Stepping down Voltage question

[hermosabeach]

[hermosabeach]

Would a simple buck converter work?

https://www.ebay.com/itm/382620018157

[ibanezfoo]

A buck converter would probably work. You can’t really just put a resistor on there. Where is the extra energy going to go? Heat. Its only 5v differential so it might just work anyway.

I’d be more concerned with using lithium batteries on a device made for nicad. Good way to start a fire. You’d have to test the current draw of the device and make sure your lithiums are able to handle it without bursting into flames. I’m assuming your 20v are lithium, if they are nicad as well ignore what I wrote.

[followingc]

You're on the right track. A buck converter would be the right way to do it.

The important consideration is how much current the vacuum requires. I bet it's a lot more than the 1.8A continuous current the device you linked can provide. There is also the startup current to get the motor moving, called peak inrush current. Sizing your converter 2x your continuous current wouldn't be a bad idea.

[hermosabeach]

I think the old one is 2 - 2 1/2 am hour battery 2000-2500mah @15.5v

The dewalt is either 3 or 5 amp hour at 20v

So should I look at a 5 amp or 10 amp buck converter?

[hermosabeach]

I can’t find published stats by shark on the vacuum on amp draw

[bohoki]

with dc i'd just use a bunch of diodes in series .7v drop each so you need like 7 of them

[starlight]

- ni-cad is shot?

- get a new ni-cad from ebay or amazon and be good for another 5 years







.

[ltranger]

Dewalt 20v batteries are 18v nominal. Your shark vaccum is just a dc motor. Just hook it up it will be fine. In fact it will run a little more rpm so it will be better, just like my daughters power wheels.

[incredablehefey]

Its not to complicated, you need to know what the max amps used during use. If the vacuum is brushless its startup current is not much at all. Once you know just buy an appropriately sized converter and use it between the battery and the tool. If you have or have access to a 3d printer you can make battery holders and electronic boxes. If you are worried about over discharge of the new lithium battery you can by a protection circuit and set it for the discharge level you want, this must be installed between the battery and the buck converter.

[followingc]

I looked up several Shark portable vacs with 15.6V NiMH batteries. Is it like the SV75?

https://www.homedepot.com/p/Shark-Cordless-Pet-Perfect-Handheld-Vacuum-SV75Z/205706173

This one draws 7 Amps according to their specs. A 2.5Amp-hour battery would run it for about 15 minutes. You'd should be ok with a 10A converter, especially if you can bolt it to some metal to distribute the heat.

[five.five-six]

Can you open up the battery pack? it's probably just 18650s with solder tabs. those things are easily found.

[hermosabeach]

The batteries are about AA size. The packs are $50 or so

The consensus is toss the item as batteries are too much money

I can buy a dewalt vacuum for $100 that runs on the new battery backs.

It would be the most reliable option

[incredablehefey]

if your going to toss it any way just give it a try without stepping the voltage down

[ibanezfoo]

The batteries are about AA size. The packs are $50 or so

The consensus is toss the item as batteries are too much money

I can buy a dewalt vacuum for $100 that runs on the new battery backs.

It would be the most reliable option