When to Retire LiPo Battery? Check Internal Resistance

by Oscar
Published: Last Updated on

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

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 :)

Expiry Date?

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).

All electrical components has resistance, even a piece of wire has resistance, so is a battery. The resistance in a battery is called internal resistance.

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 energy is wasted as heat, that’s why it gets hot 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
  • Overheat

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?

Almost of all modern LiPo chargers these days can measure IR. for example the iSDT Q6 and TookitRC M6. IR of each cell is displayed on the screen when you charge the battery. If you are getting a charger I strongly recommend getting one with IR measurement.

When measuring internal resistance, you should try to keep all conditions constant, because several factors can affect your IR readings, such as:

  • Capacity of the battery
  • Quality of the cells
  • Chemical properties
  • Age (number of discharge cycles)
  • Temperature
  • Measuring equipment
  • Voltage of the LiPo
  • Discharge rating

IR depends on the size of the cells (i.e. capacity). Larger cells have lower IR inherently. For example, 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 the winter, and it helps to warm them up in your pocket before flight.

Physical Condition

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 wildly higher than the rest, the voltage always end up unbalanced after flight, it will put more stress on other cells. Higher IR cells also generate more heat during usage.

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.

If you handle a healthy battery properly, it 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?

Edit History

  • 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”

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19 comments

Ger Hobbelt 12th May 2021 - 11:54 am

@Owen: using a multimeter to measure IR:
This is doable but is inaccurate (reckon with up to 20% off the mark) unless you are able to calibrate your measuring rig (out of scope advanced topic).

The fundamental approach is this:
– measure your battery voltage(s) before testing IR: “open circuit” measurement A
– apply a known *reasonable* load so the battery will deliver a (known) current. (I’ll get to that later)
– measure the battery voltage while load is applied. We’ll call this measurement B.
(nitpick #1: measure at the same spot along the wires as you did with the “open circuit”, preferrably at the connector)
– disconnect load
– measure the “open circuit” voltage(s) again as you did in the first step: this is measurement C.

The IR can be calculated from the difference between the average of the open circuit voltages and the voltage under load: V(IR) = ( V(A) + V(C) ) / 2 – V(B)

To calculate IR you need the load current, which we will call simply I: IR = V(IR) / I

That’s a lot of calculating to do, so to make matters simpler we will use a load of 1A (1Ampere): that means I=1 and less hassle for us as only then will our calculated V(IR) directly represent our IR.

1A load is very reasonable for many batteries and for all FPV packs: “reasonable” means your battery is rated for this load or more so you need to tweak this approach only for the tiniest batteries: your max allowed load is Capacity * C-number, e.g. a pack of 1100mAh and 15C rating can deliver up to 1100m * 15 = 16.5 Ampere! (Enter that into a “scientific calculator app” as 1100 EXP -3 * 15 to get the m milli’s in there)

For a 1A load you need a 4 Ohm resistor per cell. So for a 3S pack that’s a 4*3=12 Ohm resistor. You can get one or solder a couple other resistors in series to get at the resistance you need for your packs.

Also do note that you are loading the pack and thus producing heat in that load resistor: it will get HOT quickly!
The heat W is V*I (or V*V/R) is 4 Watt per cell. For a 3S pack you’ll be producing 3*4=12 Watts of heat at a 1A load. That’s burning your skin in seconds unless you get the resistor on a heat sink, e.g. large sheet of aluminium or copper.

The resistors are cheap, but keep in mind that the rule of thumb is that your resistor must be *rated* at AT LEAST TWICE the wattage you intend to produce in it. 10W 4R (4 Ohm) resistors (25W for 12R (12 Ohm) resistors!) are easily obtained in online electronics shops and AliExpress. Expect to see images of resistoes in a metal gold-colored (aluminium) housing with mounting holes: you can use those to screw these only your cooling plate (thick sheet of aluminium or formed “heatsink”).

If you’re not afraid of using a soldering iron and a bit of electric wiring, then, yes, you can use a multimeter to check your packs.
If that doesn’t sound like fun, you’re much better off buying a charger that does all this for you using its internal computer or looking for a ready-made load tester for LiPo packs.

Couple of notes:
I use 4 Ohm per cell as a “close enough and industry available” resistor value to get a near-1Ampere test current for a cell that’s close to 4.000V charge. Ohm’s law (wikipedia) tells you the real current, e.g. at 4.200V charge a precise 4 Ohm resistor will result in a 4.2/4=1.05Ampere load, which would be 5% off our intended load.

Aldo note that these “golden” power resistors are generally produced with a +/- 10% accuracy rating.

As the resistor load will drain your pack while connected, your pack’s charge will reduce. This is why you need to take the average if the “before” and “after” open circuit voltage measurements (A and C) as your pack’s voltage will have dropped. Taking the average is a reasonable approximation of reality, but that’s another (tiny) bit of in accuracy to reckon with.

Add to that undetermined wiring losses (which is why I said to do all voltage measurements at the same spot along the wires!), plus load resistor value change due to heating up, plus add generic multimeter inaccuracies measuring voltages at millivolt precision and you arrive at the “probably 20% off” accuracy figure I started out with. That is while using a 4 digit multimeter OR BETTER. 3 and 3.5 digit multimeters are utterly useless for this. While true 5 digit multimeters are bloody expensive…

There are other ways to measure IR (and much more accurately too) but you can’t easily do that by hand with simple tools.

BUT you CAN use a multimeter like that and the change in IR between fresh and worn-out packs is big enough to be easily noticable with this approach of “known fixed load” plus (precision) multimeter.

If you use the multimeter elsewhere this is fine, otherwise you can better spend those same bucks on a charger or tester that does all this for you on a nicely lit display. (I like the multimeter way but it sure isn’t for everybody.)

Reply
olds97_lss 30th October 2020 - 11:05 pm

I didn’t realize that high of mΩ was ok. Or does that limit drop as the capacity goes up? Per my isdt 6 plus, a pack I’m currently putting to storage charge as I type was showing 2.1-2.6mΩ when I charged it an hour ago. I considered it old/worn as it’s about 1.5yrs old. The pack is a hobbystar 6200mah 3S 50C pack.

I’m not sure what my other packs show because I usually parallel charge on other higher output chargers. My 4x100w charger doesn’t show IR at all.

When parallel charging, would the mΩ on the charger be n/cell count? So if I was charging 2x3S in parallel and was seeing 5mΩ, would that really be 2.5mΩ per cell? Wasn’t sure if IR for lipo’s was rated similarly as resistors in parallel.

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Thomas Van Driel 28th September 2020 - 12:18 am

When I charge a single 3S 450mAh lipo at 1C, IR reads in three times higher than if I charge 3 of these lipos. That is, singly IR is in the 20s, 30s and 40s. In parallel charging, IR is less than 10. Why is that?

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Sjoerd 4th August 2020 - 10:09 pm

Nice article!
Can you give some more info on different size lipo’s?

For example:
0/1000 mah xx ohm
1000/3000 mah xx ohm
3000/6000 mah xx ohm
6000/10.000 mah xx ohm
10.000 and up. Xx ohm.

Would be very helpful!

Reply
mey 21st July 2020 - 9:11 pm

Is there any charger/device that can measure the internal resistance of 1cell lipo battery? My hobbyking charger can measure the IR but only for 2cells or higher

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Oscar 31st July 2020 - 3:12 pm

you can with iSDT chargers, just need an adapter (JST-ph to XT60)

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Jeff 31st May 2020 - 5:32 pm

Other than balance charging, storage charging when not in uses, and not causing any physical damage, what other ways can one get long life uses out of Lipo batteries?

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ErlandLie 13th March 2020 - 5:53 am

About my experience LIHV batt
I have puffy 2S 450mah 45C and IR is 75, (cannot use it anymore)
And another 3S 450mah 45C and IR is 45 still use it for cinematic cinebee about 4 minutes from 4.30 percell to 3.6v percell
They same brand, fullsend..

And another brand,LDARC new battery,LIHV too
3S 530mah IR is 10 or 12

Another brand SKC
3S 520mah
And 2S 520mah have a same IR about 12 to 15

So i learnz more IR in specific battery it means lifetime.

Reply
philipp 22nd February 2020 - 11:05 am

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”. >> is this per cell or all IR of each cell added together?

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Oscar 11th March 2020 - 8:36 pm

Per cell.

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Owen 21st February 2020 - 7:42 pm

This doesn’t actually tell you how to test. This says go out and by a charger that will tell you. I was maybe expecting a explanation on how to use a digital multimeter to test. That would be a bit more helpful… or a vid if you ain’t done one already.

Reply
Roddy 21st February 2020 - 1:11 pm

Karol,
I don’t think so. 15 mOhm is a threshold for 1300~1500 mAh LiPo. If your quad uses 450 mAh then it also operates at lower current therefore at equal IR produces lower voltage sag. For 450 mAh I would estimate threshold IR at 30~40 mOhm.

Oscar,
The cell with higher IR does NOT discharge slower! The discharge current is equal for all cells thus the electric charge is equal as well. The cell with higher IR will have higher voltage sag thus it produces more heat during discharging.
It can even discharge faster – because due to wear it can have lower capacity which often goes together with increased IR.

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KarolG. 20th February 2020 - 9:36 pm

Hello,

I have got brand new Tattu HV 7.6V 450mAh 95C batteries. 3 pieces.
Internal resistance per cell is 23 Ohm each time for every out of those three.

Maybe HV batteries has higher IR? I do not know. Do you?

Have a good day

Reply
Oscar 11th March 2020 - 8:34 pm

i don’t think HV has anything to do with IR. It just uses a different chemical.

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Tanner 8th May 2019 - 9:48 pm

My charger doesn’t measure IR, but I can determine actual capacity by charging then discharging the battery. Can battery capacity help to determine battery health?

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Oscar 13th May 2019 - 2:37 pm

Maybe, but IR is more accurate and reliable.

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Jay 26th December 2018 - 2:51 am

I just crashed my fixed wing into the ocean. I had a brand new Lipo on for its first use as well, is my battery toast?

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Alex 13th November 2018 - 8:48 pm

How to know at what IR is time to drop the battery?

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Oscar 23rd November 2018 - 4:27 am

That’s why noting down IR as soon as you bought them is useful :)
It depends on how much you can tolerate the decrease in performance :) you can continue to use it if all the cells are within similar range.
The main thing you want to watch out for is if the IR of 1 or 2 cells drops much faster than the other cells, e.g. double…

Reply