BoatUS ANGLER: Do It Yourself Department
Batteries and Trolling Motors
by John C. Payne
Spring is coming and getting your boat ready to go fishing is a must. A trolling motor is now becoming one of those hard-to-live-without equipment items but getting the most out of it is a challenge. The key to trolling motor performance is the battery, and this includes initial selection, maintenance and charging.
A trolling motor requires a deep cycle battery to sustain the continuous high power demand. Ideally you need to match the discharge characteristics of the battery to the trolling motor power consumption.
To use an example, a 55 lb thrust motor unit has a peak power draw of 40 amps. If you use the trolling motor for 6 hours at average 50% load of 20 amps, the trolling motor will then require a battery load of 120 Ah. (6 hours X 20 amps=120 amp hours). A battery bank of a 240 Ah capacity rating selected as deep cycling should be kept to 50% to ensure maximum battery life. The deeper the cycling of the battery the less overall life you will get. In this case the nominal battery bank rating should be around 20 amps at the 10 hour rate.
Battery performance is also directly linked to skill at using the trolling motor. If you use the trolling-motor at above average power levels, say 30 to 40 amps, the actual available battery capacity is subsequently reduced 10-15% or even more. Conversely if you use the motor at lesser loads, say 10-15 amps, then you will get greater life.
Always try and match the trolling-motor current requirement as close as you are able to the actual battery characteristics. Sharp heavy speed changes also affect the battery performance. Gradual speed variations will cost you less power and the newer electronic speed controllers give you greater control.
Trolling Motor Battery Care
Battery problems are almost always caused by a failure to charge the battery properly. If your batteries were left on board the boat or in the garage all winter without charging, or had occasional but possibly not 100% charging, then the battery plates will have started to sulfate and you will have lost battery capacity. In fact, a battery just sitting around will slowly self discharge, even if not connected. In many cases after a day out fishing, batteries are often left discharged for a period and every day or even hour you delay starting full and complete battery charging the plates will sulfate and it will cost you both capacity and service life (sulfur molecules attach themselves to the battery plates when the battery is used and not charged. The result is “sulfate” which can kill the battery). It is worth looking at one of those on-trailer vehicle based charging systems, that way they will be charged by the time you get home. Of course don’t forget to check the water levels and top up with distilled water. In a hot summer, the electrolyte levels drop through evaporation and charging.
Charging with a cheap auto charger will never keep your trolling motor battery bank in optimum condition so invest in one of many quality chargers now on the market. A general rule is to select a battery charger that is rated at around 15-20% of rated amp hour capacity so a 100 Ah battery will need a 15-20 amp charger (rarely are you going to need a charger rated at above 30 amps).