Are You Flying Blind With Your Boat Batteries?

Are You Flying Blind With Your Boat Batteries?

You plug your batteries into a “smart” charger or maintainer every time you get off the water and figure you’re good, right? Think again. That little green “fully charged” light might be lying to you.

Modern boats rely on a bewildering mix of battery chemistries — Flooded (FLA), AGM, Gel, Lithium (LiFePO4), and even TPPL (Thin Plate Pure Lead) variants. Throw in the difference between cranking batteries and true deep-cycle banks, and it’s no wonder most boat owners are confused about what they actually have — and whether it’s healthy.

Ten years ago, life was simple: one group-24 or group-31 flooded battery did everything. Today you might have a lithium starting battery, AGM house bank, and a separate thruster/winch bank. Each type ages differently, charges differently, and fails differently.

The Real Cost of Batteries in 2025

• Cheap flooded batteries: $120–$200, 1–3 year warranty, 300–500 cycles if you baby them.
• Premium AGM: $350–$600, 3–7 year warranty, 800–1,500 cycles.
• Lithium LiFePO4 drop-ins: $600–$1,500 each, 8–12 year warranty, 2,000–7,000+ cycles.

Yes, lithium is expensive up front, but when you divide the cost by the number of usable years and cycles, it’s often cheaper in the long run — and far lighter.

Your Onboard Charger Isn’t Telling You the Whole Story

Most “smart” chargers and maintainers give you a happy-face LED or a vague percentage on an app. That’s surface-charge voltage, not state-of-health (SOH). A battery can read 13.0 V on the meter and still have only 30 % of its original capacity left.

Those handheld load testers at the auto-parts store? Great on land, useless when you’re anchored in a cove 20 miles from the nearest road.

The Solution Most Serious Boaters Use in 2025

Install a proper battery monitoring system. The gold standard is still the Balmar SG200 (now in its second generation) or similar smart shunts from Victron, Mastervolt, Wakespeed, or Simarine.

Here’s what a real monitor tells you that a $29 voltmeter never will:

• True state-of-charge (not just voltage)
• Actual state-of-health (% of original capacity remaining)
• Real-time current in/out (so you know if your alternator or solar is actually working)
• Time-remaining at current load
• Historical cycle count and depth-of-discharge data
• Low-SOC and end-of-life alerts sent to your phone

Installation is usually just four wires and 30–60 minutes with basic tools. Once it’s in, you’ll wonder how you ever lived without it.

Stop Guessing — Start Knowing

Dead batteries are the #1 cause of boating tow calls. A few hundred dollars and an afternoon spent installing a proper monitor will save you thousands in premature battery replacements and tow bills — not to mention the embarrassment of drifting helplessly while your buddies laugh over the VHF.

Your batteries are trying to talk to you. Give them a voice.

Fair winds and fully charged banks!