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I have a very old windscreen motor attached to a rotisserie which I would like to run. I was told that I need a battery charger, but most of the ones I can find are "smart" ones. I have a 2A smart charger and it doesn't work, reading a bit more it seems I need 10A capacity to start the motor while 'stalled'.

My question is does the charger have to be one that can explicitly run 12V appliances? E.g.

https://www.projecta.com.au/battery-charger-products/12v-automatic-10a-7-stage-battery-charger

or, can I get this intelligent charger with 7 modes?

https://www.supercheapauto.com.au/p/sca-sca-12v-10-amp-7-stage-battery-charger/544706.html

I have no idea what the stages in the specifications quoted below are:

SCA 12V 10 Amp 7 Stage Battery Charger
Chemistry Types: Standard Lead Acid / AGM / GEL / Calcium
Stages: 1: Primary Analysis (Desulphation) / 2: Soft Start / 3: Bulk Charge / 4: Absorption / 5:
Voltage: 12V
Start Voltage: 0 Volts
Amperage: 10A
Protections: Surge / Reverse Polarity / Overload / Short Circuit
Suitability: Batteries up to 1100CCA / 100AH
Warranty: 12 months
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    \$\begingroup\$ It sounds like you need a power supply rather than a battery charger. A smart battery charger expects to find a battery at the end of its leads. If it doesn't behave like a battery, then the charger may shut down to avoid damaging the battery. An old style battery charger was dumb, more like a power supply. \$\endgroup\$
    – Neil_UK
    Commented Jan 30, 2022 at 7:32
  • \$\begingroup\$ thanks @Neil_UK that is what I suspect. So why is it that all the smart battery chargers are much cheaper than the the battery charger that can be a power supply? \$\endgroup\$
    – Alex
    Commented Jan 30, 2022 at 8:08
  • \$\begingroup\$ Cheaper probably because they don’t use a big transformer and components used are cheap so they don’t last long. \$\endgroup\$
    – Solar Mike
    Commented Jan 30, 2022 at 8:13
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    \$\begingroup\$ You just need a 12V power supply. It should be easy to find one. They're cheap and common. A PC power supply is one example. It won't need to supply 10A since the motor won't draw that unless it has a heavy load. You'll probably need more like 3A. \$\endgroup\$
    – Drew
    Commented Jan 30, 2022 at 8:42
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    \$\begingroup\$ You don't need crocodile clamps. You can just hook up the wires with wire nuts or solder, or 100 other ways. You can even buy croc clips with a screw terminal on the back to attach a wire. Just get some electrical tape and a pair of wire strippers and wing it, you'll figure it out. \$\endgroup\$
    – Drew
    Commented Jan 30, 2022 at 8:49

2 Answers 2

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The battery smart charger takes samples of voltage and/or current, to sense the status of the battery, and uses sample data to control the charging output. This is why a smart charger is not suited to drive a motor.

Wiper motor hobby page describes typical power sources:

http://www.scary-terry.com/wipmtr/wipmtr.htm

Sites listed on the hobby page that sell power sources to drive wiper motors:

https://www.monsterguts.com/store/

https://www.frightprops.com/

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A modern, smart battery charger would be most unsuitable to run a car windshield wiper motor.

However, a simple 12V battery charger of yore, with a charging current of 3.5A or higher, would do. Such chargers would have transformers with 'high impedance' secondaries in order to limit the charging current.

Alternatively, all that would be required to rig up a power supply would be a 12 V transformer and a bridge rectifier of adequate current rating.

A car windshield wiper motor would generally draw less than 3A on load.

One would do well to measure the wiper motor current on load, when powered by a 12 V battery, before taking a decision on the required power supply current rating.

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    \$\begingroup\$ Would be better to say "measure the wiper motor current on load and the stall (start-up) current" as PSU will need to deliver that, too. Good answer anyway, upvoted. \$\endgroup\$
    – TonyM
    Commented Jan 30, 2022 at 13:47
  • \$\begingroup\$ Thank you, Tony, sure slip-up indeed! \$\endgroup\$
    – vu2nan
    Commented Jan 30, 2022 at 16:46

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