[UPDATE: The D250sa seems to have been replaced by an updated version and is no longer available on Amazon. The new D250se that seems to be replacing it, is now compatible with Lithium Batteries. I will update the links to the new D250se throughout this article.]
If you are finding this article, there's a good chance that you have probably researched your way into a fantastic migraine trying to understand the ins and outs of a dual battery system. What battery do I use? What isolator? What gauge wire is appropriate? And at the tail of each of these questions, countless forum posts containing technical jargin, debates, even scientific theory. It's a lot to take in and it's easy to blow a mental fuse. -Pardon the pun.
I was ready to beat my head against the wall. Until I stumbled across the Ctek D250sa DC to DC charger. I skimmed the downloaded user manual, looked at the diagrams and charts. Suddenly, it all became so simple. The instructions were clear, the chart showed you clearly what gauge wire to use, where to put the fuses, how to connect the wires. And once I walked myself through the setup. Boom, the system was charging my secondary house battery just like that!
The three major components to my setup, 92 amp hour deep cycle AGM battery, The Ctek D250sa, and a 500w inverter that can handle all of my battery chargers and laptop.
I'll try not to bog down into too much detail on dual battery setups in this write-up but here's some of the basics. Essentially what you want out a dual battery setup, is a system that allows you to have auxiliary power for charging devices, camp lights, running a camp fridge etc, without waking up in the morning with a flat battery and no way to start your vehicle. So this system needs to isolate the auxiliary battery or "house battery" from the vehicle's starting battery, so that no matter how much you drain your aux. battery, the car will be able to start. In the old days this was done with a manual switch that connected and disconnected the batteries. Then a device came along called the VSR (Voltage Sensitive Relay), these devises are still very popular today however most modern vehicles say 2005 or newer can not use them due to modern alternators not producing enough current consistently to charge two batteries. What a VSR does, is sense the voltage of the vehicles starting battery and open and close a relay automatically connecting and disconnecting the two batteries when the primary battery is at a safe amount of charge. These devices are great, affordable, and they work. But they don't work all that efficiently in all cases, and like I said they will not work well with newer vehicles. The primary issue is when using a VSR to charge an AGM (Absorbed Glass Mat) style house battery. These batteries are fantastic, they are tough, can handle vibration, they are sealed so they won't spill the nasty battery contents out if tipped over or mounted vertically, and they don't vent gasses under normal charging conditions. BUT, they require a specific charging profile that a vehicles alternator just isn't smart enough to do. The vehicle will leak a steady stream of power to the battery, but an AGM battery that is low on charge needs a surge of power to "wake it up" in essence so that it will be able to accept a charge more easily and efficiently. Now, don't get me wrong, an AGM will charge with a VSR setup. But not as fast or efficiently.
My setup installed in the passenger foot well of my Subaru, yes the battery box is ventilated through the firewall of the car.
So what's the answer? Today amazing devices called DC to DC chargers are on the market. I suppose the way to think of them is a smart battery charger like you would have in your garage, but it's wired into your vehicle. It knows the charging profile of different battery types like AGM's. It also acts as an isolator between the vehicle's starting battery and the aux battery. In the case of the Ctek D250sa which I am highlighting in this review, it can even act as your solar controller if you are charging your secondary battery with a solar panel! And it automatically will choose which power source is most efficient at the moment in real time. So you could be driving on a bright sunny day, and the Ctek will start pulling the charge from the solar panels instead of the alternator if it can get a better charge from the panels. You are always going to be getting an efficient proper charge to your deep cycle AGM batteries with this charger.
EDIT: Ctek now has a model of this charger that is compatible with lithium LiFePO4 batteries. The Ctek D250se
Cost. Yes, DCDC chargers are more expensive than a VSR, the Ctek D250sa sells at around the $260 to $300 mark on Amazon. But this really is the best, most efficient way to keep your secondary battery charged. And it really does simplify the whole process. After following the instructions, building and routing the cables in my Subaru, I had a working dual battery system in just a couple hours. And from there you add on your accessories like an AC power inverter, water pumps, fridge etc.
One important thing to mention is, if you are running a relatively large power bank, ie multiple batteries or a couple hundred amp hours worth of battery capacity. Ctek also sells a device that you add onto the D250sa called the Smart Pass. Essentially this "boosts" the charging performance of the D250sa allowing it to charge larger capacity batteries faster. Otherwise it would take a heck of a lot of driving to top off a large power bank like that. The smart pass itself is about the same price as the D250sa. Around $290. You are talking about a fairly significant investment. But for those of you that are living the RV or Vanlife full time and depend on your power supply heavily, it would probably pay to have things done simply, efficiently and reliably.
In summary, I've been using this system for a few months now. I have yet to see any failures, malfunctions or errors. My Subaru has started for me every time, no flat batteries. I can keep my phone, cameras and computer charged on all of my trips and adventures. The Ctek system has performed flawlessly, and I could not be happier with it.