SeaView Systems MBM-150 Multi-Battery Manager enables two or more Lithium-ion batteries (BATTERY-LI-4S-18AH-R3-RP) to be safely combined to extend BlueROV2 operational time. This can be used with hybrid and multi-battery systems. Power is drawn from the source with the largest voltage until both/all batteries equalize. A recent post described a successful application using seven MBM-150s to complete a 5.5 hour inspection. Available directly from SeaView Systems or amazon.