Short answer: it is fine to keep farm iPhones plugged in, but heat and sitting at 100% wear the battery faster. On an iPhone 15 or newer, set a charge limit of 80% (Settings → Battery → Charging). On older models, keep Optimized Battery Charging on, keep the rack cool, and check battery health every month. Retire or re-battery any phone whose back or screen starts to lift.
What wears a farm battery
- Heat. Screen on, apps running and charging all at once make heat. Heat is what ages lithium-ion cells fastest.
- Time spent full. A battery held at 100% ages faster than one held lower.
- Brightness. Farm phones keep Auto-Lock off, so the screen never sleeps.
The setting that helps
Apple's support page is direct about it: on iPhone 15 models and later, you can "choose a charge limit between 80 percent and 100 percent in 5 percent increments." Pick 80%. The phone stops charging there and runs from the charger.
On iPhone 14 and earlier there is no slider. Optimized Battery Charging "delays charging past 80%" when the phone predicts it will stay connected — you cannot force it, but leave it on (Apple Support).
| Model | What you can set | Setting |
|---|---|---|
| iPhone 15 and later | A fixed charge limit, 80–100% in 5% steps | Settings → Battery → Charging |
| iPhone 14 and earlier | Optimized Battery Charging on or off (no fixed limit) | Settings → Battery → Battery Health & Charging |
A checklist for the rack
- Charge limit to 80% on iPhone 15 and newer.
- Brightness to minimum. The automation does not need to see the screen the way you do.
- Airflow. Stand phones upright with gaps, not stacked. No cases.
- No direct sun and no enclosed shelves without ventilation.
- Check battery health monthly in Settings → Battery.
- Watch for swelling — a lifting screen or back. Unplug that phone and take it out of service.
Why not cut power on a timer?
A smart plug that switches the hub off and on to let batteries cycle sounds sensible. On a farm it disconnects every phone from the Mac at once, which ends any run in progress. Use the charge limit instead; if you must cycle, do it per phone in a window where nothing is scheduled.
Related guides
- Phone farm overheating — causes and how to cool a rack
- Phone farm UPS and power backup — what happens in an outage
- Buying used iPhones for a phone farm — inspection checklist
- Phone farm rack setup — shelves, stands, cables, labels
FAQ
Is it safe to leave an iPhone charging 24/7?
Generally yes; iPhones manage charging themselves. The cost is faster battery wear, mostly from heat and time spent at full charge.
How do I stop an iPhone charging at 80%?
On iPhone 15 and later: Settings → Battery → Charging, then set the limit to 80%. Older models cannot set a fixed limit.
When should I replace the battery?
When battery health falls to around 80% or the phone starts shutting down under load — and immediately if it swells.
Does a low battery affect automation?
A phone that loses power disconnects and stops its account's run until it comes back. Keeping the charger connected and the hub well powered avoids that. See USB hubs.