01 — Power bank math
A power bank rated at 20,000 mAh does not give you 20,000 mAh of usable charge.
Power banks have two inefficiency factors: conversion losses (typically 10 to 20 percent, since the bank converts stored DC to the USB output your device needs) and partial use of the rated capacity. A practical estimate is 70 to 80 percent usable efficiency from the rated mAh.
How to estimate phone charges from a power bank
Usable charges = (Power bank mAh × 0.75) ÷ Phone battery mAh
Example: 20,000 mAh bank × 0.75 = 15,000 mAh usable ÷ 4,000 mAh phone = ~3.75 full charges
Example: 10,000 mAh bank × 0.75 = 7,500 mAh usable ÷ 3,500 mAh phone = ~2.1 full charges
For outage preparedness, a 20,000 mAh power bank provides roughly 3 to 4 full phone charges — enough for several days of moderate use (receiving calls, checking emergency alerts, limited browsing). Keep power banks charged above 80 percent and store them at room temperature; lithium cells degrade faster when stored fully discharged or in extreme heat.
02 — Charging from a vehicle
Your car can charge phones and power banks safely — with the engine running and the vehicle parked outdoors.
A vehicle with a USB port or 12V power outlet can charge phones, tablets, and power banks during an outage. The critical safety requirement: run the engine to charge. Charging from a stationary vehicle with the engine off will drain the starter battery, potentially leaving you unable to start the car when you need it — especially in cold weather, when battery capacity is reduced.
CO risk when warming up the car to charge
Running a vehicle engine to charge devices produces carbon monoxide. The vehicle must be operated outdoors with the garage door fully open — not in a closed or partially open garage. CO from a running vehicle in an attached garage can reach lethal concentrations in minutes. Always charge with the vehicle in the driveway or outdoors, with the engine exhaust pointed away from any occupied structure.
A 30 to 60 minute run is typically enough to meaningfully charge a phone or partially charge a power bank, while keeping the vehicle battery topped up. Newer hybrid and EV vehicles have larger battery packs and may have dedicated outlets that can power devices even without running the combustion engine — check your vehicle's manual.
03 — Lithium battery fire risk
Lithium-ion batteries can self-sustain a fire through thermal runaway. The response is to leave, not to fight the fire.
Thermal runaway is a chain reaction inside a lithium-ion cell where heat released by a failing cell triggers adjacent cells to fail, releasing more heat. The reaction is self-sustaining and can accelerate rapidly. It can be triggered by overcharging, physical damage (crushing, puncturing), use of an incompatible charger, manufacturing defects, or extreme heat exposure.
- Use only the original manufacturer's charger or one specifically rated for your device. Third-party chargers that do not match the device's charging specification can cause overcharging and thermal runaway.
- Do not leave lithium devices charging unattended for extended periods — most modern chargers have automatic cutoff, but defective batteries or mismatched chargers do not.
- Inspect batteries before use for swelling, cracking, leakage, or heat during normal use. A bulging battery is a fire risk — remove from service and dispose of properly.
- Do not charge or store lithium batteries near flammable materials.
- Do not charge at extreme temperatures — below freezing or above 95°F significantly increases risk.
If a lithium battery catches fire
Leave the area immediately and call 911. Do not attempt to put out the fire — water may not stop it, standard fire extinguishers are not effective on lithium fires, and the reaction can reignite even after apparent suppression. Lithium battery fires produce toxic gases. Get out and let fire department personnel handle it.
04 — Car battery myth
A car battery is not a practical whole-house or even whole-room power source.
Common assumption
A car battery can power household devices and lights during an outage.
Reality
A typical 12V car starter battery has a usable capacity of approximately 40 to 60 amp-hours (at the low draw rates appropriate for accessories) — roughly 480 to 720 watt-hours. A single 100W light bulb running from an inverter attached to a car battery would exhaust the battery in 4 to 7 hours. Additionally, a standard car starter battery is not designed for deep discharge cycles — repeated deep discharging permanently damages the battery, which is then unable to start the car. Actual deep-cycle batteries (like those used in marine and RV applications) are designed for this use, but a standard car starter battery is not.
For practical household backup power, a portable power station (large lithium battery pack with built-in inverter and outlets) or a generator is far more capable. A car USB/12V port is useful for charging small devices while the engine is running — not for powering household loads.
Quick reference
- Power bank capacity: multiply rated mAh by 0.75 for usable charge; divide by phone battery mAh for estimated full charges
- Vehicle charging: engine running, vehicle outdoors — never in a garage; CO risk from running engine
- Rechargeable batteries: pre-charged and ready at all times; significant cost saving over disposables in high-use emergency gear
- Lithium-ion fire: caused by thermal runaway; if it ignites, evacuate and call 911 — do not attempt to extinguish
- Warning signs of a failing lithium battery: swelling, cracking, heat during use or charging, leaking electrolyte
- Car battery myth: starter batteries are not designed for deep discharge; usable capacity is limited; repeated deep cycling damages the battery
Primary sources
- NY DHSES — Lithium-Ion Battery Consumer Safety Guide: Lithium-ion battery fires spread quickly and can become explosive or reignite; do not attempt to put out the fire yourself; leave the area and call 911.
- CCOHS — Lithium-Ion Battery Charging Safety: Thermal runaway mechanism; unplug when charging complete; do not use uncertified chargers; do not charge near flammable materials; swelling/cracking/leaking indicates remove from service.