Solar LED Lighting - 24V DC

Re: Solar LED Lighting - 24V DC

Welcome to the Forum Roy!

roythoppil wrote: »

I am trying to configure an LED battery backup system using Solar Panels...

The Solar Panels are Conergy 225W, Vmp - 28.4V, Impp - 7.92A Voc- 37.1V, Isc - 8.49A

I have connected 4 of these in parallel to obtain a 900W, 24V, 32A DC System.. The batteries i am using are a Rolls 6V, 599 AH , four of them connected in series....

First warning... Most high wattage solar panels available today do not have a Vmp that matches well with 12/24/48 volt systems... For a "simple" (and inexpensive) PWM type charge controller, Vmp should be around 17.5v/35v/70v or a bit higher to insure the batteries are fully charged with high enough voltage on a hot day (Vmp as the solar panels get hot).

You panels are Vmp~28.4 volts, which will drop a few volts in full sun on a hot day. A Deep Cycle flooded cell battery bank generally uses an absorb voltage of ~14.5/29/58 volts for Absorb charging and ~15/30/60 volts for Equalization (should only be done every couple of months or so).

Normally, you would place two or three panels in series to charge a 24 or 48 volt battery bank through a MPPT charge controller (which can efficiently down convert the high voltage/low current of the solar array to the low voltage/high current need to charge the battery bank).

If you use a PWM controller (which the Samulex appears to be), one panel Vmp is too low for 24 volts (or too high for 12 volt bank), and two panels in series is too high of voltage for the 12 or 24 volt Samulex.

Is it possible for me to use Samlex TN-1500 DC-AC Inverter / Solar Charge Controller for this application... Its a Pure Sine Wave Solar Inverter/Charger Input with 24 VDC, Output:120 VAC, 1500 Watts With built-in 30Amp Charge Controller & 2.7Amp AC Charger? This is to connect the LED at the O/P and the 24V Batteries at the input..

Or can i connect a MorningStar TS60A Charge controller and a regular 24V, 1000W Pure Sine Wave inverter?

With your panels, you need a MorningStar TS MPPT 60 Amp controller for your application--Much more expensive (but it is a very good controller too):

Morningstar TriStar 60 amp MPPT solar charge controller

If someone could suggest something to me, i would really appreciate it

Thanks
Roy

I have not seen that Samulex/Meanwell inverter/charger before--It looks neat, but I have not heard anything about it or studied the manual closely yet.

Unfortunately, the (I assume) PWM internal solar charge controller is a bit difficult to match with high Wattage solar panels. Evergreen used to make Vmp=18v panels, but they are out of business. There are Kyocera ~135 watt Vmp=17.5 volt panels available (good panels, good brand)--But, the larger wattage panels are probably going to be less expensive (and less wiring/mounting hardware needed).

In the end, how much power are you going too need (average/peak wattage; Watt*Hours per day; 120 VAC or 24 VDC, etc.). Are you going to have a backup genset, etc...

-Bill

Re: Solar LED Lighting - 24V DC

I am familiar with Samulex/Meanwell--They are a pretty good brand. I have not seen the inverter/charger/solar charger before.

The problem is that the solar panels (when warm) and wiring+controller drop will get the output down to 25-27 volts or so on a hot day with those panels in parallel... You need around 28.4 to 30.0 volts at the battery bank to properly (and quickly) recharge the battery bank. Instead of 32 amps charging the battery in the middle of the day, you may only be charging a few amps instead.

You need the solar panels to be rated around 35-37 Volts Vmp minimum to get full rated current into a "24 volt" lead battery bank.

It is possible to put two of the solar panels in series with a PWM charge controller (not the Samulex--would exceed its maximum operating voltage)--But it would be operating at almost 50% efficiency of the solar array (PWM controllers are simple "on/off" type devices. When the controller is on, the battery will be at ~28-29 volts charging--even though the solar panels are capable of upwards of 58 volts or so output (solar panels are "current" mode power sources--More or less, they will output ~8 amps from zero volts to ~Vmp voltage--Above that the current rapidly falls to ~zero amps when Voc (voltage open circuit) is reached.

A MPPT (Maximum Power Point Tracking) solar charge controller is basically a "buck mode" digital switching power supply which can, sort of, behave like the DC version of an AC transformer. It can take (made up numbers) 8 amps @ 60 volts DC from the solar array and "down convert" it to ~16 amps at 30 VDC to charge the battery.

Here is a similar 240 Watt solar panel (PDF download). Look at the bottom left of page 2 for the Voltage vs current graph. From 0-30 volts you get roughly rated current (in full sun). From 30-35 volts, the panel output quickly falls to zero amps.

Panels are rated at ~25C, but will operate at ambient temperatures + upwards of 30C due to the heating effects of the sun... So, if your panel is in warm air of 35C (95F) and a 30C rise (full sun, no breeze, built against roof--54F rise), the operating temperature will be ~65C. The temperature delta of 25C to 65C is 40C rise. The voltage drop from STC is -0.400%/°C...

• 30 Vmp * (-.004/°C) * 40°C rise = -4.8 volt Vmp depression

Or, roughly, when the battery is at ~29-30 volts (should be maximum charging), there will be virtually zero current flow (Vmp-array-hot ~ 25.2 volts).

Unfortunately, your choice is to run two panels in series with a different PWM controller (higher voltage rating) and live with a ~50% loss in array performance or purchase a MPPT type charge controller for a lot more money and run the array in 2-3 panels in series to properly feed the MPPT controllers input requirements.

I guess you are from near Houston Texas--So there you will have long/hot summers to contend with. And having your panels in parallel will not supply adequate charging current for the 24 volt battery bank.

I also have some other questions/concerns--But I will make that another post below:

-Bill

Re: Solar LED Lighting - 24V DC

Roy,

I am not sure I understand the question... There are cable length/gauge/fusing-breakers/on-off switching issues to be considered in the design. As well as the size (Amp*Hour) rating of the battery bank (typically, for a flooded cell battery bank you don't want the charging/discharging to exceed ~C/8 (8 hours) rate). But 600 watts from a 24 volt battery bank is quite doable:

• 600 watts * 1/0.85 inverter eff * 1/21 volt cut-off voltage * 1.25 NEC safety factor = 42 amp minimum rated fuse/breaker/wiring

Also, many 120 VAC LED lamps have very poor power factor (down to PF~0.5--possibly even lower. PF=1.0 is "perfect")... That means that it is possible for 600 watts of LEDs to require:

• VA = Power/PF= 600 Watts / 0.5 PF = 1,200 VA (volt*amps)

Most inverters (and small gensets) have their output rated to Watt=VA ... So, even though a 1,000 Watt inverter has more than enough power to light the LEDs, the poor Power Factor of the LED lamps will push the VA rating over the 1,000 Watt (VA) maximum rating of the inverter.

-Bill

Re: Solar LED Lighting - 24V DC

vtMaps,

We use C @ 20 hour rate for all of our "rules of thumb" calculations because, in general, if you assume several days of energy storage and 50% maximum discharge for a typical off grid system--the average power draw works out to about 20 hours of energy usage (i.e., around 5 hours a night draw for 4 days until battery is dead).


But, you are correct--If you want to know how long the battery will last--You need to know the battery capacity at that usage (say 8 hour of current draw until dead) because actual battery capacity depends on current draw.

But, if we did that--then we would have to be modifying the rules of thumb based on different C rate capacities... I.e., one capacity at C/20 for average draw over time, C/8 for heavy draw (well pumping), and C/2.5 for surge capacity--And then we would have to modify the rules of thumb for "C@2.5" maximum rate would really be (for example C/1.8...etc. But, we will never C@1.8 on a data sheet--so we would need to estimate based on Peukert factor... But we don't usually know the Peukert factor either (which is also an estimate).

Sorry--the above is a an awkward bunch of hand waving. In the end--having one standard number used everywhere for basic rule of thumb calculations (like the C/20 capacity) just keeps things easier. Also, the various limits (C/20, C/8, C/25) that we use for the rules of thumb have been experimentally (and based on experience over time) determined--So even though they are not the "correct" AH capacity at that current level--They are accurate when determining the "limitations" of the over all battery bank performance.

If somebody has an application where they need, for example, huge amounts of power for a 1/4 hour a few times a year (UPS function)--Then other factors need to be considered (like AGM's which can support "operational current" at a C*4 rate). And the limitations of some chemistries like GEL which can discharge at rates over C*1 but can only recharge at C/20 rates without damage.

In the end, for the fast majority of users here, the rules of thumb hang together well, and tend to be on the conservative enough (allow for aging of components and hot/cold weather), that they gives us a quick/easy method to "design" a strawman PV Energy System. At that point, we can discuss the specific needs and see what should be changed in the design to support specific needs (like the need for running large saws in shop for a few minutes a day).

-Bill