NASA Marine BM-1 Compact 12V wiring diagram

The 12 V BM-1 Compact is intended for 5–600 Ah lead-acid banks, with a 100 A / 50 mV shunt. Do not confuse it with a 24 V variant or NASA’s lithium-specific products. The 100 A rating belongs to measured current; the display itself consumes approximately 1.5 mA.

Insert the shunt between the battery negative and every load/charger return. A direct return to the battery bypasses measurement and corrupts state-of-charge estimates. Figure 1 shows the black and white sensing wires joined at the shunt but kept separate at the display terminal block. Keep the shunt dry and isolated from nearby conductive parts; it can warm under heavy current.

Connect the fused red battery-positive lead last, then enter the correct bank capacity. The startup instructions call for applying a modest load and allowing the monitor to learn before charging immediately. With all charging and loads genuinely off, check the zero-current reading and calibrate only if needed.

Time remaining reflects current conditions, not a guarantee that tomorrow’s loads will run that long. A wrongly signed current or implausible charge percentage warrants checking shunt orientation, bypass returns, capacity and zero setting. Large inverter or thruster circuits can exceed this shunt’s 100 A limit; do not route them through it merely because their average daily consumption is small. State-of-charge estimation is not battery over-discharge protection.