Powering the transponder was a more serious problem. The system had to operate in any weather, which made conventional lead-acid batteries unsuitable due to their poor cold-weather performance. The system also had to be stored for long periods of time before being activated, which again argued against lead-acid. The solution was found to be small nickel-iron batteries that could be repeatedly and rapidly recharged in the field, and operated across a wide range of temperatures. To protect the system from capture, it was fitted with small explosives that would destroy enough of the circuitry to make it impossible to determine the exact frequencies being used. The transponders were mounted in suitcases, selected on a case-by-case basis to match common types in use in that area.