When 80 amps wasn't really 80 amps

In 2017 we set out to sail around Scandinavia. Through the Baltic, into Russia’s inland waterways and across its big lakes, north across the Arctic Circle and around the North Cape, and home along the Norwegian coast.

A trip like that depends on electricity. Navigation, communication, lights. So I designed a large lithium battery bank for the boat, charged by the engine’s alternator when needed.

The alternator was rated at 80 amps. I designed the charging around that number and let the system draw close to it.

What I hadn’t considered was the difference between what a component can deliver and what it can sustain.

A conventional alternator isn’t necessarily built to deliver its rated output for hours. And a lithium battery bank can keep accepting high charging currents long after a lead-acid battery would have tapered off.

We burned through two alternators.

The batteries did exactly what they were designed to do. The problem was everything around them. I had built a system that could demand more than the rest of the installation could handle.

It didn’t help that the boat had fallen out of its cradle the autumn before we left. The repairs took the winter and the time we’d reserved for testing, so the first real test under load happened at sea.

The trip ended up taking three seasons. One winter, with the boat laid up in northern Norway, we fitted a different alternator with a regulator I could control in software, and capped the charging at 40 amps.

Half of what the first one promised. Enough for what we actually needed.

A few lines of software could have prevented two hardware failures.

Still one of the best trips of my life.