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Author Topic:   Charging system, 1988 Johnson 150-HP
Don88outrage posted 09-09-2007 02:53 PM ET (US)   Profile for Don88outrage   Send Email to Don88outrage  
I have a question for the electrical experts out there: Is it a reasonable expectation for a 1988 Johsnon 150-HP motor's charging system, regulated, 3A at 800-RPM, 9A at 5,000RPM, to keep two batteries charged, assuming the selector switch is positioned for approximately equal time on each battery? The conditions are: both batterys are relatively new, GPS, SONAR, and VHF are always on, boat operation is about 20% crusing time, 80% drift fishing time without motor running. After about four fishing trips the batteries have a little difficulty turning the motor over, even with the selector switch set to "both" and require recharging with an external charger. Does this sound normal? I'm guessing because the same year 175-HP motor has a 15 and 35-Ampere charging system the 150's 3 and 9-Ampere system just can't keep up with the accesories being used. Appreciate your replies.


jimh posted 09-09-2007 03:35 PM ET (US)     Profile for jimh  Send Email to jimh     
If the charging system is only working 20-percent of the time the electrical load is being used, then the charging system will have to supply five times the current that the load is using if it is going to be able to recharge the battery. If you consider that the maximum charging current available is 9-amperes, this implies that the load ought to be no more than 9/5 = 1.8-Amperes. So let's examine your load.

A GPS, a VHF radio, and a SONAR combined will probably draw more current than 1.8-Amperes. So given your pattern of use, after several repetitions of this pattern your experience with the batteries eventually losing charge makes perfect sense.

You also have to consider that each time the starter motor is operated there is substantial current drain from the battery. This also requires replacement by recharging. This adds to the charging current required.

Therefore I do not think it is reasonable to expect to be able to maintain the battery system at full charge with such low-current charging and charging only 20-percent of the time the boat is in use.

My recommendation for your system is to re-wire your small boat electrical system to have two isolated batteries. Use one battery strictly for engine starting, and a second battery strictly for electronics. Use an automatic charging relay to manage the battery charging current available.

Also, for less than $100 you can install an AC-powered battery charger which will allow you to recharge your batteries when the boat is not in use. This will give you a chance to maintain the batteries in good condition without running the motor to charge them. A few hours on an AC-powered charger and you will completely recharge the batteries. Each time you use your boat you will start with a full-charge on each battery, and this should keep you going without any worries about engine starting.

newt posted 09-10-2007 08:29 AM ET (US)     Profile for newt  Send Email to newt     

As you know, the charging output available from those Johnson 150's is pretty paltry. I run the same engine, and always top the battery off on a charger after every trip. Sometimes the battery is close to fully charged, and other times it takes a while on the charger to bring it back up to fully charged.

Don88outrage posted 09-10-2007 04:58 PM ET (US)     Profile for Don88outrage  Send Email to Don88outrage     
Thank you Jim for the technical assesment and newt for your experience. The built in AC-powered battery charger sounds like the best option considering the way the boat is used and it's simplicity. With the battery's in the console, access to remove their box covers and hook up the external AC-powered charger would be a little more user friendly with a single cord to plug in.


jimh posted 09-10-2007 08:48 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is a suitable battery charger:


I have a Cabela's 8-amp dual battery charger installed on my boat. I usually let it charge and condition the batteries overnight before using the boat.

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