increasing capacity of a 12v system

Re: increasing capacity of a 12v system

camira wrote: »

from my previous post:
"with my new panels i will have 480 watts....lets assume that there is no loss at the moment so 2 hours of sunshine would give me 960 Whrs into my batteries thus making me happy since i use 728 Whrs/day

but if i look at the Amps.....my panels (wired in series) push out 103.2V (17.2 * 6) at 4.66A thus 2 hours of sunshine would give me 9.32Ahrs.....but i need 60Ahrs (728Whrs / 12V).....

OK... The simple electrical equations:

• V = I*R
• P = V*I = V^2 / R = I^2 * R
• Energy=Power*Time=P*Hours=Watts*Hours
• Amp*Hours=Energy/Volts=P*Hours/Volts=Amps*Hours

So, from a very simple point of view: Amp*Hours is simply Watt*Hours/Voltage. You will see us many times see somebody ask about Amp*Hours and we ask them what their voltage bank rating is.

Now, for a rough estimate, a lead acid battery bank Amp*Hours out = Amp*Hours in. Working in Amp*Hours gets rid of a bunch of "fudge factors" when dealing with batteries and how much energy they hold. In old boat and RV systems, they pretty much used simple filament light bulbs and DC electric motors (fans, pumps, etc.) which, for the most part use roughly the same amount of amperage at 14 volts vs 12 volts.

Now--the ugly issue. Batteries do not hold a constant voltage. But have higher voltage when charging and lower voltage when discharging. So when you need to deal with energy, you need to account for the differences in voltage when discharging and charging. So, for an example. Say you use 100 Amp*Hours at 12 volts (say 10 amps for 10 hours).

• 100 Amp*Hours out = 100 Amp*Hours to charge (near 100% efficient)

But to recharge, it takes 14.5 volts to recharge. Excluding other losses:

• 100 Amp*Hours * 12 volt load = 1,200 Watt*Hours used for load
• 100 Amp*Hours * 14.5 volt charge = 1,450 Watt*hours to charge

So, we needed about 20% more gasoline (or solar power) to recharge the battery bank after we used XX Amp*Hours / XX Watt*Hours of load.

if the panels are wired in parallel then i get 17.2V at 27.96A and with 2 hours of sunshine i get 55.92Ahrs"

Now, to look at different Vmp (series and/or parallel connected) solar panel setups.

Basically, there are two major types of Solar Charge Controllers. The older/simpler/cheaper type is a PWM (Pulse Width Modulation) which basically passes the current from the solar panels to the battery load.

You could have a 12 volt battery bank and a 17 volt solar panel at 10 amps, you will get charging the battery:

• Pbatt = 14.5 volts battery charging * 10 amps = 145 watts into battery bank

Now, if you had 10 of these panels in series with a PWM controller charging your 12 volt battery, you will still have this equation:

• Pbatt = 14.5 volts battery charging * 10 amps = 145 watts into battery bank

A PWM does not care what voltage the solar array is--as long as that Vmp voltage is greater than that of the battery bank.

The second type of solar charge controller is called the MPPT Charge Controller (Maximum Power Point Tracking). It is a "converter". The MPPT CC converts high voltage / low current from the solar panels in to low voltage / high current at the battery bank. A MPPT type Charge Controller sort of behaves like a variable transformer for an AC circuit. It uses inductors to convert the power (detail--look up Switch Mode Buck Converter power supplies). Inside, it is similar to the power supplies used in personal computers.

Basically (neglecting losses) use the Vin*Iin=Pout*Iout. Say we take my mythical Vmp=17v / Imp=10a panels and put three of them in series. Remember that they only output about 10a*14.5v=145watts into the battery. Here:

• Vin*In = 3x17v * 10a = 510 watts = Vout*Iout = 14.5volts * 35.2 amps

This conversion is possible because of an MPPT type charge controller.

You can read these two FAQ's from our host's website about solar charge controllers for more details and clearer explanations:

All About Charge Controllers
Read this page about power tracking controllers

these calculations seem weird....however, i do not need to understand the discrepancy if in the end i get a system that will work.....so, moving on.....

So, there are different ways we can use Amp*Hours and Watt*Hours / Etc. We have to use the correct formulas in the correct locations. And depending on what kind of devices we are talking about, different assumptions will have to be made.

For example, I said "simple/old" electrical systems tended to be constant current, so measuring Amp*Hours (amps over time) was good enough.

However, today, many of our loads are run on 120 VAC inverters. The AC loads see a constant 120 VAC, so their current / power consumption is constant.

An inverter is a constant Power device. Basically, The Power consumed by the inverter from the battery bank is Pin=Vin*Iin

So, for an inverter supplying a 120 VAC circuit with 10 amps of power at 85% efficiency, we see:

• Power DC side = 120 VAC * 10 Amps * 1/0.85 = 1,412 watts from battery bank

Now, lets take two voltages from our 12 volt battery bank. A) charging at 14.5 volts, and Dying battery at 10.5 volts:

• Battery Current = 1,412 Watts / 14.5 Battery Volts = 97.4 amps (charging)
• Battery Current = 1,412 Watts / 10.5 Battery Volts = 134.5 amps (near dead)

So Amps and Amp*Hours from the battery depends on the state of charge of the battery bank and charge controllers. However, the power (Watts) consumed by your AC loads remain constant.

if i go with niel's confirmation and try to send 27.96 amps through my wire.....i will need some very heavy gauge to minimize my losses (as per hillbilly's comments).....i actually crawled under the house this morning and realized (to my dismay) that i have a 45 foot run instead of 30'.....so instead of spending a fortune on wire, can i simply swap out the PWM Controller and put in a MPPT controller which (if i understand the technology) will allow me to send a higher voltage (and thus lower amperage) down my wire and the MPPT will up the amperage before sending it to the battery bank?

Yep... You are running the Solar Array at a "higher voltage / lower current" the longer distance from the solar array to the charge controller.

Put the Charge Controller close to the battery bank and use short/expen$ive wire for that run.

if that is the case, i will have 6 80W panels as soon as the 4 i have ordered come in....can i just wire these up in parallel to send 103.2V at 4.66 amps through the wire to the MPPT controller thus keeping my wire gauge relatively low

Fine--but make sure you get the correct MPPT charge controller for your application. Smaller/older MPPT type controllers tend to have low Voc input voltages from the array (75 volts or less). The larger ones (Xantrex, Outback, bigger MorningStars) tend to max out around Voc of 140-150 VDC.

Vmp at 103 is just fine for the larger MPPT controllers.

now since i have just two panels at the moment, i do not have a combiner but since i will now be at 6 panels, will i need a combiner....the panels all have a junction box....i was thinking i could make two arrays of 3 panels in series and then connect these two series strings in parallel.....does that make sense....this would give me two strings at 51.6V and 4.66A which once paralleled would give me 51.6V at 9.32A going to the MPPT controller.....i realize i am still below the 5-15% of my battery bank (415 Ahrs) but if my combiner is closer to my batteries then technically i am reducing my wire run for the higher amperage and could make it work......no?????

You really only need a combiner box + fuses/breakers if you A) have 3 or more parallel strings or you plane on adding more strings in the future.

and if this logic makes any sense....then i could connect all the panels in series at the combiner (which could be within 20 feet of the battery bank) and then i get my 27.96 amps (which is still only 6.74% of my battery bank) into the MPPT controller with only #2 wire.....

In the end, to really have a useful PV off grid system, you should be aiming at 5% or more charging current from your array (of the 20 Hour Rate battery bank capacity). You can get away with less--but you end up with a fair amount of power just keeping the battery bank charged as it gets older (old batteries tend to self discharge a fair amount--AGM's are better, but are more expensive than flooded cell type batteries).

does this make any sense??

Yep... In reality, we are always money limited in building the systems and compromises have to be made. Starting with a large bank/small array and adding panels in the future (up to 13% or a bit more) is usually the most cost effective way of doing it.

However, you will need to watch your battery state of charge like a hawk. You have very little room and can end up running the battery bank dead by accident pretty easily (deficit charging/under charging/running dead and over charging are two popular ways of killing battery banks).

A couple recommendations:

1. Get a Kill-A-Watt meter for your 120 VAC loads and/or a DC Amp*Hour/Watt*Hour meter for your DC loads.
2. Get a Battery Monitor. Will probably save you a battery bank or two over the years
3. Look at buying solar panels >100 watt rating. The larger panels tend to be cheaper per watt and require fewer electrical connections and mechanical mounting hardware than a bunch of small panels.

and could anyone recommend a good MPPT controller or are they all the same...

No, they are not all the same. There are some good, and some very bad. Our host's web store is a good place to start.

The new MorningStar MPPT line is a good place to start looking. The Xantrex XW MPPT Charge controller is also nice. And there is the Outback line tool.

Note that several of these vendors offer both PWM and MPPT type charge controllers--So be careful that you get the correct MPPT type. Also the new 45 and 60 amp MorningStar MPPT controllers (supposed to be very nice) are just rolling out now--they may not be in stock yet (just guessing--I am not connected with any store/vendor in any way).

You can also check out the Rogue MPPT controller (I don't think its input voltage is high enough for you; 60 VDC maximum Voc). Even has schematics available if you want to look at the details of how a modern MPPT charge controller works (even kits available too).

Well, that was a lot of typing. Hope at least some of it makes sense.

-Bill

Re: increasing capacity of a 12v system

Pierre,

You are very welcome--we all try to help here.

If your Imp's are within 10% of each other, you can connect them in series without much issue (the power curves are not that "sharp at the peak"--so being a little off does not change much in the end).

Similarly, being of by 10% or less for Vmp is OK for paralleling panels.

Now, you may be somewhere around Montreal Canada. And cold weather makes the solar panels run at a higher Voc/Vmp. So, while that is great for power output, you have to be careful that your weather does not cause the panels to go over voltage on a cold winter morning or under voltage on a hot, windless, summer afternoon.

On the "hot side", you will not have a problem. You are currently running a 12 volt battery bank and 17.5 volt panels (even in parallel).

On the cold side, Voc (cold) can cause the voltage to rise over the controller's maximum solar panel input rating. I don't believe you have said what MPPT controller you are going use or exactly which 80 watt panels--So, I will use some generic information to show you what to look for.

Here is the Xantrex XW MPPT Charge Controller String Sizing website.

For example:

BP 280 H solar panels:
Pmax @ STC 80 W
Pmax @ PTC 71 W
Vmp at Pmax 17.6 V
Imp at Pmax 4.55 A
Voc @ STC 22.1 V
Voltage change -80 mV/C

Lets assume your minimum practical temperature with the sun up is -20C and the maximum is +35C and this is a 12 volt battery bank. Plugging this information into the Xantrex Website shows:

Max VOC at Min Temp (cell temperature -20°C)
Min VMP at Max Temp (cell temperature 35°C)
2 Modules 51 Vdc 28 Vdc
3 Modules 77 Vdc 42 Vdc
4 Modules 103 Vdc 56 Vdc
5 Modules 128 Vdc 70 Vdc

So, the Xantrex website would recommend between 2 modules in series to 5 modules in series...

1 module would give you a minimum of 14 volts in hot weather (this assumes that the solar cell temperatures are +35C hotter than ambient air from \ heating by the sun). And the controller+wiring has a couple volt drop--so the charging voltage at the battery is ~12-13 VDC--not high enough with one panel (in very hot weather with no wind).

6 modules at -20C would give you a Voc of ~154 VDC which is over the 150 VDC maximum input voltage of the Xantrex XW charge controller.

So, for this case, since you have 6 total panels, then you would run 3 in series and put the two strings together in parallel.

Could you run 6 panels in series... Possibly. But many of the MPPT controllers will log the maximum voltage they ever see--so if you have any problems and it logged >150VDC, they may just void your warranty no matter what went wrong (and you possibly have reduced the life of your controller with the over voltage).

Of course, you panels have a little bit lower Vmp/Voc, so you may be able to put 6 in series (17.2 volts would still estimate 150.5 VDC at -20C--depending on MPPT controller you purchase--many have lower working voltages).

If you have a need for running 6 panels in series (for example a very long wiring run from panels to charge controller)--you may wire up 6 and jumper to 5 panels when you are scheduled to get any weather below -10C).

Also, you will see the string sizing website gives combinations of panel connections and their estimated power/amp output. Note there is a STC and PTC rating...

The PTC rating is based on realistic expectations (panels run hotter than STC conditions in moderate to hot weather--California uses PTC numbers for its rebate programs to predict power output) and should be used for working numbers.

The STC numbers (standard temperature conditions) are possible in your region during the winter (thanks to your new MPPT charge controller which can take advantage of rising Vmp in cold weather).

Anyway--that is what I see.

-Bill

PS: Try to keep your strings "balanced". Put the one each of the older panels + 2 each of the newer panels in each string of 3 so that Vmp is as balanced as practical. That will help the MPPT keep things optimum.

Re: increasing capacity of a 12v system

Pierre,

You have it correct. You may get a little more current (or even a little less) than you calculated... The Power curve is pretty flat so if the current is reduced a bit, the other Vmp's may go up a little bit--but in the end, those are "optimum" numbers from the manufacturers and it works out pretty well to assume that you will get about 77-80% of panel rating into the battery during warmer weather. During cold weather, with reflection from snow, you will probably do better.

The FCC class B rating can be a very nice thing to have. For the most part, you will notice improvement in analog receivers (AM/FM radios, HAM, etc.). Digital equipment using spread spectrum (cell phones, digital TV's, etc.) will usually be affected less by RF noise in strong signal areas. In weak signal areas, with lots of RF interference, you may just not get any useful reception and/or lots of drop outs. It is hard to say--plus many times the noise is very directional--so depending on your setup, your gear may be experiencing more or less noise as you walk around the property.

I also find that gear designed to meet EMI (electromagnetic interference) requirements tend to be more rugged and resist static discharge and other nearby interference sources (transmitters, lightning strikes in the area, motor starts/stops, etc.).

Lastly, for lower frequency RF (like AM radio), you just have to move the radio antenna farther from the source of the noise. Low frequency noise (like AM) is hard to reduce and causes lots of issues with any equipment for those that use AM radio for music/news away from population centers. Directional antenna and sometimes the people just turn of the charge controller when using the radio (like in an RV) so they can listen in peace and quiet.

-Bill