12v/solar question - multiple charge controllers from one solar panel? (1 Viewer)

[Kevin B]

I have a home brew lifepo battery pack with a DC/DC charger I keep in the truck, it normally charges off the alternator. I'd like to add solar to keep it topped while the truck isn't running, but if I do that I'd like to have the solar trickle charge the lead acid 12v starter battery as well.

I know just enough here to get myself in trouble but I definitely know not to try and charge two different chemistries off the same charge controller. Can I safely split the wires from the solar panel between two charge controllers, something like so? Do I need to match anything between the controllers?

 

[e9999]

never tried that. I would not take that route personally. Could you not solar charge the starter battery directly and let the DC-DC that you already have charge the other one, for instance? Or use 2 panels, it's nice to have the extra flexibility.

But, anyway, as to your specific question, my first thought is that it may be OK if one or both charge controllers are PWMs but maybe not so much if they are both MPPTs because the latter actively try to move the operating point of the solar panel, so you might have 2 competing control algorithms at play. Maybe they would play nice, I don't know, that would depend on the hardware and the algorithms used.

 

[Kevin B]

Could you not solar charge the starter battery directly

... can I? I thought the charge controller was necessary to prevent overcharging/harming the battery.

my first thought is that it may be OK if one or both charge controllers are PWMs but maybe not so much if they are both MPPTs because the latter actively try to move the operating point of the solar panel, so you might have 2 competing control algorithms at play.

The charger I have is MPPT: https://www.amazon.com/ECO-WORTHY-Batteries-Multi-Stage-Charging-Alternator/dp/B0BZCXNHDV/

I don't have a clear understanding of the difference, so I'll read up on that. Thanks for giving me something to chew on.

 

[e9999]

yes, you do want a charge controller if it's a regular size (more than a couple of W) panel. What I meant is that you can charge your starter battery directly from the solar panel (with your controller); and also use a DC-DC converter to charge the house battery from the starter battery (if it can do so with the engine off). But do you really need to keep the starter battery topped off all the time? Barring large parasitic draws or very long storage, it's probably fine.

 

[gnob]

i really don't know much about this stuff, but i tnink this basically what you're after.
this mppt has battery and load side. i kind of assume you could send the load side to your lifepo, since they typically have an mppt built in.

 

[Kevin B]

yes, you do want a charge controller if it's a regular size (more than a couple of W) panel. What I meant is that you can charge your starter battery directly from the solar panel (with your controller); and also use a DC-DC converter to charge the house battery from the starter battery (if it can do so with the engine off). But do you really need to keep the starter battery topped off all the time? Barring large parasitic draws or very long storage, it's probably fine.

I think that's what I'm suggesting, yes, a solar panel with the power wires split between a charge controller for the starting battery and the DC-DC charger for the aux. I don't HAVE to keep the starter topped, but working from home means I frequently don't drive for weeks, and camping with kids mean the truck doors are constantly open and the dome lights on, so it'd be a nice feature.

I suppose the "right" way to be what @gnob suggested, just get a charge controller that can handle charging multiple chemistries simultaneously, but those appear to be $$$$...

 

[ducktapeguy]

A Renogy DCC30S/DCC50S will do everything you listed all in one package. Automatically switch between alternator and solar, and switch between house battery (Li, SLA, AGM) and starting battery as needed. So far the only limitation I found is there is no option to select anything other than a lead acid starting battery charge profile, but it sounds like that's not a problem for your use.

Other companies probably have something equivalent but I've been using the Renogy one and it's been working fine.

 

[gnob]

I think that's what I'm suggesting, yes, a solar panel with the power wires split between a charge controller for the starting battery and the DC-DC charger for the aux. I don't HAVE to keep the starter topped, but working from home means I frequently don't drive for weeks, and camping with kids mean the truck doors are constantly open and the dome lights on, so it'd be a nice feature.

I suppose the "right" way to be what @gnob suggested, just get a charge controller that can handle charging multiple chemistries simultaneously, but those appear to be $$$$...

my controller was $120 used. morningstar SunSaver. rated to 15A
it replaced a renogy pulse width controller that died for no reason.

 

[LandLocked93]

What?? No.
Or well...doable, but...
Guessing you do not have dual engine room batts, rather just a start/main batt up front and the cabin batt in the rear?
The long term solution is a 2-position switch after the solar controller (in-line with the controller charge output). This switch can be a manual type or remote controlled.
One position routes solar controller charge to the start/main batt.
The other position routes solar controller charge to the DC/DC solar controller input.
There are other, significant factors in play here but this is the starting point to meet the need you're after.

 

[e9999]

^ the problem with a switch like that is that the batteries are different technologies, so presumably the controller would (ideally) have to be reset each time with different voltage settings, which is a pain. Not super critical to do so, but better.

It seems simple enough to have the controller outputting to one of the 2 batteries and a DC-DC charging the second battery from that first one. Heck, conceivably, in a pinch (I don't recommend this) you don't even need the DC-DC converter, you could just have the 2 batteries in parallel with big cables and just solar charge both at the same time (with the more suitable of the 2 chemistry settings).

 

[Kevin B]

It seems simple enough to have the controller outputting to one of the 2 batteries and a DC-DC charging the second battery from that first one.

It took me a bit to smell what you're cooking, but I got there eventually and think I agree. I don't need a charge controller that will charge two different chemistries at once, I just need a controller to trickle the main and provide a constant 12v to the DC-DC charger, which will do the actual charging duties for the aux. That appears to be most solar charge controllers on the market, they have one output for battery which is programmable for the various chemistries, and another output for "load" which is presumably just an unregulated 12ish volts.

The DC-DC charger I have has two inputs specifically for putting the alternator on one and a solar panel on the other, and will intelligently choose between them or isolate itself depending on how much voltage is present on which input. The "alternator" input is currently wired to the battery and doesn't kick in unless it sees over 12.5 volts, which is normally when the truck is running and the alternator is doing it's thing - when the main goes below 12.5v (ie, truck off), the charger stops pulling juice and isolates the aux. I don't know that input from the solar will push the starter battery over 12.5 volts? If it does then that'll work great. But if not, it doesn't really care about voltage on the "solar" input and will take what it can get. I can tie that "load" output to the "solar" input on the DC-DC charger and get charge off the panel that way. My only concern at this point is that it's MPPT, and you mentioned earlier that "the latter actively try to move the operating point of the solar panel" - I don't know what that means, but I'm going to find out. If the "solar" input on the DC-DC charger will take a basic 12v from the solar charge controller (instead of straight from a solar panel) without letting the smoke out, then that will work too. I have a basic cheapo controller with battery and load outputs in my parts box right now that will do the job.

If not, then I guess I'm looking at investing in a more expensive controller like @gnob and @ducktapeguy have, or playing games with switches and relays as @LandLocked93 suggested. But I'll get out the multimeter in the morning and see what that solar controller outputs, and I'll ask the manufacturer of the DC-DC charger whether I can connect the "load" output of a solar controller to the "solar" input on their charger without burning my truck down. I shall report back.

 

[ducktapeguy]

If the "solar" input on the DC-DC charger will take a basic 12v from the solar charge controller (instead of straight from a solar panel) without letting the smoke out, then that will work too. .

I don’t think it’ll work, but it’s worth a shot. The solar input needs to have a voltage higher than the battery you're charging, a few volts at least, for it to start charging, but yours may be different. Some people have said they were able to use a voltage booster to "trick" the charger into working, I tried that and didn't have good results.

What you're trying to accomplish is a pretty standard setup. Unless you're the type that likes the challenge of making things work outside their intended use, you're probably going to end up spending more money for something that "kinda" works vs. buying an off the shelf item that's already been tested. I'm speaking from experience having gone through the same thing you did. I had multiple controllers, voltage boosters, chargers, etc.. In the end, I had spent more money, had a mess of wiring I had to deal with, something that I didn't fully trust to not burn my vehicle down, and wasn't all that great even by my standards. I finally bit the bullet and just bought an off the shelf dual input controller that can charge both batteries. It was cheaper in the long run, installation was far simpler, and i only had do deal with one app to control everything.

 

[e9999]

OP, can't you disable or lower the 12.5V cutoff on the DC-DC? Mine can. Now, you may not *want* to do that, though, because you don't want the starter battery to be discharged too much, that's typically worse than having the aux one discharged. YMMV. But yes, the solar charger will eventually (or right away if the starter battery is already near full) increase its voltage above 12.5V. It is true that some charge controllers need to see a solar panel voltage higher than the battery (e.g. my Victron says it should be 5V more) to get going, but in practice that is not an issue because a suitable "12V panel" will typically have a no load voltage of over 20V even at very low insolation level, and that is plenty. That differential voltage issue may or may not be relevant with your DC-DC, though, check the specs for voltage ranges. Mine will charge the auxiliary battery even if the source battery voltage is lower than the aux batt voltage, that's the whole point of a boost + buck converter.

 

[Tesota]

This isn't gear I've used before but I wonder if something like this would be suitable to trickle charge your start batt from the aux batt

BrakeBuddy 39332 Towed Vehicle Battery Maintainer

The BrakeBuddy 39332 Battery Maintainer keeps your vehicle's battery charged while being towed behind your motorhome.

www.rvupgradestore.com

Fancier version

RVi Towed Battery Charger Plus for Flat Towing

Towed Battery Charger Plus is an on-demand system that not only trickle charges vehicles that have little draw, but can charge up to 10 amps when there is a high parasitic drain on the battery when flat towing.

www.campingworld.com

 

[LandLocked93]

I'm playing games??
Nah. Just being helpful.
Good luck!

 

[LandLocked93]

^ the problem with a switch like that is that the batteries are different technologies, so presumably the controller would (ideally) have to be reset each time with different voltage settings, which is a pain. Not super critical to do so, but better.

Good point. Missed the chem delineation, even if not super critical for short-term (i.e. base camp) charge time.
In such a case, no switch needed. Just connect solar to controller, and controller to LifePo permanently.
As you said, let DC-DC handle it all while engine runs.

I misunderstood and thought OP was after 2 diff charge strategies while also searching for a less expensive option to running large, parallel cables.
My mistake.

 

[sandcruiser]

Seems like if you take the PV output and add 2 charge controllers in parallel then the lowest voltage should pull the most power.

I think that means that the one controller will get all the power until it starts throttling due to being full, at which point the other controller will get more power.

Which is to say that the system might work fine: the power will go *somewhere* until both charge controllers stop charging both batteries.

It seems like a fun experiment. Take notes!