There are two external electronics & battery canisters on the Optima CM. They are mounted between the scrubber canister and the counterlungs in elastic sleeves on the bottom of the outer bag.
One canister contains the SOLO (SOLenoid and Oxygen) electronics board and battery. The battery is a standard 9 volt battery that powers the SOLO board as well as the oxygen solenoid. The Petrel 2 controller handset has its own internal battery and does not rely on the 9 volt battery.
The other canister contains the OBOE (Oxygen BOard Electronics) board and battery. The OBOE board controls the HUD. The battery is a standard AA type.
This battery only powers the OBOE board and HUD.
By locating the batteries in these external compartments, they are completely isolated from the head and the breathing loop. This is important because as batteries age or are damaged they can release toxic chemicals. Obviously this is not something you want to have in your breathing gas.
The batteries are accessed via a threaded cap on the end of each canister. The caps are double O-ring sealed to ensure water integrity. These O-rings must be inspected and maintained when the canisters are opened to reduce the chance of flooding. See the Battery Replacement section for additional information.
The electronics canisters connect to the controller and HUD using wet pluggable, waterproof connectors. Regular lubrication of the inside of these connectors with a light coating of silicone grease such as Molykote 111 will increase their usable life.