1. Field Sustainment Power Conditioning #TA3-04-5 (Solar Battery Chargers) Brad Lehman, Northeastern University Khalil Shujaee, Clark Atlanta University Wes Tipton, Army Research Laboratory George 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. TOC Operation area 10 mi. Spare batteries, generator, Chargers, shelter, etc. Soldiers using batteries

3. Solar Charging BB390 –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 chargeSlow charge Fast charge … DUMB –How to correctly predict charge termination for DUMB batteries like BB390 (basic research)? I V V1 I1 I2 V2 11:00am 11:15am

4. Summary: Research Outcomes Major Accomplishments:  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;  Over 50 experiments: 0% failure rate!!  Preliminary patent disclosure  Maximum Power Point Tracker (Phase II) - charger forces solar array to produce more power;  2005 IEEE PESC paper to appear in June. Prototype Charger 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.

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. Battery Voltage Current when solar array is directly connected to the battery Current when the proposed charger with MPPT is used. 4.8V660 mA720 mA 12V310 mA 24V0 mA160 mA 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) Roadmap/Relevance of Research to FFW 1.Our proposed solar battery chargers are lightweight, use renewable energy, are reliable quiet: Directly impact FFW power requirements. 2.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) 3.Year 6 – 8 outcomes hope to provide great flexibility to the soldier: i.Develop charger that can utilize various input power sources (from solar to wind) ii.Ability to recharge any battery chemistry type, such as Li-ion, NiMH, NiCd. 4.Potential commercial, dual-use, technology transfer to portable solar battery chargers for campers, RV’s, etc. US Army Natick Research Center vision of what a FFW in 2020 (left) and 2010 (right) will look like

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 A vision 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. 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. New NiMH Algorithm Lab Prototype of BB390 Charger Field Testable Prototypes Field Testing (CERDEC) Generalization to AA & Other Battery Types STO/RDECOM Solar Panel Layout Optimization YEAR 4 (Beginning of the Project) YEAR 5 Technology Transfer Roadmap YEAR 4-5