The essentials in 30 seconds
- Winter layup = optimal window for an energy refit. 4-6 weeks of work ashore, stable off-season material prices, and workshop teams available. Plan starting in October for installations before Easter.
- Skysat 7-step method: (1) current system audit, (2) target sizing, (3) BMS + battery selection, (4) alternator/DC-DC chain, (5) addition of solar/wind, (6) shunt + monitoring, (7) testing and commissioning.
- 35-45 ft sailboat refit budget: basic (battery replacement only) ~ €3,000-5,000 ex. VAT, full (lithium bank + DC-DC + monitoring) ~ €12,000-18,000 ex. VAT, integrated (+ 600 W solar + shore charger + new switchboard) ~ €22,000-30,000 ex. VAT.
- Typical ROI: lithium conversion = +20% range + -150 kg weight + 15-20 year lifespan vs lead-acid 7-10 years. Payback in 4-6 years with intensive cruising use.
- Complete energy cluster: 5 articles already published (plug & play, BMS, sizing, refit, winter layup). See our complete lithium refit guide.
Winter layup is the only time of year when the sailboat can be immobilized for 4-6 weeks without impacting the sailing program. It is the optimal window for a complete energy refit: removal of the lead-acid bank, lithium installation, alternator rewiring, addition of solar, and full monitoring. This article describes the Skysat workshop method in 7 steps.
For in-depth coverage by topic, see our energy cluster articles: lithium bank sizing, BMS comparison, DC-DC converters.
Why winter layup = optimal refit window
- Sailboat immobilized for 4-6 weeks: energy refit possible without interrupting sailing.
- Material prices: stable off-season (vs +8-12% in May-June for popular brands).
- Workshop teams available: October-March = full capacity, vs June-August = saturation.
- Seasonal electrical conditions: spring commissioning = warm conditions for battery and solar testing.
Step 1 — Current system audit
Written, documented, photo-referenced inventory:
- Current battery type (lead-acid, AGM, gel, lithium), raw capacity, age, condition (measured residual capacity).
- Electrical diagram: retrieve the original shipyard diagram or redraw if lost.
- 24 h consumption measured with a shunt over 1-2 weeks of typical sailing. See sizing method.
- Recent failures: what tends to fail? Alternator regulator, shore charger, sensors?
Step 2 — Target sizing
Theoretical calculation of target capacity:
- 24 h consumption inventory by load (see Step 1 sizing method).
- Simultaneity coefficient 0.65-0.75.
- Safety margin 30%.
- Useful DoD 80% for lithium.
- Final raw capacity = increased demand / 0.8.
On a typical 40 ft offshore cruising sailboat: raw consumption 145 Ah/24h → simulation 0.7 → 100 Ah/24h → +30% margin → 130 Ah/24h → DoD 80% → raw capacity 165 Ah.
Step 3 — BMS and batteries
BMS selection based on program (see BMS comparison):
- Standard offshore cruising: Victron Lynx Smart BMS 500 (~ €1,020 ex. VAT) + 12.8V/200Ah Smart lithium battery (~ €1,180 ex. VAT).
- Yacht with CZone: Mastervolt MLi Ultra 12/3000 (~ €2,600 ex. VAT, integrated BMS).
- Offshore racing: MG Energy 24V 304Ah (~ €2,990 ex. VAT).
Step 4 — Alternator/DC-DC chain
Switching to lithium requires alternator-side modifications (see plug & play lithium myth):
- LFP-compatible alternator regulator: Wakespeed WS500, Balmar MC-614, or Victron Orion XS solution.
- DC-DC protection: Orion XS 1400 (~ €350 ex. VAT) limits current to a safe value.
- Class T fuse: 600 A in series with the lithium battery bank (calibrated to BMS max current + 20%).
Step 5 — Solar and complementary energy sources
If the program includes extended anchoring or offshore without daily engine charging:
- Solar: 1 Wp per Ah of raw lithium capacity (standard workshop ratio). 200 Wp minimum for 200 Ah, 400 Wp for 400 Ah.
- MPPT: Victron SmartSolar MPPT 100/30 (~ €140 ex. VAT) up to 400 Wp.
- Wind turbine: only in very windy areas (>15 knots average offshore). Not relevant in the Mediterranean.
- Hydrogenerator: 500-1,000 W during continuous sailing. Excellent for offshore racing.
Step 6 — Shunt and monitoring
Essential for tracking state of charge and detecting anomalies:
- Victron BMV-712 Smart (~ €215 ex. VAT): 500 A shunt + Bluetooth + 30-day history.
- Cerbo GX: central monitoring server for the entire Victron system (BMS, MPPT, charger). Compatible with smartphone app and VRM cloud.
- Alarms configured at the switchboard for low battery voltage and BMS overheating.
Step 7 — Testing and commissioning
Before splashdown:
- Full charge with shore charger and SOC verification at 100%.
- Controlled discharge test to 80% DoD on known load.
- Engine start test with and without separate starter battery.
- Load dump test: disconnect the BMS while the alternator is charging; verify that the transient absorber functions.
- Shunt calibration and BMS programming with final thresholds.
- Documentation provided to the client: final diagram, usage manual, emergency procedure.
FAQ — Winter layup refit in practice
How much time to reserve for a complete energy refit?
3-5 weeks in the workshop depending on scope. Battery replacement only = 1 week. Full refit with solar and alternator = 4-5 weeks. Always allow a margin for unforeseen issues (ordered parts, diagram modifications).
Can the refit be done over several winters?
Yes, but experience shows the result is rarely optimal. A staggered refit = compromise on system coherence (BMS from winter 1 does not communicate with charger from winter 2 if different brands are chosen). Better: complete refit in a single winter.
Does the lithium refit affect insurance?
Yes, positively. A compliant lithium installation per IEC 62619 / ISO 16315, with external BMS, class T fuse, and professional installation by an approved technician, is more insurable than an undocumented lead-acid bank. Request a workshop certificate to keep in the boat file.
When to order for winter layup 2026-2027?
Ideally before July 2026 for workshop delivery in September. Lithium manufacturer lead times: 4-8 weeks depending on model (MG Energy racing = up to 12 weeks, Victron Lynx Smart = 2-4 weeks). Skysat workshop booking: from May for start in October.
Warranties and after-sales service after refit?
Parts: 3-5 year manufacturer warranty (Victron NG 5 years, Mastervolt 5 years, MG Energy 5 years). Labor: Skysat 12-month warranty on installation. Seasonal after-sales: possible intervention at partner ports (Vannes, Lorient, La Trinité-sur-Mer) within 48-72 h by appointment.
3 real-world cases — complete winter layup refit
Case 1 — Winter layup refit Bénéteau Oceanis 40 (2024-2025)
Scope: conversion from lead-acid bank 300 Ah → Victron 12.8V Smart lithium 200 Ah + Lynx Smart BMS 500 + Orion XS 1400 alternator protection + replacement of shore charger with MultiPlus 12/3000-120 + addition of 200 Wp solar panels + BMV-712 monitoring. Total workshop time: 4 weeks + €8,500 ex. VAT materials + €3,500 ex. VAT labor = €12,000 ex. VAT. Commissioned April 2025, estimated ROI 4 years (vs lead-acid replacement every 6-8 years).
Case 2 — Winter layup refit Lagoon 42 charter Mediterranean (2025-2026)
Broader scope: conversion to 24 V lithium bank (2 × 200 Ah Victron) + Orion-Tr 24/12 for 12 V equipment + 600 Wp solar panels + maintained diesel generator + Cerbo GX central monitoring + Touch 70 display. Workshop time: 6 weeks + €18,000 ex. VAT materials + €8,000 ex. VAT labor = €26,000 ex. VAT. Justification: intensive charter use, critical autonomy for high-season rentals.
Case 3 — Winter layup refit IMOCA Initiatives-Cœur (2024-2025)
Dedicated offshore racing. MG Energy 24 V 304 Ah HE Series bank (maximum cyclability) + 2 × Orion XS alternator protection + 800 Wp semi-rigid solar panels + Watt&Sea hydrogenerator + full Cerbo GX monitoring + satellite telemetry. Workshop time: 8 weeks + €32,000 ex. VAT materials + €12,000 ex. VAT labor = €44,000 ex. VAT. IMOCA racing standard.
Excluded alternatives — non-selected solutions
- Replacing lead-acid with lead-acid to save money: false economy. Initial differential €1,500-2,500 ex. VAT vs lithium, but lead-acid lifespan 7-10 years vs lithium 15-20 years = lithium pays off over 12-15 years. Over the typical sailboat ownership period (10+ years), lithium wins.
- Energy refit without monitoring (BMV-712): tempting to reduce the budget by €200-300. A mistake. Without a shunt + app, it is impossible to diagnose faults or optimize consumption. Do not overlook this.
- Staggered refit over 2 winters: compromise on system coherence. Better to do everything in a single winter, even if it means deferring by a year if not ready.
- Wind turbine refit: good idea in very windy areas (northern Brittany, Scandinavia) but not relevant in the Mediterranean. Prefer solar + hydrogenerator if needed.
Useful resources — during refit
- Clean electrical diagram: Excel or Visio with batteries / loads / fuses / cable cross-sections. Updated after every modification.
- BMS documentation: manufacturer manual kept on board (Victron, Mastervolt, MG Energy publish as PDF).
- Monitoring app: VictronConnect or VRM Cloud for remote monitoring during winter. Cerbo GX enables email alerts for faults.
- Maintenance logbook: record antifouling dates, cell cleaning, consumable replacements. Useful for insurance and resale.
Skysat has an integrated workshop in Carnac (Brittany) with 4 teams specialized in marine energy. This article reflects our standard method applied to 80+ sailboat refits 2022-2026. 2026 ex. VAT prices are indicative distributor rates.
