Current sensing resistors for avionics batteries

The Li-ion battery charging process goes through several critical phases to ensure the battery is charged to its maximum capacity, while at the same time following certain safety rules to avoid explosion or fire. Li-ion battery chargers require accurate current and voltage settings and monitoring, as well as a constant current and constant voltage. If these accuracy requirements are not met, the charger will undercharge or overcharge the battery. A failure to completely charge the battery results in reduced battery life, while overcharging can cause batteries to fail due to overheating.

Precision current sensing resistors are required to charge and monitor the Li-ion battery at a known current level until it reaches its final voltage. At this point, the charger circuitry switches over to constant voltage mode, providing the current necessary to hold the battery at this final voltage. For both phases, Bulk Metal Foil resistors provide higher accuracy than other technologies, such as thin film, thick film, and wirewound resistors.

Yuval Hernik, senior director of application engineering for VFR, Vishay Precision Group, comments: "Li-ion batteries are well established in portable consumer electronics and are now becoming more popular in military and advanced civilian avionics. As the scope of their applications has expanded, so have the stringent requirements they must meet, especially in terms of cell safety and lifetime. For designers, the main challenge in meeting these requirements is in finding the optimal point between undercharging and overcharging through continuous accurate and stable monitoring of input and output current."

"The need for safety was emphasized in the latest near-catastrophic incident involving the charging of Li-ion batteries, in which a Dreamliner caught fire while parked at Heathrow. Luckily, no passengers were on board. In our opinion, designers in the avionics industry have every motivation to look for more precise and reliable commercial-off-the-shelf resistors to accurately monitor the battery voltage when connected to the charger. Bulk Metal Foil current sensing resistors provide the low TCR and thermal EMF required to meet this need. They also provide wide resistance ranges, and unlike most COTS resistors, any resistance value within their given range is available with no additional cost or lead time effect," adds Hernik.

There are five important phases that need to be considered when choosing a current sense or shunt resistor for any application: connection errors, external and Internal temperature errors, frequency, drift over time, load life stability and thermal EMF. VFR's Bulk Metal Foil current sense resistors offer a four-terminal Kelvin connection for increased accuracy. They feature an ultra-low TCR of ±0.05 ppm/°C typical (0°C to +60°C) and ±0.2 ppm/°C typical (-55°C to +125°C, +25°C ref.). The surface-mount VCS1610Z and VCS1625ZP offer load-life stability of ±0.015% at 70°C for 2,000 hours at rated power while the VCS331Z power resistor features load-life stability down to ±0.005% (50 ppm) typical at 3 W and +25°C for 2,000 hours (mounted on a heatsink) and thermal EMF of 0.05 μV/°C typical.

Featuring a non-inductive (<0.08 μH), non-capacitive design, Bulk Metal Foil resistors offer a rise time of 1.0 ns with effectively no ringing, a thermal stabilization time of < 1 s to within 10 ppm of steady state value, current noise of 0.010 µVrms per volt of applied voltage (<-40 dB), voltage coefficient of <0.1 ppm/V, and resistance tolerance down to ±0.01%. 

Samples and production quantities of the Z-Foil resistors are available now, with lead times of five days for prototype samples and from two days to five weeks for standard orders at the catalog houses, depending on their shelf availability.