Battery Bank and Load Budget on a Survey Launch
A 10 m survey launch has a 12 V house bank of two 110 Ah AGM batteries in parallel, rated for 500 cycles to 50% depth of discharge. Daily loads are: chartplotter and sounder 4 A for 8 h; LED deck and cabin lights 3 A for 5 h; a fridge drawing 5 A for 10 h duty-cycled at 40%; and a laptop charger 6 A for 3 h. Charging comes from a 40 A alternator on the main engine, run about 2 h per day, plus a 200 W solar panel that averages 5 peak-sun hours. The engine start battery is separate. The skipper wants to stay out for three days without shore power.
1. Calculate the daily energy budget in amp-hours and determine whether the system can sustain three days off shore power.
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Daily consumption.
- Chartplotter and sounder: 4 A × 8 h = 32 Ah.
- Lights: 3 A × 5 h = 15 Ah.
- Fridge: 5 A × 10 h × 0.40 = 20 Ah.
- Laptop: 6 A × 3 h = 18 Ah.
- Total = 85 Ah per day.
Daily input.
- Alternator: 40 A × 2 h = 80 Ah, but charging is not linear - the last part of the charge is slow, so assume 70–80% acceptance, say 60 Ah.
- Solar: 200 W ÷ 12 V ≈ 16.7 A peak × 5 h = 83 Ah in theory, but with regulator losses, angle and cloud, assume 60–70%, say 55 Ah.
- Realistic total input ≈ 115 Ah per day, against 85 Ah load.
Net position. A surplus of about 30 Ah per day looks healthy. Over three days the bank should hold up.
But check the bank. Two 110 Ah batteries in parallel give 220 Ah total. Usable to 50% depth of discharge is 110 Ah. A single cloudy day with no engine run would draw the bank down by 85 Ah, close to the usable limit. Two such days would flatten it.
Conclusion. On the stated averages the system sustains three days, but the margin depends on sunshine and disciplined engine running. It is not a comfortable margin for a survey vessel whose loads may grow.
2. The skipper proposes to skip the engine run on the sunniest day to save fuel. Evaluate that decision.
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The proposal. Rely on solar alone on a good day, saving perhaps 4–6 L of fuel.
The arithmetic. On a sunny day solar might deliver 70 Ah against an 85 Ah load - a 15 Ah deficit. That is fine for one day, but it draws the bank down and leaves less headroom for the next. If the following day is cloudy, the deficit compounds.
Hidden costs.
- Cycle life. The batteries are rated 500 cycles to 50% depth of discharge. Deeper and more frequent cycling shortens life. A replacement pair may cost several hundred dollars, far more than the fuel saved.
- Charging inefficiency. Alternator charging from a deeply discharged bank is slow and heats the batteries. The 2 h run may not fully recover the bank.
- Operational risk. Survey equipment needs stable voltage. A low bank can cause instrument dropouts or resets at the worst moment.
- No redundancy. If the engine is needed for manoeuvring or an emergency, the charging opportunity is gone.
Judgement. The fuel saving is trivial against the risk and the battery wear. Keep the engine run as a scheduled charge, and treat solar as the variable input, not the guaranteed one. A better saving is to cut the fridge duty cycle or run the laptop from a dedicated supply. The skipper should also fit a battery monitor with state-of-charge display so decisions are made on data, not optimism.