Nearly every marine battery problem we get called out to comes down to the same thing: the boat has the wrong type of battery in it. Not a cheap battery, not an old battery — the wrong type. A cranking battery asked to run a fridge overnight will be dead in a season, and a deep cycle asked to turn over a big outboard will do it, badly, until the morning it doesn't.
The three types, and what they're actually for
Cranking (starting)
Lots of thin plates, maximum surface area, built to dump a very large current for a very short time. That's what starting an engine is. What it is not built for is being drained slowly and deeply — do that repeatedly and the plates buckle. If your boat starts, runs, and everything electrical is powered by the alternator while the motor is going, a cranking battery is the right answer and the cheapest one.
Deep cycle
Thicker plates, less surface area, built to be drained to a significant depth and recharged, hundreds of times. It will not deliver the same cold cranking amps as an equivalent-sized cranking battery. This is the house battery: fridge, lights, sounder, live-bait pump, radio.
Dual purpose
A genuine compromise, and we mean that as a description rather than an insult. It cranks well enough to start most outboards and cycles well enough to run modest house loads. For a single-battery tinnie or a runabout with a sounder and a couple of lights, dual purpose is usually the sensible call — and it's what we fit most often around Fremantle.
| Setup | What to fit | Why |
|---|---|---|
| Tinnie, sounder, nav lights | Dual purpose | One battery, light house load, needs to crank |
| Runabout with a fridge | Cranking + deep cycle | Separate the house load or you'll kill the start battery |
| Offshore boat, twin outboards | Cranking per engine + deep cycle house bank | Redundancy matters when you're 30km out |
| Yacht with a house bank | Deep cycle bank + dedicated cranking | Sustained draw and a reliable start are different jobs |
Running a single battery for both starting and house loads on a boat with a fridge. It works all summer and then strands you, usually at the least convenient moment. Two batteries and a VSR costs less than one recovery.
Why Fremantle is harder on batteries than most of Perth
Salt air accelerates terminal corrosion, and boats sit unused for weeks at a time between trips. A battery that is left partly discharged sulphates, and sulphation is largely permanent. The combination — salt, heat and long idle periods — is why marine batteries around Fremantle, North Fremantle and the harbours tend to have shorter working lives than the same battery in a daily-driven car.
- Keep terminals clean and greased — salt gets into everything down here
- Put a maintenance charger on it if the boat sits for more than a fortnight
- Fit a sealed AGM if the battery lives anywhere it can get wet
- Check the resting voltage before a big trip, not at the ramp on the day
AGM or flooded?
AGM is sealed, doesn't spill, handles vibration better and tolerates being mounted in awkward positions. It costs more. For a trailer boat that gets bounced down the freeway to Coogee or Woodman Point every other weekend, that vibration tolerance earns its keep. For a moored boat with a well-protected battery box, a good flooded battery is still perfectly sound and cheaper to replace.
If you're not sure what's in the boat now, send us a photo of the label. We'll tell you what it is and what it should be — that conversation is free and takes two minutes.
Common questions
Can I use a car battery in my boat?
You can, and people do, but it's a false economy. A car battery isn't built for the vibration a trailer boat sees or the deeper discharges a house load causes, and most aren't sealed. It will work for a while and then fail early, usually somewhere inconvenient.
How long should a marine battery last in Perth?
Three to five years is normal for a well-maintained marine battery down here. Salt air and long idle periods between trips shorten it; a maintenance charger and clean terminals lengthen it. If you're getting under three years, something else is going on — usually the battery is being cycled deeper than its type is built for.
Do you come to the boat ramp?
Yes. Boat ramps, marina pens, hardstands and driveways — that's how we work. There's no shopfront to bring the boat to. Call us with the boat and the current battery and we'll come to wherever it's sitting.