Using the same panels to charge 12V and 24V systems

Arcanist

Hello everyone.
I am setting up a solar system for a bus that I am building. It uses 24V batteries to start the engine and I have some 12V deep cycle batteries to provide my lighting and living.
I have three panels, two in parallel and one in series, these provide 40V
I have these connected to a mppt charger.

I want to add a charger to maintain the 24V batteries while I am parked up long term.
So I bought a cheap pwm controller and hooked it up but what it seemed to do was put the power from the 24V batteries back into the panels and charge the 12V batteries.
Is it possible to charge both of these systems using the same panels using maybe another mppt controller, or would the only option be to use a boost charging module from the 12v batteries to charge the 24v batteries?
A mppt controller would be the easier option but I don't really want to spend another $100 if it wouldn't work.

littleharbor2

Please provide the specs for all three panels. Are the two in series 12 volt (36 cell) panels and the single panel a 24 volt (72 cell) panel? describe your battery banks too.

As to charging two separate battery voltages you will need two controllers unless you just let the bus's alternator do the 24 volt charging. Small systems don't really benefit from MPPT controllers. You just need an inexpensive PWM controller. Depending on what your panels and battery banks are, your choice as to what controller and battery combination will depend on this info.

mike95490

MAny PWM controllers don't work well running power from battery 24V to battery 12V
You could build the entire system for 12V only, leaving your starter battery isolated, and reserved only for starting

Arcanist

All panels are 20Volt. The two in parallel are 75W panels and the one above them in series is 85W.
Through this system I get about between 4 and 5 amps through the mppt at about 40Volts
The 12V battery bank is made of one 130ah battery, one 90ah and one 70ah

The 24V bank is two automotive 12V batteries. One huge one and one smaller. I'm not sure of the specs of these batteries.

When I connect the pwm controller the voltage output of the panels drops to 24.7 volts and the amps through the mppt increases to 8.5

Thanks for the replies.
@littleharbor2 harbour that's what I thought, a mppt for the larger system as I plan on adding more deep cycle batteries over time, and a cheap pwm to maintain the starter batteries, but as far as I can see the 24v battery seems to charge the 12v batteries through the panel's positive wire.

mike95490 said:

You could build the entire system for 12V only, leaving your starter battery isolated, and reserved only for starting

I could, but I could like to keep the starters charged up as I will be parked up for a few years, and I will likely be away for months at a time.

littleharbor2

When you wreck your battery bank, which shouldn't take long, do yourself a favor and buy matching 6 volt deep cycle batteries.
As to why you have your panels wired the way you do I'm curious as to the logic behind it. You're still not clear as to what panels you have but I'm going to assume they are 12 volt, 36 cell panels. If you have two series wired why are you parallel wiring a low voltage single panel with a double voltage series wired pair?

Estragon

When connecting the panels to the 24v bank with the pwm controller, voltage from panels will be roughly Vbatt (unlike mppt, in which voltage will be roughly string Vmp).

I'm assuming you're disconnecting the panels from the mppt input, and reconnecting to the 24v pwm input?

Arcanist

How would I wreck the battery bank? Must all the batteries in the bank be matching capacity?
I am wanting to connect two controllers to charge both battery banks at the same time. Currently I haven't left them plugged in for more than about a minute but that is the idea that I can leave both plugged in.

The panels are wired like this.

Gnd--|----panel--|---panel---- V+
.........|----panel--|

They are like this in an attempt to increase the voltage and lower the current. I get the same wattage from this as when they are all in parallel.
All three panels say 17.2Vmp & 20.6Voc

Thankyou @estragon that answers part of my issue. If I use two mppt chargers then the voltage would be the same for both? I guess they would have to be the same type?

littleharbor2

Arcanist said:

How would I wreck the battery bank? Must all the batteries in the bank be matching capacity?
YES

The panels are wired like this.

Gnd--|----panel--|---panel---- V+
.........|----panel--|

They are like this in an attempt to increase the voltage and lower the current. I get the same wattage from this as when they are all in parallel.
All three panels say 17.2Vmp & 20.6Voc

Where did you get the idea to wire in series/parallel? Maybe I'm wrong, trying to decipher your wiring description.
Why aren't they all in series? This is going through an MPPT controller correct?

mike95490

If your charge controller can handle it, wire all 3 PV panels in SERIES. gives about 60ish volts, if your controller is safe with that,

Arcanist

Series is higher than the voltage allowed for the mppt. Series gives 55V or so and the max for the mppt is 50V. Higher voltage is less current.
Here is a diagram with rails A B and C

0V - A
Panel 1 (-) - A
Panel 2 (-) - A
Panel 1 (+) - B
Panel 2 (+) - B
Panel 3 (-) - B
Panel 3 (+) - C
Mppt (Solar panel +) - C
Pwm (solar panel +) - C
Mppt (ground ) - A
Pwm (ground) - A

Both sets of batteries are grounded to A
Chassis - A

The batteries are all the same voltages +/- 100mV
I thought that deep cycle lead acid batteries could go in parallel.

The starters in series are years old. They still work.

littleharbor2

Please identify your controller.

Arcanist

The mppt is this one
http://www.lumiax.com/product/mt2075mppt-20a/

The pwm is El cheapo unbranded.
It looks like this one

http://www.trademe.co.nz/1808737973

mcgivor

Reading through this thread it would seem the PWM, basically being a switch would pull the voltage down to what it needs to charge the 12V battery, this in turn reduces the required voltage for the MPPT controller to function correctly. Generally a MPPT controller should be on its own array as it needs to track the voltage, which should be higher than the nominal voltage by at least 50%-100%. The only way I can think this would work is if they are configured as to separate systems. Just my opinion others may differ.

Arcanist

So basically I use one of the panels for the pwm 24v charging and two of the panels to go to the mppt?
Or just buy a 30w panel or something to connect to it.
Thanks for your help @mcgivor

littleharbor2

What I would do is locate another 75 or 85 watt panel. Series wire two into the PWM controller for the 24 volt system. Then series wire the other two into the MPPT controller to charge the 12 volt system.

Estragon

As a matter of interest, anyone know what would happen using a pair of pwm controllers to charge separate banks from the same array? My guess is the 24v bank wouldn't get much current until the 12v bank gets full enough for its pwm duty cycle to drop, but probably wouldn't let the 24v bank discharge into the 12v one.

Not suggesting @Arcanist try this, just asking

mcgivor

Estragon said:

As a matter of interest, anyone know what would happen using a pair of pwm controllers to charge separate banks from the same array? My guess is the 24v bank wouldn't get much current until the 12v bank gets full enough for its pwm duty cycle to drop, but probably wouldn't let the 24v bank discharge into the 12v one.

Not suggesting @Arcanist try this, just asking

Since each controller would try to clamp the voltage for the battery connected, my guess would be the higher voltage controller would prevail, leaving the second 12V, in this case, at a voltage higher then it can convert generating heat in an attempt to do what it isn't designed to do, hypothetical of course because I wouldn't try it in the first place. Conversely the opposite could occur leaving the 24V battery receiving no charge, either way I can't see it working.

k_dawg1979

Just configure everything around your deep cycle voltage.  Then get a dc to dc charger that will take from one voltage to charge the other voltage

manzanita2

Do you use the starting batteries for anything else besides starting? If not, could you get a good 24v solar powered battery maintainer with its own appropriately sized solar panel to maintain the 24v side? If you could do that you wouldn't have to worry about your 12v and 24v systems interacting poorly.

I don't know if that idea is helpful, but a friend uses them on equipment that sits a lot and it keeps his batteries charged.