Should You Leave a Rechargeable Flashlight Plugged In?
Our customer asked:
Should you leave a rechargeable flashlight plugged in all the time?
TL;DR
It is better not to. A modern rechargeable flashlight has a charging circuit that stops feeding current once the lithium-ion cell is full, so leaving it plugged in will not overcharge it in the dangerous sense. Keeping the cell sitting at 100 percent day after day does wear it out faster, though, and heat makes that worse. Unplug it once the charge indicator shows full, and top it up when you have used it.
I run product innovation at LUXPRO. The charging circuit is one of the least glamorous parts of a flashlight and one of the most important. Here is what it does, what it cannot protect you from, and the habits that make a rechargeable light last for years.
What happens inside the light when you plug it in
When we design a charging circuit, we build it around how lithium-ion cells charge: in two stages. First the circuit pushes a steady current into the cell until it reaches 4.2 volts. Then it holds the voltage at 4.2 and lets the current taper off as the cell fills. When the current drops low enough, the circuit treats the cell as full and stops charging.
On our XP918v2, you can watch this happen: the indicator glows red while charging and turns blue when the cell is full, about four hours from empty. That blue light is your cue to unplug.
So why unplug if it stops charging anyway?
I give our customers two reasons.
A full cell ages faster than a half-full one. A lithium-ion cell sitting at 4.2 volts is under more chemical stress than one sitting at 3.7 to 3.8 volts. Over months, that shows up as lost capacity. A light left on the charger permanently spends its whole life at the most stressful state of charge.
Small top-ups add up. A charged cell slowly loses a little voltage. Depending on the circuit, a light left plugged in may restart charging to top the cell back up. Each of those is a small extra cycle, and the cell stays pinned at full.
I want to be clear that none of this is an emergency. A light left on a charger over a long weekend is fine. A light left on a charger on a warm windowsill for a year will have noticeably less runtime than one that was charged and unplugged.
Charging myths worth dropping
"Run it completely dead before you recharge." That advice comes from old nickel-cadmium batteries and their "memory effect." Lithium-ion has no meaningful memory effect, and it prefers partial discharges. Recharge whenever it is convenient.
"Charge it overnight the first time to break it in." Not needed for lithium-ion. Charge it fully once so you know where you are starting, and then use it normally.
"Any USB charger will damage it." The light's own circuit controls how much current goes into the cell. A normal phone charger or computer port is fine. A fast charger will not force more current than the light accepts. Do avoid damaged cables and unbranded chargers that get hot.
The habits that matter
- Unplug when it shows full. Watch for the indicator to change, as on the XP918v2.
- Recharge after use, not on a schedule. Partial top-ups are fine for lithium-ion.
- Do not charge in a hot car or in direct sun. Heat while charging is especially hard on cells.
- Do not charge below freezing. Charging a lithium-ion cell below 32°F can damage it permanently. If the light has been in a cold truck, bring it inside and let it warm up before you plug it in.
- Long-term storage: about half charged. If a light will sit for months, run it down to roughly 50 percent before you put it away, and check it every three months or so.
The emergency light problem
Here is the real tension, and it is one we think about with every light we design. For an emergency light, you want it full when you grab it. For the battery's lifespan, you want it half full and cool. Those two goals pull in opposite directions.
Here is how I handle it with my own lights:
- Charge it fully, unplug it, and store it somewhere cool. Top it up every two to three months. A good lithium-ion cell loses only a few percent a month, so it will be close to full when you need it.
- Better still, use a dual-power light with a backup. The XP920 runs on an 18650 or on two CR123A lithium primaries, which keep their charge for years. Store two CR123As in the kit beside it, and the light works even if you forgot to charge it for a year. I explain the battery options in What batteries do flashlights use?
What about lights that work as power banks?
Several of our lights, including the XP918v2 and the XP715, can charge your phone. That is useful in an outage, but every phone charge comes out of your flashlight's runtime. Charging a phone from a flashlight also costs extra energy, lost as heat in the conversion. If you will need the light later, charge the phone only as much as you need.
Signs to stop charging and replace the battery
- The light or battery gets hot to the touch while charging, beyond slightly warm.
- The indicator jumps to full almost immediately and then the light dies quickly.
- You see swelling, a bulge in the body, leaking, or smell something odd.
Stop using it, unplug it, and contact the manufacturer. If it is one of our lights, contact us and we will help. For lights with a removable 18650, a replacement cell usually restores full runtime. More on how long cells last: How long does an 18650 battery last?
Are rechargeable flashlights worth it?
If you use a flashlight more than occasionally, I think so, yes. A lithium-ion light gives you more output than the same size on alkalines, and it stops the steady spending on disposables. With the habits above, the battery will last years. I compare ten rechargeable lights, ours and our competitors', in 10 Best Rechargeable Flashlights.
Frequently Asked Questions
Q: Can you overcharge a rechargeable flashlight?
A properly designed lithium-ion light stops charging when the cell is full. The concern is not a dangerous overcharge but faster aging from sitting at 100 percent, especially when warm.
Q: How long does it take to charge a rechargeable flashlight?
Usually a few hours, depending on battery size and the charging circuit. The XP918v2 takes about four hours from empty.
Q: Should I let my flashlight battery die before charging?
No. Lithium-ion prefers partial discharges. Recharge whenever it suits you.
Q: What is the longest-lasting rechargeable battery?
It depends on whether you mean runtime per charge, charge cycles or shelf life. I break it down in What is the longest-lasting rechargeable battery?