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Author Topic:   Battery Voltage Reading
jfortson posted 02-24-2012 05:25 PM ET (US)   Profile for jfortson   Send Email to jfortson  
What is the minimum battery [terminal] voltage reading before a battery is considered suspect to hook the boat up and pull it to the landing - that is assuming the battery is otherwise good.

When first taking a new battery off of the charger, the reading was 13.27 volts. After sitting for a week with the master switch off and also running the tilt up and down several times. the reading was 12.86 volts.

I know 12.86 is good. At what reading (on a resting battery) would you say it is a "no go", that the battery needs to be charged before going to the landing?


jimh posted 02-24-2012 05:49 PM ET (US)     Profile for jimh  Send Email to jimh     
For information on how to determine a battery's state of charge from its terminal voltage, see

Battery Charge

jfortson posted 02-24-2012 06:38 PM ET (US)     Profile for jfortson  Send Email to jfortson     
Thanks. There is so much information on this site, it is difficult to know what is there.
jimh posted 02-25-2012 08:12 AM ET (US)     Profile for jimh  Send Email to jimh     
It is best to haul the boat to the launching ramp with its battery at full charge. This will give you the best chance of getting your outboard engine started at the launching ramp. If you get to the launching ramp and encounter any unanticipated problem in starting your outboard engine, having a battery with full charge will give you the most reserve capacity in the battery to continue cranking over the outboard engine. To this end, I have installed a permanent-mount battery charger in my boat. Before departing to the launching ramp, I usually plug in the battery charger for an hour of more to let it charge the boat battery. In this way I assure that my boating trip will begin with a fully charged battery.
Jefecinco posted 02-25-2012 11:19 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

That is good advice re a fully charged battery. My method is to connect a portable charger the evening before going out. Your method is certainly far more convenient but my portable charger is paid for and I am cheap and have a lot of time available. However, an on-board charger is near the top my longish list.

I also recommend a good and long set of jumper cables be carried aboard the tow vehicle. On two occasions I've used them at the ramp to help less prepared boaters get their engines started and boats out of the way of those of us waiting to launch.

I carried a portable booster battery aboard my old Dauntless 16 for many years until it finally died. Although it was not as good as the dual battery setup I finally installed it provided a low cost alternative for a while.


Swellmonster posted 02-26-2012 11:08 PM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
If another boat doesnt start at the ramp, they should not be on the water.
Prep first!!
jfortson posted 02-27-2012 07:03 AM ET (US)     Profile for jfortson  Send Email to jfortson     
The original question posted was because of sub division restrictions, the boat is stored at locked lot provided by the neighborhood association. There is no electricity there so I can not put on a charger the night before.

I was asking what would be the minimum voltage reading before towing the boat to the ramp? For excample, if the voltage is 12.0 volts, go or no go (again assuming the battery is otherwise OK)? I know 12.75 would be OK; where would you draw the line?


Jefecinco posted 02-27-2012 10:19 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
I believe there are too many variables to answer your question. Even if I knew how many engines of what size and how many batteries of what capacity, type, age, the ambient temperatures, etc I could not hazard a guess as to what voltage would guarantee to start your engines every time. Perhaps experimentation would give you that information.

In your situation I would simply use a dual battery system utilizing AGM batteries. I would wire the system in such a was as to avoid any parasitic loss to the "reserve" battery. That would provide an almost bullet proof starting system that requires almost no maintenance. If you have an off season that would be a good time to remove the batteries and put them on a maintenance charge.


Jefecinco posted 02-27-2012 10:22 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

We agree. No boat should be on the ramp that is not ready to go. Unfortunately, not everyone subscribes to this principal. In addition, things sometimes happen to even the most courteous and well intentioned boater. It never hurts to be prepared for these eventualities.


jfortson posted 02-27-2012 11:14 AM ET (US)     Profile for jfortson  Send Email to jfortson     

That is a good common sense reply. While I currently only have one battery ( Marine starting - AGM), I have installed a dual battery switch with the idea of adding the second battery. If I remember, I turn the switch off when the boat is on the trailer (remembering is a big "IF"). I do not like the thought of being at the ramp with a dead battery, much less out on the water.

The second battery will probably be a Marine deep cycle AGM since I am thinking of adding a 12 v trolling motor and will isolate the deep cycle when using the trolling motor.

In a prior boating life, I could hook up the charger and would always hook up the water hose and start the motor before leaving the driveway. The current situation does not allow that.


jimh posted 02-27-2012 12:37 PM ET (US)     Profile for jimh  Send Email to jimh     
The terminal voltage measurement shows the state of charge, but it does show the amount of stored electrical energy. There are some modern testers which try to predict the amount of stored electrical energy in a battery by making a short and not very high-current discharge. I believe these testers have some method of then predicting the amount of stored energy, which they express in units of Ampere-hours.

The battery's internal resistance also affects how much current it can deliver. Some batteries can reach a high terminal voltage indicating a full-charge state, but they also have high internal resistance. They cannot deliver very much current in that situation, and they might fail to crank over an outboard motor.

To really predict exactly how the battery will preform will require more than just a voltage measurement, and the equipment needed to take those other measurements is relatively expensive compared to the cost of a battery.

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