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Author Topic:   Charging two battery system
JimU posted 07-11-2002 10:58 AM ET (US)   Profile for JimU   Send Email to JimU  
I have just installed a two battery system in my 17 whaler with a 1, 2, Both, off switch. Question: Can I charge both batteries at the same time if I put the selector switch on "Both"? I have a two chargers, one that has an auto and manual selector. The second charger is a small "trickle" charger desingned to maintain charge.CAn I use either of these chargers to charge or maintain charge on both batteries at the same time? Any advice appreciated. JIM
where2 posted 07-11-2002 01:10 PM ET (US)     Profile for where2  Send Email to where2     
With an isolation diode, you can charge two at once. Without it, the weaker battery always sucks the extra voltage off the better battery when you have the switch in the "both" position. With an isolator, you can charge the weaker battery, until it reaches the level of the bettery battery, then you charge both.

Check the Dual Battery article in the Reference Section when JimH talks about the "both" portion of the switch.

I looked to see if West Advisor had any commentary on Battery Isolators, and they didn't.

jimh posted 07-11-2002 06:40 PM ET (US)     Profile for jimh  Send Email to jimh     
I would like to see a straight forward method of connecting batteries to outboards which involved isolating diodes--I don't think it can be done without resorting to rewiring of the under-the-cowling electrical system. Most outboard motors have the charging current source (the internal alternator or stator/rectifier) connected to the main battery lead under the cowling.

You would have to separate the charging current lead, add a new wire coming out from the cowling to the isolator, and then to the batteries.

You cannot wire two batteries through an isolator and then connect to the main battery lead to an outboard--the diodes only allow current to flow in one direction. You want the current to flow into the motor for starting, and then to flow out of the motor (and into the battery) for charging after the motor is started. The diodes allow only one way current.

Even if you did go to the trouble of re-wiring the outboard electricals, you would now have the voltage drop across the isolator diodes cutting down the charging voltage. The solution is to have a more sophisticated charger in which the sense line of the voltage regulator is connected to the battery directly, and it thus compensates for the voltage drop across the diodes.

Really, I don't think this is feasible in an outboard situation.

Even if you did go to all the trouble to accomplish this wiring, it would totally befuddle the next mechanic who looked at it and he'd probably tell you, "There's your problem."

Guts posted 07-12-2002 08:36 AM ET (US)     Profile for Guts  Send Email to Guts     
I put a perco two prong plug in the consel this dose many things. runs spot light, elec. reels and any outer 12v thing I wish too, put a male plug on. with the battery charger I have done this also. turn the switch to two on the switch and plug the charger in the perco plug. or use any plug you like? this has worked well for me. I running a newer OB and see no need to charge the battey as they never seem to be low.
jimh posted 07-12-2002 01:23 PM ET (US)     Profile for jimh  Send Email to jimh     
The Bombardier EVINRUDE outboards now seem to come with standard equipment that includes a dual battery charging system with built in isolator. That sounds like an excellent feature.
where2 posted 07-16-2002 01:08 PM ET (US)     Profile for where2  Send Email to where2     
Consider this wiring schematic: + lead running to engine runs to output lead of 1-2-BOTH switch, also attached to that terminal is a jumper going to the input segment of an isolation diode. From the 1 terminal of the 1-2-BOTH switch, you go to the + terminal of Battery #1. Off the 1 lead of your isolation diode you go to the + lead of battery #1. Off the 2 lead of your 1-2-BOTH switch, you go to the + lead of Battery #2. From the 2 lead of your isolation diode, you go to the + terminal of Battery #2.

Effect: Both batteries are always connected to the charging circuit. Which ever battery you have selected with the switch, does not have a 0.7v drop in the charging circuit due to the diode because you have shorted the diode with the switch. The battery you don't have selected will charge when the selected battery reaches a voltage level 0.7v higher than the selected battery. The batteries are effectively cut-off when you turn the 1-2-BOTH switch to OFF since the diodes are a 1-way valve. Don't use the BOTH position of the switch, unless you intend to skip the function of the isolation diode to combine the batteries in parallel.

Then, to charge the batteries at the house, you hook up a connector with your favorite style of quick disconnect plug to the + lead to the engine, and the - lead of one of the batteries. Set the 1-2-BOTH switch to OFF. The isolator does it's job, the batteries both get charged, the rectifier under the cowling prevents back flow into the stator/alternator, and the "auto" function on the battery charger at the house keeps the batteries from discharging too badly when you're not using the boat. No, they won't be as fully charged as if they were charged alone without the isolator, but if the input current paused when the combined voltage reached 13.7v, you'd still have 13.0v after the voltage drop through the isolator. If your boat won't start with 13.0v on the battery, stay home!

Alternatively, don't attach the input lead of the isolation diode to the + lead of the engine, only attach it to the + lead of your charging connector, and use the isolation diode only for charging at the house.

whitefoot posted 07-17-2002 04:33 PM ET (US)     Profile for whitefoot  Send Email to whitefoot     
I bought a Guest on-board battery charger and it works great. It has two outputs, and it charges both batterise simultaneously. Just plug the extension into the outlet I mounted to the side of the console and forget it. It charges the battery to 90% full on a 10 amp charge then goes to 2 amp to top off and maintain automatically. This was the best purchase I ever more fighting with connecting the charger to the terminals after every trip...and having to do it once for each battery(mine are located in the console so they are a pain in the butt to get to and work on).
whitefoot posted 07-17-2002 04:34 PM ET (US)     Profile for whitefoot  Send Email to whitefoot     
Oh, the model is the Guest Charge Pro 2611 if you are interested.
bsmotril posted 07-17-2002 10:47 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Your suggestion is certainly doable with the current Merc DFI outboards that use the auto style alternators. To pull it off, Merc would have to run an extra wire in the harness for the alternator/regulator voltage sense to each battery that bypasses the isolation diode in the 12V line that feeds the motor. Of course, it might cost another couple of dollars.

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