Wiring battery in "aux" position to be starting battery...

[joneseyyy]

I've searched all around and haven't seen this covered. Here's the deal, I currently run a Sears Die Hard Platinum Marine Group 31 AGM massive battery as my only battery. I am getting ready to install the IBS dual battery kit and aux battery. I have to go with a smaller Die Hard AGM battery to fit the aux tray. I'd prefer to use the smaller battery in the aux tray as the starting battery and leverage the massive capacity of the group 31 as my aux. I looked around and the stock + cable is bundled in with a bunch of other stuff, so this seems like it may be a total pain to replace completely. Is it possible to splice the OEM cable longer? If not (or not a good idea), how the heck do I go about replacing the OEM + cable seeing how it is bundled? Thanks.

 

[pawzitiv]

The easy answer is to order the ABR-Sidewinder Prado 150 cable kit and the DBi-140R battery isolator. The installation instructions cover 2 options; keep the OEM battery position as the start battery OR set-up the second smaller battery as your start battery.

I'm no automotive sparky but I'll hazard a guess that splicing battery cables is not a good idea.

 

[Titanpat57]

Why do you need an isolator? Couldn't you just hook the two together without consequence (say at the very worst, run both down?)...I mean if one battery runs the fridge for 3-4 days without starting the truck...two would give you at least 4-5 easy.

My guess is you would start the truck in that time frame, even if just for a half hour a day, which I think would just be a good practice. I'm not sure if I understand all the expense of isolators/monitoring equipment.:shrug:

Not being a wise ass..just curious...

 

[joneseyyy]

Why do you need an isolator? Couldn't you just hook the two together without consequence (say at the very worst, run both down?)...I mean if one battery runs the fridge for 3-4 days without starting the truck...two would give you at least 4-5 easy.

My guess is you would start the truck in that time frame, even if just for a half hour a day, which I think would just be a good practice. I'm not sure if I understand all the expense of isolators/monitoring equipment.:shrug:

Not being a wise ass..just curious...

For me it is more of an insurance policy than anything, even though I have been thrilled with the performance of the group 31 as a standalone, I want more capacity/redundancy, as in the next month or so I am adding a Rigid light setup and maybe an extra inverter. Add to that my winch (which if I need to use it can suck down a battery). That and I like new toys, duh!

 

[OrangeTexan]

In theory you CAN wire the two batteries together, and in practice, if you never really discharge them much beyond using them to start the truck or just a few minutes of light use, you might not see any negative effects.

The problem is that the two batteries are dissimilar, in particular the charge/discharge capacity are different. Because of this, as they discharge, their internal resistance will differ from one another, and their corresponding voltage will differ. Since they are wired parallel, they will try to "equallize", meaning the one with the higher voltage will charge the one with lower voltage until their voltages are the same. This kind of cross current between the batteries can have a significant impact on the battery life for both batteries and will also affect their performance during duty.

Additionally, as the alternator starts to recharge the batteries, if they both do not require the same amount of charge to reach full charge at the same time, the battery that reaches full charge first will still accept a small amount of charge while the other charges. However, as it continues to overcharge, its resistance will exponentially increase, and generate a lot of heat. This of course can alter the long term performance of the battery and shorten its life.

If one of the batteries ever becomes damaged enough such that it is effectively shorted, it will draw all the charge it can from the alternator. Even though the other battery may be enough to sustain the vehicle, this becomes a safety issue as this this can generate a significant amount of heat in the damaged battery and can cause a fire.

You could run 2 batteries of the same make and model together (i.e. two Sears Die Hard type 24S batteries), but as they age and their internal chemistries and resistances change, you will run into the same problems.

The isolator works such that each battery will discharge separately (unless you have the type of isolator that allows you to manually switch both batteries together for emergencies only), and allows the alternator to charge each battery separately, so neither battery stands a chance of overcharging. The benefit of the isolator then, is safety, and battery life and performance. The type of isolator you choose depends on what type of duty you expect and the flexability and features you would like in your system.

 

[Titanpat57]

In theory you CAN wire the two batteries together, and in practice, if you never really discharge them much beyond using them to start the truck or just a few minutes of light use, you might not see any negative effects.

The problem is that the two batteries are dissimilar, in particular the charge/discharge capacity are different. Because of this, as they discharge, their internal resistance will differ from one another, and their corresponding voltage will differ. Since they are wired parallel, they will try to "equallize", meaning the one with the higher voltage will charge the one with lower voltage until their voltages are the same. This kind of cross current between the batteries can have a significant impact on the battery life for both batteries and will also affect their performance during duty.

Additionally, as the alternator starts to recharge the batteries, if they both do not require the same amount of charge to reach full charge at the same time, the battery that reaches full charge first will still accept a small amount of charge while the other charges. However, as it continues to overcharge, its resistance will exponentially increase, and generate a lot of heat. This of course can alter the long term performance of the battery and shorten its life.

If one of the batteries ever becomes damaged enough such that it is effectively shorted, it will draw all the charge it can from the alternator. Even though the other battery may be enough to sustain the vehicle, this becomes a safety issue as this this can generate a significant amount of heat in the damaged battery and can cause a fire.

You could run 2 batteries of the same make and model together (i.e. two Sears Die Hard type 24S batteries), but as they age and their internal chemistries and resistances change, you will run into the same problems.

The isolator works such that each battery will discharge separately (unless you have the type of isolator that allows you to manually switch both batteries together for emergencies only), and allows the alternator to charge each battery separately, so neither battery stands a chance of overcharging. The benefit of the isolator then, is safety, and battery life and performance. The type of isolator you choose depends on what type of duty you expect and the flexability and features you would like in your system.

Thank you for taking the time to explain that..

 

[deanyuan]

I know this is an older thread, but wondering if you had figured this out. I'm starting my IBS dual battery, and had the same thoughts as you as making the passenger side battery the main starting battery, and putting in a larger battery as the aux on the driver side.