Lithium batteries for bow thrusters

Owners who have already gone lithium on the house bank usually ask the same thing next: can the thruster run on it too? The honest answer is "sometimes, with the right thruster and the right battery," and the list of batteries that qualify is a lot shorter than the list of lithium batteries for sale.

What the thruster makers say

Start with the people who write the warranty. Sleipner's FAQ recommends feeding thrusters from a bank without a battery management system, which in practice means AGM, gel or flooded. Their installation guide adds a specific warning for on/off thrusters: high-capacity lithium can supply more voltage than the motor is rated for, which shortens run time, increases wear and voids the warranty. The propeller turns faster than designed, which raises the risk of cavitation.

The same FAQ says Sleipner's proportional and speed-controlled thrusters are not affected the same way, because their controllers regulate what the motor sees. Vetus's BOW PRO line works similarly, holding thrust steady across a battery window of 10.5 to 14.4 V on 12V systems and cutting back power if current climbs too high.

Battery makers agree on the BMS problem. Victron's own blog on thrusters and lithium recommends a separate, simple thruster battery without a disconnecting BMS, and asks installers not to fit BMS-protected lithium to thrusters. Lifeline, which sells both chemistries, says the same about its lithium line and steers thruster buyers to AGM.

The numbers that matter: continuous and peak current

A 12V Sleipner SE80 is listed at around 530 A nominal draw. Bigger 12V units go well past that. Now compare a few LiFePO4 batteries that get pitched for thrusters.

Only one of those is in the right neighborhood for a mid-size 12V thruster on its own.

Mastervolt MLI Ultra 12/2750

This is the heavy-duty end of marine lithium. It pairs LiFePO4 cells with Mastervolt's BMS in a sealed case, rated at 200 Ah, 500 A continuous, and a 10-second peak listed at 1,800 A in some datasheets and 2,000 A in others. It weighs 27 kg (59.5 lb), less than a single group 31 Odyssey AGM that has about half the usable capacity.

Two things to plan for. First, Mastervolt makes a safety relay mandatory in the installation, so the BMS can disconnect the battery if it needs to. That relay is the exact mechanism that worries thruster makers, so it belongs on a system whose operator understands it. Second, nominal voltage is 13.2 V. On a proportional thruster that is fine. On an on/off thruster, Sleipner tells you to measure voltage at the motor and stay within ratings.

The MLI Ultra also talks MasterBus, CZone and NMEA 2000, so state of charge can show up on the same screens as the rest of the boat.

RELiON InSight 12V-LT

RELiON markets the InSight 12V-LT as a higher-output GC2-size battery: 120 Ah, 160 A continuous, with short-duration ratings stepping up to 400 A for 5 seconds. That is useful information for a windlass or a small thruster. For most bow thrusters on 12V it is not enough on its own, and a battery that is asked to exceed its rating will protect itself by shutting off.

Victron Lithium NG

We include Victron because it shows up on boats with Victron house systems and owners want to reuse what they know. The 12.8V/200Ah NG is rated at 200 A continuous and 400 A for 10 seconds, and it needs Victron's external BMS. Those numbers put it in house-bank territory, not thruster territory, and Victron's own advice lines up with that.

If you do go lithium

• Pick the thruster first. A proportional unit such as the Sleipner SEP or Vetus BOW PRO handles lithium voltage far better than an on/off motor.
• Size for peak, not amp-hours. The BMS continuous and peak ratings must cover the thruster's draw with margin.
• Keep it separate. Even with lithium, a dedicated thruster battery near the bow beats tapping the house bank.
• Fuse it properly. Lithium can deliver very high short-circuit current. Class T fuses are the usual choice; see our fuses and cable notes.
• Charge it properly. Use a charger with a profile the battery maker approves. ProMariner lists approved LiFePO4 support on its ProSportHD line, but check the specific battery against their list.

A worked example

Take a 12V Sleipner SE80, listed at around 530 A nominal draw. An Odyssey 31M AGM meets Sleipner's battery recommendation of 1,045 SAE CCA with room to spare and has no BMS to trip. A Mastervolt MLI Ultra 12/2750 is rated 500 A continuous with a much higher 10-second peak, so on paper it covers short bursts, but the nominal draw sits right at its continuous rating and the SE80 is an on/off thruster, which is exactly where Sleipner warns about lithium voltage. The RELiON and Victron batteries above are not in the conversation for this thruster. That is the pattern we see on most boats: the numbers only work for a handful of lithium batteries, and the thruster type decides the rest.

Weak spots, plainly

The failure mode is different from AGM. A flat AGM gives you weak thrust. An overloaded lithium battery gives you none. Cost is higher, installation needs more parts, and on on/off thrusters you may be trading weight savings for motor wear and a warranty argument.

Verdict

For a proportional thruster on a boat where weight in the bow matters, a high-current battery like the Mastervolt MLI Ultra, installed with its relay and proper fusing, is a legitimate choice. For a typical on/off thruster, the manufacturers' advice points to a dedicated AGM, and that is what we install by default. See the AGM review, or send us your thruster model through the quote form and we will tell you which side of the line it falls on.

Questions people ask