Which battery does an outboard engine need (by engine power)

If you keep a small motorboat or an inflatable with an outboard in Málaga or on the Costa del Sol, sooner or later the battery needs replacing. And the question is always the same: which battery does my engine need? It depends on the power, on whether it is pull-start or electric start, and on what you have wired up on board. Here it is, set out so you get it right first time.

Outboard battery by engine power

This table gives you a starting point. It applies to petrol engines with electric start and a 12 V battery:

These ranges are only a guide. What counts is the engine manufacturer’s manual, which states the minimum marine cranking amps and, sometimes, the minimum capacity. The battery must meet or exceed those figures: falling short means sluggish starts and a battery that does not last. Going slightly over does no harm, provided it fits the space and is properly secured.

Pull-start or electric start

A small pull-start outboard does not need a battery to get going. It is a different matter if you want to power navigation lights, a fishfinder or a bilge pump: then you do need a battery, but a house battery, not a starting one.

With electric start the battery is essential, and there is one thing worth knowing: an outboard’s charging system delivers far less current than a car alternator. On short trips, with the engine idling much of the time, the battery does not get back everything it has used. That is why a shore charger and a battery with some headroom are welcome on a boat.

CCA and MCA: what they mean

They are two ways of measuring the same thing: how much current the battery can deliver for 30 seconds to turn the starter motor.

• CCA (Cold Cranking Amps): measured at −18 °C. It is the usual figure on automotive batteries.
• MCA (Marine Cranking Amps): measured at 0 °C, so for the same battery the MCA figure is higher than the CCA figure, by roughly a quarter.

Outboard manuals usually give the minimum in MCA. Do not compare an MCA figure with a CCA figure without converting it. If in doubt, send us what the manual says and we will tell you which battery meets it.

Why a car battery is not a good idea on a boat

It fits the space and it starts the engine, yes. But it was not built for the job:

• Vibration: a boat on the plane slams into the waves over and over. Marine batteries have reinforced, better-supported plates; a car battery suffers and does not last as long.
• Tilt: a boat heels and pitches. A flooded car battery can spill acid, and next to wiring and fuel that is poor company.
• Discharges: a car battery is designed to give a burst of current and be recharged straight away. On board, the electronics keep running with the engine off, and those repeated discharges wear it out quickly.

One battery or two? Starting and house

If you only start the engine and little else, one battery, starting or dual purpose, is enough. If you run a fishfinder, GPS, bilge pump, lights or a fridge, two is the sensible choice: a starting battery reserved for the engine and a deep-cycle house battery for everything else. That way, even after hours at anchor with the fridge on, the engine will start to take you back to port.

The two are managed with a battery isolator or selector switch (the typical positions are 1, 2, both and off). It lets you choose which one you draw from, combine them in an emergency and cut everything off when you leave the boat on its mooring, so no hidden drain flattens them. On twin-engine boats, the usual arrangement is one starting battery per engine. There is more detail in our guide to boat and marine batteries.

Which technology to choose: flooded, AGM or dual purpose

Flooded lead-acid, for starting

The classic liquid-electrolyte battery: simple and fine for small and mid-size engines with few electronics. It must always sit upright in a ventilated spot. In the Varta Professional Starter range you have, for example, the Varta LFS60 (12 V, 60 Ah, 540 A) for small engines and the Varta LFS95 (12 V, 95 Ah, 800 A) for more powerful ones.

AGM

The electrolyte is absorbed in glass mat: it is sealed, does not spill when the boat heels, copes better with vibration and recharges well. It is the logical choice for large engines and fast boats. A starting example is the Optima Blue Top BTSLI-4.2 (12 V, 50 Ah, 815 A), a spiral-cell AGM that is compact and delivers a lot of cranking current for its size.

Dual purpose

It starts the engine and also withstands house discharges. It is the answer when there is only room for one battery or when you want a simpler installation. From Varta, the Varta LA80 (12 V, 80 Ah, 800 A), from the Professional Dual Purpose AGM range; from Optima, the Optima Blue Top BTDC-5.0 (12 V, 66 Ah, 765 A).

You will find every Varta and Optima model in the boat battery catalogue. We also supply Tudor and Exide and, for house and deep-cycle use, Trojan, Rolls and TAB: for those brands, ask us for a quote.

Safe installation on board: general advice

We do not fit batteries on board, but this advice holds for any boat:

• Sealed box or battery tray: it protects against spray and contains any spill.
• Firm fastening: with a strap or hold-down clamp. The battery must not move even in a rough sea.
• Protected terminals: a cover on the positive, clean and tight connections, and grease or protective spray against salt.
• Ventilation: lead batteries can give off gas while charging. Never in a closed locker with no vent, nor right beside the fuel tank.

As for delivery: we deliver batteries to private customers and trade customers in Málaga and on the Costa del Sol, marinas included. We bring it to the marina; you or your marine mechanic fit it. Ask about delivery times when you order.

Ask for a quote for your outboard battery in Málaga

To get it right we need four details: engine make and power, whether it is pull-start or electric start, what equipment you have connected and the size of the battery space or the reference of the current battery. Send them by WhatsApp or call us on +34 952 31 01 18. We are an Official Varta Distributor, with a shop at C/ La Orotava, 100, 29006 Málaga (San Luis industrial estate). If you call in at the shop, no appointment is needed: we test your current battery free of charge and take the old one for recycling.