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01-10-2020, 03:03 PM - 5 Likes   #1
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Some Pentax D-LI90 Battery Charge/Discharge Measurements

Spurred on by this recent post about battery life (A way to measure batteries or chargers? - PentaxForums.com), Iíve made some measurements of the charge and discharge behavior of a Pentax D-LI90 lithium battery.

I have three of these (genuine Pentax) batteries. They are dated 201102, 201306, and 201601, and probably arrived with my K-5 (March 2011), K-3 (March 2014), and K-1 (July 2016) respectively. Somewhere back in the K-5 era, I bought an extra, which went with the K-5 when it was sold.

The data presented here are for the 201601 battery (the newest). I will test the other D-LI90s, as well as some other camera batteries (OEM and other) from my Canon G15 and Q7 when a new data logger shows up (in the next few days; see below).

I drilled out a piece of wood so that the battery would slide part way in, making a snug fit. I drilled three small holes through which I inserted wires that make contact with the battery terminals. Using clip leads, I can connect the battery to either the Pentax charger power supply or to a resistor to discharge the battery. A voltmeter connected across the battery + and - terminals lets me monitor the voltage of the battery. I can also connect other meters to measure the current. I have not measured the charging current, though, and only checked briefly the discharge current.

Hereís a shot of the test set-up in charging mode (next to the bathroom sink!):



Initially, when I took the battery out of the K-3, it was partially discharged - it had been used for a while, but I do not know for how many frames. I charged it back up to where the charger green light went out. That gave a voltage of about 8.43 V after about 2 and a half hours. This charge cycle is represented by the red dots in the first graph. After the discharge cycle discussed below, I charged it back up to ďfullĒ again. This time the voltage maxed out at about 8.45 V when the green light went out. This charge cycle is shown by the blue dots. Since the charge state at the beginning of the original cycle was unknown, I have lined up the two charge cycles by offsetting the first cycle by an eyeballed-for-best-fit offset of 143 minutes. The two curves overlay reasonably well. For an essentially completely discharged battery, at about 6 V, the voltage jumps up immediately to almost 7 V when the charger is applied, and then takes about 4 hours to get to full charge, around 8.4 V. All 3 batteries are rated at 1860 mAh. If they can be fully charged in 4 hours, the current supplied by the charger must be at least (1860/4) = 465 mA (inefficiencies in the whole process require rather higher current). As noted, I did not measure the charging current. I will do that the next time I perform similar tests.

Hereís the graph of the charging process:




To measure the discharge curve, I placed a 53 ohm resistor (comprised of various resistors I had lying about) across the battery terminals, and measured the voltage from time to time. Upon application of the load, the voltage dropped by a tenth of a volt, and then began an almost linear discharge for about 12 hours. Near the end, the voltage dropped fairly quickly. As many have noted - a battery near the end of a charge cycle can go quickly from usable to kaput. I donít know at what voltage Pentax considers a battery to be no longer usable. When I do further testing, I will check what the camera thinks of the battery at various voltage values.

Hereís the discharge plot:




For my 53 ohm resistor, I can readily calculate what the current flow should be, using Ohmís law: I = V / R , where I = current, V = voltage, and R = resistance. Using my V and R values, the initial current should have been about 156 mA, whereas I measured 138 mA. Iím not sure why the difference, but I trust the voltage reading from my Radio Shack meter more than I trust the current reading from my Harbor Freight freebie meter!

If I take the calculated current and the time intervals between meter readings, I can total up how many milliampere-hours came out of the battery. If I do this for the discharge cycle, I get 1856 mAh - astoundingly close to the rated 1860 mAh! I donít really believe the similarity. Again, there are losses in the system, as well as some uncertainly attached to the readings themselves. But, the agreement does encourage me to believe that I have measured something close to reality.

My discharge rate was around 0.08 C (where C is the mAh capacity, assuming a one hour discharge; i.e. pulling a current of 1.86 amps from our little D-LI90). I do not know how much current a K-3 or K-1 draws during various functions, but it is probably generally more than this, perhaps approaching 0.5 C during some operations. I am trying to kludge up something to stick into the battery compartment to let me measure the current used by the camera.

As you can see from my first picture, my measurement setup is a bit of a kludge - multiple clip leads. And, I have to read the voltmeter periodically (every 10 to 20 minutes, typically), which is a drag.

I have ordered, and should receive very soon, a data logger which can store time-tagged voltages. With this, I can set up a test and then go away until it is finished (although I need to figure out a way to stop a discharge test before the voltage drops too low). Once Iíve got the logger, I will perform similar tests on all my D-LI90 batteries, repeating some to check for consistency, and to see whether age has affected them (although none have been all that heavily used - I would guess less than a few hundred cycles on any of them), as well as the other camera batteries that I have.

Stay tuned.

01-10-2020, 03:30 PM   #2
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My understanding from most manufacturers is that if the camera is to idle for a period of time, the battery should be removed. Additionally, Lithium Ion batteries should never be discharged to "0". They should have some small charge remaining. I am not sure as to the reasoning but almost every camera manual that I have read regarding battery life and maintenance strongly suggests batteries, particularly Lithium, not be completely drained.

Does anyone have an understanding as to why that would be the case?
01-10-2020, 03:58 PM   #3
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QuoteOriginally posted by Merv-O Quote
My understanding from most manufacturers is that if the camera is to idle for a period of time, the battery should be removed.
Though at the risk of discharge of the clock battery/capacitor.

( I know...I know...the manual says don't store batteries in the camera...)


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01-10-2020, 04:12 PM   #4
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IF I remember correction, there is a chip inside the camera that monitor battery voltage and even save that voltage level in the exif file when a picture is taken. Unfortunately, there is only the 3 bars indicator on the lcd, I often get caught off gard when the battery goes empty all of sudden from the last bar indicated in the top lcd of the camera. Would be nice to have a percentage of charge/discharge readable somewhere on the back lcd.

01-10-2020, 06:06 PM - 1 Like   #5
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QuoteOriginally posted by Merv-O Quote
My understanding from most manufacturers is that if the camera is to idle for a period of time, the battery should be removed. Additionally, Lithium Ion batteries should never be discharged to "0". They should have some small charge remaining. I am not sure as to the reasoning but almost every camera manual that I have read regarding battery life and maintenance strongly suggests batteries, particularly Lithium, not be completely drained.

Does anyone have an understanding as to why that would be the case?
I was looking into making a new battery pack for my DeWalt cordless drill and using lithium ion cells to do it because all the new drills use lithium ion. Apparently lithium ion cells are so great at discharging until they die, they are somewhat likely to catch fire. They are also great at burning. When manufacturers make the battery pack, they include a circuit to limit the discharge, which probably is what creates that rolloff at the right of the discharge graph. There are more details; I stopped reading when it became too complicated for a simple drill project.
01-10-2020, 06:34 PM - 2 Likes   #6
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K-3 II battery drain test and voltage measurement

QuoteOriginally posted by AstroDave Quote
I’ve made some measurements of the charge and discharge behavior of a Pentax D-LI90 lithium battery.
Interesting thread.

May I contribute some of the data from a battery-drain test I conducted several months ago?

My objective was to determine the number of shots I could expect from a Pentax OEM D-LI90 battery. Coincidentally, I was interested in the state of charge and the battery voltage under load.

In each trial series, a fully-charged battery was installed in my K-3 II. I used the camera normally during a two-week period until it would no longer 'power on' at the end of the battery's useful charge. Normal use involved the optical viewfinder, some 'chimping', and an occasional use of the rear LCD for menu adjustments. The camera was set to 'Auto Power Off' after 1 minute.

Using the application program ExifTool GUI, I captured relevant data from the EXIF data in the image files, specifically three EXIF fields found in MakerNotes:

- BodyBatteryVoltage1
- BodyBatteryVoltage2
- BodyBatteryState

I do not know what Voltage1 and Voltage2 measure in the camera system. They are offset by 0.26 to 0.18 V depending on the state of charge. I presume that V1 and V2 are measured under operational load at the time of each shot.

Two full trials were conducted using D-LI90 battery '201805'.

Series 3 (data table and plot not shown here):
At full charge: EXIF V1 = 7.45 V, V2 = 7.21 V. Battery State: 'Full' (as seen in the EXIF data of the first shot in the series).
At end of useful charge (EOC): V1 = 6.0 V, V2 = 5.81 V. Battery State: 'Almost Empty'.

Series 5: as seen in the attached table and plot.

The table also shows the 'local' delta V and predicted total shots possible based on a 6-V shut off and the number of shots to date.

From these two runs, it appears that the camera is designed to shut down when V1 reaches approximately 6 V. This threshold would correspond to a safety cut-off voltage of 3 V, which is typical for Li-ion cells.

Battery State and camera indicator: I wonder if the three bars in the top LCD battery indicator correspond to the battery states of 'full' (3 bars), 'close to full' (2) and 'running low’ (1 bar).


- Craig
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Last edited by c.a.m; 01-10-2020 at 09:10 PM. Reason: Correct voltage data for Series 3
01-11-2020, 02:25 AM   #7
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QuoteOriginally posted by c.a.m Quote
May I contribute some of the data from a battery-drain test I conducted several months ago?
Very interesting figures. The problem is what to conclude from looking at the data?

Does the three bars indicate mean?
- 3 bars = 100% ?
- 2 bar = 66% ?
- 1 bar = 33% ?

When I use Pentax DSLR, I'm fine with 3 bars and 2 bars, but I often don't know what to do when the indicator shows only 1 bar... last bar can mean 33% or 5%, it's impossible to tell. So actually I don't care if the battery indicator show 3 or 2 bars, 3 or 2 bars simply means "no concern". Now, when I pick up the camera and I see the last bar on battery indicator I have no way to know if there is still 33% of the charge or only 5%, so in case of doubt I replace the battery by a fully charged one, that also means I can seldom make use of the full battery capacity. The only way I can made use of full battery capacity without worry is when I attached the extra battery grip to the camera and set the camera to use body internal battery first, in that case the internal battery is fully used until the camera switches its power source from internal to battery grip.

Last edited by biz-engineer; 01-11-2020 at 02:31 AM.
01-11-2020, 07:32 AM - 1 Like   #8
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A lithium ion battery pack should have a balance, temp, and charging circuit. If one battery's voltage gets too low or out of balance, a circuit in the charger can indicate a bad battery pack, resulting in no charging. I have had to charge individual cells in a li-ion pack if they get out of balance to get the battery pack to charge again. This also tells me that the battery pack should not be used much longer. I have also seen a few li-ion power tools catch fire [battery burns first then the tool burns] so please use caution if attempting to diagnose or modify any li-ion battery pack. Burning batteries need to be outside as they off gas some toxic stuff....


Last edited by stihlmania; 01-11-2020 at 09:54 AM. Reason: spelling
01-11-2020, 09:01 AM - 1 Like   #9
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Great work, @AstroDave, @c.a.m!

The V1 and V2 probably represent battery voltages under two different loads such as the voltage when the camera is idle versus the lowest voltage recorded during the shooting cycle. One reason that battery indicators are so inaccurate is that it's hard to predict exactly how a battery will respond when the camera tries to draw a lot of power from it. If the camera tries to draw a lot of current from the battery and the battery cannot supply that much current, the voltage will plummet and then the camera's motors will stall and the digital circuits will glitch. There are plenty of anecdotes on PF about camera problems associated with discharged, weak, or aging batteries.

Draining a Liion battery too far causes damaging chemical changes to its internal structure. For battery packs with more than one cell (the D-LI90 contains two cells), this is even worse than it first appears. During discharge, the stronger cell(s) in the pack can drive the weaker cell down to damaging voltages. Thus the V2 voltage in the camera might be 5.7V but the cell voltages might be 3.5 for the stronger cell but only 2V for the weaker one. Each time an imbalanced battery pack is drained completely, the weaker cell gets damaged further.


If you want the most shots per charge, run the battery down to nothing but don't be surprised if the battery stops holding much of a charge, the battery indicator becomes erratic, or the camera glitches during shooting.

If you want the most shots possible over whole life of the battery, recharge it often, keep it cool, and never leave a depleted battery sitting in the camera or camera bag.
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