Various useful tables of electrical information.

You can use these figrues to estimate the % charge of a battery but you should do it only after the battery has been left for a couple of hours without being charged or discharged.

When working out the resistence of a cable run, remember to use the total length; there and back again. The maximum safe current is limited by the heating of the cable but on a boat you often need to use thicker cables in order to reduce the voltage drop. Work out the voltage drop using the resistence figure and Ohm's law.

where V is the Voltage drop, I is the current (Amps), R is the resistence peer unit length (Ohm/metre) and L is the run length (there and back) in metres.

For example, if you wire a 150 Watt headlight on a 20 metre long narrowboat the expected current is 150 Watt/12 Volt = 13 Amps. But if you used 13 Amp rated cable, the resistence would be 0.0125 Ohm/metre * 2 * 20 Metres = 0.5 Ohm, so the Voltage drop will be 13 Amp * 0.5 Ohm = 6.5 Volts!