Winter, the Silent Enemy of Your Battery
During the winter months, your jet ski sleeps on its trailer or in its garage. The same goes for your WILLIAMS tender, in the boat's garage or its shed — but its battery continues to slowly discharge. An AGM lead-acid battery naturally loses 3 to 5% of its charge per month at room temperature, and even more in cold, damp weather. Without a maintenance charger connected all winter, the voltage gradually drops.
The critical threshold never to cross: 8 to 10 volts. Below this limit, the lead plates undergo irreversible sulfation: lead sulfate crystals deposit on the active plates and can no longer be dissolved by a simple recharge. The battery may appear to regain correct voltage after charging, but its actual capacity is permanently reduced — it will no longer hold a charge under load and will leave you stranded at the worst possible moment.
💡 Pro tip: Connect a maintenance charger (float charger or smart charger type) as soon as winter storage begins. A 1-2A charger is sufficient to keep the battery fully charged all winter without risk of overcharging. This is the most cost-effective way to double your battery's lifespan.
Why Spring is the Key Time to Check Your Battery
Spring start-up is the ideal — and often the last — moment to detect a failing battery before it lets you down at sea. Start by measuring the open-circuit voltage with a multimeter:
- 12.7 V or more: fully charged battery ✅
- 12.4 – 12.6 V: partially discharged battery, recharge before use
- 12.0 – 12.3 V: weak battery, monitor closely
- Less than 12.0 V: probably sulfated battery — replacement recommended
- Less than 10 V: irreversibly damaged battery — immediate replacement essential ⛔
Caution: a battery that reads 12.6 V after recharging but quickly drops under load (engine start, accessories) also needs to be replaced. Open-circuit voltage is not enough — test it under load or have it tested by a professional.
AGM Deep Cycle: The Technology Adapted for the Marine Environment
Not all batteries are created equal, and the marine environment is particularly demanding. AGM (Absorbed Glass Mat) batteries are now the standard for jet skis and motorized tenders, for several reasons:
- Sealed and maintenance-free: the electrolyte is absorbed in fiberglass separators — no risk of acid leakage on board, even in case of shock or tilt.
- Vibration resistant: the plates are held under pressure in the casing, making them much more resistant to shocks and vibrations than classic flooded batteries.
- Deep Cycle: designed to withstand repeated deep discharges (powering accessories, frequent starts) without premature plate degradation.
- Low self-discharge: a quality AGM battery retains its charge much longer than a conventional battery during winter storage.
⚠️ Do not confuse AGM and gel batteries: while both are VRLA (maintenance-free) technologies, AGM uses fiberglass separators impregnated with electrolyte, while gel uses a silicated gel electrolyte. Both are suitable for marine environments, but AGM Deep Cycle batteries are generally better suited for combined starting and deep cycle applications, such as on a jet ski or tender.
Not All Batteries Are Compatible With Your Model
It's a common mistake: buying a generic "12V" battery without checking the specifications. However, each Sea-Doo or WILLIAMS tender model has precise requirements in terms of capacity (Ah), dimensions, and terminal type. An undersized battery will wear out prematurely and struggle to start the Rotax engine in cold weather. An oversized battery simply won't fit in the compartment.
For WILLIAMS Minijet 280, Sportjet 285/325/385/445 tenders (pre-2019) and Sportjet 345/395/435 (Rotax, from 2019), as well as for the vast majority of SEADOO Jet Skis (GTI, GTX, GTR, RXP, RXT, Wake Pro, Fish Pro — excluding SPARK), the reference battery is the LEOCH LDC12-25 (12V 25Ah).
👉 Discover the LEOCH LDC12-25 battery compatible with WILLIAMS & SEADOO
Our Tips for Extending Your Battery's Lifespan
- Connect a maintenance charger as soon as winter storage begins and keep it connected all winter (1-2A smart charger).
- Disconnect the negative terminal if you cannot connect a charger, to limit self-discharge due to parasitic loads.
- Store in a cool, dry place, away from frost (ideally between 10 and 20°C). Intense cold accelerates discharge and can crack the casing.
- Check the voltage monthly during winter storage with a multimeter. If it drops below 12.4 V, recharge immediately.
- Clean the terminals before each season and apply anti-corrosion grease (such as technical Vaseline or terminal spray) to prevent oxidation.
- Avoid repeated deep discharges without recharging: each discharge below 50% capacity reduces the battery's lifespan.
The LEOCH LDC12-25 Battery – Our Recommendation for WILLIAMS & SEADOO
After testing several models on the market, we have selected the LEOCH LDC12-25 (12V 25Ah AGM Deep Cycle) as the reference replacement battery for WILLIAMS tenders and SEADOO Jet Skis equipped with BRP Rotax engines. It meets OEM specifications, installs without modification, and its AGM-GEL technology guarantees a longer lifespan than entry-level batteries — even after several winters.
👉 Order the LEOCH LDC12-25 battery – Delivery to France & Europe
Also consult our guide on starting your Sea-Doo in spring and our article on flushing the engine after each saltwater outing.
