checking current

How much current a circuit draws will tell you how long a fully charged battery will power a particular device. An average multimeter will only measure small currents and it is easy to damage one with a current in excess of 10A and typically this probe socket will be fused as shown. Here I want to install a cabin light for reading but first want to check how much power it will draw. To do this I must break the circuit so the current will pass through the meter to be measured. First, I select amps DC. Then I connect the red probe to the power lead feeding the device and the black probe from the multimeter to the positive terminal on my light. The negative lead to the light remains connected to the battery and when I switch the light on I see it will draw just over 0.9A.

Compare this with my more powerful deck light. The circuit is passed through the meter and it reads just over 3.4A. So what do these two readings mean in regard to power consumption? Well, at three hours, for example, my reading light at 0.9A will consume 2.7Ah compared with my spotlight, which would consume 10.2Ah (amps x time.) These figures are good to know in regard to appliances that will be drawing current when the engine is not running. A fully charged boat systems battery rated at 80Ah will deliver 8A for 10 hours before becoming completely flat. Marine batteries should not be discharged below 50% before needing to be recharged.