Wiring batteries in series AND parallel

[dougw1515]

have a loaner boat that has a 36v trolling motor. The batteries are rigged in series in the rigging compartment. I'm having "issues" with the cranking battery - sorta. It has never failed to start the engine but @ 14 miles up the river or 20 miles down lake... I don't wanna take that chance. The batteries are charged every time I return and prior to heading out for a tournament. Then they are charged every time the boat comes off the water. But.... With two livewells going 8-10 hours a day the cranking battery is getting hammered. On my GPS/Sonar display I've seen as low as 9.5v. It's not really that low but close. From time to time cranking the engine kills the electronics. Sooooo.... I'm trying to devise a rather simple solution. I want to move everything off of the cranking battery but the engine. My plan "was" to wire the 3 trolling motor batteries parallel and move all the accessory wiring to the trolling batteries.



When I tired to implement that plan I was attempting to hook positive to positive but generated a rather healthy spark(s) in the process. Disconnecting the trolling motor plug solves that but... what happens should I wire batteries in parallel that are also wired in series. Heck is that even doable without blowin' something up?



As a note there were small sparks coming from the loose wiring lug even before attempting to hook the batteries up in parallel.



Long story short. I'm wanting to connect three batteries in parallel that are currently wired in series. Can I do this? Just as an aside what is the battery charger going to think about this set-up, if it's do able?

 

[Barnacle_Bill]

Re: Wiring batteries in series AND parallel

Yes you can rewire them in parallel. You probably got a spark because you didn't disconnect everything first. Your on-board charger will still work as wired if all the leads are for 12V batteries. Keep in mind that if you do this your trolling motor WILL NO LONGER WORK. A simpler solution would be to carry a set of jumper cables just in case you need them or install a battery selector switch connected to the starting battery and one of the TM batteries.

 

[dougw1515]

Re: Wiring batteries in series AND parallel

I can't "re-wire"... was wanting to do parallel AND series.

I've been diddling around with a multi-meter and it appears if I reverse the polarity of the leads I'm wanting to move to the trolling batteries I can leave it wired in series(36v) and still utilize the capacity of two out of three of the trolling batteries.

By saying I'm reversing the polarity... Between two batteries I can still get a 12v reading but... appears that the polarity reverese due to them being wired in series. Such that I'd have to put the negative leads from the accessories/pumps to the positive post of one battery and the positive leads from the accessories/pumps to negative post of another battery. When I connect the volt meter to the two seperate batteries I get a reversed polarity reading if I have the positive probe to the positive post and the negative probe to the negative post.

 

[RickJ6956]

Re: Wiring batteries in series AND parallel

You can't get 36 volts from two batteries, no matter how they're wired.

There is no reversal of polarity. There is only "reference to zero volts", which depends on where you put your meter leads.

Black lead to the negative of the 1st battery and red lead to the third battery = 36 volts. Move the black or red lead to the second battery and you'll see 24vdc. Any one battery will read 12vdc. This is exactly how you can use them as 36, 24, and 12 volt supplies. Charging with the alternator is not possible, though.

I would buy a fourth battery. Use one for cranking the engine and keep it separate from all other systems. Connect the other three in series = 36 volts. Use the one that's connected directly to ground as a 12v "house" battery. Connect a three-bank charger to the batteries as if they were standalones. Let the engine charge the cranking battery.

If for some reason the engine battery ever drains use jumper cables to one of the others.

 

[dougw1515]

Re: Wiring batteries in series AND parallel

Well aware I can't get 36v from 2 batteries....

The 36v connection is made at the trolling motor plug. 24v is max in the rigging compartment.

As stated in my original post this is a loaner boat. It's a Gambler 2007 Intimidator - there is absolutely no room for an additional battery. Making/adding anything that cost more than a few bucks to implement isn't something I'm willing to do.

The polarity may not be reversed but... when I connect my test leads from one batteries positive post to the adjacent batteries positive post I get a 12v reading. Also.... with the black test probe contacting the negative post of one battery and the red test probe contacting the positive post of another battery I get a reading of -12.36v. If I reverse the leads the reverse polarity indicator goes away.

 

[Silvertip]

Re: Wiring batteries in series AND parallel

You cannot have simultaneous parallel and series connections in the 36 volt system. You can however has separate 12 volt and 24 volt feeds off that system. From what I understand of your description, you are attempting to power accessories from the troller system to save the starting battery. Below is a 36 volt series system -- pretty basic. Below that is a 24 volt troller system with a separate 12 volt feed. You can do the same thing with the 36 volt system. For safety purposes, always take the 12 volt feed from the same battery that has the 36 volt ground cable attached to it.

That 12 volt feed can be interconnected with the starting battery if you wish but since isolation is what you are after, simply move the "boat harness" from the starting battery to this 12 volt feed. Be sure to move both the +12 volt and ground sides of that feed.

 

[dougw1515]

Re: Wiring batteries in series AND parallel

Thanks SilverTip n' everyone else that responded...

I was hoping to be able to at least utilize 2 of the 3 trolling batteries but it's pretty self-evident that that ain't happening... I was aware of the 12v feed option but looking for a way to distribute the load a bit more over the trolling batteries.......

Thanks again!

Doug W.

 

[Silvertip]

Re: Wiring batteries in series AND parallel

Nope, the minute you try to share a ground with the other batteries things go POOF. Again, what you can do is drop 12 volts off each battery just like the the example. BUT -- there had never ever be any chance that any and I do mean any of the components or any part of the engine electrics get shared. Again I use the example of a stereo head unit on one battery and an amp on the other. both will power up nicely but the grounds on the cables that interconnect the two devices will create the dreaded short that ends your day and toasts your equipment. Take a good look at the 12 volt drop from battery #1. Then mentally draw the same two lines on battery #2, You will see that the ground on battery #2 is also +12 volts on battery #1. I hope you understand the ramifications.

 

[dougw1515]

Re: Wiring batteries in series AND parallel

Yup... I got it. I just moved everything but the engine leads to the tolling battery that provides the ground for the 36v system(which is battery #1 in your 36v diagram) and gave up trying to spread the 12v load across multiple batteries.