Next up in our “Ask Drone Girl” series is about using old lipo batteries. If you have a question for Drone Girl, contact her here.
I have two LiPo batteries with about 40 recharge cycles that I’ve been storing for about 4 years in an ammo box. To be honest, I forgot about them. They were stored at the correct storage voltage. Are they safe to use, or should I get rid of them? How long do LiPos last if stored at the correct storage voltage? And how could I test them to see if they are ok?
While LiPo batteries are one of the safest forms of R/C battery, they can still be extremely dangerous if charged, discharged, stored, maintained, or handled improperly. So, you’re already doing the right thing to be concerned about using old LiPo batteries!
LiPo batteries don’t have expiration dates, but they can see a drop in performance after excessive use or passed time.
“LiPo batteries show a significant drop in performance after 12
to 18 months,” said Sarah Petrova, a hardware engineer at Intel.
But that doesn’t mean you can’t use them.
“Four years of storage is not enough to ruin a properly stored battery,” said Mathayi Abraham, a professional electronics and telecommunication engineer, and founder of the site Robot Powered Home. “My LiPo batteries which had been stored for more than eight years worked fine when I checked recently. However, remember to slowly balance charge the battery before you use it again.”
What voltage to store your LiPo battery at
A fully charged LiPo cell is 4.2 volts, while a dead cell
is 3.3 volts.
“If you plan on letting your battery sit for a few weeks (or four years), storage charge it to 3.8V per cell,” Abraham said. “For example, if your battery has three cells, you should charge it to 11.4V for storing it.”
Assuming you stored them at the correct storage voltage, your battery is probably safe to use. But LiPo batteries are sensitive and while they can last more than four years sitting on a shelf, temper your expectations.
“A conservative estimate for battery life would be 18 months to over two
years, with only a slight voltage drop, when stored at room temperature
and at a voltage of around 50% of full charge (3.6 to 3.7v),” said tech entrepreneur Phil Strazzulla.
What you need to know about recharge cycles
While LiPos are great because they can be recharged, they won’t last forever.
“The maximum charge cycles for most LiPo batteries tends to be around 300,” said Thomas Bradbury.
But even 300 recharge cycles is optimistic. Bradbury said that only applies to higher quality batteries. “Many LiPo batteries will not even last for 300 charges,” he said.
The metric that matters most: internal resistance
While age and recharge cycles have an impact on your battery’s health and safety, the real metric in determining whether your LiPo battery is safe to use is internal resistance.
Internal resistance is one of the best indicators of battery health by determining how effectively a battery can deliver current.
Higher internal resistance means lower performance. You can measuree internal resistance through most standard LiPo chargers.
“In my experience for typical 1300mAh to 1500mAh packs (no matter the amount of cells), I consider under 10mΩ to be brand-new condition, 10mΩ to 15mΩ to be good, 15mΩ to 20mΩ to be limited performance, and everything over 20mΩ to be ‘retired,” Petrova said.
And for what it’s worth, Petrova said she replaces batteries after a maximum of three years.
“I would not advise anyone to keep a LiPo battery in operation for four years, even if it was stored correctly and not used very often,” she said, adding that the electrolytic solution of the battery goes into decomposing mode after too long a period of inactivity.
Strazzulla said he also tests to check if old batteries in storage are still usable by measurinig their voltage before being hooked up to a charger.
“If they have dropped below 3v or show no voltage at all, they are unusable and should be discarded.”
How to keep LiPo batteries in peak condition
With proper care, you can make the same LiPo battery last longer than someone else’s.
- Never run down the battery to a point where it is completely dead
- Never overcharge your batteries (and never over-discharge them, too)
- Avoid pushing the battery too hard by discharging at a current higher than it’s rated
- Don’t let batteries overheat
- Store, charge and discharge them in fire proof LiPo safety bag, metal ammo box, or other fire proof container (it’s always worth investing the $10 in a explosion-proof LiPo bag or ammo can).
And often, you don’t even need an expert to know if a LiPo battery is safe to use. Just use your eyes — and your common sense. If you see signs of swelling or puffing, dispose of your LiPo battery safely and immediately.
“Swelling of LiPo batteries happens due to the gas generated by the decomposition of the battery’s electrolytic solution,” Petrova said.
If LiPo batteries come with these risks, then why do we use them?
In short, they’re powerful. A LiPo battery cell has roughly four times the energy of density of nickel cadmium batteries, which are commonly used in cordless and wireless telephones, or emergency lighting. They also have roughly four times the energy density of nickel metal hydride batteries, a type of rechargeable battery commonly found in digital cameras.
LiPos are also fairly common. They power items that require high amounts of energy, meaning not just drones, but likely the next generation of electric vehicles too.