Petrea Heathwood explains how to check voltage drop in your power system.
It can pay to check the voltage drop (or resistance) in the charging circuit wiring between power sources (engine alternator, solar panels, wind generator) and your storage batteries at least once a year. Any high resistance will decrease the power being delivered to the storage batteries. If one exists, clean up all the terminals and plugs in the wiring, inspect everything for corrosion and check the resistance again. If it still exists, replace the wiring with a heavier gauge, taking the opportunity of deleting any joins or plugs in the relevant circuit to reduce future problems.
For the electrically challenged the tool to use is a multimeter. The setting you need is the resistance one. There are commonly a number of resistance scales offered on a multimeter, even the cheapies from Dick Smith. The usual way of doing things is to select a mid–range one and then keep changing to more sensitive ones until you get a reading.
Basically you need to disconnect the wire leading to the power source at both ends and connect the meter across them. Obviously if the ends are a long way apart, you will need to use an extension wire. This must be a heavier–gauge wire than the circuit wire, so that it offers little resistance to the measuring process.
You are chasing the resistance of the wire, and only the wire. Theoretically you want zero resistance to get maximum charging effect. If you have something else in the charging circuit between the generator and the battery (eg a controller), you have to measure the resistance of the individual wire lengths before and after the something else and add them together to get the whole.
Corrosion of the wire (reduces its cross-section) and poor terminal contacts will increase the resistance of the circuit. Obviously if you undo the terminals you should spot any potential problems but you won’t spot wiring corrosion hidden by insulation etc, so in effect it is a check on the fitness of the wiring as well as being able to reveal whether the charging current might not be getting through due to resistance.
Corrosion of the wire makes the current work harder to get through (like calcium deposits in your water pipes or fat build up in your arteries). As well as lowering the charge rate, it causes the wire to heat up with the consequent danger of a fire. This is why you should size your fuses to match the cabling — not the power consumption of what’s connected in the circuit.