Adding a 2nd battery considerations

[redhat]

I want to add a 2nd battery to my 2005 Tahoe. We plan to do a lot of 4 wheeling (ATVs), camping off the grid, etc. Truck is NOT our primary electrical source, however, the extra battery capacity would not hurt for different items.

Truck already has a place for it that can be populated with the 2nd battery and corresponding wiring.

I want to connect these in parallel, hot to hot, ground to ground, effectively doubling my system reserve and CCA -- well two batteries worth. I have seen some online who do this have isolators installed and am not sure why.

Am I missing anything here on my hookup? Do they have an isolator to potentially ALWAYS keep one battery Good/Charged if they accidently drain their main? I guess I don't see the need.

Additionally, that isn't any harder on my alternator, right? The way I'm interpreting this is that it would, in theory, just act like a bigger battery.

 

[DriveHard]

I added a second to my 2011 Silverado.

I see it done darn near everyday in off highway equipment without isolators. If you replace batteries in pairs with about the same manufacturing date, I don't believe you have anything to worry about.

I replaced an expense single battery on my Silverado with two $48 wal-mart specials...they lasted 7 years, and were still going strong when I sold it. No isolators, nothing special, just wired in parallel. I abused them a bit, with a 1500+ watt stereo and tailgating...still never an issue.

 

[atikovi]

Do they have an isolator to potentially ALWAYS keep one battery Good/Charged if they accidently drain their main? I guess I don't see the need.

Accidentally or if one battery develops a bad cell and dies. The weak battery will drain the good one to the point neither with start the truck. You don't see the need? You're in the middle of nowhere and can't start it would be a big need.

 

[mk378]

GM diesel pickups were fitted with two batteries at the factory. It's a very simple setup where they are permanently in parallel. A positive cable runs across the top of the radiator to connect the positive circuit. Each battery is grounded on its side of the truck.

 

[64bawagon]

Isolating it would depend on what you ultimately want it used for. If youre looking for general increase in capacity then whats been suggested works fine. If you want the battery for use when the vehicle is off and still maintain your first battery then thats another story.

 

[WobblyElvis]

As mentioned, you have not made it clear what you want to use the extra battery for. If you are afraid of accidentally running the battery down to the point of no start, you want the batteries to remain separated so you still have a starting battery. If you are trying to double the use of the battery for some light electrical use, you can still accidentally run them both down to nothing. A booster pack might be a good thing to consider.

 

[redhat]

I want a 2nd battery for additional electrical reserve, lights, maybe extended radio usage, nothing too crazy.

I already have a new 2nd battery. Also, I do carry a NOCO boost pack at all times. So I think running them in parallel would suit me fine.

 

[twX]

Additionally, that isn't any harder on my alternator, right? The way I'm interpreting this is that it would, in theory, just act like a bigger battery.

Two batteries will draw twice as much power from the alternator as they are charged. At a full state of charge, charging current will be low, maybe around 3-5 amps each for two 100 Ah batteries. However, at a low state of charge, the batteries will draw much more power, perhaps over 50 amps each, which could put a lot of stress on the alternator.

That said, one battery at 60% state of charge will draw about the same charging current as two batteries each with 80% charge, so the alternator will only be more stressed if you are really deeply discharging both batteries.

If you plan to deeply discharge the batteries, you might want to instead just bring along a second lithium or deep-cycle lead acid power battery when you think you'll need it. A portable battery booster could fill this role while also allowing you to jump start the vehicle. Installing a second battery in the vehicle will add unnecessary weight, and the additional charging current will cause a slight drop in fuel economy.

 

[Fitter30]

Battery isolator is needed. They come in different amp ratings and price ranges.

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[dogememe]

I had dual batteries on several older GM trucks over the years. A 99 Suburban and a 77 Suburban. Just simply connected both positives together with a fat cable and grounded the second battery to the chassis. Never had any alternator issues.

 

[DriveHard]

I had dual batteries on several older GM trucks over the years. A 99 Suburban and a 77 Suburban. Just simply connected both positives together with a fat cable and grounded the second battery to the chassis. Never had any alternator issues.

They are made just for this.

 

[Chris142]

Only real negative I see is that if the batteries got ran low the alternator will take a very long time to bring them back up.

 

[atikovi]

GM diesel pickups were fitted with two batteries at the factory. It's a very simple setup where they are permanently in parallel. A positive cable runs across the top of the radiator to connect the positive circuit. Each battery is grounded on its side of the truck.

I remember having one and buying a new battery for it. 6 months later I found out if I let it sit for a week, it wouldn't start. The older battery still in the car had a bad cell or something, and was sucking power from the new one.

Also have a limo with an auxiliary battery in the trunk. It's somehow isolated from the car's main electrical system but there is a switch under the dash I can press to connect the two. I put a new cheap 26 battery in the trunk and drove it for over a year with a bad main battery that couldn't start the engine by itself when cold.

 

[eljefino]

The best way to do this is to buy two new batteries simultaneously of the same brand so they match each other in discharge characteristics.

The best reason to add a second starting battery in parallel is to get more cold cranking amps. Your usage implies deeper discharges for which you should get marine/RV style batteries to match.

My GMT900 cuts the radio out after umpteen minutes on ACC, IIRC.

My RV had the isolator, and a little "instant jump start" button in the cab that momentarily connected them if necessary.

You also have the possibility of adding solar to your little bug-out rig. A 2x4 foot panel is good for 100 watts, which is substantial.

My concern for your alternator is if you're plodding around at low RPMs and low speeds off road, not getting air through the hood to cool it off. Your mechanical (?) fan should help a little, but its shroud probably doesn't line up with the alternator specifically.

 

[Donald]

So my F250 PSD has two batteries in parallel mainly to start the diesel engine in the dead of winter.

Now my boat has two batteries, a starting battery and a house battery. Battery switches. And a battery "combiner". It connects the two.batterirs when the alternator raises up the voltage to be a charging voltage.

A battery isolator is an old way of handling two batteries and there is a 0.7V voltage drop using the isolator due to a silicon diode.

Your setup should be more like a boat than a two battery truck.

 

[meep]

I wouldn’t worry about alternator fears…. Even in parallel the batteries won’t absorb huge amounts of current at normal running voltages. Even with a single battery, whatever you use, the alternator will put back. Your truck probably has a 160amp or so unit, it’s not going to break a sweat.

I would avoid the diode-based isolator, as you’ve got to run a separate voltage sense wire to the ECU, or whatever is sensing the determined battery voltage. The diode type isolators lose about 0.7 volts, which is a lot when it comes to charging strategy.

a manual battery join switch like in marine use would be easy and effective. Leave it in “both” under normal use, then isolate the starter battery when parked and run the toys off the secondary battery. Jump back to both when it’s time to head out.

or, use a high amp contactor (relay) to bring them to parallel when the ignition is on, and drop one off when it’s not.

 

[JHZR2]

GM diesel pickups were fitted with two batteries at the factory. It's a very simple setup where they are permanently in parallel. A positive cable runs across the top of the radiator to connect the positive circuit. Each battery is grounded on its side of the truck.

That’s how my dodge Cummins is set up. IIRC it is 1/0 from battery to battery across the radiator (each grounded to the body on their side), and then 2/0 to the starter.

For OP Id probably consider going the opposite way, leave the OE starter wiring, but get the next size larger wire and use that for the cross-connect. If the OE setup is reasonably priced it may be a good option, otherwise get a decent crimper and lugs and DIY. Not hard…

An isolator is often used to allow a starter and hotel battery to exist in tandem. While it sounds like OP may like a hotel battery, I’m not sure running it that way is the best bet.

 

[tessaseveride]

An isolator along with moving all accessories to the secondary battery is what I would do.

 

[Elkins45]

My 98 Chevy has had an intermittent phantom drain almost since it left the factory. I installed a second battery but put a switch on the ground connection so it stays isolated. Letting the phantom drain both batteries defeats the purpose of the second one.

About once a month I switch it on and let it charge while driving to keep it functional, then switch it back off when I get home. This is the switch I used: https://www.autozone.com/ignition/switches/p/hella-battery-main-disconnect-kill-switch/341155_0_0

 

[BHopkins]

I'm trying to understand how two batteries would put more load on an alternator. Unless I'm missing something, the only time that the alternator would see more load is if the batteries are drained quite a bit, while the engine is not running, so the alternator is not charging. When the engine was started, then there would be a big pull on the alternator.

But under normal circumstances. the load on the alternator should be no more than when one battery was in use.

For me, I don't see much advantage to the second battery, unless it is isolated, to give you protection if/when the primary battery is dead/drained. That is, unless you are looking to run a lot of accessories when the engine is off. If that is the case, then I would think you probably need to look at getting a bigger alternator, so you can bring the batteries back up to full charge again.