Solar Battery Chargers for Field Sustainment Power Conditioning

1. Field Sustainment Power Conditioning#TA3-04-5 (Solar Battery Chargers)Brad Lehman, Northeastern UniversityKhalil Shujaee, Clark Atlanta UniversityWes Tipton, Army Research LaboratoryGeorge Frazier, SAIC (Year 5 - helping with technology transfer) Presentors: Khalil Shujaee (CAU) and Florent Boico (NU) Presented at CTA Conference: June 2, 2005

2. Motivation/Background • Background • Dept. Army recently mandated that all training exercises must use rechargeable batteries; • Estimated to save $70M annually (versus non-rechargeable); • About 75% of Army rechargeable batteries are BB390 NiMH (4lbs). (BB390 has 2 x 12V legs and can be used as either 24V or 12V battery.) • Motivation for Solar Chargers • Soldiers carry four BB390 batteries (= 16 lbs) for portable electronic equipment; • Forward field observers, scouts, special ops, are constrained to stay within 10 miles of TOC (Tactical Operation Center where there is a charging facility shelter); • Portable solar arrays carried by soldier (~1lb) reduce number of batteries carried and eliminate the need to stay near TOC. Operation area Soldiers using batteries 10 mi. TOC Spare batteries, generator , Chargers, shelter, etc.

3. Solar Charging BB390 Cloudy at 11:15am Sunny at 11:00am 11:15am 11:15am V I V2 I2 I1 V1 11:00am 11:00am • Companies have attempted to work with CERDEC to build NiMH solar chargers: • Chargers failed: They falsely terminate charging before completion; • CERDEC refused to use any of these chargers. BUT batteries become damaged or have reported reduced life-cycle when connecting directly to the solar array. • Known charging algorithms are applicable to constant power source: • Termination for “dumb” NiMH batteries (BB390) occurs based on battery V, dV/dt, time, and sometimes temperature T or dT/dt. • Solar arrays produce varying current sources depending on clouds (fools known chargers) • Fast charge Slow charge Fast charge … • How to correctly predict charge termination for DUMB batteries like BB390 (basic research)?

4. Summary: Research Outcomes Prototype Charger Major Accomplishments: • Must Be Sure Algorithm: • Immune to changing illumination or temperature conditions • Permits a little overcharging to « make sure » battery is full SOC • Hardware: • Monitors voltage, current and temperature for each leg (thermistors inside BB390) • Differential temperature measurements between battery legs determine when charge is complete. • Explained that performance degradation of batteries when charging with solar array is due to high temperature overcharging; • Designed, built and tested Phase I solar array NiMH battery charger control algorithms; wOver 50 experiments: 0% failure rate!! wPreliminary patent disclosure • Maximum Power Point Tracker (Phase II) - charger forces solar array to produce more power; • 2005 IEEE PESC paper to appear in June.

5. Phase II: Maximum Power Point Tracking (MPPT) • We have built preliminary Phase II chargers that include MPPT: • Adjusting the duty ratio of the Up-Down converter forces the solar array to operate at its maximum producing power point; • MPPT adaptively optimize charging to different NiMH batteries (12V, 24V, 9.6V, etc.) • Bypass switch improves power efficiency when MPPT not needed. Higher Charging Current is Achieved with MPPT

6. Relevance to Army’s Vision • Army Future Force Warrior (FFW) • Power Vision: “72-hour continuous autonomous team operations, high density, low weight/volume, self-generating/re-generating, reliable, safe power source/system.” (from FFW home web page) US Army Natick Research Center vision of what a FFW in 2020 (left) and 2010 (right) will look like • Roadmap/Relevance of Research to FFW • Our proposed solar battery chargers are lightweight, use renewable energy, are reliable quiet: Directly impact FFW power requirements. • We are attempting to transition technology developed: • Negotiating with vendors to implement the algorithms on their existing chargers; • Discussing with CERDEC on “convincing” their vendors to use new algorithms; • Two recent meetings with US Army Natick Research Center (they seem to deal with larger (kW) power systems, but have been open to discuss new projects) • Year 6 – 8 outcomes hope to provide great flexibility to the soldier: • Develop charger that can utilize various input power sources (from solar to wind) • Ability to recharge any battery chemistry type, such as Li-ion, NiMH, NiCd. • Potential commercial, dual-use, technology transfer to portable solar battery chargers for campers, RV’s, etc.

7. Proposed Research Activity For Year 6-8 • VISION: Design and build a smart Future Force Warrior power system architecture capable of: • Sensing and adapting charge algorithms according to battery chemistry • Running from primary source, rechargeable battery or from combination thereof • Ability to recharge battery with virtually any primary source (AC, DC, solar wind, etc.) • Programmable output voltage. • Handling input voltage range of up to 5:1 Avision of a Future Force Warrior power system architecture

8. Appendix • Supporting slides for presentation that will not be shown in the limited 10 minute presentation

9. Roadmap YEAR 4-5 YEAR 5 YEAR 4 (Beginning of the Project) Generalization to AA & Other Battery Types Technology Transfer STO/RDECOM Solar Panel Layout Optimization New NiMH Algorithm Lab Prototype of BB390 Charger Field Testable Prototypes Field Testing (CERDEC) • YEAR 4 Achievements: • Designed a robust solar NiMH battery charge control algorithm. • Built charge controller that clamps onto BB390 with embedded algorithm. • YEAR 5 Expectations/Objectives: • Transfer charger algorithms to existing CERDEC vendors to implement within their BB390 chargers: target timeframe to achieve this is by the end of GFY 2005. • Conduct field testing of the prototypes. • Technology Transition: • Extend algorithm to other NiMH batteries, e.g., commercial AA cells • STO program: Help optimize layout of solar cells in for applications at US Army Natick Soldier Center.