AGM batteries for bow thrusters

A bow thruster is the rudest load on a boat. It sits there doing nothing for weeks, then asks for several hundred amps for ten seconds while you are trying not to hit a piling. The battery that feeds it does not need to be big in amp-hours. It needs to deliver a lot of current without sagging, recover between bursts, and never decide on its own to switch off.

That is why our standard install puts a dedicated AGM battery as close to the thruster as the boat allows, with a small onboard charger keeping it full from one AC circuit. The thruster stops borrowing from the house bank, the heavy cable gets short, and the voltage at the motor stays higher. This review covers the AGMs that show up most often in that job.

Why AGM, and why not just any AGM

Sleipner's own guidance is to feed thrusters from a battery bank that is not protected by a battery management system, which in practice means lead-acid, AGM or gel. A BMS can trip on the thruster's current draw and leave you with no thrust at the worst moment. An AGM just sags a bit and keeps going, which you can feel at the joystick.

The catch is that thrust depends on the voltage the motor actually sees. Sleipner rates its on/off thrusters at 10.5 V on a 12V system because cable, switch and fuse losses eat into what the battery delivers. A battery with low internal resistance and a high cranking rating holds that voltage up better. That is the spec to shop on, more than amp-hours.

Odyssey Extreme 31M (ODX-AGM31M, formerly 31M-PC2150)

This is the one we see in thruster compartments most. It is a group 31 dual-purpose AGM rated at 103 Ah, 1,150 CCA and 1,370 MCA, with a 5-second pulse rating of 2,150 amps and a listed internal resistance of 2.2 milliohms. It weighs 77.8 lb and has both SAE posts and studs, which makes lugging heavy thruster cable straightforward.

For scale: Sleipner's installation guide lists a 12V SE80 drawing about 530 A and asks for a battery of at least 1,045 CCA (SAE). One 31M clears that. Bigger 12V units, like the SE100 with its 740 A draw and 1,425 SAE CCA recommendation, call for two in parallel.

The weak spot is charging. Odyssey's technical manual says that for best results the charger should supply at least 40% of the battery's 10-hour capacity, and never less than 10%. The 31M's 10-hour figure is 98 Ah, so a 6-amp maintainer is under Odyssey's floor. Odyssey also wants around 14.7 V absorption for cycling use. Plan the charger with the battery, not after.

Odyssey ODS-AGM6M (PC2250)

If the compartment has room for a wider, squatter box, the PC2250 is a group 6T battery at 126 Ah, 1,225 CCA, 1,550 MCA and 2,250 pulse amps, weighing 86 lb. It is a single-battery answer for some thrusters that would otherwise need two group 31s. Check the footprint first: it is 11.3 by 10.6 inches, which does not fit everywhere a group 31 does.

Lifeline GPL-31T

Lifeline builds AGMs aimed at deep cycling. The GPL-31T is 105 Ah, 600 CCA and 750 MCA, at 64 lb. Lifeline quotes a self-discharge rate of about 2% per month at 77°F, which suits a battery that may sit for weeks between uses.

The cranking numbers are the issue. On a thruster where the manufacturer asks for 1,000-plus CCA, one GPL-31T does not get there, so you are into two or more batteries or a larger case. Where Lifeline earns its place is a smaller thruster, or a boat where the same battery also does a windlass and sees deeper discharges.

Lifeline's technical manual calls for 14.3 V absorption and 13.3 V float at 77°F, and at least 0.2C of charge current for repeated deep cycling. Check that your charger's AGM profile is close to that, because some chargers run AGM hotter.

Lifeline GPL-8DL

The 8D is the big-boat answer: 255 Ah, 1,350 CCA, 1,675 MCA, and 156 lb. One of these can feed a large 12V thruster without a parallel pair. It is also a two-person lift into a bow compartment and needs a platform that can take the weight. Most small onboard chargers, including ProMariner's ProSport line, say they are not for 4D and 8D batteries, so budget for a larger charger.

What matters at install time

• Distance. Every foot of cable between battery and motor costs voltage. Sleipner's cable tables count the whole loop, positive and negative, and step up cable size as the loop grows. Mounting the battery near the thruster is the cheapest performance upgrade available.
• Hold-down and venting. AGMs are sealed but still need a proper box or tray and strapping, and Odyssey's 31M has an external vent fitting.
• Fuse and switch. Put the main fuse and a battery switch close to the positive terminal. See our fuses and cable review.
• Charging. A dedicated charger keeps the thruster battery independent of the house system. Our notes on the ProMariner ProSport cover sizing it to the battery.
• Same age, same type. If you parallel two batteries, buy them together and keep them matched.

How AGM stacks up against lithium

Lithium saves weight and holds voltage better, which sounds perfect until you read the thruster manuals. Sleipner warns that lithium can push an on/off thruster above its rated voltage, raising wear and voiding warranty, and that a BMS cut-out under load is a real risk. Their proportional thrusters handle it better. We go through the details in lithium vs AGM and the lithium battery review.

Verdict

For most on/off thrusters, the Odyssey 31M is the default pick: the cranking numbers line up with what thruster makers ask for and the case fits where a group 31 fits. Pair it with a charger that can put out at least 10 amps on that battery. If the thruster is small or the battery also runs a windlass, a Lifeline GPL-31T is a calmer, easier-charging choice. For big 12V thrusters, compare two 31Ms against one 8D on weight, space and charger cost before deciding. If you are unsure what your thruster needs, send us the model and we will match the battery in the quote.

Questions people ask