Charge controler and amps

doggyjumper · February 2010

Hi

I am still confused regarding the amps rating of solar charge controllers (not MPPT).
As far as I understand Amps are drawn but will me limited to the max a panel can provide ? so the rating is based on the panels ?

ie one panel capable of 7.5 Amps (18V 133W) .. 10 amps controller
3 of above panels in parallel = 22.5 Amps so 30 Amps controller

be gentle

thanks
Guy

doggyjumper · February 2010

Re: Charge controler and amps

Sorry for the stupid question as a found an answer:

How to Size a Solar Charge Controllers

Solar charge controllers are rated and sized by the solar panel array current and system
voltage. Most common are 12, 24, and 48-volt controllers. Amperage ratings normally
run from 1 amp to 60 amps, voltages from 6-60 volts.
For example, if one module in your 12-volt system produces 7.45 amps and two modules
are utilized, your system will produce 14.9 amps of current at 12 volts. Because of light
reflection and the edge of cloud effect, sporadically increased current levels are not
uncommon. For this reason we increase the controller amperage by a minimum of 25%
bringing our minimum controller amperage to 18.6. Looking through the products we
find a 20-amp controller, as close a match as possible. There is no problem going with a
30-amp or larger controller, other than the additional cost. If you think the system may
increase in size, additional amperage capacity at this time should be considered.

G

audredger · February 2010

Re: Charge controler and amps

Amps out needs looking at too. I have 12 175watt panels 3x4 into an 80amp controler. looks like this: 106.2v 19.8 amps in, 24volt battery bank = 77.8 amps @ 27v. It's the 77.8 amps out that dictates an 80amp controler not the 19.8amps in.

I would encourage you to look at a MPP controler. I had two non mpp controlers to start and replaced them with MPP's. Much better performance in my system!!

audredger · February 2010

Re: Charge controler and amps

In your case; 3*133watts = 399 watts, 399wats / 12v = 33.25 amps, need a 40 amp controler or; 399 watts / 24v = 16.625 amps, need a 20 amp controler or; 399 watts / 48v = 8.3125 amps, a 10 amp controler would work.

That help?

BB. · February 2010

Re: Charge controler and amps

No--Guy has it correct.

He is looking at PWM controllers which cannot control/down convert current and voltage--Not MPPT which can.

-Bill

audredger · February 2010

Re: Charge controler and amps

Can a PWM controle convert up? 18v to 24v?

BB. · February 2010

Re: Charge controler and amps

Mike,

A PWM charge controller is just litterly a "switch" that opens and closes. There are no energy storage devices (typically inductors) which can raise, or lower, voltage.

A MPPT controller is typically a buck mode converter which can only down convert voltage (and increase current) efficiently.

To increase voltage efficiently, a boost mode converter could be used (only can regulate voltage higher). Can work fine--but is less efficient than a buck mode converter.

To regulate output voltage (energy) both higher, equal to, and lower than input voltage. Generally you have to put both types of regulators together (buck/boost).

I am not sure--But I think I remember reading that the long awaited Classic Controller from Midnite Solar has the ability to increase voltage--looking forward to reading the specifications when it is released.

-Bill

audredger · February 2010

Re: Charge controler and amps

If a PWM is only a switch then: a 18v panel @ 7.5 amps in a 12v system charging at say 14.4v, the "excess" 7.5 amps @ 3.6volts would be wasted? That's 27 watts per panel dow the tube?

Did I get that right?

Please be patient with me, seem the more I learn the less I know!?

Most of my learning has been graphic like, spitting up wind.

RCinFLA · February 2010

Re: Charge controler and amps

Yes that's right.

Panel is illumination based current source clamped by the inherent diodes of each cell in the panel. An 18v MPP panel has 36 cells.

Once the battery comes up to the full charge points (absorb, or float) the switch opens and closes putting panel's burst of current into the battery while the controller monitors the battery voltage. When the battery rises above the absorb level the switch opens, the battery voltage starts to decline below the absorb level and the switch closes again. Controller keep the voltage, +/- some small delta around the absorb point. After some programmed time, or rate at which battery voltage increase for a given applied current, the controller reverts to float voltage level.

All accomplished by switching the panel on and off to the battery. This is the pulse width modulation name.

An MPPT controller has a buck switcher. This is a power converter so it allows the panels voltage rise to the maximum power point and converts this voltage down to battery voltage. This results in a current gain over what the panel's current is putting out. If the panel is putting out 8 amps at 18vdc (144 watts), and if the battery is at 13 vdc then the charge current output from the MPPT controller into the battery is about 10 amps (130 watts). This includes about a 90% efficiency in the power conversion.

BB. · February 2010

Re: Charge controler and amps

Don't get us wrong--a properly configured set of solar panels and a PWM type controller is not a bad setup.

PWM controllers cost less to make and use less power to operate their internal electronics. Overall, on average, a MPPT controller can generate roughly 10-15% extra power based on that spread of 18-14.5 volts.

Since Vmp rises with cold weather, the average power increase by an MPPT controller is more in very cold weather.

For systems less than 400 watts or so, a PWM controller may be a better choice (there is the very good MorningStar 15 amp MPPT charge controller that can run ~200 watts of solar panels very nicely on a 12 volt bank--but, they are not cheap). The difference between the PWM and MPPT 15 amp controllers is ~$101 to $235...

Where a MPPT controller stands out is for larger systems and/or systems where the solar panels are a good distance from the battery shed/charge controller.

You can, very efficiently, run your solar array upwards of Vmp=100 VDC the long distance and use the MPPT controller to down convert to the 12-48 Volt battery bank. Just like using high voltage/low current to transmit power from the utility then using a transformer to drop the voltage/increase the current just outside your home.

Grid Tied inverters (battery less solar panel->120/240 VAC house wiring connection) can even run in the ~200-600 VDC range using MPPT type technology.

A couple of Charge Controller FAQs:

All About Charge Controllers
Read this page about power tracking controllers

-Bill

RCinFLA · February 2010

Re: Charge controler and amps

Yes, the 20% gain sounds good but this only applies when in bulk mode. The overall gain depends how much time the system spends in maximum charging mode. Once the battery is charged, and the panel's output is not fully used then the MPPT controller offers little advantage over PWM.

The general feeling around here is MPPT is worth it if you have a PV array that produces over 1000 watts. Below this the extra expense of MPPT controller is not worth the gain.

RCinFLA · February 2010

Re: Charge controler and amps

You may wonder why 36 cells in series with 18v MPP is needed to charge a 12 v battery.

The 18v MPP is at 25 deg C. The inherent diode of each cell has a voltage drop dependent on temperature. It gets lower at high temps and higher at cold temps.

On a hot summer day, the panel may get to 60 deg C in the sun. The MPP voltage drops to about 15 vdc. This, with some voltage drop for wiring and controller, barely makes the required absorb voltage level required for a 12v lead acid battery.

In this situation, the MPPT controller has no advantage over a PWM controller.

doggyjumper · February 2010

Re: Charge controler and amps

Thanks Bill and RC

your wordings made so much more clear

guess the next part is to make sure the battery is capable of receiving the amps with the controller basically being a switch ?

Guy

BB. · February 2010

Re: Charge controler and amps

Solar panels are "current mode" power sources... The difference between Imp (max power) and Isc (short circuit) is not very much (current is almost the same).

So, looking at batteries--the basic rule of thumb is that the charging source should be between ~5% to 13% of the 20 Hour Rating (Amp*Hours) of the battery bank.

Below 5%, batteries may not properly charge (need enough current to mix electrolyte). Above 13%, flooded cell batteries can start to overheat (and charging is less efficient)...

So, working backwards--say you have 22.5 amps of charging current at 12 volts, then the appropriately (and useful) sized battery bank would be around:

22.5 Amps / 0.13 = 173 AH rough minimum 12 volt bank
22.5 Amps / 0.05 = 450 AH rough maximum 12 volt bank

By the way, assuming a sunny summer day of 5 hours of full sun and 3x 133 watt solar panels and 52% end to end efficiency (flooded cell batteries and an AC inverter):

3x133 watts * 5 hours of sun * 0.52 = 1,037 Watt*Hours

Enough to run a 100 watt load for:

1,037 WH / 100 Watt Load = 10.3 hours per day

-Bill

mike95490 · February 2010

Re: Charge controler and amps

audredger wrote: »

If a PWM is only a switch then: a 18v panel @ 7.5 amps in a 12v system charging at say 14.4v, the "excess" 7.5 amps @ 3.6volts would be wasted? That's 27 watts per panel dow the tube?

Did I get that right?

Watch out guys, this is a sharp one!

You just saved yourself 3 weeks of back & forth posting - PWM and MPPT differ by more than one letter. But, that's the gist of it, the PWM just discards the mismatch. MPPT converts it pretty efficiently, but once the batteries get 80% charge, they cant accept the extra current, so some still gets wasted. (but that's the time to get the house vacuumed, dishwasher run, bread machine and such, when you have excess solar.

Charge controler and amps

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