Bathmaster Power Distribution

Contact: Paula Kesler for questions

General Article

Technical Note

Design:

The current and voltage requirements of the computer, sonar, and global positioning system, GPS, were derived via a combination of equipment specifications and direct measurement. The GPS and sonar were measured at 1.0 Ampers, A, on 12 volts direct current, VDC. The laptop computer was measured at 1.3 A on 18 VDC.

The original 12 volt lead acid rechargeable battery was limited to 4 Amper-hours, AH, which gave a total of 8 AH for the pair of them. This would run the sonar and GPS with no margin for supplying the computer. With a 50% design margin to account for partial battery charge, deterioration, and other problems, the minimum for a 12 VDC battery would be 12 AH.

To develop computer power, an 18 VDC source was desired. Three light duty, 0.60 A at 6 VDC, commercially available voltage dividers were acquired. They are installed with the ground wires floating at the 12 VDC point of a pair of 12 VDC, 12AH batteries making a 24 VDC stack. The computer is powered between the lower battery’s minus terminal and the positive side from the voltage dividers. Diodes were installed at each divider’s output to prevent current from reversing into a failed, or failing divider.

Power Combiner

The 12VDC for sonar and GPS power is also routed through the "Power Combiner" black box. This line terminates in twin 12VDC sockets. A fourth voltage divider is switched to 12VDC to provide conditioned power for the sonar. The second line has no conditioner and feeds the GPS.

All dividers have self-contained 3A fuses. The 12VDC line from Master Power switch to Power Combiner box has a 5A fuse. The 24VDC line from the upper battery to the Power Combiner has a second 5A fuse.

The Power Combiner is a junction box for power distribution all voltages except solar power for charging the lower battery are routed through it. Solar power is routed to the lower battery through the Master Power switch. This means the switch must be closed to charge the lower battery. As the solar panel has a dedicated Battery Charge Regulator, no voltage divider is used to condition the panel’s output. 

Operation:

The solar panel is capable of powering the GPS and sonar while it is in full sun. The "off switch" for the solar panel is its cover. The laptop is capable of running on its internal battery, for a few hours. The two 12AH batteries may be switched into the circuit by switching on the Master Power. The switch is oriented like a common wall plate light switch. When Master Power is "on" the solar panel’s Battery Charge Regulator "Activated" light, the three computer circuit voltage dividers’ tell tales, and the sonar’s divider tell tale will illuminate. This presents a drain on the batteries even when the sonar, GPS, and computer are switched off individually. However, it is the only condition in which the lower battery may be charged via solar power. The upper battery must exchange places with the lower battery if the upper battery is to be charged by the sun.

The Computer Power switch, located on the Power Combiner black box, interrupts power to the laptop only. It is switched off when there is sufficient charge in the computer’s internal battery to operate. Overcharging the computer by leaving it connected to power all day merely generates heat and wastes power. No computer battery need be installed to operate, however, please check the computer manufacturer’s instructions.

If there is no power to the equipment when the switches are "on", please verify that all connections are solid between the batteries, wiring, fuses, switches, and equipment plugs. Or, check the fuses located in the appropriate line. Remember the voltage dividers are fused with 3A and the 12VDC and 24VDC lines are fused with 5A fuses. The Power Conditioner black box does not contain a fuse.

The total amp-hours will be sufficient to operate for a full day’s work. The wall charger is capable of providing 0.5A to charge one 12VDC battery. The lower battery should be OK since it is getting a solar charge. The upper battery will need a wall charge. I am estimating about 12 hours to bring the upper battery back to operating condition, but this will need to be field tested for a more definite result.