Why the manual number wins An outboard battery lives in a box in the weather, takes wave pounding all season, and has to spin a big engine in cold spring air. Mercury sets a cranking requirement for each engine family and prints it in the manual. That number is the minimum....
Quick answer: Your owner's manual sets the battery spec, and the number that matters is the cranking rating. A current Mercury 150 FourStroke calls for at least 1,000 MCA (800 CCA) and 180 Ah. Smaller motors need less, and older Mercury guidance roughly doubles the requirement for cold-weather starts on mid-range motors. Lithium cranking batteries are allowed, but only on Mercury's approved engine list and only if the battery meets Mercury's published spec.
Why the manual number wins
An outboard battery lives in a box in the weather, takes wave pounding all season, and has to spin a big engine in cold spring air. Mercury sets a cranking requirement for each engine family and prints it in the manual. That number is the minimum.
If your manual lists MCA, buy to MCA. If it lists CCA, buy to CCA. Don't guess between them, and don't let a parts counter talk you into "close enough." A battery that barely meets spec in July often won't in October.
Our boat battery guide covers chemistry, wiring and battery switches in depth. This page is the model-specific piece: what your Mercury actually asks for.
MCA, CCA and Ah in plain English
| Rating |
What it measures |
Why it matters |
| MCA (marine cranking amps) |
Cranking amps for 30 seconds at 0°C (32°F) |
The marine standard. Most outboard manuals use it. |
| CCA (cold cranking amps) |
Cranking amps for 30 seconds at −18°C (0°F) |
Tested colder, so 800 CCA is a bigger ask than 800 MCA. |
| Ah (amp-hours) |
Stored energy |
Runs electronics with the engine off. |
| RC (reserve capacity) |
Minutes the battery can supply 25 amps |
Another way of expressing stored energy. |
If the manual gives both MCA and CCA, meet both. A battery rated 1,000 MCA / 800 CCA covers the 150 FourStroke on both counts.
Battery specs by Mercury model
The 150 FourStroke line comes from Mercury's current owner's manual. The smaller-motor lines come from an older Mercury electrical FAQ that's widely quoted by owners. Treat those as a starting point and confirm against the manual for your serial number, because specs have changed over the years.
| Mercury engine |
Minimum cranking spec |
| 9.9 / 15 / 25 hp FourStroke |
465 MCA or 350 CCA |
| 30 to 50 hp FourStroke |
465 MCA / 350 CCA above freezing; 1,000 MCA / 750 CCA below freezing |
| 75 / 90 hp FourStroke |
465 MCA / 350 CCA above freezing; 1,000 MCA / 750 CCA below freezing |
| OptiMax 2.5 / 3.0 L |
1,000 MCA / 750 CCA |
| 150 FourStroke (current manual) |
1,000 MCA / 800 CCA / 180 Ah |
The cold-weather column is the one that surprises people: below freezing, the mid-range requirement more than doubles. In Ontario we launch onto water that was ice a few weeks earlier, so the cold column is the one to buy for.
That same older guidance suggested 70 to 100 Ah for mid-range motors. If a chartplotter, livewell and sounder run off your cranking battery, buy toward the top of that range. Better still, put house loads on a second battery. A cranking battery for the main motor plus a house battery (and one for the kicker on a triple-rigged walleye boat) means a dead house bank never strands you.
Group sizes: what fits
"Group" is a case size, not a power rating:
- Group 24: the common mid-size case, typical in smaller aluminum boats
- Group 27: longer case, usually more Ah
- Group 31: the big case, most capacity
Measure your battery box before you shop, and match the terminal layout to your cables. A group 31 that doesn't fit the box is a paperweight with posts.
Flooded, AGM or lithium?
| Chemistry |
Strength |
Watch out for |
| Flooded lead-acid |
Cheapest up front, dependable |
Needs charging discipline; can spill if tipped |
| AGM |
Sealed, vibration-resistant, maintenance-free |
Costs more |
| Lithium (LiFePO4) |
Light, long cycle life |
Only approved by Mercury on specific engines and specs (below) |
Mercury's lithium rules
Mercury has published a service bulletin on lithium cranking batteries, and it's refreshingly specific.
Approved engine families (all serial numbers):
- 2.1 L 75 to 115 hp FourStroke, Pro XS and SeaPro
- 3.0 L 150 hp FourStroke, Pro XS and SeaPro
- 3.4 L V6 and 4.6 L V8 175 to 300 hp FourStroke, Pro XS, SeaPro and Verado
- 2.6 L L6 200 to 400 hp Pro and Verado
- 7.6 L V12 500 to 600 hp SeaPro and Verado
Every other Mercury outboard, all MerCruiser engines and Mercury Diesel are not approved for lithium cranking batteries. If your engine isn't on the list, buy a quality AGM or flooded battery that meets the manual spec.
What the battery must meet on an approved engine:
| Requirement |
Mercury's spec |
| Chemistry |
Lithium iron phosphate (LiFePO4), built for marine cranking |
| Cranking |
At least 800 A for 8 seconds at −7°C (20°F) |
| Peak charge acceptance |
165 A for one minute |
| Maximum charge current |
150 A |
| Maximum charge voltage |
14.8 V |
| Reserve capacity |
135 minutes (RC25) |
| Sealing |
IP67 or better |
A bargain lithium that can't take the alternator's charge current, or isn't sealed to IP67, doesn't meet the spec. The first battery Mercury evaluated and approved under this standard was the RELiON RB100-HP: 100 Ah LiFePO4, rated 800 A for 8 seconds, IP67, with a 10-year warranty.
Two things worth knowing:
- This is about the cranking battery. A lithium house bank for electronics is a separate conversation.
- Per Mercury's bulletin, a battery that meets the spec on an approved engine doesn't void the engine warranty. Anything off-spec, or lithium on a non-approved engine, is a different story.
Not sure which side of the line your motor is on? Ask us through the service form, or when you build a repower quote. We'd rather answer it before you buy a battery than after.
Sources
About the author
Jay Harris runs Harris Boat Works in Gores Landing, Ontario, a third-generation family marina on Rice Lake since 1947 and a Mercury dealer since 1965.
