Background: Researching dual battery systems to implement in my 4thGen. My setup will include the stock starter battery and a NorthStar AGM 31m I picked up from the recent GroupBuy.

I love all the options between straight solenoid, smart solenoids, auto voltage relays, smart isolators, etc. I like the concept of having a device that will cut-in and cut-out the batteries automatically based on voltage, but that price-point is much higher.

The deciding factor for me will lie within the answer to my concerns with the different types of batteries I have. My batteries are different chemical types and thus have different recharging profiles, and I'm wondering just HOW important it is to have one way charging flow from the alternator to the starter battery through to the aux battery.

I am specifically looking at the BlueSea ACR's / Littelfuse smart solenoids / enerdrive VSR's versus the Piranha isolators or similar.

What are your guys (girls) thoughts on charge rates with the AGM Batteries? Am I thinking too much on this? Is there any scientific evidence supporting one way or the other? Thanks.

possibly answered my own question:

according to this article on bluesea's website: https://www.bluesea.com/support/arti.../ACR_Operation

the ML-ACR's take care of the functionality i wanted: charging the starter battery before the aux battery in the event of a voltage mismatch (ie using the aux battery until it reads < starter battery voltage by some significance)

The ML_ACRs will top off / charge the starter up to a customizable preset, then connect the two batteries to charge / top off the aux battery. This reduces total load on the alternator (don't plan on swapping it for better until it goes 'tits up') and *hopefully* allowed the aux AGM to charge to full capacity.

anyone that still wants to chime in please do.

"How smart does it need to be"

One wire, and a fuse on each end.

Nope. With the setup you suggest the (AGM) battery will only ever charge to around 80%, it doesn't account for isolating the cranking battery from the aux battery and it doesn't handle charging the cranking battery first either. While it may 'work' this approach isn't valid as I purchased a battery that retails for close to $400 and would cause potential large loads on the stock alternator, among other life-shortening aspects.

AGM batteries require different stages of charging to properly reach full capacity.

Bulk charge: apply slowly ramping voltages to the battery just like most vehicle systems. This applies varying voltage and current to the battery until it reaches ~80% capacity. Stock alternator will accomplish this no problem.

Absorption charge: apply a constant, higher voltage (14.4v < typical < 14.8v) and slowly back off the allowed current to the battery until full. This is akin to topping off any container with liquid perfectly, without spilling.

Float charge: small top-offs over time, (13.5v < typical < 13.8v), akin to trickle charging. I imagine the stock alternator will also take care of this without any 'smarts'.


The Absorption charge is the complex part I'm curious if anyone is handling properly. Without this stage, I imagine the battery is used at only 80% capacity and cycles of charging are between 50% (whatever you drain it to) and 80%. AGMs like to be charged full from what I read, and why would you sacrifice 20% of the your available mAh? Considering how much these batteries cost, that doesn't seem worth it to me.

The main problem you'll run into is supplying 15-15.5V constantly with enough amperage to keep the AGM floating above 15V when in operation, the only way to do that reliably would be to have a custom alternator built with a voltage controller setup. And you would be at a risk of overcharging the AGM since you're walking a fine line. By keeping the charging voltage at stock levels you're only losing that small amount of extra AH capacity, which for any automotive use will most likely never be needed anyway when you're talking multiple batteries above the 70AH mark. The most important thing in an automotive application is keeping the batteries at a proper resting voltage, so a voltmeter is a must so you don't drop down into the mid 11's and risk shortening the life of the batteries. At most the only thing I'd run is an isolator if I planned on draining the aux battery way down, anything else is an unnecessary expense IMO.

In my Tacoma I had a simple dual battery setup. I used a regular starting battery and a regular deep cycle battery and had great results. The dual purpose AGM battery from Carquest was absolute junk.

Anyway for my setup, I used a 200a continuous duty solenoid wired in to a 3 way switch which was set up as Auto-Off-On. For Auto I got power from the DRL circuit which only supplied voltage when the truck was running. This way the truck started off the starting battery only, and once the truck was running both batteries were connected and being charged by the alternator. When the truck was off all my accessories and lights were drawing power from the deep cycle. The switch in Off meant the batteries were separated. 'On' meant power from the deep cycle would trigger the solenoid to join the batteries together which allowed me to jump start myself if my starting battery was dead. If my deep cycle went dead all I had to do was start my truck and then jump the connection on the solenoid to get it to start charging the deep cycle, which usually meant just bridging the connection with my pocket knife.

I'd like to see a pic/wiring diagram of your set up, if you don't mind.

I'm looking to do dual batteries, do the big 3 upgrade and install another fuse/relay block, to feed my couple of accessory lights, and a winch (still looking at winch options atm).

I saw the group buy for the smart voltage system awhile back, almost jumped in, but it seemed like more than I needed.

I'm planning on doing two of the same batteries, I'd just like to have one that can be isolated easily, so that if I'm winching for an extended period, I still have an isolated battery than I can use to either keep the truck running, or start it when I'm done winching.

I probably have the most basic system there is, and I love it.

I worked this up a few years ago for someone on here.
it works VERY well. my main reason for doing it was similar to yours. winching and heavy electrical draws.

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I came to the same conclusion that just linking the batteries will technically work but will likely lead to premature failure of the secondary battery, especially AGM. There are many battery isolators/links on the market and many people have had great success with them. That would likely be the best short term budget friendly option. But if you are willing to spend the coin, I would suggest using an actual charging system that can modulate the charging voltage and current appropriately for the given battery type. I decided to go with the CTEK D250S and Smartpass system. It has a few significant drawbacks but a ton of really excellent features. More details here.

Underfire, you're right, overcharge is the risky part. i see your perspective now. voltage controller setup would be ideal, i've found a few that will handle a proper charge for AGM, but at almost $300 I can just buy another battery when this one craps out.


still working on electrical drawings in CAD. i'm a chronic over-researcher and preparer for mods.
If you're going with two of the same battery and just want boost for heavy loads, then a straightforward isolation solenoid, like @nevada posted, will work perfectly.

Have you checked out something like this?

National Luna Dual Battery System Kit with Surface Mount Controller

my man.

i'm surprised i didn't find your build while tearing through the site looking into this. would have saved me a bunch of research. i like the setup. what are you using to make your diagrams? i've been doing mine in CAD and its total overkill.

i have. compared the luna and the IBS systems. both are great products but don't do much to help properly charge the AGM battery.

had i gone the route of a different deep cycle battery i would have just slapped one of these on and called it a day. thanks.

I've ran my Interstate 100AH AGM for around 3 years now, and it has yet to fail me once in the slightest. I was running a stereo that would pull my system down into the mid 12's while the engine was running for a while and also had it directly linked to a wet cell for about half of the time. At this point I've upgraded the starting battery to a Stinger AGM that I bought used and haven't had any problems with it either, I think AGMs are more robust than they used to be.

Sorry, I've had the setup for a while but only recently got around to the write up! Adobe Illustrator is my tool of choice for diagrams, haha. I mostly leave the CAD at work.

Let me know when you get your setup designed and installed, I'd like to see it.