Field Sustainment Power Conditioning #TA Brad Lehman, Northeastern University Khalil Shujaee, Clark Atlanta University Wes Tipton, Army Research Laboratories Presented by: Florent Boico Dept. Elect. & Comp. Engin. Northeastern University May

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. TOC Operation area 10 mi. Spare batteries, generator, Chargers, shelter, etc. Soldiers using batteries

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)

NiMH Overcharge Detection 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. Full State Of Charge

Solar Charging BB390 –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 chargeSlow charge Fast charge … DUMB –How to correctly predict charge termination for DUMB batteries like BB390 (basic research)? I V

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

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

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

PIC µC ADC Battery thermistors voltage leg 1&2 leg1 leg2 BB390 leg1leg2 Serial connection for data logging (optional) Prototype charging current Phase 1 Charger Prototype (2004) A prototype of the charger has been built. Its characteristics include : charge monitoring by sensing : -voltage & current across each leg - cell temperature clamps on the BB390 battery. optional RS-232 connection for data logging and evaluation by a computer. algorithm fully upgradable.

Charge, n=n+1 sense Vbatt(n) & Ibatt(n) |dV/dt|>thsld1 Imax-Imin>thsld2 Reset : Vmax=0 dpos=0 Vmax-Vbatt>0.1V & dpos=1 Trickle charge dV/dt>thsld3 dpos=1 yes nono nono Vmax=Vbatt(n) Vbatt(n)>Vmax yes nono nono nono Initialisation : Vmax=0 dpos=0 Voltage Charge Control Algorithm (2004) If the voltage suddenly drops or if strong discrepancies in the current is detected, the algorithm is reset to prevent false overcharge detection Minimum and maximum current over a period of 5 minutes After reset the algorithm waits for a positive voltage slope to allow overcharge detection

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

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

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!

When battery overcharges, it heats up Differential Temperature Algorithm Battery or leg 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 can also heat up due to external causes (e.g. : sun) but is not fully charged yet Differential Temperature Algorithm

How can one differentiate between overcharging and external heating ? 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 Differential Temperature Algorithm thermistors Two Independent Legs

2005 Differential Temperature Algorithm thermistors T1-T2> thresholdT1-T2< threshold Overcharge No Overcharge

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 Differential Temperature Algorithm Charge leg 1&2 dT1/dt > thsld1 Charge leg 1 Stop leg 2 timer1=0 d(T1-T2)/dt >thsld2 Timer1>10minincrement timer dT2/dt > thsld1 Charge leg 2 Stop leg 1 timer1=0 d(T2-T1)/dt >thsld2 Timer1>10minincrement timer leg1 fully chargedleg2 fully charged no yes no yes Switch to trickle charge in each leg

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 Thsld reached, Entering potential overcharge mode. Experimental Results Differential Temperature Algorithm External heating does not fool new algorithm

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