(This article was posted previously in the Pentax DSLR Discussion forum. It has been edited slightly for readability.)
The Pentax Battery Level Indicator -- What it actually shows 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 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 type 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:
https://exiftool.org/TagNames/Pentax.html)
- 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 – so, it’s time to swap batteries.
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