per battery.

Filled from 2900 to 4500 in 8 minutes on battery, and pulled one bar off the four segment battery indicator.

I should get 4 fills per pair of batteries, not great but better than pumping or spending a bunch of time running the car.

Moving up to a 13.8 volt converter might bring me so greater efficiency, so I may do that, but having trouble finding one that has a 17 or 16 volt advertised lower input. The converter I bought listed a 15 volt lower limit. This is important because when it hits that limit, it might bypass and send battery voltage straight to the output. Depends on the design.

[sb327]

One of those ‘so far, so good’ times. Sounds like it is working like it’s supposed to. Nice progress and kudos for following through on an idea. Sounds like overall expense was kept within reason too.

Dave

[Greg_E]

Expense is going up a lot, I need 2 more batteries, but at least I can use them for other tools too. If you remove the batteries, this is like a $30-$40 mod.

I really want to find a switching regulator, Ill have to look harder.

[Greg_E]

Not sure if anyone is interested, but I built a 24 to 13.8 system for my GX CS4. Just in testing mode with the battery clamps.

[Greg_E]

Running into problems with the CS4, measured 689 watts to start compressor and around 600 watts above 4000psi. This might not be a good candidate for battery power.

[Pellethuntr]

That's what like 60amps on your "12v" system? That seems like a lot of amps after the initial start up. You can put a capacitor in to help with the amp draw at start up but that woln't help you while its filling.

[Greg_E]

My meter said 52.25 amps peak.

I started looking at car stereo capacitors, "dang" things are BIG! I may pull a dead computer supply apart and dig out the largest filter caps present and see what happens. A user at AGN said he used a killowatt and measured 370 watts at the AC plug.

[Greg_E]

Used dual 24-12 40a converters and it runs fine. 50 amps or 600 watts and voltage only sagged to 11.98 with a normal high of 12.21 volts. Looks like it just needs more capacity because the manufacturer averaged the draw, not giving a peak.

FYI, the 650 watt AC supply sagged down to around 10 volts when it started, so instantaneous peak might be more than 650 watts and my meter just wasn't fast enough to catch it.

And all that said, 600 watts is a lot for most idling cars, make sure to let the battery charge a bit before shutting the engine off.

[Pellethuntr]

Used dual 24-12 40a converters and it runs fine. 50 amps or 600 watts and voltage only sagged to 11.98 with a normal high of 12.21 volts. Looks like it just needs more capacity because the manufacturer averaged the draw, not giving a peak.

FYI, the 650 watt AC supply sagged down to around 10 volts when it started, so instantaneous peak might be more than 650 watts and my meter just wasn't fast enough to catch it.

And all that said, 600 watts is a lot for most idling cars, make sure to let the battery charge a bit before shutting the engine off.

Nice work Greg and thanks for keeping things updated!

I would like to see the start cycle on that unit with an oscilloscope. It is probably more amps than you think. Might be worth putting a start capacitor in. It would probably gain you a little more run time and be less stress on your batteries long term.

[Greg_E]

I was looking at caps for car stereos, man they are big and some are expensive. What I decided was to buy a second 13.8 converter and I'll just run the pair for now. A 60a or 80a version was way over $125. With the dual 13.8 the amps should be down below 50 which should give a lot of overhead on these cheap converters. They are rated for 5xx watts each, not sure if that is peak or continuous. Probably peak based on what I saw. Triple the price and things were rated continuous 40 amps with peak to 80 amps, these expensive ones probably would have worked.

Back to the car stereo caps, they also scare the daylights out of me. Exposed terminals, massive current, just seemed like an explosion waiting to happen with my use case.

I'll eventually look back into motor start caps, I think one would extend the life of the motor by a lot.

[BSJ]

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