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Posted: Mon Dec 19, 2016 5:25pm

Post Subject: Bow Thruster Battery Charging

I have a problem with charging the batteries for the bow thruster. When I checked the charging voltage going to the battery, it showed only 7 volts. The charging cables are fed from a diode split charge unit (cannot remember the make, but from memory it looks like a sterling model). The voltage coming out of the split charge unit is around 14 volts. Between the split charge unit and the bow thruster batteries we are losing at least 7 volts. The battery is reading 12.2 volts when connecting the multi meter direct to the terminals. When I checked the voltage of the battery from the end of the wires by the split charge relay it showed 12.2 volts. I did isolate the wires from the split charge unit so the reading I received was from the bow thruster batteries. Iâm a bit confused as the voltage from the battery to the end of the wires has no drop, yet in the opposite direction there is a drop. The cables for both positive and negative are at least 6mm. The distance from the split charge unit to the bow thruster batteries is approx. 17m. The batteries, wires and diode split charge relay are all new. My next plan is to buy some larger cable and see if that works. Any other ideas? Thanks Hugh


Posted: Mon Dec 19, 2016 10:10pm

Post Subject: Bow Thruster Battery Charging

Right, this can be explained fairly easily assuming you actually mean 6 sq mm conductor cross sectional area. Volt drop is proportional to both cable length AND current flow. It is inversely proportional to the cable's conductor cross sectional area. When you measured the voltage at the split charge diode end of the cables the only current flowing was whatever the meter drew so as the meter has an exceptionally high resistance the current would have been all but zero amps so the volt drop would also be virtually zero amps - hence the high voltage reading. The other way round you could have almost the full alternator output flowing (no data given on alternator size or length of boat) so you have double the length of the boat in out and back cable run and a high current. Lets say 50 amps and a 57 ft boat. This gives a cable length of around 53 meters. Using the calculation:- (0.0164 x 50 x 53) / 6 sq mm CCSA = a volt drop of around 7 volts. This leaves about 7 volts trying but not succeeding to charge the battery. No there is something I do not understand. With the bow thruster battery reading 12.2 volts the reading at the input terminal of the bow thruster should also have been 12.2 volts UNLESS the bow thruster was operating. Then it would be much lower. Unless you have dirty or loose terminal (like the battery post clamps) there is no way you should loose voltage down a pair of short cables (probably fat ones to boot) with only a voltmeter loading at the end so that reading should also have been battery voltage. I suspect that as the battery gradually discharged eventually the charging current flow caused such a volt drop the battery simply stopped charging because of lack of volts. Then it discharged even faster. I suspect you have a very flat battery. Take it out and charge it at home or put a shoreline with a mains charger on it to get it as fully charged as possible. The re-measure the charging voltage. I suspect that providing the battery has not been destroyed the low charging current would cause a very low volt drop so everything woudl appear to be OK until the state of discharge demanded a higher current. There are two ways forward if my diagnosis is correct. If you run an inverter for long periods or spend a lot of time on a shoreline just use a dedicated mains charger located close to the battery. Otherwise you are right, you need thicker cables, much thicker. I would guess something getting on for 50 to 60 sq mm CCSA and they will cost a lot of money unless you can find a job lot. The other thing is a conventional split charge diode will lose over half a volt through the diodes. You can get electronic zero volt drop ones or use a split charge relay. I suspect I would try a voltage senate one paralleling with the engine battery.

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