Moderated Discussion Areas
  ContinuousWave: Small Boat Electrical
  Parallel Cables For Batteries

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   Parallel Cables For Batteries
jtms posted 05-09-2010 06:38 PM ET (US)   Profile for jtms   Send Email to jtms  
I have two batteries on my Montauk but only one runs off the engine so the other battery doesn't have any way to recharge. I want to run cables so both batteries can charge off the engine. Today I ran a red wire from the red on battery 1 to red on battery 2. I did the same for the black wire. Will this do the trick? I want to make sure I am not doubling the voltage of the battery. I know you can do perko switches that are great, but I am looking for something easy.
jimh posted 05-09-2010 08:08 PM ET (US)     Profile for jimh  Send Email to jimh     
You have connected the batteries in parallel. The combined voltage will now tend to be closest to the battery which has the lower voltage.
jtms posted 05-10-2010 09:34 AM ET (US)     Profile for jtms  Send Email to jtms     
If both batteries have the same voltage then I should be okay. I am just wanting to ensure that I did the "parallel" correctly? The reason I did this was the second battery is solely for running the trolling motor, but whoever rigged it did not attach this battery to the main engine so there isn't any way to recharge it. Ultimately, this is what I am trying to accomplish.


jimh posted 05-10-2010 11:48 AM ET (US)     Profile for jimh  Send Email to jimh     
Electrical wiring does not understand a motivation. You have connected the batteries in parallel. They now work as one battery. Both batteries will now provide current to all loads.

I do not recommend connecting 12-volt lead-acid batteries in parallel for normal operation.

There are much better methods for implementing the charging of a second battery from a single charging source than to wire the two batteries permanently in parallel.

Jerry Townsend posted 05-10-2010 03:20 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
As JimH - connecting batteries permenantly in parallel is not good - as it can lead to problems you would not like.

Instead, use a solenoid (a good old-fashioned starter solenoid of yesteryear works well) with each of the two main contacts connected to the positive post of each battery. Connect the solenoid contact to the alternator/generator of the engine. Using this technique - the batteries are connected in parallel ONLY when the alternator/generator is operating. ---- Jerry/Idaho

Jerry Townsend posted 05-10-2010 03:24 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
You can get solenoids from a RV supply store or a automotive parts supply store. --- Jerry/Idaho
jimh posted 05-10-2010 11:30 PM ET (US)     Profile for jimh  Send Email to jimh     
What is the gauge of the wire that was used to connect the two batteries in parallel?
Jerry Townsend posted 05-11-2010 02:04 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
The wire guage becomes important only if one is starting an engine with the batteries connected.

Using a solenoid, as I mentioned above, the batteries are connected only when the alternator/generator is operating. With the engine not running, the batteries are NOT connected - but, if the engine is running, the solenoid is energized and the connection between the batteries is made.

Using the solenoid, the connecting wire between the two batteries (positive post - as the negative posts are connected permenantly) only has to carry the charging current - so a reasonable wire - something in the 10 - 12 guage range would work well. ---- Jerry/Idaho

sternorama posted 05-11-2010 10:52 AM ET (US)     Profile for sternorama  Send Email to sternorama     
jtms-as you probably know, now that you have the batteries connected in parallel, a long trip on the trollong motor could reduce the voltage on both batteries to the point where you might not have enough juice left to start the main engine.

And be careful using skinny wires when connecting batteries-they can get hot and start to burn!

jtms posted 05-11-2010 05:38 PM ET (US)     Profile for jtms  Send Email to jtms     
Jim H-not sure on the size of the wires. They are the same width as the original wires and are copper (the wide ones you get at WestMarine). I guess I need to add a solenoid. I will try and tackle that in the next week or so. I understand that running the trolling motor might kill the main battery that starts the engine. I don't really see that this is any different than running the radio all day and killing the battery. I would imagine a quick remedy for this would be to crank the engine up every couple hours to recharge both.

Right now, I can run the trolling motor off the battery, but there isn't any way to charge the battery unless I plug it into shore power. I guess I could leave it as is and find a shore power cord that fits the female end. It has a special two prong male end that I have not been able to find. Is this a Whaler specific housing? If so, anyone know where I could find such a beast?

Jerry Townsend posted 05-11-2010 07:03 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Your trolling motor will discharge your main battery (the one connected to your engine) - ONLY IF the batteries are permenantly wired in parallel. Using the solenoid eliminates this possibility - as the batteries are connected ONLY when the engine is running - which charges both batteries.

Another suggestion - install a voltmeter on your trolling motor battery - and when it drops to some reasonably low voltage (in the 8 - 10 volt range) - or your trolling motor starts to slow down or drag a bit, then start your engine - which THEN connects both batteries and charges both batteries - if you are using a solenoid. ----- Jerry/Idaho

whale posted 05-11-2010 07:11 PM ET (US)     Profile for whale  Send Email to whale     
I used [Blue Seas [automatic] charging relay. [An automatic charging relay] lets you charge one battery; once that [is] full, then relay kicks in to charge the other one. Mine came with a switch that lets me to connect the batteries just in case my starting dies off.
jimh posted 05-11-2010 08:24 PM ET (US)     Profile for jimh  Send Email to jimh     
As far as I know there is nothing specific about a Boston Whaler that would dictate a special plug is used for electrical wiring. Most electrical wiring on a boat uses standard marine plugs.

I suggest you visit the Blue Seas website and read about their automatic charging relay devices. An automatic charging relay would be a good solution to your problem.

As others have mentioned, with both batteries wired in parallel, you are at risk for discharging them both with the electrical trolling motor and stranding yourself when you cannot crank over your main engine.

If the two batteries you have connected in parallel are not of the exact same type, same manufacturer, and of the same age and general condition, you will find that when operated in parallel one battery will tend to be stronger than the other. The effect of the parallel wiring, however, will be to make the composite battery behave more like the weaker of the two.

jtms posted 05-12-2010 10:16 AM ET (US)     Profile for jtms  Send Email to jtms     
Thanks for the tips. I will check out the Blue Seas website.

Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.