Remote Battery Wiring

A separate battery to run a winch or bow thruster is cheaper, safer and works better than running a heavy cable from your starting or house bank.

This installation requires some electrical skills and since errors could cause an electrical fire you should not attempt it unless you feel confident. This project has not been checked for compliance with ABYC or USCG standards and the author presents this as a guideline to be interpreted by qualified installers. No warranty or guarantee is given or implied.

THE THEORY BEHIND THIS PROJECT

Winches and bow thrusters, located in the bow, are usually a long distance from the starting or house batteries. These items draw a heavy current for a short period of time. Unless you use very heavy cable for the long run, the voltage drop by the time it gets to the device, will have reduced the available power.
What is worse, however, is that very heavy copper cable is an invitation for an on-board fire if it gets shorted so for safety you have to add a large fuse - large enough to take the stalled motor load of the winch without giving you a nuisance blow - yet small enough that a short at the far end can draw enough current through that long cable to blow the fuse.

In many installations, these requirements conflict making practical current protection impossible.

The current drawn by the stalled motor is often so close to the current drawn by a short at the remote end that any fuse or circuit breaker that can protect against the short, will give nuisance blows just when you really need it.

PROVIDING A REMOTE BATTERY IS SAFER, CHEAPER AND WORKS BETTER

Instead of installing a very expensive heavy gauge copper line up AND BACK, and the expensive fuse to protect it, install a battery in the forepeak to provide local power. Now, the heavy wire is very short with very little voltage drop, and you can keep the battery charged with an economical charging line. Depending on the size of the device to be powered you can usually get by with a low cost automotive starting battery. These are made to take heavy loads for short periods of time and remain on float charge the rest of the running time. Since the charging line is low current, it is easy to provide fuse or circuit breaker protection. By installing a 50 amp combiner in series with the charging line, the remote battery will be maintained at full charge without any diode drop yet it will not discharge back into the house or starting battery.

NOTE:- Since there is a battery at each end of the charging line you need a fuse or circuit breaker at EACH end.

INSTALLATION

Parts List:-

• Automotive battery with enough capacity to run the device for the longest job. (Multiply AVERAGE current by time in hours to get amp-hours required, then double this figure. Example, 80 amps x 30/60 hours x 2 = 80AH ) For charging compatibility it should be the same type (lead-acid or gel) as the starting battery.
• 50 amp combiner The combiner should be mounted with the remote battery.
• Two fuses or circuit breakers. I prefer the automatic reset thermal breakers that are typical in the automotive industry. 
• A good quality battery box. 
• Red and Black cables for the charging line - 10 gauge is adequate.
• Conduit to protect charging line - essential on a metal boat, desirable on others.

Select a location for the forward battery. It should be ventillated because the battery will be on charge much of the time and producing small amounts of hydrogen that could be explosive if allowed to concentrate. Larger installations might consider an automatic power vent that only runs when dangerous gasses are being produced. A gel type battery is much less of an explosive gas problem but it is still present if overcharged.
Provide a SOLID mounting for the battery and battery box. Keep in mind that the bow takes much more of a pounding than the traditional battery locations so go overboard (not literally) on strengthening the mounting. The battery (box) must be rock solid when strapped down. Due to the motion and stresses, I would not use the conventional webbing strap they supply as the buckles provided are usually poor. Good quality ratcheting strap(s) of a material that is acid proof would be better, or provide a stainless retainer with through bolts and wing nuts.
Connect the device to the battery following manufacturer's suggestions. Connect the charging line, following this schematic and the installation instructions supplied with the combiner. If you use our thermal circuit breakers, you must use crimp on ring terminals to connect the wire - the screws are too small to adequately secure 10 gauge stranded wire, even if tinned.