4 DIY Solar Cabling Mistakes That Can Cost You — And How to Fix Them

If you’re setting up a solar battery system yourself, small wiring mistakes can turn into big performance issues or safety hazards. Todd from Big Beard Battery walks through four critical wiring missteps many DIY installers make—and how you can avoid them. Let’s dig deeper into the “why” behind each one.

1. Unequal Cable Lengths (Balance the Race)

Todd illustrates that positive and negative battery cables must be the same length so electricity flows evenly—like both sides of a steering wheel. When using multiple batteries, mismatched lengths create imbalance: the shorter cable does more work and stresses the system. This imbalance can show up in just months.Tip: Measure cable lengths carefully, including any bus bar runs, and adjust custom cables to compensate for even minute differences.

Supporting insight: A Solar Electric forum thread confirms that “overall wire lengths connecting battery to common bus must be the same on each battery” to avoid imbalance.

2. Incorrect Wire Gauge (Avoid Overheating & Voltage Drop)

Todd emphasizes choosing wiring gauge based on current to the load, not battery size. For instance, a 3,000W inverter on 12V draws ~250A—you need cables rated for at least that, and preferably higher (e.g., 400A). Undersized cables heat up, create resistance, and waste energy as heat.

From Victron’s "Wiring Unlimited" guide: select appropriate cable thickness based on current, keep voltage drop below recommended limits, and use their toolkit or tables to guide selection.

3. Improper Torque (Secure Connections Matter)

Todd points out the importance of correct torque: metallic contacts should be tight enough to create a solid electrical connection without over‑torquing. Loose connections cause heat and resistance, while overtightening risks damage.

A marine safety case demonstrates how improper torque led to a battery case melting—not because of the nut, but because a stainless washer was incorrectly placed under the lug, increasing resistance and heat.

4. Misplaced Washers (Order Matters)

Todd teaches the correct order: bolt → lock washer → flat washer → cable lug → battery terminal. Washers that contact the battery between the lug and terminal introduce poor conductivity and high heat.

Supporting technical consensus: From DIY Solar forums, discussions clarify that any washer between the lug and terminal can impede conductivity—opt for the correct sequence to prevent heat build‑up