Presented by: Florent Boico Dept. Elect. & Comp. Engin. Northeastern University

1. Field Sustainment Power Conditioning#TA3-040-5Brad Lehman, Northeastern UniversityKhalil Shujaee, Clark Atlanta UniversityWes Tipton, Army Research Laboratories Presented by: Florent Boico Dept. Elect. & Comp. Engin. Northeastern University May 31 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); • Soldiers like rechargeable batteries so much that they are bringing them into combat also; • 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. Motivation/Background • Portable Solar Chargers • Being field tested by CERDEC C2D Army Power Division; • Best solution: soldier directly connects solar array to battery and lets charge all day while on a mission; • Companies have attempted and failed to build power electronic charge regulators to control the charging. • Issues when Solar Charging • Experiments show reduced battery capacity; • Batteries sometimes overheat and vent: can no longer be used; • Batteries are constantly being recharged by soldier, even if battery lightly discharged • Leads to reduced life cycle of battery. Three different portable solar arrays charging three NiMH batteries. (Picture courtesy of Dennis Lane, CERDEC, C2D, Army Power Division)

4. NiMH Overcharge Detection Full State Of Charge Conventional (known) NiMH charge control algorithms stop charging (or switch to trickle charge) when battery voltage begins to decrease or when rate of cell temperature begins to substantially rise.

5. Solar Charging BB390 V I • Companies have attempted to work with CERDEC to build solar chargers: • Chargers failed: They falsely terminate charging before completion; • CERDEC refuses to use any of these chargers. • 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 • Fast charge Slow charge Fast charge … • How to correctly predict charge termination for DUMB batteries like BB390 (basic research)?

6. Typical sunny day measurement The negative voltage slope shows that the battery is overcharging

7. Typical cloudy day (scattered clouds) Changes in the solar array current cause changes in the battery voltage. Conventional charge control algorithm falsely terminate charging !

8. Variation of Temperature Throughout the Day Temperature inside the battery pack Time (min) Large variations of the temperature can falsely trigger charge termination on temperature based algorithms.

9. Phase 1 Charger Prototype (2004) Prototype BB390 • A prototype of the charger has been built. • Its characteristics include : • charge monitoring by • sensing : • -voltage & current across each leg • cell temperature leg1 leg2 voltage leg 1&2 leg1 Battery thermistors ADC leg2 PIC µC • clamps on the BB390 battery. • optional RS-232 connection for data logging and evaluation by a computer. • algorithm fully upgradable. charging current Serial connection for data logging (optional)

10. Voltage Charge Control Algorithm (2004) Initialisation : Vmax=0 dpos=0 After reset the algorithm waits for a positive voltage slope to allow overcharge detection Charge, n=n+1 sense Vbatt(n) & Ibatt(n) If the voltage suddenly drops or if strong discrepancies in the current is detected, the algorithm is reset to prevent false overcharge detection Vmax=Vbatt(n) yes Vbatt(n)>Vmax dpos=1 no yes |dV/dt|>thsld1 Reset : no yes no Vmax=0 dpos=0 yes Imax - Imin>thsld2 dV/dt>thsld3 Minimum and maximum current over a period of 5 minutes no no Vmax - Vbatt>0.1V & dpos=1 yes Trickle charge

11. 2005 Results : • Refined Maximum Power Point Tracker • Differential temperature based charge control algorithm developed

12. 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

13. New Charge Charge Control Algorithm Developed in 2005: Differential Temperature Method • The algorithm is based on measuring the difference in the temperature of each of the two legs of the BB390. • When overcharge occurs in one leg, it can be detected by comparison with the other leg. • Method gives improved robustness • Not sensitive to changing illumination conditions • Not sensitive to changing ambient temperatures • 100% success rate after dozens of experiments!

14. 2005 - Differential Temperature Algorithm • The algorithm is based on measuring the difference in the temperature of each of the two legs of the BB390. • When overcharge occurs in one leg, it can be detected by comparison with the other leg. Battery or leg When battery overcharges, it heats up.

15. 2005 - Differential Temperature Algorithm Battery can also heat up due to external causes (e.g. : sun) but is not fully charged yet.

16. 2005 - Differential Temperature Algorithm How can one differentiate between overcharging and external heating ?

17. Two Independent Legs thermistors 2005 - Differential Temperature Algorithm • Supposing one leg is charged and the other is left open : • In case of overcharge the temperature will rise in one leg only • If the pack is heated from outside, the temperature will rise in both legs

18. thermistors 2005 Differential Temperature Algorithm T1-T2> threshold T1-T2< threshold No Overcharge Overcharge

19. 2005 - Differential Temperature Algorithm • The algorithm functions as follows : • Detects a rise in temperature in any of the legs. • Keep charging the leg that has been detected as potentially overcharging • Measure the slope of differential temperature measurement to detect overcharge • After a certain time if no overcharge is detected, charge is resumed in both legs. • The differential sensing of the temperature reduces the effect of external heating and ambient temperature on the measurement. • Overcharging detection of one leg via temperature using this method is more robust. • Thresholds are current dependant. Charge leg 1&2 no no dT1/dt > thsld1 dT2/dt > thsld1 yes yes Charge leg 1 Charge leg 2 Stop leg 2 Stop leg 1 yes yes timer1=0 timer1=0 no no Timer1>10min increment timer increment timer Timer1>10min no no d(T1 - T2)/dt d(T2 - T1)/dt >thsld2 >thsld2 yes yes leg1 fully charged leg2 fully charged Switch to trickle charge in each leg

20. 2005 - Differential Temperature Algorithm Experimental Results Thsld reached, Entering potential overcharge mode. External heating Normal charge resumed, Internal balancing takes place. Thsld reached, Entering potential overcharge. Overcharge detected on leg1, Charging process over. Algorithm based on simple derivative fails New algorithm delivers accurate full SOC detection External heating does not fool new algorithm

21. Conclusion • More robust voltage charge control algorithm (2004) • Differential temperature charge control algorithm (2005) • Maximum Power Point Tracker (2004-2005) • Algorithms are being implemented inside the • prototype charger. • The phase one prototype is stand alone and mechanicaly compatible with BB390 batteries