Do duel batteries need a bigger alternator?

[jwill226]

I ordered a full wiring harness for my 78 k10 350 from this site and while I'm waiting on it to come in the mail I'm planning things out.  The truck has 2 battery trays but the PO only used one.  I'm going to take this opertunity to replace all the battery cables and drop in 2 fresh batteries and keep the other as a spair for an unrelated project.

I have never added a battery to a system.  I understand how to wire it and not to do it backwards and make a 24v system.  My question is will there be any extra load put on the alternator by doing this because it needs to keep 2 batteries charged?

I don't plan on using tons of high power items.  I have a 100w sub behind the seat already (no plans to go bigger)and later I plan on adding plenty of offroad lights.  All of them will be LEDs.  200-300w for driving (engine running) at the most and maybe up to 150w worth for lighting up around the campsite/work area as needed (engine off).  The lights would see minimal use but on the days they are used it could be for hours at a time.

[Irish_Alley]

not 100% but you might hurt the starter. you will be putting double the amps to the starter. the charging shouldnt be a problem since you wont be charging 2 dead batteries, think of it like jump starting another truck. yes you will make the alt work harder to bring the other battery up to charge but once your there you really wont be using the battery for anything since your alt will be putting out enough power to power everything. but most people with amp will just put a bigger alt in and be done. and thats all i would do unless you find the alt cant keep up with everything

[jwill226]

Thanks.  I didn't think about the starter.  Is there any way to protect it?

[bd]

The starter will not be affected by a second battery.  The starter draws the current it draws, whether connected to one or 20 batteries wired in parallel.  Adding a second battery will increase available cranking time, depending on how the auxiliary battery is connected into the existing circuit.

• The second battery can be cabled to augment cranking by connecting directly in parallel with the existing battery, w/o an isolation circuit, or
  
  
• The auxiliary battery can be isolated through a relay or diode isolator, so that accessory loads tied to the auxiliary battery will not discharge the primary cranking battery.
  

But, there are specific details that must be considered to enjoy trouble free success with either approach.

An alternator will work harder with additional batteries and charging will take longer.  Depending on which approach you choose, alternator wiring may require a subtle, but important change.

[bake74]

     As you stated, if you plan on using lights and acc's around the campsite, I would suggest bd's second method.

The auxiliary battery can be isolated through a relay or diode isolator, so that accessory loads tied to the auxiliary battery will not discharge the primary cranking battery.

     This is how I did it on all my off road rigs while my children were growing up and never had to get a jump from somebody.  I on the other hand had to jump many a stranger and friend.  The best part of this way is if your drive back from camping is long enough you recharge the battery by the time you get home.