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I have a 12V 19 watt DC water pump (800LPH), A 12 V DC step down converter, and two Dewalt batteries.

One at 18 V 5 Ah, other 18-54 V 9 Ah.

I've connected pump to step down to battery correctly, but when trying to power with either of the Dewalt batteries, the pump only makes a jolting movement and doesn't start or run.

I'm almost sure the Dewalt batteries should be sufficient to power the tiny pump, what is going wrong?

Step down converter 12V 5A

enter image description here

Pump model AW500

enter image description here

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  • \$\begingroup\$ You never say the specs of the 12V step down converter. Maybe it can't handle the load. How much you estimated as the motor start current, 20A, 10A? Does it read on the motor? \$\endgroup\$
    – Justme
    Commented Sep 3, 2023 at 16:53
  • \$\begingroup\$ Please provide technical data sheets for the devices. \$\endgroup\$
    – Andy aka
    Commented Sep 3, 2023 at 17:08
  • \$\begingroup\$ Added two pics of each spec, unfortunately these chinese pumps don't state startup surge, only that is 19watts \$\endgroup\$
    – David jones
    Commented Sep 3, 2023 at 17:46
  • \$\begingroup\$ @Justme if the stepdown is rated 12v 5a it should be capable of 60watts correct? I would have thought it's adequate with a safe margin to run the 19watt pump \$\endgroup\$
    – David jones
    Commented Sep 3, 2023 at 17:49
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    \$\begingroup\$ @Davidjones If you give the pump something like a 2 m or 3 m head of water on the input, does it start? That head of water would contribute to reducing the starting current requirement, so if it works with that assistance, it would show that the buck converter is inadequate. \$\endgroup\$
    – Andrew Morton
    Commented Sep 3, 2023 at 18:32

3 Answers 3

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The pump is 12v at 19 watt. The current to run the pump is about 1.6 amp. Try connecting the pump to a 12v car battery to test it to see if the pump is good. If so the issues are with the bats or the converter. Good Luck

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  • \$\begingroup\$ You forget that the pump can take a huge amount of current when starting up. Using a car battery to test that pump works does not mean the converter is bad, it just means it is a bad idea to use a 5A converter to power something that requires more that 5A to start. \$\endgroup\$
    – Justme
    Commented Sep 4, 2023 at 5:40
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I've used the mini360 before ans it's pretty impressive but it has a hard limit of 3A current. exceed that and it will go into shutdown. the mini 560 looks to be a large version of the 360 with a 5A limit.

your pump has a 1.6A motor so you should be fine, but The motor needs like 16A to start.

the cheap solution is to use a DC-DC converter supply with a current limit instead of an overcurent shutdown.

search "DCDC" "CVCC"

Else measure the DC resistance of the pump and divide 12 by that to get the peak starting current for your pump, then buy a large enough DC-DC converter to handle that..

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  • \$\begingroup\$ Thanks for the education, it makes sense. Would this module be sufficient Amazon.co.uk/dp/B07V6H8DFQ \$\endgroup\$
    – David jones
    Commented Sep 5, 2023 at 9:40
  • \$\begingroup\$ the problem I'm having is the dcdc cvcc converters cost £30, the pump is only £10, surely there's a more cost effective solution to this? \$\endgroup\$
    – David jones
    Commented Sep 5, 2023 at 10:28
  • \$\begingroup\$ icstation.com/…, I've had similar from aliexpress for lower prices. \$\endgroup\$
    – Jasen Слава Україні
    Commented Sep 6, 2023 at 3:23
  • \$\begingroup\$ @Davidjones "a more cost effective solution" - can you get an 18 V pump? \$\endgroup\$
    – Andrew Morton
    Commented Sep 9, 2023 at 9:41
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Genericly speaking, you overestimate the buck converter you are using. At 12V output it is limited to "< 4 Amps" and 14V to 20V input range. At 12V out you need a heatsink at 3 Amps (or less). Frankly you should add one anyway even if you only pull above 1.5A after startup.

The quality of the caps of a generic module will be less than ideal, you may need some stabilization caps on the output.

Your motor may only take 1.5A under load, but the start up surge will be much higher. Online you can find answers ranging from 2 times the nominal current to 20 times the nominal current. Ideally you'd have a datasheet but you could measure it out by connecting it to a large 12V supply and using an ammeter (ideally with peak detection).

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  • \$\begingroup\$ So a good rule of thumb is times the normal load by 10 - 20 fold when choosing a step down conveter(or other types for that matter) \$\endgroup\$
    – David jones
    Commented Sep 5, 2023 at 9:43

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