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About Mainspring
Mainspring Energy manufactures and delivers fuel-flexible, low-emissions local power solutions that rapidly add new capacity and deliver reliable, affordable, and sustainable electric power. The company began commercial shipments of its Mainspring Linear Generators in 2020 and today has hundreds of megawatts in advanced development and field operations for leading Fortune 500 companies, data centers, and utilities. Mainspring also partners with global energy leaders including AEP, NextEra Energy Resources, Schneider Electric, and more.
The core values that ground our work, guide our decisions, and connect us together:

• Pragmatic Optimism
• Excellence without Ego
• Proactive Collaboration

About the Role
Mainspring Energy is seeking a Staff Mechanical Engineer - Battery Design to lead battery pack and module development for current and future energy platforms.
As Mainspring's internal battery technical authority, you will help define next-generation battery architectures for future products while improving systems already in production. You'll partner closely with battery suppliers to influence pack and module designs and drive solutions that balance safety, reliability, manufacturability, performance, and cost. You'll influence key decisions across cell integration, thermal management, safety, and system architecture while working at the intersection of mechanical, electrical, and battery engineering.
Our battery systems do far more than just store energy. They are the dynamic core of our linear generator platform. Tightly integrated into every sub-system, they act as the ultimate performance driver-sparking to life during grid-islanded startups, taming dynamic transient loads, and delivering resilient long-duration standby. This role is your chance to tackle complex battery engineering challenges head-on, drive our supplier strategy, and ensure our systems deliver rock-solid, scalable reliability in the real world.
The ideal candidate combines deep lithium-ion battery pack expertise with strong supplier-management experience and the ability to translate system requirements into robust production-ready designs.
Key Responsibilities

• Serve as Mainspring's technical lead for battery pack and module design.
• Partner with battery suppliers to review, improve, and influence pack and module architectures.
• Own battery hardware from requirements definition through production launch and sustaining engineering.
• Evaluate battery designs for safety, reliability, manufacturability, thermal performance, lifecycle, and cost.
• Lead technical reviews of cell integration, interconnect systems, thermal solutions, and safety features.
• Support supplier selection, design reviews, validation activities, and manufacturing readiness.
• Develop battery architectures for future generator platforms.
• Collaborate with Electrical Engineering, BMS, Reliability, Manufacturing, Test, NPI, and Supply Chain teams.
• Lead root-cause investigations and corrective actions for production, test, and field issues.
• Provide technical leadership on complex battery-related challenges.
• Travel to supplier sites as needed to support development and issue resolution.

Qualifications

• BS or MS in Mechanical Engineering or a related discipline.
• 10+ years of engineering experience with significant focus on lithium-ion battery pack or module design.
• Deep understanding of battery pack and module architecture.
• Experience working directly with battery suppliers and contract manufacturers.
• Strong knowledge of cell integration and interconnect design, including bus bars, current collectors, voltage sensing, harnessing, and cell terminal joining methods.
• Experience designing series and parallel battery architectures.
• Working knowledge of cell construction, dielectrics, fusing, compression systems, and thermal interface materials.
• Experience with battery thermal management and thermal runaway propagation mitigation.
• Understanding of battery aging, lifecycle performance, and charge/discharge behavior.
• Knowledge of environmental durability, vibration, grounding, creepage/clearance, and EMI-related packaging requirements.
• Experience supporting validation testing, manufacturing launch, and sustaining engineering.
• Experience with UL certification of battery systems.
• Proficiency with SolidWorks and PLM systems.

Preferred Qualifications

• Experience with prismatic-cell battery systems.
• Knowledge of BMS integration, contactors, relays, and protection systems.
• Experience with high-voltage battery or power systems.
• Experience with battery thermal protection materials and thermal interface design.
• Familiarity with FEA tools such as Ansys.
• Experience with products deployed in harsh outdoor environments.
• Background in EVs, energy storage systems, industrial electrification, power electronics, or other ruggedized energy platforms.

$172,000 - $203,000 a year
This position is onsite at our Menlo Park HQ. The salary will be adjusted to reflect local market conditions based on employee location as well as the experience of the employee. Along with the base salary, Mainspring offers pre-IPO stock options + benefits.
Before you apply, a quick note on your resume
We care about the substance of your work and want to make sure we learn as much as we can about your design experience. To help us evaluate your experience accurately, please ensure your resume includes:

• What you actually built or worked on - name the real product, subsystem, or hardware (not just "components" or "systems"). "Redesigned the bracket mounting the compressor to reduce vibration failures" tells us far more than "designed mechanical components."
• What you personally did, separate from what your team did. If you contributed to a group project, tell us your specific role and contribution.
• The engineering fundamentals you applied - tie your skills (CAD, FEA, GD&T, thermal/fluid analysis, testing, etc.) to the actual problem you used them on, rather than listing them as standalone skills.
• What happened as a result - did it pass validation/testing, resolve a failure, ship to production, and if so, at what volume or scale (prototype quantity, low-rate production, high-volume manufacturing)? This helps us understand whether your experience is with one-off/prototype hardware or product that shipped at scale.

One more thing: please don't feel obligated to keep your resume to one page. If accurately describing your relevant projects and contributions takes more space, that's completely fine - we'd rather see the detail than have you compress or omit meaningful work to fit a page limit.
Thank You!
Does your experience not meet all of our posted requirements? Studies have shown that some people are less likely to apply to positions unless they meet every listed requirement. At Mainspring, we are committed to building a diverse, inclusive, flexible, and collaborative environment, so if you want to help us transition the world to clean and affordable electricity, and don't meet all posted requirements for a particular role, we'd still love to hear from you. Mainspring can sometimes be flexible enough to shift responsibilities for the right person, or otherwise identify open or upcoming roles that may better fit your professional background.
In more traditional words, Mainspring Energy, Inc is an Equal Opportunity Employer. All qualified applicants will receive consideration for employment without regard to race, color, religion, sex, sexual orientation, gender identity, national origin, or protected veteran status and will not be discriminated against on the basis of disability.
This compensation and benefits information is based on Mainspring Energy's estimate as of the date of publication and may be modified in the future. We generally do not negotiate on salary once we have made an offer. The level of pay within the range will depend on a variety of job-related factors that may include location, relevant prior experience and/or education, or particular skills and expertise. New hires joining the company tend to be paid within the starting base pay range noted above, with opportunities to increase pay over time based on development of additional skills, competencies, and company-specific knowledge.
We may use artificial intelligence (AI) tools to support parts of the hiring process, such as reviewing applications, analyzing resumes, or assessing responses and identifying potential inconsistencies or verification signals in application materials based on available information. These tools assist our recruitment team but do not replace human judgment. Final hiring decisions are ultimately made by humans. If you would like more information about how your data is processed, please contact us.