Current Draw Batteries

Current Draw Batteries

by Conrad & Judy Kreuter

Q: Dear Boat Talk: I am in the process of adding a GPS unit and a new depth finder to my boat. I am concerned that my battery system will not be adequate. What advice can you give me? JG, Holtsville, NY

A: Dear JG: You didn’t mention whether you had a single or dual battery system. If you have a single battery system the current draw on the battery, which is measured in amperes, is equal to the total current required by all of the electrical systems on your boat. These may include the GPS, fish finder, navigation lights, baitwell pumps, bilge pumps, radio, etc. In most cases, the alternator on the engine which recharges the battery, is capable of powering these devices as well as supplying sufficient current to keep the battery at full charge. Of course, the alternator is only charging when the engine is running.

The amount of current supplied depends on engine speed. At idle or slightly above idle, the alternator output will only be a few amperes. At high speed, the alternator is capable of supplying its rated output. If you have more current draw than the output of the alternator, you will slowly drain the battery even while the engine is running. This is the condition you are worried about.

It is easy to figure out how much current your boat electronics draws. Just add the current requirements of each device. This information can be found in the user manuals. The owner’s manual for the engine will list the maximum current output of the alternator, which is always given at the highest RPM. At lower speeds the alternator output will be less. If the total current draw exceeds the available current output from the alternator, the battery will discharge. If this condition continues for a period of time, the battery will be totally discharged or dead. If your boat has this problem, the only alternative will be to turn off unneeded electronics to conserve battery power. You can also, at great expense, retrofit a more powerful alternator to the engine. Some engine manufacturers make kits for this purpose.

Many boaters often worry that they will find themselves with a dead battery when far from shore. In this instance a dual battery system is a solution. This gives the boater a spare battery for backup.

In a dual battery setup, a battery switch is installed. The switch will have four positions: “only battery one”, “only battery two”, “both batteries” and “off”. The engine will only charge the battery that is selected. By using a battery switch, you can alternate the use of each battery so that you can always count on having two fully charged batteries. When using the “both” positions the batteries are connected together in parallel which gives you twice as much starting power. There should be no reason to use the "both” position if your are alternating the use of battery on and two. The battery switch should be left in the “off “ position when leaving the boat. This will insure that there is no drain on the battery because of electronics items being left on.

Battery switch systems are commonly misused. Most people only select battery “one” each time they use the boat. This means battery “two” never gets used. As a result of non-use, battery two may discharge and not be at full charge. If this is the case, switching to the “both” position of the battery switch will result in two marginal batteries. The reason this happens is because the fully charged battery will try to recharge the discharged battery as soon as the switch is moved to the “both” position lowering the charge in the good battery.

A useful device in a two-battery system is a battery isolator. This device allows the charging of both batteries regardless of the position of the battery switch. The battery isolator will allow each battery to recharge up to its full capacity without any interference between the batteries.