# Can I drive a 12V water pump with a 3.7V battery and capacitor.

#### JPU

May 19, 2012
282
Hi All

Is it possible in some way to use a 3.7V battery to charge a Capacitor and therefor power a small 12V water pump for about 2 seconds. Thats just enough to fire a small burst of water out of a tube.

The design I am thinking of would use a a Picaxe chip and motion sensor powered by a 3.7V battery. When the sensor is activated I want the pump to then fire of a small burst of water through a jet. The only problem is that I do not want to use another power source other than the 3,7V battery as each burst only last a second. I intend to use 12V windscreen wiper water pumps as they are readlily available and reliable.

Any help or ideas would be apprecited,

Many Thanks

JPU

#### Gryd3

Jun 25, 2014
4,098
Hi All

Is it possible in some way to use a 3.7V battery to charge a Capacitor and therefor power a small 12V water pump for about 2 seconds. Thats just enough to fire a small burst of water out of a tube.

The design I am thinking of would use a a Picaxe chip and motion sensor powered by a 3.7V battery. When the sensor is activated I want the pump to then fire of a small burst of water through a jet. The only problem is that I do not want to use another power source other than the 3,7V battery as each burst only last a second. I intend to use 12V windscreen wiper water pumps as they are readlily available and reliable.

Any help or ideas would be apprecited,

Many Thanks

JPU
Yes and no...
Even if you fully charge the capacitor the 3.7 is too low to power the motor. So No.
You 'can' get a boost converter to bring the voltage up to 12V to charge the cap. Then Yes.

The size of the capacitor may be an issue depending on the requirement of the pump.
1 F = 1 As / V
1A for 2 seconds @12V
? F = 1A 2s / 12V
160mF will 'just barely' do it, but the voltage will drop off quite quickly.
Perhaps someone has a little more intel on a more suitable size.

Additionally, the boost converter will drain the 3.7V battery almost 4 times faster.
So if you want to charge the capacitor at 200mA @ 12V, you will find that you are pulling 700-800 mA from the little 3.7V battery. So be sure to pick one that can last. Otherwise you should but a 12V battery and simply step it down to 3.7 for your other electronics.

#### Viktory2k1

Apr 14, 2015
41
I would say no, depending on the exact desired outcome. By the time you do the math and find a cap that will fit your needs, you could have already made/bought a 12v power supply. You need the 12v and desired amprage, even for 2 seconds. A pump like that, if I can remember right will use around 500Ma at 12v, like a fishtank pump. You might have a PS laying around already for this. I think a ps with 500ma would be more then what the pump wants so even better. Check out some old cordless phone chargers or old printer ps's in the basement to see if they meet your needs. Old printer power supplies usually are much higher voltage though. I wouldn't mess with a cap like that.

#### Kiwi

Jan 28, 2013
473
You got cat trouble?

#### Gryd3

Jun 25, 2014
4,098
I would say no, depending on the exact desired outcome. By the time you do the math and find a cap that will fit your needs, you could have already made/bought a 12v power supply. You need the 12v and desired amprage, even for 2 seconds. A pump like that, if I can remember right will use around 500Ma at 12v, like a fishtank pump. You might have a PS laying around already for this. I think a ps with 500ma would be more then what the pump wants so even better. Check out some old cordless phone chargers or old printer ps's in the basement to see if they meet your needs. Old printer power supplies usually are much higher voltage though. I wouldn't mess with a cap like that.
To be fair, you can buy some double layer, or super-capacitors for \$10-15 quite easily to get a capacitor array of 1F and well over 12V. I simply don't like the capacitor idea for powering a motor due to complexities. You also have to concern yourself with the fact that the pump will draw plenty more current on start-up so a 500mA supply may be undersized.

*Considering it sounds like it needs to be portable, I still think the best option is a small 12V Rechargeable Sealed Lead Acid and a small 3.7V regulator.
The batteries can be quite small, and are often used in home alarm systems inside the panel. The 3.7V regulator would be simpler to build and install than a 3.7V to 12V boost converter.

#### JPU

May 19, 2012
282
Thanks all for your very help full answers. I think Ill go with using a 12V batter and regulator.

Kiwi, Not cats, much bigger than that, LOL!

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