How to measure capacity of a Lithium-ion battery
I bought a Lithium-ion battery for a camera (much cheaper than the brand replacement but non unreasonably cheap compared to AAA Li-Ion batteries with similar charge). I however have doubts that it has the capacity it claims on the package (in mAH). Is there a simple way for me to roughly verify the claim on the package which is more precise and less time consuming that to run some benchmarks with the camera and a comparison battery? I.e. is there some device / method that given a a Lithium-ion battery which is supposedly fully charged determines:
- Is it actually fully charged
- What is its current charge in mAH
Assessing full charge is the easy part.
Method (a) A fully charged Lithium Ion single cell battery will have an open circuit voltage of about 4.2 Volt*. (4.1 to 4.2 OK. 4.0 not quite there. 4.3 - a bit high.) Some cameras use two cells - double the expected voltages. Laptops and other larger devices use 3 or more cells. The voltage should be a multiple of the above voltage. [*There are variants that allow higher voltages. Unless you are CERTAIN that this includes your one, assume that it doesn't. Getting it wrong can be 'upsetting'.
(ie N x (4.1 to 4.2V))
Method (b) Use a good quality charger (eg one supplied by camera manufacturer or one of known quality) which has a "charging light.
Place "charged battery on charger". Depending on how long since it was last charged the charge light should either flash or perhaps remain on for a minute or two and then go off.
Remove battery from charger. Wait 10 seconds. Place battery back on charger. Charge light should flash very briefly and go out.
Assessing capacity is harder, but not hard.
(a) you can get some indication, for nominally equal batteries, from the weight. A significant part of the weight in a LiIon battery is actively involved components whether electrically or mechanically (separators, conductors, electrolyte & (of course) Lithium metal. Two batteries of the same nominal capacity should have similar weights. I'd guesstimate that a 10% difference may be due to happenstance and construction, but beyond that I'd be suspicious. In larger & heavier batteries this test will work better than for very small batteries.
For interest, for AA NimH cells this is an excellent indicator. Modern high capacity AA's which claim 2500 mAh + capacity should be in the high twenty gram range - say 26 grams plus with some just over 30 grams. Anything under 20 grams is a complete dud and anything 25 grams or below is suspect.
(b) For any sort of accuracy you need to discharge the battery to an "end point" and measure capacity. No other method reasonably available to you is available. There are other methods such as measuring the change in voltage over a given time under a given load and trying to assess where you are on the discharge curve. This is difficult to get right and needs experience and a degree of luck. Measuring discharge time is "easier".
Best is a constant current load, which can be made very easily with eg an LM317 and one resistor, but I'll assume for now that you don't want to do that. Ask if interested.
A discharge resistor that takes at least one hour to discharge should be used. You could use a motor or lamp or camera or ... but a resistor has some advantages.
R minimum ~= (Cells_in_battery x 4000) / mAh
eg if you have a 1 cell battery (Voc=~4.2V) of 1500 mAh capacity then
- R = cells x 4000 / mAh = 1 x 4000/1500 = 2.666 ohm ~= 3 ohm or 3.3 ohm (std value)
Use the next largest resistor than the value calculated.
Up to Several times larger is OK BUT it will take proportionally longer.
Resistor power rating: Resistor power = V^2/R = (4 x number of cells)/R
eg for the above single cell and 3 ohm resistor the minimum wattage rating is
- 4 x 1 / 3 = 1.333 Watt.
Use a 2 Watt or greater resistor.
I'll describe this briefly as I don't know your experience level. This may be easy to follow or hard. If hard, ask more questions.
- Attach temporary wires to battery terminals. Two paper clips bent at end resting on terminal is flat and accessible and held with weight or tape. Wires inserted into connector id not openly accessible. Some batteries will not provide power until you give them secret handshakes. but most will.
Battery with accessible terminals.
Below: Harder to access terminals. Two dress making pins or two wires can work here BUT DO NOT SHORT TOGETHER !!! IF YOU ARE NOT COMFORTABLE DOING THIS DON'T DO IT.
- Monitor battery voltage throughout. Multimeter connected to battery wires and set to appropriate range.
- Connect resistor to battery leads. Start a timer. Monitor voltage. Stop at 3.2V per cell. DO NOT DISCHARGE BELOW 3 VOLTS PER CELL. STOPPING AT 3.2V IS A "GOOD IDEA". A LiIon battery may be damaged badly by very deep discharge. Set a timer. DO NO leave this running and walk away.
Below: Typical lithium Ion 1 cell 'battery' discharge curve.
Best method is to do this with genuine and clone batteries and compare times.
- Method (c) Easiest :-).
Use a camera. Set to video or timed photos. Note start and end frame times. Compare.
Major advantages are
"set and forget
no playing with battery connections
UPDATE - January 1st 2013 - Happy New Year.
I've just been asked offlist by somebody about the LM317 circuit I mentioned for constant current discharge. Here is an example. I copied this from the very useful and relevant webpage on LED driving - here and they in turn copied it from an LM317 data sheet.
The offlist query said
- You mentioned a way by using LM317 to determine battery capacity. I need to check a lithium ion battery with about 1700mAh capacity.
What do you recommend to me to measure this kind of battery capacity in a reasonable time like 3-4 hours.
A 1700 mAh battery would be discharged in 3 hours by 1700/3 =~ 570 mA and in 4 hours by 1700/4 ~= 425 mA. So using about 500 mA and seeing how long it takes will give a measure of battery capacity.
The current of the3 load in the circuit above is
Iout = Vref/R1 so
R1 = Vref/Iout
For an LM317 Vref = 1.25V so for 500 mA
R1 = V/I = 1.25V / 0.5A = 2.5 Ohm.
Power in R1 = I^2 R = 0.5^2 x 2.5 or about 0.7 Watt.
A 1 Watt resistor would probably survive this - a 2 Watt or 5 Watt would be better.
The LM317 will dissipate V_LM317 x I = (Vbattery - Vref) x I = (4.2-1.25) x 0.5 =~ 1.5 Watt. So a heatsink or piece of Aluminum or other thermally conductive material on the LM317 will be "a good idea". I use 4.2 V for the battery voltage. It will drop as the battery discharges.
Note that in many cases a 1700 mAh LiIon battery can be safely discharged at up to 1C rate - = 1700 mA in this case. Safer is C/2 = 850 mA. Actual max allowed rate should be set by the manufacturer. Use Imax = C/2 if no data available. This will usually be safe but "caveat emptor" / "YMMV" ... . If using a higher rate the power dissipation in the resistor and LM317 will be higher and changes will be needed. Some LM317 will handle 1A max. Some will handle 1.5A. (Some smaller pkgs < 1A) . See data sheet. The LM350 is a big brother version of the LM317 that works at several amps.
The battery endpoint voltage should be the endpoint Voltage that you will use in your system. As per my comments above, this MUST NOT BE below 3.0V to prevent battery damage, and higher is safer. You need either to keep a close eye on this if stopping discharge manually OR set up an automatic cutoff system. How you do this and how you time the discharge period is up to you.
Great, detailed answer. I'll give that a go. Regarding 1b, I tried that but the results are odd. Both the external charger I used and the camera internal charger I used claim fully charged at the end of charging and agree with each other. When inserted into the camera and switched on (non charging mode) the camera claims that the battery is only about 1/3 charged. I'll try and verify voltage...
What brand and model of camera and battery? Some batteries have internal electronics that talk to the camera and report capacity. The capacity claim may be accurate OR the battery may be fully charged BUT the clone battery may not have emulated the original protocol accurately enough. // An often reasonably continuous load on a camera battery is to put it into USB mode. I have several cameras which do not shut down if there is no activity in this mode but simply drain the battery. This has the advantage of not overdsicharging the battery. You still need to manually time it.
the camera is a Sony HX9V and the battery is some no-name brand off e-bay. Not bottom of the heap though, cost $20 (which sounds reasonable for a 1400mAh battery), came in a well made package with grammatically correct English, boasts double IC circuit protection, knows the secret Sony handshake, and claims cells made in Japan. The USB tip is great. Normal camera mode is very unpredictable in battery drain as these fancy cameras spend a lot of energy in processing to find faces, adjust exposure, make sure everybody smiles, do GPS, ... which is hard to control.
The failure mode of a cheap battery is more likely to be low cycle life than low capacity. It's a design tradeoff and a scammer would probably choose to make the battery test well in a handful of cycles and fail after a month instead of a year. By then they've changed their seller ID on eBay.
My extensive professional experience is with NimH and not LiIon. I have good LiIon experience as a keen photographer with numerous Sony / Minolta system batteries. **IN MY EXPERIENCE:** Clones have slightly lower capacity than genuine. Some clones fail to extremely low capacity long before they should. Clones that are good in capacity tend to have poorer calendar lives than original batteries. Overall clones are inferior in a range of ways with the original batteries always being the best performers. But, value for money clones are good enough on average to be worth their price.
There is no *metallic* Lithium in Lithium-*ion* cells, e.g. see the Wikipedia article Lithium-ion battery. This is a widespread misconception, so it would be well worth the effort to fix it here, to help alleviate that.
@BillDubuque I'm not sure what part of the answer you are referring to. The closest I can see to what you say is "A significant part of the weight in a LiIon battery is Lithium metal." This is a correct statement as it stands and is the manner in which the battery Li content is referred to in eg IATA and similatr airfreight regulations. It was not meant to be addressing the form in which the Lithium metal is present. | Avoiding metallic Lithium as "bare metal" is an important object of end of charge arrangements. Terminal voltages above 4.2V tend to plate out metallic Li - a VERY bad idea.
Yes, that statement is incorrect. **There is no Lithium *metal* in Li-ion batteries** (except trace amounts in rare exceptional conditions when it plates out due to over/too-fast charging). This (widespread) confusion causes many problems, e.g. firefighting techniques differ for cells containing Lithium metal (Lithium *primary* cells) vs. those that contain no metallic Lithium (Lithium-ion *secondary* cells). Please read the linked Wikipedia article to learn more about Li-ion batteries.
Some further corrections: (1) 4.2V is not necessarily 100% state-of-charge, e.g. many modern cells are charged to 4.35V not 4.2V. (2) Many chargers turn on their "completely charged" indicator when the CV phase starts, which can be anywhere from 80-90% state-of-charge.
(3) It is crucial to use the correct current and termination-voltage when attempting to duplicate the manufacturer's capacity claims. Normally Li-ion cells are rated at C/5 discharge, terminated between 2.5-2.75V, but it is possible that cells intended for lower-drain devices (e.g. cell phones) may be quoted at lower drain rates. Consult the datasheet to be sure.)
Further, your (false) claim that "A significant part of the weight in a LiIon battery is Lithium metal" is inconsistent with your answer here. A typical 18650 weighs about 45g, but there you claim it has about 0.75g Lithium content, hardly a significant part of its weight. Even if you mean the nebulous "equivalent Lithium content", whatever its definition, surely it is not meant to be more than an order of magnitude different from the *actual* Lithium metal content.
Yes, you need a dummy load and a way of measuring voltage (i.e. multimeter)
It can be as simple as a resistor, but the current will drop alongside voltage, so mAh is a little "harder" to calculate. A constant current dummy load is probably preferable. This can be bought as an IC, or easily made with an opamp, or a couple of transistors (you can use one but not as accurate, a thing called a cascode is better)
Either way, say your battery starts off at 4V fully charged (usually around 4.1 to 4.2V really), and you place a 40 ohm resistor across it. This will cause 100mA of current to flow. If you measure the voltage regularly, you can calculate the current flowing. When the battery is flat you can integrate the readings gathered to give you battery capacity in mAh. So if you were to read hourly and you get 10 readings of 100mA before the battery is considered flat (around 3V usually, be good to be whatever manufacturer tests to, but voltage will drop quite steeply at end of capacity), you have a 1000mAh battery (In reality the current will drop unless you use a constant current load)
Note that batteries will usually have less capacity at higher currents, so if you draw 1A the overall mAh will be lower. I would select a value that is low enough for the battery to perform reasonably (whatever the manufacturers tested at, if there is a datasheet available - probably quite low)
You should end up with something like this when you plot your results (note the x axis is in Amp hours:
C is the discharge rate needed to discharge in 1 hour, so for a 1000mAh battery 0.1C is 100mA, 1C is 1A, 18 C is 18A. You can see how the capacity drops at higher discharge rates. NOTE - (as Russell mentioned) do not attempt to discharge at large currents (e.g. much above > 1C) unless you are absolutely sure the battery can handle this (e.g some camera batteries and RC batteries can discharge at huge rates) The above pic is only meant to be an example of discharge curves.
Also make sure the resistor (if you use one) is accurate (1% or less if you can) and rated for whatever power you need it to handle. For the above example at 0.1A and 4V, you have 0.4W, so you need at least a 0.5W 40 ohm resistor.
Do not discharge standard lithium Ion batteries at faster than the 1C rate eg not more than 1500 mA for a 1500 mAh battery. Some standard batteries will allow 2C discharge and some special batteries much more **BUT** trying to discharge a standard battery at say 10C may be the last discharge it ever does. "Vent with flame" is the standard euphemism.
For info: I've been measuring lithium ion cells with 100mA constant current and 3.5 minimum shutoff voltage. Found the general, inexpensive 14500 cells stamped with 4000 mAhr show about 150 to 325 MaH capacity. The larger 16850 also stamped 4000 mAh show about 1000 to 1600 mAh capacity. I recently dismantled old laptop batteries and found they were unmarked 16850 type but all had capacities of 1760 mAh. Running higher discharge rates drastically reduce the capacity as mentioned in this article. It is best to quantify battery capacity requirements according to your circuit design to determine actual time of use. The difficulty is finding batteries to buy that state "honest" ratings.
I don't know if anyone is ever going to read this response, since it's been years since the last one, but here is my approach, havind had the same question: what's my LiIon battery's capacity.
Wel I figured waht I was really interested in was: how much charge is delivered between a fully charged state and the battery shutting down. Shutting down, in my case, meant that the equipment started beeping, meaning that it wanted to be recharged.
To charge the battery I use a simple usb cable, but with a device that measures voltage, current, time, and total milliAmp-hours. These gadgets can be found on the internet and are cheap.
So, to determine the capacity of the Li-Ion battery at hand, just let it die down and recharge it. The amout of mAh shown in the recharge dongle is the effective capacity.
I understand that Li-Ion batteries do not get hot or anything, and any mAh going in during charging, is available for discharge in operation as well. So, the number you find fully charging it should give you the answer.
Not really, if you measure the power going in some will be wasted as heat and is not recoverable when pulling energy back from the battery. The way to determine how much charge you have is usually done from the batteries characteristic curve.
"Charge efficiency is about 99 percent and the cell remains cool during charge". This I read on the "http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries" page. Any heat wasted is wasted outside the battery, i.e. the voltage difference between charger and battery times the charge current.
I've recently bought one of these USB testers and you have to understand what's happening under the hood to interpret results correctly. Charging is done with 5V (as you said you use USB), but the battery's internal voltage is between 3.7V and 4.2V. Therefore you'd need to correct for the voltage conversion: the same amount of charged energy (expressed in Wh) yields a smaller mAh value displayed by the USB tester gadget equivalent to an approximately 25% higher mAh value received by the battery. Same for discharging: a booster circuit increases a power bank's voltage output.
Good point: know what's going on under the hood. In smaller devices, the "voltage conversion" from 5 to 3.x Volts is performed by a diode and resistor in series, with a level detector that says "enough already" by means of a led. In that case mAh registered at the USB output is the actual charge. I fully appreciate that that's not the best way to charge a Li-Ion battery, but it was the situation I was dealing with when I wrote my original response. Thanks for the comment.