device to only draw from panel and not from bank?

browningbuck

I was curious if you can use a diode or some other circuit to allow a device to use energy from the panel when it is available but not draw on the battery bank?

example is a small 12v 100w panel with trojan 27tmx battery and controler. most the time the battery system is full only led lighting currently drawing on the system at night. however i would like to be able to wire in a 12v fan to run constantly when the sun is out however if a cloud or storm or what ever comes by and there isnt energy from the panel available it will not draw from the bank. what would i need to build? or just the proper term for me to search as "diode and solar panel" is not the correct string of words to look for. thanks for any help in advance.

BB.

Re: device to only draw from panel and not from bank?

Yes, you could--but this can raise other issues.

A diode will have between 0.2 and 1-2 volts drop across it when passing current (depending on diode type and ratings).

So, we tend not to recommend using diodes on solar arrays because they drop siginificant power (voltage drop * current = lost power) and the voltage drop reduces the charging voltage available to the battery bank.

It is doable, just need to understand your needs and components (Vmp/Imp rating of panel, how hot it gets in your area, will you have a solar charge controller, size/rating of fan, etc.).

Today, we usually recommend using some sort of charge controller between the solar panels and the battery bank. This helps prevent the battery bank from being over charged (boiling water dry, etc.).

If you use a charge controller, any modern controller does the "diode" function anyway... So, you can place the fan between the array and the charge controller input. The fan (use appropriate rated fan for array voltage) will run whenever there is enough sun, and the charge controller+battery bank will take the rest of the current (as needed) from the array.

For larger solar systems, we generally use a voltage controlled switch (i.e., battery voltage > 13.x volts, turn on fan) or (some higher end) charge controllers can be programmed to turn on when the battery bank is charged and there is excess charging energy available (or when the battery is above "gassing voltage" and need to pull hydrogen away from battery box).

Lastly, another option is to just purchase a separate 10 watt (or whatever) solar panel and connect directly to the fan... Will work fine and avoid any issues with charging/operating your battery bank. Very common for vent fans, solar hot water circulating pumps, etc. where operation is only needed when the sun is up.

-Bill

solarix

Re: device to only draw from panel and not from bank?

should be able to connect your dc fan directly to the pv panel. The charge controller already acts like a diode and prevents current from flowing backwards from the battery.

browningbuck

Re: device to only draw from panel and not from bank?

Bill thank you for your response, the system is in design phases currently so thats why i am asking before building. The set up will be a tear drop trailer for traveling. the idea of using an individual panel for the fan isnt as cost effective as id like to do this, do you have a link or a search string so i can better educate myself so that i can possibly ask better and more informed questions on this topic?

browningbuck

Re: device to only draw from panel and not from bank?

solarix, i see what your saying and that actually makes sense to me.... now is that ok for the system? and thank you again everyone for your help.

BB.

Re: device to only draw from panel and not from bank?

Yes, the problem is smaller panels are much more expensive ($$$/Watt) than larger panels... There is not one good answer--You pretty much have to do paper designs (and possible experiments) to figure out the best solution for you.

I suggest we start at the beginning and figure out a paper design based on what you have so far.

First you have a 105 AH @ 12 volt battery (we use the 20 Hour discharge rates for our rules of thumbs). To properly recharge such a battery we use 5% to 13% rate of charge:

• 105 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 99 Watt array minimum
• 105 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 198 Watt array nominal
• 105 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 257 Watt array "cost effective maximum"

You have a 100 Watt panel--First, what is its Vmp/Imp rating... For a 12 volt battery bank we look for Vmp~17.5 to 18.6 volts or so for a "12 volt" panel. If the Vmp rating is too low, then it will not fully/quickly recharge a 12 volt battery bank on a hot day (with controller+wiring+panel derating when hot combined losses).

A 98 Watt panel is the minimum we would recommend for recharging your battery. This means that any power you tap for running the fan needs to be pretty small (5 watts or so maximum). If you put a 20 Watt fan on the array, you will be drawing too much energy and never fully recharging the battery except when you have AC shore power/generator power available (kind of common for a small RV--limited space for solar panels and batteries).

So, this becomes the question of how much energy you will actually be using... A 5% rate of charge system should be committed to charging during the day and discharging at night. If you have significant daytime loads (radio, laptop computer, etc.) and don't want to use a genset, I would be concerned that you do not have enough panel to be full time "dry camping" (i.e., living off grid).

If the trailer is weekend use/seasonal, you can bend the rules a bit. The battery may not last quite as long--But it really does not matter (i.e., battery may die from age vs deep discharging a few dozen times a year).

So, if you are dry camping in the southwest, and have reasonable sun (avoiding winter/monsoon season), you will get ~4 hours of sun per day:

• 100 watts * 4 hours of sun * 0.52 AC system efficiency = 208 Watt*Hours per day (of AC power)
• 100 watts * 4 hours of sun * 0.61 DC system efficiency = 244 Watt*Hours per day (of DC power)

All depends on your needs.

-Bill

bill von novak

Re: device to only draw from panel and not from bank?

browningbuck wrote: »

I was curious if you can use a diode or some other circuit to allow a device to use energy from the panel when it is available but not draw on the battery bank?

Get a small 6V panel. (You can get one for $9.99 from Radio Shack.) Connect it to the coil of a 5V relay that connects the fan to the main system. When the sun is out the fan will run, and when you lose the sun it will shut down.

browningbuck

Re: device to only draw from panel and not from bank?

thank you both of you! i knew i was cutting that size battery fairly close with regards to panel size, i understand max discharge and will do everything in my power to avoid it including forgetting to battery tend it. as for the design aspect of things i think i REALLY like the idea of a relay system a NO relay with a 6v mini panel is creative to say the least. Bill i think i will take the advice and also up size the panel to a 240W~ or such. i am still a little unsure of what/ how much energy ill use while dry camping. so thank you for everything both of you.


looking at http://www.solar-electric.com/eoplly-ep125m-72-190w-monocrystalline-solar-panel.html

http://www.solar-electric.com/solarworld-sunmodule-sw245-polycrystalline-solar-panel.html

http://www.solar-electric.com/trina-solar-multicrystalline-240-watt-tsm-240pa05.html



thought on them or other suggestions?

BB.

Re: device to only draw from panel and not from bank?

None of us moderators work for, or are connected to our host NAWS (Northern Arizona Wind & Sun).

Some general things to look at...

Shipping--It is very expensive to ship solar panels... And the fees can be higher for the larger panels (over ~140 Watt). And panels much over 175 Watts usually need two people to move/install them safely.

Also, several of those panels are Vmp=36 volts, and other is Vmp=30 Volts... Gets into your battery bank voltage (12 volts at this time). All three panels will work with a 12 volt battery bank. The 30 volt panel will only work with a 12 volt bank (unless you get a second panel to place in series).

Then, you have lean very quicly about solar charge controllers. A PWM type--You need to "match" the panel Vmp to the battery voltage (around Vp=17.5 to 18.6 volts) to "efficiently" couple to the 12 volt battery bank.

An MPPT type charge controller will "efficiently" down convert from High voltage, Low current of the solar array to the low voltage high current needed to charge the battery bank.

If you were to connect a PWM controller to your proposed 240 watt panel--you will lose around 50% of the panels effective energy.

An MPPT controller will not have the problem.

PWM controllers are $100 +/-, MPPT controllers are $300-$600 or so +/-...

I have to go right now, but here are some quick links:

http://www.roguepowertech.com/index.htm
MidNite Solar Kid MPPT Solar Charge Controller
Morningstar SunSaver 15 Amp MPPT Solar Charge Controller
Morningstar TriStar 30 amp MPPT Solar Charge Controller
Morningstar TriStar 45 amp MPPT Solar Charge Controller

All are good companies and good products. You only "need" a 15 amp controller with a single 240 Watt panel. The "Kid" is due out any time (call NAWS for details). Rogue has been a very popular and reliable controller too (build in Oregon).

Do your paper design first and ask questions... This stuff is a bit like building a jigsaw puzzle. It is easy to buy parts that don't fit together properly.

I would also suggest you look at a Battery Monitor (such as the Trimetric, or Victron is another good brand). Makes understanding your battery usage/charging much easier. But they are not cheap.

The Kid+Wizbang Jr (remote current shunt) may make the suggestion for a Battery Monitor suggestion redundant (not sure, you need to research).

-Bill

browningbuck

Re: device to only draw from panel and not from bank?

Bill, quite informative, however it seems that this is starting to get way out of my desired price range. I was hoping to put together 100w-200w system for ~$400 ish i think i may have been a bit mislead by the things ive seen around the webs.

so i know you get what you pay for... that being said im really trying to keep this cost effective. so thoughts?

100w panel
$128
Optimum Operating Voltage (Vmp): 18.9 V
Optimum Operating Current (Imp): 5.29 A
Open - Circuit Voltage (Voc): 22.5 V
Short- Circuit Current (Isc): 5,75 A
Maximum Power at STC: 100 W
Operating Module Temperature: -40°C to + 90°C
Maximum System Voltage: 715 VDC
Maximum Series Fuse Rating: 15A

MPPT Controller
$12
http://www.ebay.com/itm/20A-MPPT-Solar-Panel-Battery-Regulator-Charge-Controller-12V-24V-Auto-Switch-/261323217168?pt=LH_DefaultDomain_0&hash=item3cd813d910


then the Trojan 27tmx battery
$157

BB.

Re: device to only draw from panel and not from bank?

It is difficult... Smaller panels (~140 watt and less) cost a lot more money ($$/Watt).



Kyocera KD140GX-LFBS

$265.00
$1.893 140 Watt Vmp~17.7 volts






SolarWorld SW-260 Monocrystalline

$285.00
$1.096 Vmp~31.6 volts @ 260 watts




The least expensive MPPT controller (without going to "off shore" units) is the MorningStar:

Morningstar SunSaver 15 Amp MPPT Solar Charge Controller $225.00

Vs a decent PWM controller:

ProStar 30 Amp 12/24 Volt Solar Charge Controller $118.00



2*$265 panels + $118 pwm controller = $648
1*$285 panel + $225 mppt controller = $510

Add shipping (if you cannot pick up panels locally). And the controllers have optional digital displays and Remote Battery Temperature Sensor (RBTS highly recommended for the Sunsaver 15 MPPT).

Plus wiring, optional displays, panel mounting, fuses/breakers.

Of course, you can go price shopping and see if you can by somethings local (contact a local solar installer and see if you can buy a panel locally to save shipping, etc.).

-Bill