Electronic – Car battery spot welder. How to reduce battery power

In place of the LM7805, using a Low Drop Out (LDO) regulator such as MIC29300-5.0WT would allow operation at as little as 5.37 volts, providing 3 amperes at that voltage, enough for your Beagleboards but not the motors.

The more expensive MIC29500-5.0WT at $5.46 apiece on Digikey would provide the 5 amps you might want instead.

• Tapping between batteries is best avoided with unbalanced loads.
• And yes, the batteries loaded higher would deplete earlier, with unpredictable outcomes.
• In stepping down 6 to 7.5 volts to 5 volts at 5 amps using an LDO, up to 12.5 watts of heat would necessarily be dissipated at the regulator: Power dissipated as heat = voltage difference * average current = 2.5 * 5

However, realistically the batteries won't be at 7.5 volts under load, and not for long anyway. Also, the continuous current drawn would not be 5 amperes through the regulator. Your design would ideally provide the motors with unregulated power, so your "step down about 4-5 amps" deserves a revisit. Besides, voltage is stepped down, not current, at least in this context.

Switching (buck) regulators would waste less heat, roughly around 15% that of the LDO, but they usually need more voltage headroom to operate. YMMV, searching DigiKey might yield what you need.

Why is your hub needing to be powered from the same power as your car motors? Your hub should be at your base station, the car would just carry a WiFi dongle of some sort, ideally?

Suggestions:

1. Edit: [15-Oct-2012] Provide regulated power only to the logic parts, drive the motors directly off the battery, and add a hefty decoupling capacitor before the regulator to work around motor inductive spikes. If your batteries can handle 4 motors of 2 amp each, they can handle the initial surge the capacitor will need.
2. Provide a diagram of some sort, of what is being attempted, and some clarity on how the powered hubs got aboard the moving car. This will enable more helpful answers.

When the battery is supplying power (discharging) to, e.g., the starter motor, the direction of the electric current is out of the positive terminal through the load and into the negative terminal.

Within the wire and frame, the electric current is due to electron current which is in the opposite direction of the electric current.

Within the (lead-acid) battery, the electric current is primarily due to proton (hydrogen ion) current which is in the same direction as the electric current.

So, there are at least three currents to consider: the abstract electric current (flow of electric charge), the electron current (flow of electrons, a carrier of negative electric charge), and the proton current (flow of protons, a carrier of positive electric charge).

Note that when the battery is charging, the electric current is into the positive terminal and out of the negative terminal.