Battery How To?

[cdpsrjc]

I want to add a second battery to my boat for all the accessories (stereo, depth finder, lights etc).



What do I need to do so my starting battery and accessory battery both charge off the alternator while under way?



When we're stopped, engine off, I want the acc/deep cycle battery to run everything on the boat and have the starting battery isolated from the load. I'm confused about isolators, and the mind boggling array of fancy expensive battery chargers on the market. What do I need (I'd like it to be automatic), and what should it cost? I expect trailers have the same issue. Solutions?



Richard

 

[HBarlow]

You might want to determine the charging capacity of the boat motor's alternator first. If it doesn't have the capacity to recharge two batteries under normal usage the idea may not be feasible.



If the alternator has sufficient charging capacity you will need a battery isolator. I think it will come with instructions for connecting the two batteries to permit charging both but discharging only one while keeping the starting battery in reserve.



Harvey

 

[Reddog1]

This gives me the opportunity to impress someone with my 12-volt automotive electrical knowledge.





If I have done this correctly, here is a couple of web sites that has information on what you are trying to do:



http://www.phrannie.org/phredex.html



http://bart.ccis.com/home/mnemeth/tech.htm (click on the “The 12 volt side of life”)





Both of these sites are great.



Let us know if you are impressed.



Wayne

 

[L_Rutigliano]

Adding a second battery is an easy modification to your boat. In my opinion you have three options:



The first thing you need to do is get a second battery box. Ideally, it should be mounted as close as possible to the existing battery to keep the cables short, but on my boat I added the 2nd battery to other side of the transom to keep things balanced. Remember, you adding 50lbs to the back of your boat, so if you already have a planing issue, this is going to make it worse. You can get additional cables at any marine (expensive) store, or any auto parts store. You can also get them custom made to the correct length at any battery shop. Don't forget to add convoluted plastic tubing to the cables if they are long and mounted across the transom to protect them from rubs and chaffing.



3 options:



1) Connect them in parallel, ie + to + and - to -. Advantages: very simple install and more battery capacity than single battery. Disadvantage: no isolation.



2) Connect a Perko battery switch. Advantages: still a very simple install w/added battery capacity. Isolation of starting and deep cycle battery. Disadvantage: need to manually select which battery to run off of, start engine with, charge, etc.



3) Connect an isolation module to your alternator (typically a Surepower module). Advantage: complete automatic isolation. Disadvantages: very confusing to install. The instructions are clear as mud. If you have a Ford motor (ie-Motorola alternator), forget about it.



I did option 1 for a year with no problems at all except it bothered me that I couldn't isolate. Then thought about option 3, but did option 2.



My $0. 02 worth.



Hope this helps.

 

[Prairie Dog]

lrutigliano, good info. The only thing I would add is that batteries have different internal resistance from each other, especially if they weren't made at the same time or are diff sizes. Because of this they tend to discharge each other fairly quickly in storage. Much quicker than a single battery will. One way to minimize this is to replace both batteries at the same time so their as close in resistance as possible. The other way is to use a battery isolator which tend to die at the most inconvenient time. You can also buy a switch that selects battery #1, #2 or Both. This is probably the most robust method for isolating.



Scotty

 

[bmoeller]

They have lots of different battery goodies- http://www.jcwhitney.com/catsubpic.jhtml?CATID=3111