Bow and stern thruster battery calculator: the maker’s figure

The short answer

• Size the bank in cranking amps. A thruster draws hundreds of amps for a few seconds at a time. So the makers rate the bank in cold cranking amps. Amp-hours measure how long a small load can run, and the makers do not size a thruster bank on them.
• Each maker has its own rule. Sleipner prints a minimum for each model and voltage. VETUS prints a minimum and a maximum. Sideshift gives a total for each series. Lewmar asks for crank capacity at least equal to the thruster's current draw.
• Some models carry no cranking figure. The picker has 251 entries, one for each model at each voltage, and 222 of them carry one. For the rest, the page prints what the maker does give, such as amp-hours, an example battery or a suggested type. It uses the maker's own words and never turns them into a number of its own.
• Using the house bank. Sideshift's FAQ says a deep-cycle bank with the right cold cranking amps “may be an option”. VETUS and Sleipner recommend a separate battery near the thruster. The catch either way is the cable, because a bank aft of a bow unit means a long, heavy run.

Check your bank in four steps

1. Pick your thruster. Choose the model in the calculator at the top of the page. Only models whose maker publishes a current draw are listed. For any other thruster, type the draw from your manual instead.
2. Read the maker's figure. Most makers give a minimum in cold cranking amps. Some print amp-hours, an example battery or a rule, and the page shows that in their words. The section “Why the figure is cranking amps” explains the unit.
3. Type the bank you have. The cold cranking figure is printed on a starting battery's label. Add the figures together for batteries wired in parallel. A 24 volt pair in series counts once.
4. Type the cable run. Enter the one-way distance from the bank to the thruster, and the result sizes the cable. The section “What this does not tell you” covers where the bank should sit and what the calculator leaves to the battery page.

Why the figure is cranking amps

A thruster loads a battery the way an engine's starter motor does. It pulls hundreds of amps for a few seconds, then nothing.

What decides whether the motor gets its voltage is how hard the bank can push for those seconds. That is exactly what a cold cranking rating measures. An amp-hour rating measures how long a small load can run. The makers do not size a thruster bank on it.

The battery page sets out each maker's rule with the figures. It also gives what they say about starting, deep cycle, AGM and lithium in their own words.

What this does not tell you

• Whether the bank is where the maker wants it. Sideshift's manual puts the batteries within a short cable of the controller. Its FAQ allows a longer run for each series. If the bank sits aft of a bow unit, sort out the wiring first. The battery page's section “Where the bank lives” has the makers' distances, and the wiring calculator sizes the cable.
• What a tired bank costs in thrust. The makers publish two thrust figures. One is at full voltage and the other is at the sag they rate for. The battery page works out the loss from those two published figures.
• How to charge it. The battery page's section “Charging a bank that lives in the bow” has what each maker says about the charge feed.