Archangle:Deep Cycle Battery Backup

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This is my page of notes about using a deep cycle marine battery as backup power for CPAP machines.

Basic Design Notes

This is for using a deep cycle lead acid "marine" battery about the size of a car battery. There are other solutions, but this is the one I'm discussing here.

NOTE: The fuse is IMPORTANT for safety. Do NOT omit the fuse.

Parts List

  • Inline fuse connected directly at the battery. Do NOT omit the fuse
  • Deep cycle "marine" battery
  • Battery trickle/float charger
  • Battery box
  • Cabling to connect to the battery
  • Adapter to power CPAP machine from the battery


Inline Fuse

Lead acid batteries, even small once can produce a LOT of current. If you have a short in the wiring, there is considerable risk of fire.

It is important that any cable you connect to a CPAP battery have a fuse located right at the battery terminal. You will probably be using this battery setup for a long time, so there will be many opportunities for the wiring or connectors to get damaged.

I found alligator clip to cigarette lighter adapters with built in fuse holders at my local auto parts store. If you use other types of cables, be sure they include a fuse at the battery, even if you have to add it yourself. A 5 amp fuse will probably work well.

Deep Cycle "Marine" battery

Get a "deep cycle" battery.


This wiki discusses wet cell deep cycle "marine" batteries about the size of a car battery. There are other options.

There are also "gel cell" or "AGM" batteries. The main advantage of these batteries is that they are not spillable. They do cost considerably more than a "wet cell" deep cycle battery.

Every lead acid battery, even deep cycle batteries, are damaged a little by every discharge cycle. The deeper the discharge, the more the damage done by each cycle. Deep cycle batteries have thicker lead plates and don't damage as much when discharged, but they can't provide as high a current.

Be careful when buying a "marine" battery. There are "deep cycle" marine batteries, which is what you want. Deep cycle batteries may also be labeled as "trolling" batteries. There are marine "starting" batteries, which will work, but will wear out more quickly if you discharge them deeply. There are also "dual purpose" or "starting/trolling" batteries.

Lead acid batteries do not "need" to be discharged periodically like some batteries like Nickel Cadmium batteries may.