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I'm going on a camping road trip and need to power my CPAP. Instead of running one long cable from the car, which has its own issues, (dead battery/car, shorts, tripping) I want to get a 2nd car battery to take into the tent to then power the CPAP. Problem solved? Nope!

This trip is for more than a week and we will be moving camp and traveling during the day. I want to be able to charge the battery during the day while driving so it doesn't die.

Is there a device to plug into the cigarette lighter that can charge this second car battery?

TIA!

  • I'm guessing your second battery is a deep cycle. How many amp/hours is it rated for and what type (NiCd, NiMH, Lead Acid, Lithium Ion, Lithium Polymer, etc) is it? – user16128 Jan 31 at 1:03
  • Hi @Mark, what is a CPAP, and what are its energy requirements? – GdD Jan 31 at 8:16
  • en.wikipedia.org/wiki/Positive_airway_pressure it is a medical device. – trond hansen Jan 31 at 8:30
  • There are many split charge devices available to charge and properly control two batteries, once you know the specifications of your equipment you can choose a suitable one. – Solar Mike Jan 31 at 9:34
  • Could drive on one battery till its charged, then pull over, swap to empty battery and jump start with the full one, then use that one during the night - repeat – Chris Feb 3 at 13:43
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You want a deep-cycle battery

A lot of people just go "car battery" for any large, wet-cell auxiliary battery. Using an actual car battery would be a mistake.

Lead-acid batteries are full of compromises (comes with the acid), and one of those compromises is that the battery can be good at high-amperage engine starting, OR, be good at delivering low current for long periods. Pick one. Using one for the other will quickly destroy it: car batteries aren't designed to be deep discharged. So using a literal car battery for a CPAP will disappoint, unless it's a one-time thing and you're willing to heave-ho the battery at the end. You want a deep-cycle type battery. These are noted by little screw-down lug terminals in addition to normal battery lugs.

However, "deep cycle" is still not license to drag the battery down to flat. This will prematurely age the deep cycle battery, taking weeks off its life and capacity everytime you do. (as opposed to car batteries, which takes months off their life and capacity).

This thing is easy

It's little more than a plug going into the cigarette lighter. When I make them, I intentionally make them excessively long and with thin wire. Because I want to increase the wire resistance to reduce maximum current, so I don't blow the fuse on the cigarette lighter.

It can be as simple as simply plug it in direct into the cigarette lighter while the car is running and let it charge over time. However you should disconnect it when the engine is off so you don't drain the car's battery, and you MUST MUST MUST disconnect it when starting the car engine (otherwise this battery will try to "help" and will overload the connection). You must also never reverse the wire connection.

So aftermarket devices that help you do this job are basically there as a protection circuit, to prevent it from overloading in any direction.

Lithiums are different

Note that if you attempt this with a lithium battery, all of the above is void. You must use protective circuits with a lithium battery, unless you like explosions. On the upside, lithium batteries don't mind being discharged through their full range.

You may be better off using a so-called "Solar Generator"

If you want all these features bundled in a consumer friendly package, then the google term is "Solar generator". Note that you should avoid (as in turn off/disable) the 120/230V inverter function, since you want 12V. The inverter burns a lot of power just being "spun up" with no loads, so make sure you get one that allows it to be turned off.

You don't need to use solar panels with a "solar generator" but that feature is included. You can just charge it from a cigarette lighter, and it has the necessary protective features.

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You need to contact the suplyer of the equipment to find a charger made for your model of cpap,most of the mobile cpap machines can be connected to the car electrical system as a backup power suply and for charging.

What type of charger you need depends on the make and model you have,do not take any chances with the equipment and your own health.

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  • Also, if you do use this battery, it may be a good idea to keep the battery outside the tent. It can be hazardous sleeping in closed quarters with it. – Jupiter Jan 31 at 10:45

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