Duffy Battery Replacement
A Duffy is only as good as its battery bank. When an evening cruise turns into an anxious hour watching the gauge, it's usually the bank — and usually the whole bank, not the one battery that tested worst.
Batteries in a Duffy live a hard life: deep discharges, long sits, and a charger that may or may not be set correctly for the chemistry it's feeding. Three to five years is a normal life for a flooded lead-acid set in this service. When they go, they go together, and that's the honest reason we replace them as a matched set rather than picking off the weakest one.
What's included
- Load test of the existing bank and a charger output check
- A matched set of new deep-cycle, AGM, or lithium batteries
- New cables, lugs, and terminal hardware throughout
- Battery boxes, hold-downs, and ventilation inspected and corrected
- Charger profile set for the new chemistry
- Old batteries hauled away and recycled at no charge
- Full charge cycle and verified run-time before we hand it back
Flooded, AGM, or lithium?
Flooded lead-acid
The cheapest way to get your boat back. They need topping up with distilled water every few months and they don't love being left half-charged, but they're proven, they're recyclable, and for a boat that gets used most weekends they're perfectly good. Expect three to five years.
AGM
Sealed, no watering, no spill risk, and more tolerant of sitting partly charged. Roughly 40 to 60 percent more than flooded up front for a similar capacity. This is the sensible middle choice, and what we fit most often.
Lithium (LiFePO4)
Half the weight, close to twice the usable capacity for the same rated amp-hours, and eight to ten years of life. It needs a compatible charger and a battery management system, so it's a bigger up-front job — but per year of service it's often the cheapest option, and the boat is noticeably lighter and quicker.
Why the whole bank goes at once
Batteries in series are only as strong as the weakest one. Put a new battery next to five tired ones and the new one is dragged down to their level within months — you've spent money and bought yourself very little. It feels wasteful, and it isn't.
Getting the most out of a new bank
Charge after every single use, even a short one. Don't leave the boat sitting at half charge for weeks. Keep the terminals clean and greased. Do those three things and you'll get the top end of the life range instead of the bottom.
Not sure the batteries are the problem? We load test before we sell you anything. Sometimes the bank is fine and the charger, or a corroded cable, is the real fault — that's a much smaller invoice.
Typical pricing, installed
| Bank | Price |
|---|---|
| Duffy 18 · six-battery flooded set | $1,290 |
| Duffy 21 / 22 · eight-battery flooded set | $1,690 |
| AGM upgrade, either size | Add $520–780 |
| LiFePO4 conversion, including charger | $4,900–7,400 |
| Cables & terminal rebuild only | $180–420 |
Prices include batteries, new cabling, installation, charger setup, and recycling of the old set. Dockside installation across the service area at no extra charge.
How it works
Four steps, no surprises on the invoice
Test
We load test the existing bank and measure what the charger is actually delivering, rather than assuming.
Recommend
Chemistry and capacity matched to how you really use the boat — Sunday afternoons and Thursday sunsets need different answers.
Install
New cables, clean terminals, correct torque, correct fusing, and hold-downs that actually hold down.
Set & verify
Charger profile set for the new chemistry, full cycle run, and the run-time confirmed before you get the boat back.
Common questions
Battery Replacement, answered
How long should a set last?
Three to five years for flooded lead-acid in typical Duffy use, five to seven for AGM, and eight to ten for lithium. Charging habits move that range more than brand does.
Can you do it at my slip?
Yes. A standard bank swap is a two-to-three hour dockside job, and it's included in the price across our service area.
Do you take the old batteries?
Always, at no charge. They go to a certified recycler — please don't put them in a dumpster.
Can I replace just one battery?
You can, and we'll do it if you ask, but we'll tell you first that the new one will be pulled down to the level of the old ones within a few months. On a bank more than two years old it's usually money poorly spent.
Is lithium worth it on a Duffy?
If you keep the boat more than four or five years, usually yes — lighter, faster, longer range, and it outlives two or three lead-acid sets. If you're planning to sell soon, it's hard to get that money back at resale.
Ready when you are
Tell us your boat and your harbor, and we'll come back with a free written estimate — usually within one business day.