Solar controller placement

ducktapeguy

Adventurer
This may be a basic question but I did a quick search of this forum but couldn't seem to find the answer. Can I run a solar controller off the main battery/alternator with a solar panel and have it charge my aux battery? Not at the same time, but if they were wired in with a switch is that going to cause an issue?

Background. I have an DIY aux battery pack to run my fridge. It's stored in the rear cargo area next to the fridge. It's connected through a relay box to the main battery cable so it charges whenever the car is started. I've recently added a 100W solar panel with 10A renology controller to charge the battery. The panel is mounted to the roof and the wires run into the engine compartment. What I'd like to do is switch between solar and alternator whenever the car is parked/driven., What I would like to do put the solar charge controller near the battery in the rear cargo area, however, I only have one set of 10ga wires leading the the rear cargo area so it would have to use that for both the battery and panel.

When the car is parked, the panel wires connect through the relay to the charge controller to the battery. All is fine on that end and it works as designed. But, when the engine is running, the alternator output would be connected to the input side of the charge controller. I'm assuming power is power, whether it comes from the alternator or solar panel, the charge controller shouldn't know the difference. But in reality is there going to be a problem using the charge controller in this manner? The alternator voltage doesn't get much higher than 14V, where as the panel can output 20V. Not sure if there is a certain voltage threshold or delta the controller needs before it starts charging, and I don't know if this might cause problems with the alternator charging circuit. Right now i've got the relay disabled so it's only being used for solar, but I'd like to have them switch automatically.

The alternative is to locate the controller in the engine compartment and have the relay switch between the output/battery side of the controller instead of the input. Eventually I may do it that way when I relocate the battery but for now it's going in the cargo area.
 
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Herbie

Rendezvous Conspirator
Generally, you want the controller close to the battery being charged, especially when you're actually pushing a lot of current. This is a must if you don't have a remote temp sensor.

The controller has the sensor, and adjusts the charge current based on temperature. A large temperature differential between battery and controller could theoretically damage the battery.

If you have to choose, better to put the controller near the house battery that's likely to need serious recharging, and let the starter battery be father away, since that's likely just to get a trickle charge, under normal circumstances.
 

ducktapeguy

Adventurer
Thanks. That's what I am trying to do, putting the controller in the same box as the house battery so it can be a self contained unit. Not too worried about the current side since it's only 100W panels and it'll probably never see more than 8A.

However, since i only have one set of wires going to the controller, the alternator and solar panels will have to "share" those conductors, switching between them depending if the car is on or off. Essentially the solar controller will be acting as a DC/DC charger for those times when the car is on and charging from the alternator. I just wasn't sure if that was going to cause issues with either the controller or the alternator regulator. Seems like if it worked, more people would be using a cheap solar controller vs. an expensive DC/DC charger.
 

Fatboyz

Observer
This can work. But realise, your alternator (for this context) has basically unlimited current. If the battery is ’extra low’ it may want to accept more current than the solar controller capacity.

Some controllers if exceeded, will simply shut down.
Some may release magic smoke.

Were it my project, would connect solar to battery as usual, connect alternator to battery ’downstream’ of the solar.

My thoughts also. Put the solar controller next to the house battery. I'm assuming you have a set of wires (I think you said 10ga) from the house battery to the road battery. If you had a battery ACR there it would allow 2 way current to your road battery. Power coming from the panel through the hose battery to the road battery, and when the engine is running, power from the road battery through to the house battery.
 

ducktapeguy

Adventurer
This can work. But realise, your alternator (for this context) has basically unlimited current. If the battery is ’extra low’ it may want to accept more current than the solar controller capacity.

Some controllers if exceeded, will simply shut down.
Some may release magic smoke.

Were it my project, would connect solar to battery as usual, connect alternator to battery ’downstream’ of the solar.
Thanks. At some point I will be relocating the aux battery to the engine compartment so it will be connected in a standard switchable configuration through a 5 pin relay. But for now I have it stored in the cargo area because it gets used in multiple vehicles.

In normal use I've never seen the aux battery pull more than 7-8 amps when directly connected to the alternator, so I'm assuming that'd be the max I'd see with the controller also. I should mention it's a PWM controller, I don't know how much of a difference that makes. I've never used these solar charge controllers before, so I'm not sure how it works and whether it would even allow more than the rated current or just limit it. I guess it's cheap enough I could give it a try and see what happens.
 

dreadlocks

Well-known member
PWM is the only tha'd work w/an alternator I think, MPPT's usually require few more volts more input than output to work, mine requires +5v to just startup, then +1v after.. PWM's often dont have any limiting built in other than what circuit can handle before you let the magic smoke out.. MPPT's will limit output, and since input is higher voltage that means input amps is less so it wont take in more amps than it can handle, none of that exists in PWM, its just switching on/off at high frequency to manage voltage and not paying any attention to current.
 

Peter_n_Margaret

Adventurer
PWM is the only tha'd work w/an alternator I think, MPPT's usually require few more volts more input than output to work, mine requires +5v to just startup, then +1v after.. PWM's often dont have any limiting built in other than what circuit can handle before you let the magic smoke out.. MPPT's will limit output, and since input is higher voltage that means input amps is less so it wont take in more amps than it can handle, none of that exists in PWM, its just switching on/off at high frequency to manage voltage and not paying any attention to current.
Not so.
These PWM controllers are current limiting. http://www.plasmatronics.com.au/ They can also switch other accessories (like auxiliary generators), accommodate shunts and battery temperature compensation.
I have a PL60 that is now 15 years old.
Also, there are some top end MPPTs that can boost low voltages to that required for charging.
Cheers,
Peter
OKA196 motorhome
 

dreadlocks

Well-known member
Ah yes but thats a $300 USD controller, whereas OP has a $16 Cheapo controller as most people using PWM on this forum.. I mean yeah it can be done, but at that point your well past just getting separate DC charger and solar charger and getting better results.

I forgot about those boost controllers, Genasun has one but they are rather expensive for the low output at 12v.. makes a lot more sense for like a 48v bank where it could handle a rather large 27v house panel..
 
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