Your M777A2 howitzer operates on a 24‐volt system with two (2) 12‐volt batteries. Those batteries are crucial to operating the howitzer. Unfortunately, howitzers can spend many days sitting in the motor pool between uses. The batteries slowly drain until they're at a low state of charge (SOC). Then when it's time for the next mission, your digital fire control equipment system (DFCS) won’t be ready.
Of course, your options are essentially to connect your prime mover to the howitzer, hook up the W-3 cable and run the vehicle (700 rpm for the FMTV and about 850 rpm for the M939-series 5-ton) until the batteries are charged. Since the solar battery charger is available in your howitzer’s BII, it should be used when the howitzer is in long-term storage to ensure the battery is maintained at an adequate state of charge.
To assist the solar charger in maintaining proper battery condition, the battery state of charge should be around 80 percent or greater before attaching the solar charger.
Note that if you use the solar panel and not the AC plug, the unit will deliver a trickle charge to the batteries to maintain them with a limited amount of charge. It’s very important to start off with a charged set of batteries for the solar charger to work properly.
For a quicker charge rate, plug the unit into a regular AC outlet. This results in the batteries charging quicker due to a consistent and reliable input power source, as compared to the sun that is only up during the daytime.
You'll find the charger listed as Item 83 in WP 0098-38 of TM 9-1025-215-10 (Sep 19).
There are three NSNs available. Get the charger, case and solar panel with NSN 6130-01-540-3380. Get the charger and solar panel, but no case, with NSN 6130-01-521-1387. The charger with no case or solar panel comes with NSN 6130-01-521-1329.