Electronics basics

Ohm's law

The one relationship worth knowing is Ohm's law: voltage = current × resistance (often written as V = I × R). Voltage is the push, current is the flow, and resistance is how much the path fights back.

Power fits in too: power (watts) = voltage × current (P = V × I). A 24V pump drawing 5A is pulling 120W. If the wiring adds resistance, some of that voltage gets eaten before it reaches the pump, and the pump sees less than 24V even though the battery is fine.

That is why your multimeter matters. Measure at the battery, then measure at the load under the same conditions. If there is a big gap, you have too much resistance somewhere: thin wire, a bad crimp, a corroded connector, or a fuse holder that is not making proper contact.

Try the sliders

The defaults match the example below: 24V at the battery, 0.2Ω in the wiring, and a load that ends up drawing about 10A. Nudge the wire resistance up or drop to 12V and watch what happens to current, voltage at the load, and power.

A quick example

Say you have 24V at the battery and a load that draws 10A. The cable plus connectors add 0.2Ω of resistance total. Ohm's law says you lose 10 × 0.2 = 2V in the wiring. The load only sees 22V. For a pump or motor that might mean less flow or harder starting. For a charger it might mean it simply will not run properly.

Halve the current by running at 24V instead of 12V for the same power, and that same wiring only drops 1V. That is a big part of why I push 24V where I can.

Cable quality and length in DC systems

DC is less forgiving than mains AC in a backyard setup. There is no transformer stepping voltage down at the other end, and your battery can deliver a huge surge current into a short. Good cable and short runs matter more than people think.

Length and voltage drop

Every metre of cable adds resistance. Longer run, more drop. Higher current, more drop. There is no magic: look up a wire gauge chart for the current you are carrying and the length of the run, then size up one step if you are on the fence. I would rather spend a few dollars on thicker copper than lose power in the walls of the shed.

Keep high-current runs as short as you can. Battery to fuse, fuse to bus, bus to load. Put the MPPT and battery close together where practical. Run the panel cable once and terminate it properly rather than joining halfway with whatever was in the scrap box.

Quality matters

Use copper cable rated for the job. Thin speaker wire or cheap figure-8 flex is fine for a LED strip, not for a pump on a 24V bank. For anything that carries real current outdoors, use proper stranded copper with insulation that suits heat and UV, and terminate it properly with Anderson or MC4 connectors or decent lugs, not twisted wire and electrical tape.

Fuse close to the battery positive side on each circuit. The fuse protects the cable from a short, not the other way around. A cable that is too thin for the current and length will get warm before the fuse blows, which is a smell you only want once.

What I do in practice

• Size wire for the worst-case current and the actual run length, then go one size up if unsure.
• Prefer 24V loads over 12V for the same power so current and drop stay lower.
• Keep battery, fuse and main loads in one enclosure or a short cable hop, not scattered across the yard.
• Check voltage at the device, not just at the terminals on the bench.
• Replace any run that gets warm under normal load. Warm wire is telling you something.

Tim and I cover multimeters and batteries in more depth in the Jarramali video. For hands-on connector work, the crimp walkthroughs on the connectors page are the next step after you understand why the wire size matters.