Two solar charge controlers on the same battery

[richard cabesa]

Dear Solar Heads,

I have a solar panel mounted on top of the Sprinter that runs through a Morningstar MPPT charge controller to an 8D 12V battery. This keeps me topped up most of the time.

As long as I park in the sun.

I want to park in the shade. I'm thinking a second panel on a cord to place out in the sun would do the trick.

The panel on the roof is too large to carry another matching along for this set up and run through the existing controller. My limited electrical knowledge tells me that I cannot run two different voltage panels into one charge controller and that the second panel would need it's own.

So the questions are, can

Can I just hook two different controllers to the same battery?

Special wiring , switching, solenoid ?

Even though the panels would be different size, should the controllers be of the same type?

I'm looking at some of the 100w type panels that come with a controller and folding legs already fitted and those controllers are not MPPT. Maybe a bare panel and a less expensive Morningstar PWM controller.

Watt am I missing?

Signed,
Electrical Dummy

p.s. there is also a Magnum MMS charger/inverter in the system by I don't want to plug in sometimes

 

[offero]

What's the nominal system voltage on your existing solar panel? 12V? (Will typically be labelled 19-20V open circuit).

 

[richard cabesa]

It is a Kyocera KD205GX-LP

Electrical Specifications at STC
Maximum Power: 205 watts
Tolerance: +5/-5%
Maximum Power Voltage: 26.6 volts
Maximum Power Current: 7.71 amps
Open Circuit Voltage: 33.2 Voc
Short Circuit Current: 8.36 amps
Maximum System Voltage: 600 volts
Series Fuse Rating: 15 amps
Temperature Coefficient of Voc: -1.20x10-1 V/°C
Temperature Coefficient of Isc: 5.02x10-3 A/°C

 

[con kso]

I use a ZAMP 120 watt portable solar system with attached controller. It's got a cable that's about 10' long- and that cable comes with two interchangeable attachments- an SAE type (I think that's what it's called- I have it on my motorcycle heated gear and battery tenders- you push it together) and alligator clips to manually attach to battery.

I think you could very easily just attach alligator clips to your battery and deploy the ZAMP to charge up in the shade. I just used my system for 21 days in southern Baja and I was very happy with it's performance. I really beat on the ZAMP deploying it in the dirt and on sandy beaches- it was great and stood up to the Baja test as far as I was concerned.

You might want to be a little slick and do what I did- I ran 10 guage wire down from my battery terminals to an SAE terminal in an unused fog light port on the front of my truck- I bought a thru-hull type fitting from ZAMP and now I don't have to pop the hood, I just push the panels plug into it.

I wrote a review of my system in this section of the forum that has lots of pics (in the thread about single battery systems and AAA)- here's my setup:

 

[dwh]

You can hook up umpty-zillion chargers to the same battery. It's like having multiple air compressors to the same air tank, each with its own pressure switch. Each charger will supply power until the battery reaches whatever the voltage set point is of the charger.

That's what happens anyway when there is sun on your solar panel and you are driving the truck - two different charging systems active at the same time.

Have at it.

(And you can generally hookup non-identical PV modules to the same charge controller without issues as long as the Vmp is within 10%.)

 

[meental]

What is your charge controllers rated input? I would just get a 2nd panel and a long extension and hook up the 2nd panel in line with the existing panel. If your parked in the shade, your 200 watt panel won't be producing very much power leaving plenty of headroom for the 2nd panel when plugged in. If your really worried you could throw a blanket or cover the main panel when the 2nd panel is plugged in.

 

[richard cabesa]

Thanks guys,

Keep any more input coming if you've got it.

A couple of my buddies call me Smoke Fingers when I work on auto electrical :ylsmoke:

 

[offero]

One more question: what's the current rating on your Morning Star MPPT charge controller? It looks like their lowest rated MPPT model is 15A, which might push you over the edge, as you said in your original post, if add another panel.

The other comment about adding a second charge controller makes sense if you think that you'll overload your current controller. I'd add a like controller though so that if both the roof-mounted panels and the portable ones are producing power, they're both MPPT and they both use the same charging algorithm. Mixing PWM and MPPT controllers doesn't sound like a great idea, though I'm sure it would work.

 

[dwh]

One more question: what's the current rating on your Morning Star MPPT charge controller? It looks like their lowest rated MPPT model is 15A, which might push you over the edge, as you said in your original post, if add another panel.

The other comment about adding a second charge controller makes sense if you think that you'll overload your current controller. I'd add a like controller though so that if both the roof-mounted panels and the portable ones are producing power, they're both MPPT and they both use the same charging algorithm. Mixing PWM and MPPT controllers doesn't sound like a great idea, though I'm sure it would work.

Doesn't matter.

An MPPT controller does the MPPT magic on the solar side. MPPT sets the load voltage on the solar loop to max out the watts available from the solar at any given moment, but it does nothing in terms of charging a battery.

After it maxes out the wattage input on the solar side at whatever the optimum voltage of the moment is, it down-converts the voltage through a buck converter - and by stepping down the voltage also steps up the amperage which is the Viola! magic of MPPT. (MPPT marketing droids call that "boost", but really, it's just a sort of electrical transmission that downshifts to get more torque.)

It then feeds the result into a PWM charge control circuit to actually charge the battery.

Mixing MPPT and PWM charge controllers to the same battery isn't a problem. Best, as you say, if they are both programmed to the same charge profile - for example 14.4v bulk, 14.2v absorb and 13.6v float - but it won't matter if they have different charge profiles...it just means that averaged over time one will end up doing a bit more of the work of charging the battery than the other.

 

[offero]

Doesn't matter.

An MPPT controller does the MPPT magic on the solar side. MPPT sets the load voltage on the solar loop to max out the watts available from the solar at any given moment, but it does nothing in terms of charging a battery.

After it maxes out the wattage input on the solar side at whatever the optimum voltage of the moment is, it down-converts the voltage through a buck converter - and by stepping down the voltage also steps up the amperage which is the Viola! magic of MPPT. (MPPT marketing droids call that "boost", but really, it's just a sort of electrical transmission that downshifts to get more torque.)

It then feeds the result into a PWM charge control circuit to actually charge the battery.

Mixing MPPT and PWM charge controllers to the same battery isn't a problem. Best, as you say, if they are both programmed to the same charge profile - for example 14.4v bulk, 14.2v absorb and 13.6v float - but it won't matter if they have different charge profiles...it just means that averaged over time one will end up doing a bit more of the work of charging the battery than the other.

Makes sense. Thanks for explaining this things in an easy-to-understand way... always enjoy your solar posts.

 

[dwh]

Thanks for explaining this things in an easy-to-understand way... always enjoy your solar posts.

No worries. Thanks for the props - they are always appreciated. Helps to balance out the slings and arrows I have to dodge when I go walkabout off in conspiracy kook territory.

 

[fratermus]

Can I just hook two different controllers to the same battery?

Yes. with a couple edge-case caveats:

* if the controllers don't talk to each other AND you are relying on trailing amps to terminate Absorption you could get premature termination
* with monster installs it could be possible to end up with more Bulk charging current than one intends.

On the latter point: I charge my LiFePO4 at ≤0.4C. In some perfect storms I can exceed that charge rate so I disable or derate one of the charging sources.

 

[Step-Hen]

* if the controllers don't talk to each other AND you are relying on trailing amps to terminate Absorption you could get premature termination

It's not intuitively obvious to me why that would be. Can you explain? I'm not challenging the assertion, I'm just dumb.

 

[fratermus]

Can you explain? I'm not challenging the assertion, I'm just dumb.

Let's say you need to see 0.02C at Abs voltage to terminate, so 4A for a 200Ah lead bank. You set the controller to watch for 4A trailing amps to trigger the end of Absorption and the beginning of Float. At 6A it's still Absorping because 6A > the 4A setpoint. Things are going according to plan.

Same scenario but with two charging sources. The bank is still accepting 6A, but it's being supplied by two separate controllers (3A each for purposes of the discussion). The bank is accepting >4A but each controller sees only its 3A contribution and does what we told it to do: terminate Absorption when trailing amps fall below 4A.