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01-15-2020, 06:02 PM - 12 Likes   #1
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The Pentax Battery Level Meter -- What it actually shows

The Pentax Battery Level Indicator -- What it actually shows

Have you ever wondered what your camera’s battery meter is really telling you?

This article presents my study that aimed to establish a quantitative relationship between the Pentax K-3 II battery level indicator and the battery's state of charge. In other words, how much battery charge is left as indicated by the meter?

Happy to receive any comments.

- Craig



Summary

The Operating Manuals for various Pentax cameras provide scant information on the meaning of the Battery Level Indicator. The analysis reported here has determined that the indicators on the Pentax K-3 II, using an original D-LI90 battery, represent the battery charge levels shown in Figure 1, which has not been documented in the manuals or other Pentax literature.

Importantly, the findings reveal that a ‘3 bar, full’ indication can be illuminated at as low a charge as half-full, while the ‘2 bar’ indication is not nearly “close to full” as stated in certain operating manuals.






Introduction

Many Pentax cameras implement a battery level indicator, either on a top-panel LCD, a rear information screen, or both. Typically, the top LCD indicator illuminates up to three segments or 'bars', depending on the charge state of the battery. The indicator in the rear screen uses colour – commonly, green, yellow or red. By themselves, the battery indicators provide only a coarse qualitative cue of the state of the battery's charge. Furthermore, operating manuals for various Pentax cameras do not provide any details to guide a user to accurately understand the battery level or how many shots are left.

Operating manuals for certain older cameras, such as the K-30, K-50, and K-5 II, provide a brief description of the Battery Level Indicator. Unfortunately, more recent manuals, such as those for the K-3, K-70 and K-1, omit such information entirely, so those users have little insight into the meaning of the indicator. There doesn’t seem to be any evidence that the implementation or precision of the battery indicator have changed over the years, so it likely functions similarly across the older and newer camera models. Therefore, it is assumed here that the information that is available in some manuals may be applied generally.

According to the Operating Manual for the K-5 II, the LCD indicator represents the following 'battery levels':
- 3 bars: Full
- 2 bars: Close to full
- 1 bar: Running low
- no bars, but illuminated: Almost empty
- no bars, blinking: empty, or end of useful charge.
The following analysis quantifies the four battery levels and relates the state of battery charge to the indicator status. This study did not intend to analyze the number of shots possible from a fully-charged or partially-discharged battery. Also, the scope of this article precludes a thorough description of battery ‘fuel gauge’ implementations or an analysis of the dynamic discharge characteristics of the Pentax battery under various operating modes.


Li-Ion Battery Discharge Characteristics


As shown in a post by PF member @AstroDave ( Some Pentax D-LI90 Battery Charge/Discharge Measurements - PentaxForums.com), and illustrated in various manufacturers' technical data sheets, the voltage across a Lithium-Ion (Li-Ion) cell decreases with its discharge state; the more energy drawn from a battery, the lower its voltage. A battery’s State of Charge (SOC) is defined as the fraction of the maximum possible energy charge present in a rechargeable battery, commonly expressed as a percentage. For example, a fully-charged battery has, by definition, an SOC value of 1.0 or 100%; a battery that is discharged halfway has an SOC of 50%.

Estimating a battery’s SOC accurately and precisely is difficult because few relevant physical parameters are measurable while a battery is in use. A simple scheme may measure the battery’s voltage to infer the SOC, but the relationship between voltage and remaining capacity is not strictly linear. However, under typical discharge conditions, a healthy Li-ion battery exhibits a roughly linear, sloped voltage-capacity profile across most of its discharge cycle until it is nearly exhausted. The voltage decreases approximately proportionally to the SOC over most of a discharge cycle, so the SOC may be inferred from the measured voltage. Using the open-circuit (no-load) voltage (Voc) generally provides more accurate results than the closed-circuit voltage (Vcc, i.e., under operating load), which is affected by a cell’s variable internal resistance and the circuit’s operating current.

SOC estimates based on Vcc can have errors of 10% or more, especially at the ‘tail end’ of a discharge, but this may be an acceptable trade-off for this practical and relatively simple method. Using Vcc is suitable for determining the relative state of charge, but is not appropriate for measuring the battery’s actual capacity in mAh.

The Pentax D-LI90 battery consists of two cells, almost certainly model 18500, from an unknown manufacturer. The battery has a specified nominal voltage of 7.2 V (3.6 V per cell), a maximum charge voltage of 8.4 V, and a typical end-of-discharge cut-off voltage of 6 V. A new, fully-charged battery in its relaxed state will show an open-circuit voltage of approximately 8.4 V. It has a rated capacity of 1860 mAh.


Image files contain battery voltage data


Unlike the Exif data for most other camera brands, fortuitously the Pentax Exif data include three tags related to the camera battery (Ref: Pentax Tags, Pentax Tags , revised 11 December 2019):
- BodyBatteryVoltage1 (‘V1’)
- BodyBatteryVoltage2 (‘V2’)
- BodyBatteryState.
The battery voltages are interpreted in volts, while the state is registered as an integer that is encoded as follows:
1 = Empty or Missing
2 = Almost Empty
3 = Running Low
4 = Close to Full
5 = Full
This information may be readily extracted by using an Exif reader application such as ExifTool or ExifToolGUI. Typically, the battery information appears in the Makernotes section of the Exif structure.

It is not known at which pick-off points in the camera the two voltages are measured, or what parameters they actually represent. Lacking a design specification or model to relate the two voltages, this study used Voltage1 to characterize the discharge profile, assuming that it measures the operating closed-circuit voltage.


Transition Levels for the Battery State

The basic methodology in this study involved looking at the Exif data in hundreds of my existing image files, in particular sets of images that included sequences of many tens of images taken at short intervals during single photo outings or lens focus-calibration sessions. ExifToolGUI was used to scroll through the files while looking at the battery tags in Makernotes. Stable transitions in BodyBatteryState and their associated battery voltages were noted, i.e., Full --> Close to full, Close to full --> Running low, etc. Some 20 such discrete transitions were identified. Each set of similar 'transition voltages' was averaged to find a valid value pertinent to each battery state.

Across the images, three D-LI90 batteries had been used arbitrarily in the camera: date stamped 201805, 201509, and 201207. The two older batteries are believed to be satisfactorily healthy despite their age. A battery grip was not employed.

In addition, the study also examined two special sets of image files, in which controlled battery life tests had been performed on a single battery (dated 201805). Besides incorporating the image files from these two tests into the broad Exif inspection, they were used to establish the value of Voltage1 at full charge (7.46 and 7.45 V) and full discharge (6.0 and 5.9 V), which are key parameters in this analysis.

The voltage transition points are indicated in Table 1. Assuming the linear discharge profile mentioned above, the state of charge was determined as follows:
Full charge: V1max = 7.455 V (average 7.46, 7.45)
End of useful charge (EOC, i.e., empty): V1min = 5.95 V
State of charge (SOC) = 1 - ( (V1max - V1) / (V1max - V1min) )



Corroborating Tests

One of the special test sets, called 'Series 3', was used for a different methodology to relate the battery state indication to the state of charge. Series 3 started with a full battery and achieved 370 shots before the battery was depleted. The number of shots associated with each Body Battery State was determined from the Exif data, and their percentages give an estimate of the SOC divisions. For example, as shown in Table 2, 47% of the images had a Battery State of 'Full', which correlates well with the estimate determined from the overall image sets. While the findings do not agree perfectly across the two methodologies, they are reasonably close.




To further validate the methodology and corroborate the findings, another dedicated battery discharge test was conducted using continuous video recording segments. Recording was started with a freshly-charged battery. At various times, the recording was stopped, a still image was taken immediately, and the open-circuit voltage was measured on the extracted battery using a digital voltmeter. The recording test was continued until the battery indicator was observed to transition solidly to ‘2 bars’.

The pertinent data from this test is given in Table 3, while Figure 2 shows the open-circuit voltage and BodyBatteryVoltage1 (V1) as function of operating time. Of note is the full-charge V1 voltage of 7.49 V, which is about 0.5% higher than the values seen in the other two series (7.46, 7.45 V). This suggests that the battery had achieved a slightly higher full charge.

The transition to 2 bars was registered at a Voltage1 value of 6.74 V, compared to the average of 6.67 V and a maximum reading of 6.71 V seen in the main data set.








Conclusion

The overall findings are summarized in Figure 3. The vertical bars indicate the Voltage1 values associated with each battery state, while the maximum state of charge (SOC) of each state is given above each bar.




In summary, the Battery Level Indicator on the K-3 II, using a genuine Pentax D-LI90 battery under normal operating conditions, indicates the following battery charge status:

- 3 bars: battery is charged 100 – 50 %
- 2 bars: 50 – 33 %
- 1 bar: 33 – 15 %
- no bars, but illuminated: less than 15% charge remaining.

If a user sees 3 bars, the camera has at least 50% of the battery charge remaining – it’s not below half. Two bars indicate at least 33% charge – still good for perhaps a couple hundred more shots under modest battery consumption. A single bar could mean as little as 15% battery power remains. No bar means that there is still some 'juice' left, but the camera will shut down soon, especially under power-hungry functions such as Live-View – it’s time to swap batteries.


Last edited by c.a.m; 01-15-2020 at 06:28 PM.
01-15-2020, 06:20 PM   #2
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Nice work! Especially your last plot. I agree that at ~6 volts, your battery status is just about toast! Time to stick it in the charger.

I've got my data logger, and hope to report some more info on discharge curves for various vintages of D-LI90s (when I get done reviewing some NASA proposals!).
01-15-2020, 06:51 PM   #3
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QuoteOriginally posted by c.a.m Quote
- 3 bars: battery is charged 100 – 50 %
- 2 bars: 50 – 33 %
- 1 bar: 33 – 15 %
- no bars, but illuminated: less than 15% charge remaining.

If a user sees 3 bars, the camera has at least 50% of the battery charge remaining – it’s not below half.
Thanks for all of your hard work. Your conclusions reflect my working practice with the K-3, that being if the gauge shows less than three, seriously consider swapping it out before doing or continuing an extended session. Corollaries include starting the day with a fresh battery and taking care to have several (in my case, five) fresh replacements in the bag. I often end up swapping out one, have never had to do two, and pray that if I ever run through the full stack, there is something good on the card.


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01-15-2020, 07:25 PM   #4
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Wow, this is an excellent piece of work. I had unsubscribed from the original "how long will a battery last" thread and missed @AstroDave's excellent follow-up thread. Frankly, I'm blown away by the insightful posts and the grunt work to support those posts that the two of you have done.
QuoteOriginally posted by c.a.m Quote
No bar means that there is still some 'juice' left, but the camera will shut down soon, especially under power-hungry functions such as Live-View – it’s time to swap batteries.
I don't think it is a coincidence that the "safety cut-off" point is somewhat above 5V, many digital IC's operate on 5VDC and if the voltage in those digital circuits deviates too much from that, uncorrectable errors start showing up. In terms of power efficiency, it is much better to continuously step down voltage than to step it up.

The other comment that I will make is that it also isn't a coincidence that precise voltage readings are included in the EXIF maker's notes, not for the camera user, who doesn't need that information after the photo has been taken, but for the people developing the camera. Therefore, the skewed weighting used for the battery indicator that you discovered is by design, not by accident. From the standpoint of the user, the worst possible outcome is to have a camera fail to take a photo when the shutter is pressed, the second worst possible outcome is to stop taking pictures because the user isn't confident the battery will last long enough. The camera designers need the battery indicator to guide the user's behavoir, so the user doesn't get ticked off from changing batteries too often and also doesn't get ticked off because he didn't get adequate warning that a battery swap was going to be necessary.

01-15-2020, 11:11 PM   #5
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That is great. Thank you for sharing that information. Now if I could just have the something similar for when my gas gauge light comes on, LOL!
01-15-2020, 11:16 PM   #6
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This is really helpful, and explains why I always felt that batteries can go from "close to full" to "almost empty" very quickly.
From the description in the manual I have believed that battery meter was more linear (100-75-50-25-0), but it is clearly not.
It is almost like they designed the meter in "stops" (1 - 1/2 - 1/4 - 1/8).

As meter showing "close to full" could actually be down to 1/3, or that "full" could be down to 1/2, will change how I use my batteries.
01-16-2020, 12:36 AM   #7
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My recent experience, especially with my KP, fully supports the findings that a battery can be only half-charged, or thereabouts, whilst still showing a "full" indication.

I never had a problem with my K-5 and the relatively enormous battery capacity of the D-Li90, the spare was only ever insurance and a convenience for when the other battery was being recharged.

With the K-70, the D-Li190 spare got used very occasionally out of necessity, but mostly still just insurance and convenience.

Now, with my KP, the battery gets replaced every time I come home, almost irrespective of how many shots I've taken, and the battery from the camera goes on the charger. A second battery in the bag is a necessity, so a third has become my "insurance".

Far too often do I check my battery level, see "all-green" then seemingly just a few minutes later the whole thing goes dead in my hands


Last edited by kypfer; 01-16-2020 at 12:38 AM. Reason: grammar
01-16-2020, 12:38 AM   #8
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Thank you so much. This is really helpful information.
01-16-2020, 08:48 AM   #9
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QuoteOriginally posted by AstroDave Quote
Nice work!
Thanks! I see that you're hunting for dead D-LI90s.

QuoteOriginally posted by stevebrot Quote
Thanks for all of your hard work. Your conclusions reflect my working practice with the K-3, that being if the gauge shows less than three, seriously consider swapping it out before doing or continuing an extended session.

Thanks for your comment. During my research, I found quite a few anecdotes about the battery gauge and surprising rapid depletion of batteries.

QuoteOriginally posted by RGlasel Quote
Wow, this is an excellent piece of work.
Thanks!
01-16-2020, 08:54 AM   #10
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QuoteOriginally posted by RGlasel Quote
it also isn't a coincidence that precise voltage readings are included in the EXIF maker's notes
Actually, I did not find voltage tags in the Exif data for Canon, Fuji, Nikon, Olympus or Sony cameras. The Pentax Exif appears to be far more detailed than the others.

QuoteOriginally posted by david94903 Quote
Thank you for sharing that information. Now if I could just have the something similar for when my gas gauge light comes on, LOL!
LOL How true.

QuoteOriginally posted by Fogel70 Quote
As meter showing "close to full" could actually be down to 1/3, or that "full" could be down to 1/2, will change how I use my batteries.
I was somewhat surprised at the findings. "Full" and "Close to full" are not entirely accurate descriptors!

QuoteOriginally posted by kypfer Quote
fully supports the findings that a battery can be only half-charged, or thereabouts, whilst still showing a "full" indication.
This is good to know. Thanks.

QuoteOriginally posted by Pentaxrookie Quote
This is really helpful information.
Thanks. Glad to hear that it's useful.

- Craig
01-16-2020, 02:20 PM   #11
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QuoteOriginally posted by c.a.m Quote
Thanks! I see that you're hunting for dead D-LI90s.
Yeah - then I can put some wires in an LI90 case and insert it into the camera, in parallel with an external battery, and make some real-time, under-load measurements!

A dead charger would be nice, too.
01-16-2020, 03:14 PM   #12
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Wow, this is thorough and quite instructive. Greatly appreciate your work, Craig.
01-17-2020, 02:26 PM   #13
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QuoteOriginally posted by Madaboutpix Quote
Wow, this is thorough and quite instructive. Greatly appreciate your work, Craig.
Thank you for your kind words.

Having an engineering background, I enjoyed doing the tests, collecting the data, and writing the article. Sometimes my wife (gently) suggests to me that I spend more time on the technical parts than taking pictures!


The other thread that @AstroDave started also includes interesting data. He's got a data logger now, so we'll probably gain additional insight from his work.

- Craig
01-17-2020, 02:44 PM   #14
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QuoteOriginally posted by kypfer Quote
My recent experience, especially with my KP, fully supports the findings that a battery can be only half-charged, or thereabouts, whilst still showing a "full" indication.

I never had a problem with my K-5 and the relatively enormous battery capacity of the D-Li90, the spare was only ever insurance and a convenience for when the other battery was being recharged.

With the K-70, the D-Li190 spare got used very occasionally out of necessity, but mostly still just insurance and convenience.

Now, with my KP, the battery gets replaced every time I come home, almost irrespective of how many shots I've taken, and the battery from the camera goes on the charger. A second battery in the bag is a necessity, so a third has become my "insurance".

Far too often do I check my battery level, see "all-green" then seemingly just a few minutes later the whole thing goes dead in my hands
This is almost identical to my experience with the KP as well. Now I know why. I always have several batteries with me just in case.
01-18-2020, 05:49 AM - 1 Like   #15
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Impressive effort and nice article!

QuoteOriginally posted by c.a.m Quote
- 3 bars: battery is charged 100 – 50 %
- 2 bars: 50 – 33 %
- 1 bar: 33 – 15 %
- no bars, but illuminated: less than 15% charge remaining.
Fits well with my experience with the K-5/K-3/K-1. Somewhat less scientific, but I have always treated 2 bars as a "swap as soon as is convenient" signal, and 1 bar as "swap as soon as possible".
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