Marine starting battery comparison
Optima D31M vs Century Dual Force+ 31X MF AGM
Both are AGM candidates, but the 31X MF lists higher Ah, CCA and reserve figures. The D31M is narrower and lighter in the Australian distributor's listing. Compare those trade-offs with charging compatibility and application-specific warranty terms.
Specification-based comparison. We have not bench-tested these models. Prices are dated observations, not live offers. Read the manufacturer sources and editorial policy.
Which battery should you shortlist?
Option 1
Optima BlueTop D31M
D31M / Australian product ID 139352
Consider D31M if its narrower footprint and published ratings suit the existing installation and your required duty.
Option 2
Century Dual Force+ 31X MF
31X MF / 148118
Consider 31X MF if the additional rated capacity is useful and its wider, heavier case fits the boat with compatible charging.
Battery specifications compared
Manufacturer-published figures, checked 2 October 2026. Missing or contradictory details are identified explicitly.
| Specification | Optima BlueTop D31M | Century Dual Force+ 31X MF |
|---|---|---|
| Exact model | D31M / Australian product ID 139352 | 31X MF / 148118 |
| Application | AGM marine starting and accessory use | AGM starting and shallow cycling |
| Rated voltage | 12V | 12V |
| Rated capacity | 75Ah | 100Ah at the 20-hour rate |
| CCA at −18°C | 900A | 950A |
| MCA at 0°C | 1125A | Not stated in the cited table |
| Published reserve capacity | 155 minutes (supplier RC figure; test conditions to confirm) | 200 minutes |
| Dimensions (L × W × H) | 324 × 165 × 237mm; 237mm total height | 332 × 172 × 210mm; 232mm total height |
| Weight | 27.1kg | 28.5kg |
| Terminals | Twin; check supplier diagram | Standard / wing nut; check diagram |
| Published warranty | 36 months in Australian supplier listing; conditions apply | Brochure lists 24/12 months by application; confirm terms |
Optima BlueTop D31M: Figures follow the Australian distributor's D31M listing. Overseas D31M documents can show slightly different dimensions and mass; match the actual supplied unit and terminal drawing rather than mixing specifications from several markets. Check manufacturer details ↗
Century Dual Force+ 31X MF: The website headlines 24 months; the marine brochure lists 24/12 months for starting/cycling applications. Confirm which warranty applies to your intended use. This model is included in Century's marine brochure even though its product page sits in the 4WD category. Check manufacturer details ↗
Australian prices and quote checks
Optima BlueTop D31M
Current price not verified
The Australian distributor specification page does not quote a selling price. Request a local quote for product ID 139352 and check warranty coverage.
Check price source ↗Century Dual Force+ 31X MF
Current price not verified
Ask for a current quote for item 148118, including installation if required. No assumed price band is used.
Check price source ↗Source links on this page are direct references, not affiliate tracking links. Compare delivered prices on the same day and obtain a separate scope for installation, charging changes and protection.
Two AGM batteries with different capacity ratings
AGM describes construction, not one universal performance level. The D31M and 31X MF have different published capacity and starting figures, so compare the exact models rather than assuming either badge establishes equivalence.
Higher nameplate Ah can be useful, but the actual accessory runtime depends on demand, discharge conditions and the reserve retained for starting. Neither battery should be selected solely by dividing its label capacity by a guessed daily consumption.
A narrower footprint can decide the installation
The D31M's listed width is smaller, while the 31X MF has a lower published total height. A narrow box and a low access lid therefore present different trade-offs. Check all three dimensions, not just the longest edge.
Both installations need suitable restraints and accessible terminals. Confirm the actual terminal arrangement before ordering, especially where existing cables have limited reach. A replacement that forces strained cables is not equivalent merely because it fits the floor of the compartment.
Evaluate warranty and recharge requirements
The local D31M listing and the 31X MF sources use different warranty wording. Confirm the term for the intended application rather than comparing the largest headline number. Keep the written supplier terms with the invoice and commissioning information.
Existing charging equipment must be checked against the selected battery's current instructions. The shared AGM label does not establish identical settings or permitted duty. Compare complete quotes and avoid turning promotional durability claims into an unsupported ranking.
Common battery comparison questions
Are both batteries AGM?
Yes. The cited Australian supplier sources identify both as AGM products.
Does 31X MF list more Ah than D31M?
Yes. The source values are 100Ah for 31X MF and 75Ah for D31M. This is a specification comparison, not a measured runtime test.
Sources and comparison method
We matched the exact model codes to the manufacturer documents below on 2 October 2026. The selection advice is our interpretation of those specifications. We have not measured capacity, durability, waterproofing or service quality. We do not turn manufacturer cycle claims into a tested lifespan ranking.
- Century Yuasa: Australian Optima D31M specifications ↗
- Optima Australian range brochure ↗
- Century 31X MF specifications ↗
- Century Australian marine range brochure ↗
Specifications, offers and warranty terms can change. Match the supplied unit to its current manual before purchase. Report a correction with the model code and source.