Lithium vs AGM for bow thruster batteries

Lithium has won the house-bank argument on a lot of boats. The thruster battery is a different job, and the answer flips more often than people expect. A thruster asks for a huge current for a short time, a few times a day, and the one thing it must never do is stop without warning. Those requirements play to AGM's strengths and expose lithium's one serious weakness.

The case for AGM

An AGM battery has no electronics between the cells and the terminals. Ask it for 600 amps and it delivers, with voltage sagging as the load and depth of discharge rise. As the battery tires, the thruster gets weaker in a way you can feel, which is a warning. Sleipner's recommendation is to feed thrusters from a bank that is not protected by a BMS, typically AGM, gel or flooded, for exactly this reason.

The numbers fit the job. An Odyssey Extreme 31M is rated at 1,150 CCA and 2,150 amps for 5 seconds from a group 31 case. That clears Sleipner's battery recommendation for a 12V SE80, which draws around 530 A. AGMs are sealed, cheaper than lithium per battery, and charged by any decent charger with an AGM profile.

The costs are weight and voltage sag. Two Odyssey 31Ms weigh about 156 lb, and in the bow of a 30-footer that is noticeable. And a sagging battery means less voltage at the motor, which means less thrust. That is why our installs put the AGM close to the thruster, so the cable run does not add to the sag.

The case for lithium

Lithium iron phosphate holds a flat, high voltage under load and recharges fast. A Mastervolt MLI Ultra 12/2750 stores 200 Ah in 59.5 lb, and Mastervolt rates it for 500 A continuous with a 10-second peak in the 1,800 to 2,000 A range depending on the datasheet revision. On a proportional thruster, that means consistent performance from the first maneuver to the last.

The problem is everything else on the lithium shelf. A RELiON InSight 12V-LT is rated at 160 A continuous and 400 A for 5 seconds. A Victron Lithium NG 12.8V/200Ah is rated 200 A continuous and 400 A for 10 seconds. Those are good batteries for house loads and far short of a mid-size 12V thruster's draw. Exceed a BMS limit and the battery protects itself by disconnecting.

Where lithium goes wrong on thrusters

• BMS cut-out. Victron's own blog describes the risk of a sudden cut-out mid-maneuver, and asks installers not to fit BMS-protected lithium to thrusters. Lifeline says the same about its lithium line.
• Overvoltage on on/off thrusters. Sleipner warns that lithium can drive an on/off thruster above its rated voltage, spinning the prop faster than designed, increasing wear and cavitation, and voiding warranty. Its speed-controlled thrusters are not affected the same way.
• Shared banks. Hanging a thruster on a lithium house bank combines both problems and adds the risk of dropping the house loads too.

Charging and protection differences

AGM wants the right voltage and enough current. Odyssey, for example, says charge current should never fall below 10% of the battery's 10-hour capacity, so a 6-amp maintainer is too small for a 31M. Lithium wants a charger profile the battery maker approves, and often temperature limits on charging.

On fusing, ANL fuses are standard on AGM thruster circuits. With lithium, the battery can deliver far more fault current, which is why Class T fuses with their 20,000 A interrupt rating are the usual pick. See our fuses and cable review.

A worked example: Sleipner SE80 on 12V

Sleipner lists the 12V SE80 at about 530 A nominal draw and recommends a battery of at least 1,045 SAE CCA.

• Odyssey 31M (AGM): 1,150 CCA, so one battery meets the recommendation. No BMS, so no cut-out risk.
• Mastervolt MLI Ultra 12/2750 (lithium): 500 A continuous with a much higher 10-second peak. Short bursts are inside its ratings, but the nominal draw sits at its continuous limit, and the SE80 is an on/off thruster, which Sleipner warns about with lithium.
• RELiON InSight 12V-LT or Victron Lithium NG 200Ah: 160 A and 200 A continuous respectively. Not suitable for this thruster on their own.

Swap the SE80 for a proportional thruster and the lithium case gets stronger, because the controller manages motor voltage. Keep the on/off thruster and the AGM is the straightforward answer.

Storage and self-discharge

A thruster battery can sit for weeks. Lifeline quotes about 2% self-discharge per month for its AGMs at 77°F, which is low for lead-acid, and an onboard charger keeps it full anyway. Lithium self-discharge is also low, but batteries with built-in electronics draw a little standby current, so check the maker's storage guidance before leaving one disconnected over a long winter.

Which to choose

• On/off thruster, any size: AGM, dedicated, near the thruster.
• Proportional or speed-controlled thruster, weight-sensitive bow: lithium can work, with a battery rated well above the thruster's draw, the maker's relay or protection, Class T fusing and a matched charger.
• Lithium house bank already aboard: keep the thruster off it. A separate AGM thruster battery is the clean answer.

The bottom line

For most boats, a dedicated AGM thruster battery with its own charger is the right answer, and it is our standard install. Lithium belongs on proportional thrusters where the weight savings matter and the battery's ratings genuinely cover the load. For model-by-model detail, read our AGM review and lithium review, or ask us to spec the battery with your thruster.

Questions people ask