DC/DC charger + Solar musings

[wandererr]

So I have a 115AH house battery that primarily runs the fridge, solar and an 8AWG going back to the main battery (I'm using an old run that I had for the newly added house battery).

Just recently I have learned that connecting the power from the main battery or the alternator directly to the house battery is not the most efficient as the alternator might be dropping its output on as needed basis and might not recognize the house battery draw as needing "charging" but rather will see it as another "draw". To combat that, there are DC/DC charger, but the least expensive one I found was Renogy in Amazon Warehouse for $100 (20A model). It is said that you set it up for your type of a battery and it takes variable DC input (from 9V to 16V) and optimizes the output for charging.

So now I'm wondering - couldn't I use the solar charge controller to do the same? The solar charge controller has circuitry that adjusts the charging voltage/current to the battery type that is connected, it deals with a wide range of inputs and let's face it - my charging input will never be below 12.8V coming from the alternator. So could I use it as my DC/DC charger? Any issues there? Am I missing something obvious?

Oh yeah, when parked my solar charger controller would be fed by the 160W panel that I have.

 

[LivinDaDream]

You'll have to check your manual, but solar charge controllers require a minimum voltage above their charging voltage to be able to operate and provide the required charging voltage to the battery...typically 15V or more, if my memory serves me correctly. You're unlikely to see that high of a voltage from your vehicle charging system.

Depending on the vintage and model of your vehicle, the charging system can be pretty sophisticated and looks at current draw, system voltage and often if certain high current loads are activated in the vehicle, they'll boost charge voltage substantially, often briefly above 15V. I would do some data gathering on your current system and monitor voltages at your vehicle battery and your house battery and see what the voltage drop is at various charging states. As your battery charges, you'll draw less current and the voltage loss in your cable will decrease providing a higher voltage at your house battery as it nears full charge. This will also vary due to loads in your camper that might be active while driving and trying to charge...refrig, etc. I'd certainly recommend something larger than 8GA for your run.

Is your solar panel mounted on your vehicle and useable while driving? No problem to leave it connected and charging in addition to your vehicle.

 

[wandererr]

You'll have to check your manual, but solar charge controllers require a minimum voltage above their charging voltage to be able to operate and provide the required charging voltage to the battery...typically 15V or more, if my memory serves me correctly. You're unlikely to see that high of a voltage from your vehicle charging system.

Good point - I will check, but I believe that it charges even as the voltage drops.

Depending on the vintage and model of your vehicle, the charging system can be pretty sophisticated and looks at current draw, system voltage and often if certain high current loads are activated in the vehicle, they'll boost charge voltage substantially, often briefly above 15V. I would do some data gathering on your current system and monitor voltages at your vehicle battery and your house battery and see what the voltage drop is at various charging states. As your battery charges, you'll draw less current and the voltage loss in your cable will decrease providing a higher voltage at your house battery as it nears full charge. This will also vary due to loads in your camper that might be active while driving and trying to charge...refrig, etc. I'd certainly recommend something larger than 8GA for your run.

2012 JK - I also have a voltage monitor in cab and it usually hovers around 13.8 while driving. Regarding the 8gauge - I ran it for auxiliary power in the back couple years in the back and while I plan to replace it with something heavier, it's an issue of time

Is your solar panel mounted on your vehicle and useable while driving? No problem to leave it connected and charging in addition to your vehicle.

At the moment no - I opted to go with "suitcase" style folding out 160W panel as it allows me to park the vehicle in shade and still charge the battery with the solar panels out in the sun.

 

[Bear in NM]

While not specifically answering your questions, I would add that yes, you can do some "trickery" with various parts of a system. One of the little "tricks" I have experimented with is using a laptop charging brick run through solar charge controllers. Folks do this with bench chargers all the time. The upside to the laptop charger is that they can be had to push 20 volts with models of varying amperage. They are small, and you can get them for a few bucks at your local computer repair place, which will have a box of chargers with no home. Huh, 20 volts and 5 or so amps sure sounds like a 12v nominal PV panel.

A little more to your query, I have tested this through older PV controllers, and more recently through a victron smart controller in a LiFePo4 battery box that I built. The victron can be configured to the penny for settings, via their phone app. This is no brainer for lead acid batteries. A little more picky for LiFePo4.

As noted above, if you understand exactly the amps and volts that are needed or that are being pushed, or that you can control the pull, you can certainly get creative. I picked up a Victron dc-dc for my van, and am still in the pondering process. My 2004 Quigley e-350 seems to bounce around quite a bit as to pushed volts when under way. I use a scangauge II to monitor volts, just need a way to figure out if I can get amps output from the alternator. In my van, the last thing I want to do is fry my alternator, as changing it out would be a major pain. As is almost any repair on a van motor.

Craig

 

[Rando]

This isn't going to work, at least not in the way you are proposing.

There are two types of solar controllers, MPPT and PWM. MPPT controllers typically need to see a few volts above the battery voltage on their input in order to start charging. For the most common (and recommended) Victron MPPT charge controllers, they need to see 5V above battery (so about 17V) on the input to work, and they will never see that from your alternator. The vast majority of MPPT chargers will also only buck (reduce) the input voltage to provide battery voltage, so even if you can get it to start, it will only reduce the amount of charge your battery sees.

A PWM charger is just a switch. For the majority (bulk) charge, that switch is always on, so the charger won't do anything. Once the battery reaches the absorption voltage (say 14.4V) then the switch starts to turn on and off to hold the voltage at that level. However, if you are feeding the charge controller from your alternator that is outputting 13.8V, then you will never get to absorption, and the charge controller will never actually do anything.

This will work if you (carefully) add a DC-DC boost converter between the alternator and charge controller to boost the alternator up to say 24V, then use an MPPT solar controller to buck that voltage back down to battery charging voltage. If you do this you have in effect made a buck-boost charger which is what a normal DC-DC charger is. But given the time, cost, and fuss involved, you would be better to go with a purpose built DC-DC charger, or just use your alternator.

So I have a 115AH house battery that primarily runs the fridge, solar and an 8AWG going back to the main battery (I'm using an old run that I had for the newly added house battery).

Just recently I have learned that connecting the power from the main battery or the alternator directly to the house battery is not the most efficient as the alternator might be dropping its output on as needed basis and might not recognize the house battery draw as needing "charging" but rather will see it as another "draw". To combat that, there are DC/DC charger, but the least expensive one I found was Renogy in Amazon Warehouse for $100 (20A model). It is said that you set it up for your type of a battery and it takes variable DC input (from 9V to 16V) and optimizes the output for charging.

So now I'm wondering - couldn't I use the solar charge controller to do the same? The solar charge controller has circuitry that adjusts the charging voltage/current to the battery type that is connected, it deals with a wide range of inputs and let's face it - my charging input will never be below 12.8V coming from the alternator. So could I use it as my DC/DC charger? Any issues there? Am I missing something obvious?

Oh yeah, when parked my solar charger controller would be fed by the 160W panel that I have.

 

[ducktapeguy]

What type of house battery do you have? If it's SLA then you can just connect straight to the starting battery with no controller. There will be some losses in the wire but it will work if you drive long enough. It's not the best way of doing it but it works. If it's a different type than the starting battery then you need some type of controller.

Went through the same thing just recently when I started this thread.

https://expeditionportal.com/forum/...er-recommendations.225333/page-2#post-2929570

Let just say if it were as simple as hooking up a solar charge controller to the battery then DC/DC chargers wouldn't exist. To skip to the solution, I ended up going with a Renogy DCC30S DC/DC charger+controller for a simple hassle free all in one solution and wasn't much more expensive than trying to piece things together myself.

MPPT controller won't work directly with alternator because of the minimum voltage requirements. I tried a boost converter +MPPT controller but there were some strange behaviors that I couldn't figure out and it wasn't worth the effort to diagnose. Also tried a separate Renogy DC/DC charger + Victron controller, but decided to just bite the bullet and get something that was specifically designed to do what I needed.

 

[broncobowsher]

Let the alternator do the major charging. That straight 8AWG is fine. Even if the modern alternator doesn't fully charge the battery, it is still very capable of doing a lot of bulk charging.
Put some solar on it and that will take care of topping off the battery just fine.

What I am putting together is an automatic relay to connect house to starter batteries. Solar on the house battery.
The auto relay will connect the 2 batteries when there is charging going on. Either alternator or solar has enough to get over the turn on voltage. But once the charge is off it will disconnect and keep them isolated. If I leave it parked outside the solar will keep the house full and also the starter battery topped off. Driving at night, alternator is keeping the house battery going (even if not all the way, but still better than nothing)