Prosecution Insights
Last updated: April 19, 2026
Application No. 17/708,512

PARTITION FOR ELECTROCHEMICAL APPARATUS, ELECTROCHEMICAL APPARATUS, AND ELECTRONIC APPARATUS

Non-Final OA §103§112
Filed
Mar 30, 2022
Examiner
VAN KIRK, DUSTIN KENWOOD
Art Unit
1722
Tech Center
1700 — Chemical & Materials Engineering
Assignee
Ningde Amperex Technology Limited
OA Round
4 (Non-Final)
76%
Grant Probability
Favorable
4-5
OA Rounds
3y 4m
To Grant
87%
With Interview

Examiner Intelligence

Grants 76% — above average
76%
Career Allow Rate
13 granted / 17 resolved
+11.5% vs TC avg
Moderate +11% lift
Without
With
+10.6%
Interview Lift
resolved cases with interview
Typical timeline
3y 4m
Avg Prosecution
31 currently pending
Career history
48
Total Applications
across all art units

Statute-Specific Performance

§103
61.2%
+21.2% vs TC avg
§102
20.8%
-19.2% vs TC avg
§112
13.5%
-26.5% vs TC avg
Black line = Tech Center average estimate • Based on career data from 17 resolved cases

Office Action

§103 §112
DETAILED ACTION Notice of Pre-AIA or AIA Status The present application, filed on or after March 16, 2013, is being examined under the first inventor to file provisions of the AIA . Continued Examination Under 37 CFR 1.114 A request for continued examination under 37 CFR 1.114, including the fee set forth in 37 CFR 1.17(e), was filed in this application after final rejection. Since this application is eligible for continued examination under 37 CFR 1.114, and the fee set forth in 37 CFR 1.17(e) has been timely paid, the finality of the previous Office action has been withdrawn pursuant to 37 CFR 1.114. Applicant's submission filed on 29 September 2025 has been entered. Status of Claims Claims 1, 3-5, and 8-21 are currently pending Claims 1, 8, and 13 are amended Claims 6-7 have been cancelled New claims 19-21 have been added Status of Amendments The amendment filed 27 August 2025 has been fully considered, but does not place the application in condition for allowance. Status of Objections and Rejections of the Office Action from 27 June 2025 The 102 rejections over He are withdrawn in view of Applicant’s amendment. The 103 rejections over He in view Amine are maintained in view of Applicant’s amendment and have been modified to address the new limitations and claims. Claim Rejections - 35 USC § 112 The following is a quotation of 35 U.S.C. 112(b): (b) CONCLUSION.—The specification shall conclude with one or more claims particularly pointing out and distinctly claiming the subject matter which the inventor or a joint inventor regards as the invention. The following is a quotation of 35 U.S.C. 112 (pre-AIA ), second paragraph: The specification shall conclude with one or more claims particularly pointing out and distinctly claiming the subject matter which the applicant regards as his invention. Claim 16 is rejected under 35 U.S.C. 112(b) or 35 U.S.C. 112 (pre-AIA ), second paragraph, as being indefinite for failing to particularly point out and distinctly claim the subject matter which the inventor or a joint inventor (or for applications subject to pre-AIA 35 U.S.C. 112, the applicant), regards as the invention. In the present instance, claim 13 defines the partition as comprising packaging layers, comprising a polymer material, and an intermediate layer, comprising three structural layers each comprising at least a carbon material. Claim 16 then specifies that the partition is selected from at least one of the listed metals. It is unclear how much of the partition this selection applies to, which pieces of the partition this selection interacts with, the intermediate layer or the packaging layers, and whether this selection manifests as an extra layer in either piece separate from those already claimed in claim 13 or as an alloy of the selected metal with the carbon material of one or more of the structural layers or the polymer material of one or more of the packaging layers. The following is a quotation of 35 U.S.C. 112(d): (d) REFERENCE IN DEPENDENT FORMS.—Subject to subsection (e), a claim in dependent form shall contain a reference to a claim previously set forth and then specify a further limitation of the subject matter claimed. A claim in dependent form shall be construed to incorporate by reference all the limitations of the claim to which it refers. The following is a quotation of pre-AIA 35 U.S.C. 112, fourth paragraph: Subject to the following paragraph [i.e., the fifth paragraph of pre-AIA 35 U.S.C. 112], a claim in dependent form shall contain a reference to a claim previously set forth and then specify a further limitation of the subject matter claimed. A claim in dependent form shall be construed to incorporate by reference all the limitations of the claim to which it refers. Claim 16 is rejected under 35 U.S.C. 112(d) or pre-AIA 35 U.S.C. 112, 4th paragraph, as being of improper dependent form for failing to further limit the subject matter of the claim upon which it depends, or for failing to include all the limitations of the claim upon which it depends. In the present instance, claim 13 defines the partition as comprising packaging layers, comprising a polymer material, and an intermediate layer, comprising three structural layers each comprising at least a carbon material. Claim 16 then specifies that the partition is selected from at least one of the listed metals, which broadens the range of options that the partition may comprise, rather than further limiting the range already set in claim 13. Applicant may cancel the claim(s), amend the claim(s) to place the claim(s) in proper dependent form, rewrite the claim(s) in independent form, or present a sufficient showing that the dependent claim(s) complies with the statutory requirements. Claim Rejections - 35 USC § 103 The following is a quotation of 35 U.S.C. 103 which forms the basis for all obviousness rejections set forth in this Office action: A patent for a claimed invention may not be obtained, notwithstanding that the claimed invention is not identically disclosed as set forth in section 102, if the differences between the claimed invention and the prior art are such that the claimed invention as a whole would have been obvious before the effective filing date of the claimed invention to a person having ordinary skill in the art to which the claimed invention pertains. Patentability shall not be negated by the manner in which the invention was made. Claims 1, 3-5, and 8-21 are rejected under 35 U.S.C. 103 as being unpatentable over He et al (US 20160020481 A1), hereinafter He, in view of Amine et al. (US 20020164441 A1), hereinafter Amine. Regarding claim 1, He teaches a partition 4 and 14 for an electrochemical apparatus (Fig. 1), comprising: an intermediate layer 4 and packaging layers 14, wherein the packaging layers 14 are located on upper and lower surfaces of the intermediate layer (Fig. 1); wherein a material of the intermediate layer comprises at least a carbon material [0046]; and a material of the packaging layers comprises a polymer material [0032]. He does not specify that the partition is ionically insulative or that a temperature at which the packaging layers start to soften is at least 10°C lower than a temperature at which the intermediate layer starts to soften. However, He teaches an intermediate layer comprising Cu, Al, graphite plate, or carbon fiber plate [0046] and a packaging layer comprising one of polypropylene, polyethylene, polystyrene, or polyurethane [0032] which are included in the list of suitable materials disclosed in the instant specification at [0040-0043]. Therefore, it is inherent that a temperature at which the packaging layers start to soften is at least 10°C lower than a temperature at which the intermediate layer starts to soften and that the partition plate is ionically insulative. When the structure taught by the reference is identical or substantially identical to that of the claims, the claimed properties or functions are presumed to be inherent. See MPEP 2112.01. He is silent as to a second structural layer located on one side of the first structural layer and a third structural layer located on the other side of the first structural layer, and one of the packaging layers being on the surface of the second structural layer and/or another one of the packaging layers being on the surface of the third structural layer. However, Amine teaches a partition wherein the intermediate layer comprises a first structural layer 13 (Fig. 1) and a second structural layer 15 located on one side of the first structural layer. Amine teaches a preference for having two layers of thin metal foil rather than a single thicker metal foil [0028]. It would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to further modify this teaching to include a third structural layer on the other side of the first structural layer in order to enhance the expected benefits. Further, Amine teaches the structural layers as comprising aluminum foil [0010]. He teaches aluminum foil as being an art recognized equivalent to graphite plate and carbon fiber plate [He 0046]. Therefore, the structural layers of Amine are functionally equivalent to the intermediate layer of He. He and Amine are both considered to be analogous to the claimed invention because they are in the same field of partition walls enclosing electrode assemblies in electrolytic solution. Therefore, it would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to modify the intermediate layer of He with the teachings of Amine to have three structural layers with the packaging layers of He being arranged on the surfaces of the second structural layer and/or the third structural layer. Doing so would have improved the solvent impermeability of the partition, made the partition easier to heat seal, and prevented foil cracking [0028]. Regarding claim 3, modified He teaches the partition according to claim 1. Amine further teaches a packaging layer 17 overlaid along 100% of the inside of the metal foil intermediate layer 13 and 15 (Fig. 1 of Amine). It would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to modify He to fully overlay the intermediate layer with the packaging layer, as taught by Amine. Doing so would have trapped any acid released by the electrolyte and prevented degradation of the integrity of the structure, in particular the intermediate layer [0037]. Regarding claim 4, modified He teaches the partition according to claim 1. He further teaches the packaging layers 14 covering peripheral edges of two surfaces of the intermediate layer (Fig. 1). Regarding claim 5, modified He teaches the partition according to claim 1. He further teaches one of the packaging layers 14 being on at least one surface of the intermediate layer (Fig. 1). Regarding claim 8, modified He teaches the partition according to claim 1. He further teaches the carbon material comprising at least one of carbon felt, a carbon film, carbon black, acetylene black, fullerene, a conductive graphite film, or a graphene film, in this case carbon fiber plate or graphite plate are considered to be equivalent to carbon film or graphite film [0046]; and the polymer material comprising polypropylene, acid anhydride modified polypropylene, polyethylene, ethylene and copolymer, polyvinyl chloride, polystyrene, polyether nitrile, polyurethane, polyamide, polyester, amorphous a-olefin copolymer, or a derivative of the foregoing substances [0032]. Regarding claim 9, modified He teaches the partition according to claim 1. Amine further teaches a foil layer 13 varying in thickness from about 6 μm to about 120 μm [0038]. Three foil layers would produce a thickness range of about 18 μm to about 360 μm. In the case where the claimed ranges "overlap or lie inside ranges disclosed by the prior art" a prima facie case of obviousness exists. In re Wertheim, 541 F.2d 257, 191 USPQ 90 (CCPA 1976); In re Woodruff, 919 F.2d 1575, 16 USPQ2d 1934 (Fed. Cir. 1990). It would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to modify the thickness of the intermediate layer foil of He with the thickness of Amine. Doing so would have prevented easy cracking of the foil and allowed for easier heat sealing of the structure [0038]. Regarding claim 10, modified He teaches the partition according to claim 1. He does not disclose wherein the temperature at which a material of the intermediate layer starts to soften is greater than 130°C. However, He teaches an intermediate layer comprising graphite plate or carbon fiber plate [0046] which are included in the list of suitable materials disclosed in the instant specification [0043]. Therefore it is inherent that the temperature at which the material of the intermediate layer starts to soften is greater than 130°C. See MPEP 2112.01. Regarding claim 11, modified He teaches the partition according to claim 1. He does not disclose wherein the temperature at which a material of the packaging layer starts to soften is 120 °C to 240 °C. However, He teaches a packaging layer comprising one of polypropylene, polyethylene, polystyrene, or polyurethane [0032] which are included in the list of suitable materials disclosed in the instant specification at [0043]. Therefore, it is inherent that the temperature at which a material of the packaging layer starts to soften is 120°C to 240°C. See MPEP 2112.01. Regarding claim 12, modified He teaches the partition according to claim 1. Amine further teaches a foil layer 13 varying in thickness from about 6 μm to about 120 μm, in certain embodiments from about 5 μm to about 50 μm, and in other embodiments from about 9 µm to about 18 µm [0038]. Amongst these ranges, three foil layers could produce a thickness range of about 15 μm to about 54 μm. In the case where the claimed ranges "overlap or lie inside ranges disclosed by the prior art" a prima facie case of obviousness exists. In re Wertheim, 541 F.2d 257, 191 USPQ 90 (CCPA 1976); In re Woodruff, 919 F.2d 1575, 16 USPQ2d 1934 (Fed. Cir. 1990). Examiner notes that Amine further teaches a preference that the combined thickness of the multiple thinner foils be less than the thickness of the one thicker foil [0028], indicating that three foil layers should preferably have a combined thickness equal to or less than that of two foil layers at possibly about 10 µm to about 36 µm. It would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to modify the thickness of the intermediate layer foil of He with the thickness of Amine. Doing so would have prevented easy cracking of the foil and allowed for easier heat sealing of the structure [0038]. Regarding claim 13, He teaches an electrochemical apparatus (Fig. 1), wherein the electrochemical apparatus comprises at least one partition 4, at least two electrode assemblies 5a and 5b, an electrolyte [0056], and outer packaging 1, and the electrode assemblies 5a and 5b are located in a separate sealed cavity 2 and 3, wherein, the partition 4 and 14, comprises an intermediate layer 4 and packaging layers 14, wherein the packaging layers 14 are located on upper and lower surfaces of the intermediate layer (Fig. 1); wherein a material of the intermediate layer comprises at least a carbon material [0046]; and a material of the packaging layers comprises a polymer material [0032]. He does not specify that the partition is ionically insulative or that a temperature at which the packaging layers start to soften is at least 10°C lower than a temperature at which the intermediate layer starts to soften. However, He teaches an intermediate layer comprising Cu, Al, graphite plate, or carbon fiber plate [0046] and a packaging layer comprising one of polypropylene, polyethylene, polystyrene, or polyurethane [0032] which are included in the list of suitable materials disclosed in the instant specification at [0040-0043]. Therefore, it is inherent that a temperature at which the packaging layers start to soften is at least 10°C lower than a temperature at which the intermediate layer starts to soften and that the partition plate is ionically insulative. When the structure taught by the reference is identical or substantially identical to that of the claims, the claimed properties or functions are presumed to be inherent. See MPEP 2112.01. He is silent as to a second structural layer located on one side of the first structural layer and a third structural layer located on the other side of the first structural layer, and one of the packaging layers being on the surface of the second structural layer and/or another one of the packaging layers being on the surface of the third structural layer. However, Amine teaches a partition wherein the intermediate layer comprises a first structural layer 13 (Fig. 1) and a second structural layer 15 located on one side of the first structural layer. Amine teaches a preference for having two layers of thin metal foil rather than a single thicker metal foil [0028]. It would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to modify this teaching to include a third structural layer on the other side of the first structural layer in order to further enhance the expected benefits. Further, Amine teaches the structural layers as comprising aluminum foil [0010]. He teaches aluminum foil as being an art recognized equivalent to graphite plate and carbon fiber plate [He 0046]. Therefore, the structural layers of Amine are functionally equivalent to the intermediate layer of He. He and Amine are both considered to be analogous to the claimed invention because they are in the same field of partition walls enclosing electrode assemblies in electrolytic solution. Therefore, it would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to modify the intermediate layer of He with the teachings of Amine to have three structural layers with the packaging layers of He being arranged on the surfaces of the second structural layer and/or the third structural layer. Doing so would have improved the solvent impermeability of the partition, made the partition easier to heat seal, and prevented foil cracking [0028]. Regarding claim 14, modified He teaches the electrochemical apparatus according to claim 13. He further discloses a separator 12 used to isolate positive and negative and only allow the passage of lithium ions [0020]. He does not disclose the outermost layer of the electrode body comprising a separator, and the separator being adjacent to the partition. However, it has been held that rearrangement of essential working parts of a device is prima facie obvious. See In re Japikse, 181 F.2d 1019, 86 USPQ 70 (CCPA 1950) see also MPEP 2144.04 IV.C. Regarding claim 15, He teaches the electrochemical apparatus according to claim 13, wherein an outermost layer of at least one electrode assembly 5a and 5b comprises a current collector 8’and 9’, and the current collector is adjacent to the partition (Figs. 2 and 3). He does not disclose wherein an outermost layer of at least one electrode assembly comprises a separator, and the separator is adjacent to the other side of the partition. However, it has been held that rearrangement of essential working parts of a device is prima facie obvious. See In re Japikse, 181 F.2d 1019, 86 USPQ 70 (CCPA 1950) see also MPEP 2144.04 IV.C. Regarding claim 16, modified He teaches the electrochemical apparatus according to claim 13. He further teaches an outermost layer of the electrode assembly 5a and 5b comprising a current collector 8’and 9’, the current collector is adjacent to the partition (Figs. 2 and 3), and current collectors of electrode assemblies on both sides of the partition have opposite polarities, in this case 8’ is positive and 9’ is negative. He teaches the partition comprising Cu, Al, graphite plate, or carbon fiber plate [0046], but is silent as to the partition being selected from at least one of Ni, Ti, Ag, Au, Pt, Fe, Co, Cr, W, Mo, Pb, In, Zn, or stainless steel. However, Amine teaches a metal foil partition comprising aluminum, stainless steel, nickel, copper, and alloys thereof [0028]. It would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to select a combination of stainless steel or nickel and graphite plate or carbon fiber plate for the partition material because the combination of Amine and He teaches aluminum, stainless steel, nickel, copper, graphite plate, carbon fiber plate, and alloys thereof as being art recognized equivalents for metal foil partitions (MPEP 2144.06). Regarding claim 17, modified He teaches the electrochemical apparatus according to claim 15. He further teaches the packaging layer 14 being of electronic insulation, in this case made of insulating and sealing pieces [0032], an outermost layer of the electrode assembly 5a and 5b comprising a current collector 8’ and 9’, and the current collector being adjacent to the partition (Fig. 3). He is silent as to the whole partition being electronically insulative. However, Amine teaches fully overlaying an intermediate layer 13 with a packaging layer 17. It would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to modify He to fully overlay the intermediate layer 4 (of He) with the packaging layer 14 (of He), as taught by the layout of Amine. Doing so would have trapped any acid released by the electrolyte and prevented degradation of the integrity of the structure, in particular the intermediate layer [0037]. This would also overlay the intermediate layer with an electronically insulative packaging layer, producing an electronically insulative partition as required in claim 17. Regarding claim 18, modified He teaches the electrochemical apparatus according to claim 13. He further teaches an electronic apparatus, wherein the electronic apparatus comprises the electrochemical apparatus according to claim 13, in this case a vehicle [0001]. Regarding claim 19, modified He teaches the electrochemical apparatus according to claim 1. Amine further teaches a packaging layer 17 overlaid along 100% of the inside of the metal foil intermediate layer 13 and 15 (Fig. 1 of Amine). It would have been obvious to someone of ordinary skill in the art before the effective filing date of the claimed invention to modify He to fully overlay both surfaces of the intermediate layer with the packaging layer, as taught by Amine. Doing so would have trapped any acid released by the electrolyte and prevented degradation of the integrity of the structure, in particular the intermediate layer [0037]. Regarding claim 20, Modified He teaches distinct first and second structural layers and they are considered different layers. modified He teaches the electrochemical apparatus according to claim 1. He further teaches the structural layer comprising graphite plate or carbon fiber plate [0046]. Therefore, it would have been obvious for one of ordinary skill in the art to select different materials for each structural layer, such as graphite plate for the first structural layer and carbon fiber plate for the second structural layer. Regarding claim 21, He teaches the electrochemical apparatus according to claim 1. He further teaches the partition being configured to separate a first electrode assembly 5a from a second electrode assembly 5b (Fig. 1) [0030]. Response to Arguments Applicant's arguments filed 27 August 2025 have been fully considered but they are not persuasive. Applicant’s argument that the adhesive of Amine does not qualify as a structural layer is moot in view of the modified rejection above. However, Examiner respectfully notes that the structural layer is not defined in the claims and the adhesive layer of Amine is used to hold together layers 13 and 15, which aids in the product structure. Therefore, the adhesive layer could be considered to be a structural layer. Applicant argues that He in view of Amine does not teach the partition separating two electrode assemblies because layer 14 is not initially taught to be between the two electrode assemblies. Examiner respectfully disagrees because there is no requirement that all parts of the partition be positioned in between the electrode assemblies and the partition of He is still able to separate the two electrode assemblies, even with the layer 14 being initially positioned along the perimeter of the intermediate layer before the introduction of the teachings of Amine. Applicant argues that Amine only teaches packaging for an exterior portion of a battery with only a single packaging layer being disclosed. Examiner respectfully notes that only the interior portion of the packaging for Amine is used for teaching, which is designed to face the same conditions as the interior of He and may be applied to both sides of the intermediate layer of He. Conclusion The prior art made of record and not relied upon is considered pertinent to applicant's disclosure. Matsunaga (JP 2013004563 A) teaches an electrochemical apparatus comprising two metal plates with adhesive in between and on either side that could potentially be used to read on three carbon containing intermediate layers and two packaging layers. Minamitani (US 20190348644 A1) and Minamitani (US 20160248052 A1) both teach an electrochemical apparatus that could potentially still read on the partition separating two electrode assemblies with the electrode assemblies being positioned on left and right sides of the partition rather than top and bottom sides, as desired by the instant, but not claimed. Any inquiry concerning this communication or earlier communications from the examiner should be directed to DUSTIN KENWOOD VAN KIRK whose telephone number is (703)756-4717. The examiner can normally be reached Monday-Friday 9am-5pm EST. Examiner interviews are available via telephone, in-person, and video conferencing using a USPTO supplied web-based collaboration tool. To schedule an interview, applicant is encouraged to use the USPTO Automated Interview Request (AIR) at http://www.uspto.gov/interviewpractice. If attempts to reach the examiner by telephone are unsuccessful, the examiner’s supervisor, Niki Bakhtiari can be reached at (571)272-3433. The fax phone number for the organization where this application or proceeding is assigned is 571-273-8300. Information regarding the status of published or unpublished applications may be obtained from Patent Center. Unpublished application information in Patent Center is available to registered users. To file and manage patent submissions in Patent Center, visit: https://patentcenter.uspto.gov. Visit https://www.uspto.gov/patents/apply/patent-center for more information about Patent Center and https://www.uspto.gov/patents/docx for information about filing in DOCX format. For additional questions, contact the Electronic Business Center (EBC) at 866-217-9197 (toll-free). If you would like assistance from a USPTO Customer Service Representative, call 800-786-9199 (IN USA OR CANADA) or 571-272-1000. /DUSTIN VAN KIRK/Examiner, Art Unit 1722 /NIKI BAKHTIARI/Supervisory Patent Examiner, Art Unit 1722
Read full office action

Prosecution Timeline

Mar 30, 2022
Application Filed
Oct 25, 2024
Non-Final Rejection — §103, §112
Jan 23, 2025
Response Filed
Apr 03, 2025
Final Rejection — §103, §112
Jun 10, 2025
Response after Non-Final Action
Jun 25, 2025
Final Rejection — §103, §112
Aug 14, 2025
Applicant Interview (Telephonic)
Aug 14, 2025
Examiner Interview Summary
Aug 27, 2025
Response after Non-Final Action
Sep 29, 2025
Request for Continued Examination
Sep 30, 2025
Response after Non-Final Action
Feb 20, 2026
Non-Final Rejection — §103, §112 (current)

Precedent Cases

Applications granted by this same examiner with similar technology

Patent 12592416
SOLID-STATE ELECTROLYTE FILM AND SOLID-STATE BATTERY
2y 5m to grant Granted Mar 31, 2026
Patent 12590175
HYDROPHILIC POLYMER, METHOD OF PREPARING THE SAME, AND LITHIUM SECONDARY BATTERY CONTAINING THE HYDROPHILIC POLYMER
2y 5m to grant Granted Mar 31, 2026
Patent 12580247
BATTERY PACK APPARATUS
2y 5m to grant Granted Mar 17, 2026
Patent 12573688
COOLANT PORT ASSEMBLY
2y 5m to grant Granted Mar 10, 2026
Patent 12567643
Battery Housing With Valve Device, Battery and Motor Vehicle
2y 5m to grant Granted Mar 03, 2026
Study what changed to get past this examiner. Based on 5 most recent grants.

AI Strategy Recommendation

Get an AI-powered prosecution strategy using examiner precedents, rejection analysis, and claim mapping.
Powered by AI — typically takes 5-10 seconds

Prosecution Projections

4-5
Expected OA Rounds
76%
Grant Probability
87%
With Interview (+10.6%)
3y 4m
Median Time to Grant
High
PTA Risk
Based on 17 resolved cases by this examiner. Grant probability derived from career allow rate.

Sign in with your work email

Enter your email to receive a magic link. No password needed.

Personal email addresses (Gmail, Yahoo, etc.) are not accepted.

Free tier: 3 strategy analyses per month