Category Archives: Boat Battery Wiring

test post


this is a test post about one of my products

Memory Issue

Hi Kevin,

I just purchased a 1986 Chaparral 235xlc, from a guy in Maryland. It was a salt water boat powered buy a 350 Chevrolet with a SEI (alpha) outdrive.

Alpine stereo

The problem...

I put a new battery into the boat, put it into the water, running for about a hour max. Took boat home, checked battery voltage...12.28 volts. Went back a few days later, battery was stone dead...4.3 volts.

Upon looking in the bilge area, I see that a lot of the boat wiring is just cut off, with no idea of to where they were coming from. I also noticed that there is five extra small wires attached to the battery terminal. I would like to trace their locations and remove them if possible.

To solve the battery drain, I added a battery cut-out switch to the boat.

Is there any way of getting a wiring diagram for my boat? Any idea what would be draining the battery in my boat?

Thank for the help,


Your best bet for a wiring diagram would be directly from Chaparral. Unfortunately, the problem probably has nothing to do with the original wiring.

The usual suspect for a battery drain is a stereo memory. I would trace this first. After running for an hour the battery voltage should be well above 12.28 volts.

It is possible that the alternator is not functioning properly. It may not be charging and also creating the draw down when the engine is off.

Hope this helps

Switch Switches

Good morning Ez,

Blue Seas Boat Battery Switches

I’m interested in installing one of your Smart Boat Battery Switch Systems. Could you please tell me if it would work with my current battery switch? I have a dual batter that was built by Blue Sea Systems. It is their model# 9001E.



The system would replace your existing 9001 switch, but the change comes with a big advantage.

Instead of having a 1/2/both/off switch you would have an On/Off with the ability to emergency combine. This means that you can still start the boat if you should run down the starting battery. But, by losing the "BOTH" option from the switch, it makes running that battery down much harder to do.

Hope this helps,


Boat Battery Isolator or VSR?


My boat wiring question is simple.

Voltage Sensitive Relay or a Battery Isolator?

I have a 2004 PolarKraft 165SC with a single 50 HP Yamaha 2 stroke. I also use a Guest 2-bank onboard AC changer. I really hate having to drag my accessory boat battery out to recharge it when away for a two week vacation in Canada. so one or the other seems to be something I could really use.

Boat battery isolator and VSR

The VSR seems to be a more complex solution - at least from what I've seen on the net. The boat battery isolator seems to do the same thing quite simply.

I've seen some isolators that have large heat sinks and some that don't (which are smaller and more expensive - but reviewers say they are better and it's worth it..) I think I just saw something about an ACR - is that another option?

Can you offer any advice ?? I've read a lot of your online advice and I'm still not clear on this. I think I read that this idea is not applicable to my boat? Also saw a nice diagram of the wiring, but you used both a VSR and an Isolator. I'm confused.

Have I oversimplified the issue/decision, or perhaps I've missed something and it's more complicated.


I would definitely use a voltage sensitive relay system over an isolator in your application.

The VSR would connect between you battery's positive posts with a ground cable connecting the negatives. When either battery goes above 13.7 volts the VSR engages and both batteries are charging.

If you used an isolator you would need to first isolate the alternator output wire from you outboard engine. Usually this is difficult. It is not just the positive battery cable. Since the alternator on the outboard is connected directly to the battery, if you added an isolator you would need to account for the voltage drop induced by the isolator. This could be as much as 1 volt and will cause your batteries to not fully charge.

An isolator is much easier to install on an inboard engine. The alternator wiring is much easier to access.

As for the confusion in terminology, BEP Marine calls their device a VSR. Blue Seas call the device an ACR. Both do the same thing and connect the same way.

Hope this helps

Small Pontoon Boat Battery Wiring


Hope this is an easy one!

Playbuoy pontoon

I just completed a pontoon rebuild and want to have this setup for the boat wiring:

  1. Starting crank battery – hopefully charged/maintained by my 110 outboard
  2. House deep cycle batteries – hooked to an onboard charger for when AC is available


  1.  I want to have the option to utilize my two house batteries as a backup in case my starting battery fails. What kind of battery switch/setup would work best for this?
  2. For the two house batteries, do I connect them in parallel for best reserve use for my 12 volt accessory needs?
  3. If I do this, what kind of onboard charger should I get if I wanted to maintain all three of my boat batteries? Two bank? Three bank? Something else altogether?

Any other advice you could offer would be great. I have plans for a marine electrical shore power system as well, so keep that in mind if it changes your answer.

As with my previous projects, I plan on getting all my stuff from EzAcDc, so if you want to provide part numbers, all the better!

Thank you,


Any idea the charging output of your 110 hp motor?

Also, are the house batteries also going to run a 24v trolling motor or strictly 12v stuff on the boat?



Hi Kevin,

No trolling motor for now. Just 12v accessories.

I have been told that 6 amps is the output of my outboard.

Also, I wanted to add that I thought I would start with one house battery and add another later. My concern would be that the batteries would drain one another.

Thank you,

Hi Bryan,

I would use our manual boat battery switch system.

Batteries 2 and 3 will be wired in parallel to increase your reserve capacity, but keep the voltage at 12 volt. These would be connected to the BAT2 position on the battery switch.

With only 6 amps of available output, your engine will never fully recharge your 3 battery bank. While running long distances, you can put the battery switch in the BOTH position to charge all three but when the engine is not running I would turn it to any other position than BOTH.

Since you will basically have two batteries, one big and one small, you can use a 2 bank boat battery charger.

Size the charger based on how long you can wait to recharge your bank. If you have 2 batteries that have 100 Ah of capacity connected in parallel, you have 200 Ah to recharge. Most of the time you do not run batteries below 50%, so you would realistically be replenishing 100 Amps. A 5 amp charger would take about 20 hours to recharge. A 10 amp charger would only take 10 hours.

Hope this helps,


Boat Battery Cable Question

Dear EzAcDc,

I am planning on buying the Guest dual boat battery switch plus the wiring for my application.

Dual boat battery switch system

I have two questions before I put in my order:

  1. I am running the boat battery wiring from the switch to my two batteries. The boat is a 16 foot with a 50 HP Johnson. The batteries are a standard starting and deep cycle trolling (620-720 cca). From the switch I am running two 3 foot cables to the positive terminals and one 2 foot to connect the negative terminals battery to battery. In this application would you recommend 4 AWG cable?
  2. I am ordering the switch and the cables separately, will I still get the installation instructions?

Thanks for your help!


Based on your description, your battery cable lengths will be fine using 4 AWG wiring.

We try to get instruction sheets to everyone we work with, but we occasionally screw up. Just in case, here is a copy of the boat wiring for the battery switch system.

Hope this helps,


Best Battery Location

Hi Kevin,

I just placed an order for your Automatic Smart Boat Battery Switch. It seems to be exactly what I need to manage my batteries.

boat battery lugs

I do have one concern. In looking over the boat wiring diagram for the system, it appears that in your drawing the batteries are fairly close together. But, in my boat the cranking battery is near the motor and my trolling battery is closer to the front of the boat - about twelve feet away.

I’ve been told that the charging batteries should be close to the charging system. Is this true?

Can I wire the system in a way to keep my batteries in their current configuration? What size of wire would I need to do this? Or, should I try to re-arrange and put the batteries closer together?

Thank You,


A big concern in marine electrical is circuit protection for the battery connections.

In your case, the safer approach would be to put the batteries near each other in the stern, use the provided 2 AWG battery cables, and increase the length of your trolling motor leads to reach the rear of the boat. Then install the trolling motor circuit protection at the battery connection.

Hope this helps,


Nothing But The Reel Thing


I bought one of your VSR based automatic smart boat battery switch systems a little over a year ago. It was an easy install, works great, and really makes using the boat a lot more fun.

Shimano electric reel

I am now considering adding a second house battery to my boat. It will be hooked up in parallel and will be used to help power an electric reel.

The reel is a very powerful one and has a big power draw. Will there be any problem adding this to my marine electrical system along with the automatic smart switch?

Thank you


The smart switch will work great for this application.

You will want to make sure that all batteries are the same type. Lead acid, agm, gell, etc.

Hope this helps


Smart Switch Chatter?


I have a 1995 Chapparal 1930 SS Aniversary Edition with a 4.3LX V6.

Chapparal 1930 with wakeboard tower

We are adding 3 mile speakers to the ski tower and a high powered amp will be included. We need to add a second battery for the stereo and I want to use your Smart Boat Battery Switch Systems.

However, on another web site I saw customers asking questions about why their VSR was chattering and the answer was the charging system is too small. Are you familiar with this scenario and if so?

Please provide me with your thoughts on this.



Great question. The answer is that you shouldn't have any problem.

Here's why....

The brains of any good automatic smart battery switch is the VSR - and first generation of these from 5 years ago were terrible for chatter. It didn't matter how big your alternator was. With a large enough amp you could easiler make the vsr cycle or chatter several times per second.

The current vsrs need a low voltage for ten seconds to cycle which eliminates the chatter. Any alternator larger than 20 amps works great.

Hope this helps


Don’t Do It.

Hello Ez,

I’m looking to replace some corroded boat battery wiring cables on my boat that has a 260 Mercrusiser, i.e., a Chevy 350.

Right now my positive cable is three feet long and the negative cable is just over six feet.Don't do it.

The boat’s starter was submerged many years ago over the winter, so it needs good voltage to kick over on cold mornings, otherwise it’s fine. Plus, my cables are a bit corroded so that’s not helping.

I’m going to keep a HUGE battery in the boat, 1000 cca, and would like the cables to be able to pass that current with minimal loss. I’m thinking a 2/0 AWG is the way to go. What would you recommend? Also, what size connector do I need for my starter? Is it 3/8”.



Based on your description, I believe that something in this marine electrical system is bound to fail. Because of that, I’m not comfortable making any recommendations except that you find a good marine electrical specialist in your area.

I hope you understand. Good luck.