… Making sense of the numbers

‚‚The NP-W235 is the new battery for the recently released Fujifilm X-T4 camera. The following compilation of battery characteristics is taken from “nameplate” specifications (on the battery label), and published specifications (mainly X-T4 Owner’s Manual),  as well as some independently derived specifications. Explanations of the interpretation and significance of the various specifications are also given. For convenience, key points are noted for most sections.

Given that the new battery’s double-curved face is not very suitable for printing, most information is contained on the under-surface of the battery (unlike the NP-W126S battery which was able to use both front and back surfaces). The NP-W235’s under-surface displays specifications and safety warnings, and some standards compliance information, with different sections in English, Japanese, Korean, French, and Chinese. Because of limited space, only the safety warnings tend to be duplicated in the different languages. Other standards compliance symbols are impressed on the top and bottom ends of the battery.

 

CONTENTS

[ 1 ]  Battery Name
[ 2 ]  Component Cells
[ 3 ]  Capacity
[ 4 ]  Voltage
[ 5 ]  Nominal Energy
[ 6 ]  Dimensions
[ 7 ]  Volume
[ 8 ]  Weight
[ 9 ]  Volumetric Energy Density
[ 10 ]  Gravimetric Energy Density
[ 11 ]  Charging Temperature Protocols
[ 12 ]  Operating Temperatures
[ 13 ]  Production Date
[ 14 ]  Summary
Appendices

 

[ 1 ]  BATTERY NAME

Fujifilm have continued to follow their naming scheme for multi-cell battery packs, where the battery model number is derived from the battery’s capacity. Some older battery models do not conform to this scheme.

The apparent discrepancy between the NP-W235’s derived 2350mAh capacity specification, and the 2200mAh actually printed on the battery, will be explained below (refer: Section [ 3 ]  CAPACITY).

 

[ 2 ]  COMPONENT CELLS

The B-profile (or “B” cross-section) of the NP-W235 battery is an immediate clue to what shape cells are inside. In contrast, previous X-series and GFX-series batteries have had rectangular profiles/cross-sections, indicating rectangular-prism shaped (admittedly, with rounded edges) internal cells.

However, the battery also carries a standardised alpha-numeric descriptor string (2INR19/50), which indicates the internal composition of the battery pack, by cell type and size. Note that the descriptor string makes no reference to capacity.

You can find this same 2INR19/50 alpha-numeric string on similarly sized batteries from other camera makers, such as Canon’s EP-L6, Nikon’s EN-EL15, and Sony’s NP-FZ100, for example, although older batteries may not carry this descriptor string.

The NP-W235 battery (like its NP-W126S and NP-T125 siblings), is manufactured by Panasonic Energy, in its Wuxi, China, plant.

The internal cells are most likely Panasonic’s NCR18500 cells (model variant still to be identified), which fit the descriptor string. A retrieved copy of the data sheet for this cell, quoted a Minimum Capacity of 1900 mAh, but this data sheet (NNP Series, NCR18500, 2010B-1) was from February 2010. It is not unreasonable to expect the current model variant to be in the 2200 mAh capacity range, after a further 10 years of development.

 

[ 3 ]  CAPACITY

Capacity indicates the battery’s capability for sustained transfer of electrical charge. It specifies current per period of time, as milli-Amp hours (mAh). Past models of Fujifilm batteries commonly stated two different capacity specifications on the battery: a Typical Capacity, and a Minimum Capacity.

With the NP-W235 battery, Fujifilm have used a revised terminology for stating capacity (when using English language), which is to specify a Nominal Capacity and a Rating Capacity. These two specifications can be found in the X-T4 Owner’s Manual (English language PDF, page 338). We presume that this change only represents a difference in the naming of the specifications, but not in the underlying specifications that are indicated. The terminology differences are contrasted in the following table:

The NP-W235 battery only carries one printed capacity specification, 2200mAh, which is the Rating Capacity. The capacity value which would be determined from the battery name (a value of 2350mAh on the basis of the “235” in the name), is the Nominal Capacity.  Note that the label’s second occurrence of the 2200mAh value is preceded by the Chinese characters 额定容量 which generally have the meanings indicated in the Term (2) column of the table below.

Term (2) is commonly rendered as “Nominal”. (Try copy-and-pasting “translate 额定容量” into your browser, and check the various results). However Fujifilm’s NP-W235 English language usage, now renders Term (1) as “Nominal” (instead of “Typical”). The current Fujifilm usage essentially conflates the terms “Nominal” and “Typical”. Because of this, we must be careful when making specification comparisons.

The practical difference between the two Capacity specifications (Rating or Nominal), is largely related to intention. The Rating Capacity (or Rated Capacity) can be seen as a more formal expression of the Capacity, and might be used where the intention is to specify a reference class, or to indicate conformity, in situations pertaining to regulations (examples of such would be, safety, statutory, or excise contexts). The Rating Capacity is also the proper specification to use whenever a capacity is required for the determination of other specifications (for example, to determine Energy Capacity). In contrast, the (now called) Nominal Capacity, (formerly referred to as the Typical Capacity), is appropriate for contexts where the intention is to inform user expectation. This capacity value can reflect optimal results obtainable under ideal usage conditions.

The reason Nominal Capacity and Rating Capacity differ in their values, is because they were determined under different measurement conditions. Controlled measurement conditions include discharge rate, discharge termination voltage, and reference temperature. Changes to any of these conditions will affect the outcome of the Capacity test. Recent Fujifilm batteries (such as the NP-W126 class batteries) used different reference temperatures for the Typical and Rated Capacity measurements of their internal cells, with 25°C being used for determining Typical Capacity, and with 20°C being used to determine the Rated Capacity.

Within the standard operational range, the measured battery capacity will be positively affected by increased ambient temperature, and negatively affected by increased discharge rate. This effect is known as capacity offset.

Differences due to testing conditions (in this case, the reference temperature) adequately account for the capacity variations when comparing Nominal Capacity and Rating Capacity values.

However, the main reason for the user to differentiate between Nominal Capacity and Rating Capacity, is to avoid making invalid comparisons between different battery models. For example, make sure that the Typical Capacity of the NP-W126S is compared with the Nominal Capacity of the NP-W235, and that the Minimum Capacity of the NP-W126S is compared with the Rating Capacity of the NP-W235.

 

[ 4 ]  VOLTAGE

Voltage, or electromotive force, is the potential difference that the battery is able to effect across an energised circuit. There are two voltages printed on the NP-W235 battery: a Nominal Voltage, and a Maximum (or Charging) Voltage. The Nominal Voltage (7.2 Volts) is followed by the “solid line over dashed line” (Unicode symbol U+2393), which simply indicates that it is a Direct Current voltage. This 7.2 Volts is the expected Nominal Voltage for a twin cell Lithium-ion battery. The Maximum Voltage (8.4 Volts)  is preceded by the Chinese characters 充电限制电压 meaning “Charging Voltage Limit”.

Although Fujifilm does not specify a Minimum Voltage, this voltage is implied from the Maximum and Nominal Voltages, since (by definition) the Nominal Voltage lies halfway between the Maximum and Minimum Voltages.  Because the battery uses two cells connected in series, the battery voltage specifications are double those of a single cell.

Battery voltage is not a constant, but continuously varies according to the battery’s state of charge. Stating a Nominal Voltage is a way of attributing a virtual “fixed” voltage to the battery.

Note that, because the relationship between the battery’s voltage and capacity, is non-linear, the Nominal Voltage does not necessarily coincide with the battery’s 50% capacity point.

 

[ 5 ]  NOMINAL ENERGY

Nominal Energy, or Energy Capacity, is the rating measure for the amount of energy that is stored in the battery. The Nominal Energy of the NP-W235 battery is 16Wh (Watt hours). It is determined by multiplying the Rating Capacity in Amp-hours by the Nominal Voltage of the battery.

Significantly, the NP-W235’s Nominal Energy is even higher than that of the GFX system’s NP-T125 (14Wh) battery. The big increase that came with the NP-T125 battery, was its ability to power a higher voltage (10.8 Volt) system.

The NP-T125 (GFX series battery) has been shown for comparison, while noting that the stated voltage and capacity do not produce exactly 14Wh. There is a another possibility, which is, that the NP-T125’s Nominal Energy is actually 13.3Wh, and has been rounded up to 14Wh. This is reasonable if the specification is understood in the sense of “The battery’s nominal energy does not exceed 14Wh”, an appropriate assertion when set in a safety assessment context.

The International Air Transport Association (IATA) safety regulations, require lithium-ion batteries to be marked with the battery’s Watt-hour rating on the outside case. The Watt-hour rating is the measure by which lithium-ion batteries are regulated. Batteries, power banks, and devices with batteries inside, are assigned to different risk categories on the basis of the amount of energy they hold (energy which could be released as heat, fire, or explosion). In this context, 100Wh is the significant threshold for being categorised into a higher, more stringently controlled hazard class. The NP-W235 battery (at 16Wh) falls well below this threshold.

When comparing different batteries solely on the basis of mAh Capacity, differing Voltages can be a confounding factor, reducing the validity of the comparison. Nominal Energy is a better specification for comparing the ability of batteries to drive a system, because it takes into account the Nominal Voltage of the battery, as well as the battery’s Coulombic Capacity.

 

[ 6 ]  DIMENSIONS

The battery’s Dimensions specify its maximum extensions along various axes. The following figures are from the published dimensions of the NP-W235 battery. Of course, whether you consider the longest dimension to be a Height or a Length measurement, depends on the observational orientation of the battery.

This information, however, is of limited usefulness, because of the complexity of the battery’s shape, when compared to that of a simple prismatic shaped battery such as the NP-W126S. For this reason, we would like to also take a closer look at the battery’s Volume.

 

[ 7 ]  VOLUME

The battery’s  Volume is not specified by Fujifilm, so it has been derived independently. Volume indicates the amount of space appropriated by the battery. However, with respect to total camera size, the idea that “smaller is always better” may be overly-simplistic, when we consider that a more “substantial” camera, can contribute to a more satisfying (and more sustainable) hand fit, and feel. Knowing the battery’s Volume is advantageous, because it can be used to determine Volumetric Energy Density.  (refer: Section [ 9 ] VOLUMETRIC ENERGY DENSITY).

Although the Volume can be measured by displacement methods, I have used a graphical method to estimate the battery’s Volume. In contrast to a prismatic battery pack, where it is a simple matter of multiplying its three dimensions, the Volume of a battery pack with complex shape details can be tedious to derive based on linear dimensions. However, if we ignore micro-details (deeming them trivial, in terms of their contribution to the total volume), and look only at the larger-scale shape of the battery, the Volume assessment process can be simplified considerably. If we can determine a cross-sectional area, it is only necessary to multiply it by the height (or length, depending on how you look at it) in order to derive a good approximation of the battery’s Volume.

A distortion-free photographic image of the end of the battery, has been used as a surrogate for this cross-sectional shape. Applying a grid method for estimating the area of an irregular shape, a scaled 2 x 2 millimetre grid was overlaid, so that each grid cell represented 4 square mm.

The grid cells were each assessed as to whether they enclosed 0, 1, 2, 3, or 4 square mm (rounded to the nearest square mm) of the shape. Critical squares could be further divided in four equal squares or four equal triangles, as appropriate, in order to facilitate the assessment. In this way, the area of the image shape was estimated.

Based on this grid method, the cross-sectional area is approximately 776 square mm (1.2 square inches). Multiplying this area measurement by the length of the dimension perpendicular to it (52.3mm), gives a volume of approximately 40585 mm³.

The Volume estimate of 40585 mm³, is only 87% of the rectangular-prismatic volume (46386 mm³) which would be obtained by simply multiplying together the Height, Width, and Thickness.

With irregular shape profiles (such as this B-profile), space savings (due to contoured “form fitting” surfaces, for example), may not be able to be fully realised. For instance, the available space may be in locations where, in practical terms, it is of little use. Furthermore, to the extent that a camera’s battery chamber might still take a basically rectangular shape, the reduced (due to contours) volume of the battery might be effectively negated. However, apparent “wastage” of potential free space (as air spaces or voids), may still be contributing to design efficiency in other ways, such as heat transfer control. For these reasons, the battery’s space demands (how much bearing it has on the size of the camera), probably fall intermediate between the minimum footprint (based on the contoured shape), and the maximum footprint (based on the extreme dimensions of the virtual rectangular-prism shape).

Compared to the NP-W126S battery, the NP-W235 increases Rating Capacity at the cost of some “loss of compactness”. Importantly, the Capacity gain offsets and exceeds the loss. Note that the NP-W235’s true (not prismatic) Volume was used.

 

[ 8 ]  WEIGHT

Battery weight contributes to the required long term effort involved in supporting the camera during use. However, the idea that “lighter is always better” is possibly overly-simplistic, when we consider that the extra inertia of an appropriately “weighty” camera, can contribute to stability in hand-held usage. The NP-W235 battery weight (from published specifications) is 79g, or 2.8oz. The (presumed) 18500 cells should weigh approximately 34g each, with the balance of the battery weight (about 11g) due to battery case and circuit board. For comparison, the NP-W126S battery weighed 47g total, with each internal cell weighing 20.3g, and the balance of the battery weight (about 6.4g) being due to the battery case, internal separator, and circuit board.

Compared to the NP-W126S battery, the NP-W235 increases Rating Capacity at the cost of some “loss of lightness”. Importantly, the Capacity gain offsets and exceeds the loss.

 

[ 9 ]  VOLUMETRIC ENERGY DENSITY

The battery’s  Volumetric Energy Density is not specified by Fujifilm, so it has been derived independently. The Volumetric Energy Density reflects the efficiency impact of the battery’s spatial footprint on the camera size. It is specified in Watt-hours per litre (Wh/l). Provision of increased battery power, typically comes at the cost of increased camera size. If a new battery model has an increased Volumetric Energy Density value, that increase tends to justify any growth in camera size (due to larger battery).

The Volumetric Energy Density is calculated by dividing the Nominal Energy of the battery pack by its Volume. (For reference, one cubic millimetre is 1.0E-6 Liters). It is worth remembering that the total increase in camera size from the X-T3 to the X-T4, is not only due to increased battery size, but also to the inclusion of the new IBIS mechanism.

 

[ 10 ]  GRAVIMETRIC ENERGY DENSITY

The battery’s  Gravimetric Energy Density is not specified by Fujifilm, so it has been derived independently. The Gravimetric Energy Density reflects the efficiency impact of the battery’s mass on the total weight of the camera. It is specified in Watt hours per kilogram (Wh/kg).  The provision of increased battery power, typically comes at the cost of increased camera weight. If a new battery model has an increased Gravimetric Energy Density value, that increase tends to justify any rise in camera weight (due to heavier battery).

The Gravimetric Energy Density is calculated by dividing the Nominal Energy of the battery pack by its Weight. It is worth remembering that the total increase in camera weight from the X-T3 to the X-T4, is not only due to increased battery weight, but also to the inclusion of the new IBIS mechanism.

 

[ 11 ]  CHARGING TEMPERATURE PROTOCOLS

In order to avoid cell degradation during the charging process, the Fujifilm charging systems (both in-camera and external), switch to different charge rates for different temperature ranges, or even suspend charging if the temperature is outside the allowable range. I will refer to this behaviour as implementing Charging Temperature Protocols. These Protocols are required by the battery, but implemented by the charging device. As an example of how this works, when charging the NP-W126S battery in the BC-W126S charger, if the thermistor resistance (read from the T terminal of the battery) reaches 27 kΩ (kilo-Ohms) or higher, it indicates that the (inversely related) battery temperature is 10°C (50°F) or lower. This temperature is too low for optimal charging, and triggers a charge current reduction from the normal 720 mA (a rate of 0.6 C), to the low-temperature charge current of 420 mA (a rate 0.35 C). These numbers were verified experimentally. The low rate is specified by Panasonic’s data sheet for the NP-W126S’s internal cells (NCA673440), where it states “At temperatures below 10°C, charge at a 0.35C rate”. We expect the charging of the NP-W235 battery to be managed in the same way. The X-T4 Owner’s Manual (English PDF, page 309) states that “Charging times will increase at ambient temperatures below +10°C (+50°F) or above +35°C (+95°F)”. The reason for the increased charging times, is the reduction in charging rate, for these low and high temperature ranges.

In the past, third-party chargers have not replicated this temperature referenced behaviour, so we wonder, in the near future when we foresee third-party “NP-W235” batteries and chargers becoming available, whether they will observe these temperature based Charging Protocols. Whatever the case, the extent to which third party options are able to implement strategies to avoid cell degradation, will be of importance in determining the suitability, safety, and wisdom of choosing such options.

 

[ 12 ]  OPERATING TEMPERATURES

There is an expectation that Lithium-ion powered digital cameras, like the X-T4, will be operated in a wide range of environmental temperatures. Within this range, the Lithium-ion battery should be able to perform efficiently and safely. The X-T4 Owner’s Manual (English language PDF, page 338) specifies the battery’s Operating Temperature range as 0°C to +40°C. However, this specification is too conservative in the low temperature range, where Lithium-ion batteries are able to operate at temperatures significantly below freezing (Note, this is for operation, that is, discharge, not charging). If we consider the NP-W126S battery (also specified as 0°C to +40°C by Fujifilm), we find that Panasonic’s data sheet for the internal cells, states a discharge temperature range of -20°C to +60°C. We would expect a similar range for the NP-W234 battery. The upper limit of this range aligns with the maximum temperature of +60°C printed on the battery.

Adding to the perplexity of this matter, on page 337 of the X-T4 manual, under “Power supply/other > Operating Conditions”, a temperature range of -10°C to +40°C (+14°F to +104°F) is stated. I would suggest that the 0°C to +40°C range is a recommended Operational Temperature range (and possibly an amalgamation of the safe discharge temperature range, and the safe charging temperature range), while the -20°C to +60°C is the Absolute Environmental Temperature range, for the battery. In general, Lithium-ion batteries can be discharged well below 0°C (although the available capacity will be reduced). When Lithium-ion batteries operate at very low temperature, a typical decrease in efficiency is: while at -10°C the battery can deliver only about two-thirds of its normal capacity, and while at -20°C the battery can deliver only about half of its normal capacity. Although these very low temperatures are not ideal, the batteries can be utilised (but not charged) at these temperatures, and photographers do successfully use them down to -10°C and lower.  Operating Temperature range is a specification area where Fujifilm could make further clarification.

A subset of the environmental temperature range is the Storage Temperature range. The X-T4 Owner’s Manual (p. 309) states this range as +15°C to +25°C (+59°F to +77°F). This temperature range is to avoid cell degradation while the battery is not being used for extended periods of time. Although not explicitly stated by Fujifilm, the optimal storage temperature is the lower value in the range, about +15°C (+59°F), and the optimal state of charge during storage, is about 40 – 50%.

 

[ 13 ]  PRODUCTION DATE

Mainly for the purpose of tracking production batches, OEM batteries are typically marked with a production number, which can identify the production lot, and possibly also, the date of production. Production lot information may be required for warranty issues, as well as for product safety notifications and recalls. Some standards, such as Korean Certification, require the production year, month, and lot number, to be provided. The new NP-W235 battery (unlike the NP-W126 series batteries, which required a decode-chart in order to determine the production date), follows the NP-T125 (GFX cameras) practice of clearly displaying the production date on the battery.

The first two digits indicate the year, and the second two indicate the month. Production date is relevant to the user because lithium-ion batteries typically give their best performance within a two year period of the time of production. Knowing the production date can assist in predicting when a performance drop-off is likely to begin, and in devising a replacement plan.

Although the X-T4 camera’s “battery age” display function, might make knowing the production date superfluous in terms of battery replacement planning, the production date information can still be a useful unique identifier to distinguish between multiple batteries.

 

[ 14 ]  SUMMARY

The Fujifilm X-T series mirrorless cameras have been renowned for their reduced size and weight when compared to DSLR cameras. With the release of the X-T4 and its bigger battery, there is widespread approval of the longer operation times available, but also some reservations about the effects on camera size and weight due to both the IBIS and the larger battery. The following table compares important properties of the NP-W126S and NP-W235 batteries, which affect performance against size/weight trade-offs.

Compared to the NP-W126S battery, the NP-W235 is more efficient, as is evidenced by its higher Volumetric and Gravimetric Energy Densities. This equates to more power for the package size and weight. The modest down-side costs of increased weight and size, are offset by a significant gain in Capacity, at advantage ratios that could not be achieved by simply designing the new camera to hold two NP-W126S batteries, for example.

This survey did not look at shots-per-charge, or video-capture endurance, since, as a matter of not only battery efficiency but also camera efficiency, such investigations are beyond the stated scope of the present study. However, We look forward to being able to test the new battery, in real world situations, where we expect the results to be different from those of the standard CIPA testing. I hope the present work will give users of the new X-T4 camera, a better appreciation of the NP-W235 battery, and contribute to realistic performance expectations.

 

APPENDICES

TEXT INFORMATION SOURCES

Fujifilm NP-W235 Operating Instructions:  BL00005069-100
(日本語, English, Français, Deutsch, and Español)
and
Fujifilm NP-W235 Istruziono d’uso:  BL00005069-300
(Italiano, and Pусский)

Fujifilm Digital Camera X-T4 Owner’s Manual:  BL00005063-201
(English)

 

TRANSLATION OF THE JAPANESE SECTION
The Japanese printed sections of the battery label read:

電式バッテリー
Electric battery

富士フイルム株式会社
Fujifilm Corporation

使用後は、リサイクル協力店へ
After use, take to a recycling center.

危 険 :
Danger:

大険・ 発熱、発炎、破裂の原因となるので、火・水中投入、加熱、分解、ショートなどしないこと。
Do not throw into fire / water, heat, disassemble, or short-circuit as this may cause serious injury, heat generation, fire, or rupture.

高温での充電、 使用、放置をしないこと。
Do not charge, use or leave at high temperature.

専用充電器以外で充電しないこと。
Do not charge with any device other than the dedicated charger.

 

TRANSLATION OF THE CHINESE SECTION
The lower four lines of the battery label (written  in Chinese), read:

富士胶片株式会社
Fujifilm Co., Ltd.

锂离子电池组
Lithium-ion battery pack

型避:NP-W235
Type: NP-W235

中国组装
Assembled in China

充志限制电压 :8.4V
Charging voltage limit: 8.4V

额定容量 :2200mAh
Rated capacity: 2200mAh

注意:
Note:

禁止拆解、手击、挤压或投入火中。
Do not disassemble, strike, crush or throw into fire.

请勿置于高温环境中。
Do not place in high temperature environment.

电池浸水后禁止重用!
It is forbidden to reuse the battery after immersion in water!

[Last revised: 18/05/2020]

 

A NOTE OF CAUTION

The Internet is full of misleading and outright wrong “information”. I make every effort to ensure that the information I present is factual, as best I am able to establish. However, there is always the possibility that I have made a mistake, or that my sources are in error. If you are going to use the above information as the basis for important decisions, or use it as a reference source for other informative works, I recommend that you exercise due diligence and seek independent verification of the accuracy of the information presented here.

Thanks for reading . . .

Fuji NP-W235 Battery

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