When the grid goes down, power becomes a matter of life and death for those dependent on CPAP machines or oxygen concentrators. In this critical reliability guide, we calculate the Watt-hour requirements for humidified vs. non-humidified therapy, explain why 24V native DC connection is mandatory for efficiency, and how to build a fire-safe LiFePO4 backup that fits under the bed.

Sleep Safety in a Blackout

For millions of people with Sleep Apnea or COPD, a power outage isn't just an inconvenience; it is a health emergency. While you can buy "Medical UPS" units, they are often overpriced lead-acid bricks that last only a few hours. A custom-built Lithium Iron Phosphate (LiFePO4) system can provide 3 to 7 nights of autonomy in a compact, portable package.

Designing for medical backup is different from designing for camping. Reliability and Fire Safety are the only metrics that matter. We do not use "recycled" cells here. We do not use "bypassed" BMS units. We build systems that work every time.

1. The Load Analysis: Humidifier is the Enemy

The power consumption of a CPAP machine (like a ResMed AirSense 10 or 11) varies wildly depending on one setting: The Heated Humidifier.

• Pump Only (Pressure): ~5 to 10 Watts.
  Consumption (8 hours): 40Wh - 80Wh.
• Pump + Humidifier + Heated Hose: ~40 to 60 Watts.
  Consumption (8 hours): 320Wh - 480Wh.

The Math: Turning on the humidity increases power consumption by 500% to 600%.
If you are designing a backup for a multi-day blackout, the most effective "battery upgrade" is simply turning off the humidifier. However, if humidity is medically required, you must size the battery bank to handle roughly 500Wh per night.

2. The Efficiency Trap: DC vs. AC

Most people plug their CPAP AC brick into an inverter. This is incredibly inefficient.
1. Battery DC -> Inverter AC (90% eff).
2. Inverter AC -> CPAP Brick DC (85% eff).
Total Efficiency: ~75%. You lose 25% of your battery capacity to heat.

The Solution: Native DC Cables.
Most CPAP machines run on 24V DC internally (ResMed) or 12V DC (Philips). You can buy a "DC Output Cable" for your specific machine for $30.
By connecting a 24V Battery directly to a 24V CPAP, efficiency is near 99%.
Note: Even if you have a 12V battery, using a 12V-to-24V DC converter (95% eff) is still far better than using an AC inverter.

3. Chemistry Selection: Why LFP is Mandatory

Since this battery will likely live under your bed or on a nightstand, Thermal Runaway is an unacceptable risk. Standard Li-Ion (NMC) batteries (like those in e-bikes) catch fire violently if abused.
LiFePO4 (Lithium Iron Phosphate) is chemically non-combustible. Even if punctured or shorted, it does not release oxygen. It might smoke, but it won't turn your bedroom into a furnace. For medical indoor use, LFP is the only ethical choice. (See Chemistry Safety for details).

4. Oxygen Concentrators: The High-Power Challenge

Unlike CPAP machines which sip power, Oxygen Concentrators (like the Invacare Perfecto2) are compressors. They draw 300W to 500W continuous.
A 100Ah 12V battery (1200Wh) will run a 400W concentrator for only 3 hours.
For oxygen users, a small battery is not enough. You need a Solar Generator Strategy. You need a large battery bank (2kWh+) coupled with solar panels to recharge during the day. Alternatively, consider "Pulse Dose" portable concentrators (like Inogen) which use far less power (~40W) than continuous flow home units.

5. Building the "Blackout Box"

For a dedicated CPAP backup:

• Battery: 12V 100Ah LiFePO4 (1280Wh). This provides ~2 nights with humidity, or ~14 nights without.
• Case: A plastic ammo can or Pelican case.
• Ports: Install a 12V Cigarette Socket (Marine Grade). Most DC CPAP cables use this plug.
• Fusing: Use a 15A resettable circuit breaker. If the CPAP cord shorts, you want the breaker to pop instantly.

6. UPS Functionality

You want the system to take over instantly when the grid fails.
Connect a high-quality LiFePO4 Charger (10A) to the battery and leave it plugged into the wall. Connect the CPAP to the battery.
The charger keeps the battery full. When power cuts, the charger turns off, but the battery continues powering the CPAP with zero interruption.
Warning: Ensure your charger handles "Load Switching" well and doesn't over-voltage the CPAP. Using a specialized DC-UPS controller is safer than leaving a generic charger attached 24/7.

Summary

Medical backup power is about peace of mind. By moving to a native DC connection and utilizing the safety of LiFePO4 chemistry, you can build a system that is smaller, lighter, and longer-lasting than any hospital-grade UPS. Test your system monthly by unplugging the wall power for a night. Knowing your equipment works is the best sleep aid available.