How to Test Voltage Drop in Boat Battery Wiring

Equipment and information required

Load the circuit first
Voltage drop is invisible at rest. The circuit has to be carrying its normal working current before any measurement means anything — this is why a fault checks out fine with everything switched off.
Measure at both ends of the same conductor
Source terminal to load terminal on the positive, then repeat on the negative. Measuring against a distant ground point mixes in drop from other runs and gives a misleading figure.
Check the negative as carefully as the positive
The return path carries the same current and drops the same way. Negative-side drop, often at a shared ground stud, is the half people skip.
Test across each connection individually
Measure across a terminal, a fuse holder, a switch or a crimp while loaded. Any meaningful reading across a single joint is a bad connection, not cable resistance.
Decide whether it is cable or connection
Drop spread evenly along a run means the cable is too small for the current or the distance. Drop concentrated at one point means a connection to remake.
Re-check charging runs specifically
A charging run with excessive drop means the charger reaches its target voltage while the bank does not, and the bank never finishes charging. This is a frequent cause of a bank that sits stubbornly below full.

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