We previously discussed how to dispose LiPo batteries, in this article we will talk about how to determine when to throw them out. Apart from physical damages, the most important factor is internal resistance (IR), which indicates the health and performance of the LiPo battery.
Table of Content
- Discharge Cycles
- Expiry Date
- Internal Resistance Explained
- How to Measure IR?
- Physical Condition
- “Is my battery still safe to use?”
The Average Lifespan of a LiPo Battery – Discharge Cycles
If you are lucky enough not to break your LiPo battery before its end of days, it should have an average lifespan of about 400–500 cycles.
Of course this also depends largely on factors such as how much “abuse” you put your batteries through, and how you handle them on a daily basis.
500 cycles might sound a lot, but for us flying mini quad, it’s extremely likely that we damage them way before we hit that number :)
LiPo batteries don’t have an expiry date printed on them, but from my personal experience, new batteries almost always perform better than old batteries, even when they just sit there and not being used much.
I generally find batteries that are 12 to 18 months old to start showing noticeable drop in performance. I usually replace my batteries when they are around 2 to 3 years old, even when they might look completely normal on the outside.
It’s helpful to label and date your battery packs when you first get them in.
Internal Resistance Explained
One of the most useful battery health indicators would be internal resistance (IR).
As explained in my LiPo battery beginner’s guide, IR determines how effectively a battery can deliver current to your quadcopter. Higher IR means lower performance, and more heat generated during charging and discharging.
IR increases over time, and it can rise more rapidly when:
- Over-discharging and over-charging
- Pushing the battery too hard by discharging at a current higher than it’s rated for an extended period of time
For typical 1300mAh to 1500mAh packs (regardless how many cells), I would consider under 10mΩ to be great condition, 10mΩ to 15mΩ to be fine, 15mΩ to 20mΩ to be old, and over 20mΩ to be “time to retire”.
If one of the cells has noticeably higher IR than the rest (e.g. 100% higher), it’s probably not safe to use and should be thrown out, as that cell will supply less current and heat up more than it should.
IR has to do with discharge rate too, for example if your battery is designed for low current applications with 10C rating, IR will inherently be higher. 18650 Li-ion batteries have much higher IR than typical LiPo batteries, so do not worry!
How to Measure IR?
When measuring internal resistance, you should try to keep all conditions constant, because several factors affect your IR readings, such as:
- Quality of the cells
- Chemical properties
- Measuring equipment
- Discharge rating
IR depends on the size of the cells (i.e. capacity). Larger cells have lower IR inherently, when you parallel charge, the IR will appear lower than when you charge those batteries individually.
Note that IR increases at lower temperature, that’s why LiPo batteries perform worse in cold winters, and it helps to warm them up in your pocket before flight.
A visual examination of your LiPo battery can help determine if it should be retired.
LiPo batteries used on drones can easily become deformed in a crash since they are exposed on the outside. It’s risky to use dented batteries, especially during charging.
Further Reading: Here are some tips to protect your LiPo batteries from physical damage
Your batteries can also become “puffed” after some abusive uses, or when they are getting old.
Unbalanced Cell Voltages
It’s pretty normal that the cell voltages are slightly different after a flight, e.g. 3.55V, 3.59V, 3.61V. The point being, they should all be within reasonable range.
When the internal resistance of particular cells are higher than the rest, they discharge slower and tend to have higher voltage at the end of the flight. Eventually this might lead to over-discharging of other cells and cause swelling. Higher IR cells also generate more heat during usage.
This is why you should always “balance charge” your batteries in case of over-charging.
Pay Attention to Performance
Battery performance decreases when they get older:
- Not holding charge, voltage drops after charging, and flight time reduced
- Voltage sag is noticeably worse
Another thing to keep in mind is the temperature of the LiPo after a flight. If a battery is getting way hotter (can’t hold it in your hand for more than 10 seconds) than others, it’s also a sign of aged battery.
Check Temperature While Charging
Battery getting warm during charging (charging at 1C) is an extremely important sign that your battery has a problem, and you should stop immediately. LiPo batteries shouldn’t get noticeably warm when charging at reasonable rate. The common cause is internal short and it can be extremely dangerous.
“Is My Battery Still Safe to Use?”
If you ever ask yourself this question, the answer is probably no.
A healthy battery handled properly should never set on fire. But when you have a battery with a dented corner, or one of the cells with unusually high IR, the risk increases exponentially.
Nobody can tell you if a battery is going to explode, but would you risk your house over a damaged $20 Lipo battery?
- Sep 2017 – Article created
- Nov 2018 – Added effects of high IR and comments about “check bat temp while charging”
- Mar 2019 – Added “is my LiPo still safe to use”
- Feb 2020 – Added “how to measure IR”