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This question already has an answer here:

I have a problem similar to Suresh @ 12V Constant voltage regulator for Car? (I want to power an electric dog fence from my car battery), and like Trevor_G suggests, I probably don't have the smarts to do a DIY solution. I get that you can't get 12v output if you only have 10v input. Trevor_G suggests instead an "off-the-shelf automotive 12v power supply." I'm not sure what type of device Trevor_G is recommending - one of those 12v rechargeable lithium battery "portable power supplies" that are supposed to power your 110v shaver while charging 6 cell phones simultaneously? The 110v AC-12v DC power supply that came with the dog fence provides 12v DC at 750mA. Assuming the unit draws 750mA, how big of a battery (watt/hours?) do I need to power it for 12 hours on one charge? (I get confused witht he math) Is this even practical? How long would I have to run my car to recharge the unit? Manufacturers don't give any data on this. Thanks. Forgive me for being an electronics pre-schooler.

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marked as duplicate by laptop2d, winny, Dmitry Grigoryev, Marcus Müller, MCG May 29 '18 at 11:58

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  • \$\begingroup\$ 12 hours or 24 - is it switched off at night? Have you considered a solar panel? Most electric fence systems tend to state the battery size required... \$\endgroup\$ – Solar Mike May 25 '18 at 5:13
  • \$\begingroup\$ Your problem is different than the linked post, if I understand properly. In the linked post, the user wants to use the 12V from the battery car within the car. Because the supply is also wired to everything else in the car (alternator, glow plugs, ...), it can't be considered stable. In your case, you want to power a dog fence. I guess it means the battery is not within a car, but used standalone (unless the dog fence is within the car?). If this is the case, it is much easier because the supply will be subject to much less fluctuation. So please clarify. \$\endgroup\$ – dim May 25 '18 at 7:11
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get that you can't get 12v output if you only have 10v input.

Indeed you can't when using a "regulator" but you can with a boost converter.

But in your case (you want 12V output while the input can vary between 10 V to 14 V) what you actually need is a buck/boost module.

Here's an example: enter image description here

Note the light blue rectangular component, it's a potmeter. It has a screw you can turn and that will adjust the output voltage. You will need a voltmeter or multimeter to measure the output voltage and turn the screw such that you get 12 V at the output.

The 110v AC-12v DC power supply that came with the dog fence provides 12v DC at 750mA.

That means that this power supply can deliver up to 750 mA. The actual current consumption of the device will actually be a lot lower. The only way to know the real value is to measure it.

But assuming it is consuming 750 mA (0.75 A) at 12 V how long will a car battery last? Simple: An average car battery is for example 40 Ah meaning it can deliver 1 A for 40 hours. Or at 750 mA: 40 Ah / 0.75 A = 53 hours. I'm ignoring any losses from the buck/boost module here to keep things simple. We just want a rough indication of the runtime.

In practice I expect that the dog fence unit will consume much less than 750 mA. A more realistic estimate might be 100 mA (just my guess). That would mean that a fully charged 40 Ah car battery will power the fence for at least 2 weeks.

If you keep the battery in your car (and also use it for your car) while it powers the fence I would not run it for longer than a week as you might be unable to start the car if you run down the battery too much!

To fully charging the car battery again you should be driving around for an hour or so at moderate to high speed. I am unsure if the car will charge the battery fast enough when it is running idle.

If you power the dog fence regularly from the car I would get a mains powered car battery charger to charge the car battery instead of burning fuel for that.

Also consider using a solar panel + battery solution instead of the car's battery since the dog fence device should not consume that much power so a solar panel might provide more than enough power.

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  • \$\begingroup\$ Think Bimpelrekkie got essence of problem, & thank you for your advice. Didn't know what device to look for. Sounds like buck/boost module is what I need. Was thinking if system works, would get solar panel & dedicated battery. But good to know I can test using car battery. Will the module absorb voltage spikes as much as 40v if someone accidentally starts car without disconnecting system from battery? Do I need inline fuse before or after buck/boost module? \$\endgroup\$ – Joel May 26 '18 at 20:34
  • \$\begingroup\$ A fuse is always a good idea since car batteries can supply sh#tloads of current and you want to break that current in case of a fault. So yes: use a fuse ! A 5 A fuse will do the job. The XL6009 chip on this module is rated for only 32 V so 40 V might kill it if that happens. I'm unsure how common 40 V spikes are, yes they can happen when the motor is running but the battery disconnected. Not sure if they also occur when starting. I'd just buy two modules and if starting the engine kills the module, take extra precautions to limit the spikes to the module. \$\endgroup\$ – Bimpelrekkie May 26 '18 at 20:49

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