# Battery failure on DC powered exhaust fans

3 Posts
Aug 15, 2009 02:43 pm
Battery failure on DC powered exhaust fans

Hello,
I currently have two 24 volt, 50 watt exhaust fans operating 24 hours on wind/solar thru two sets of 12 volt (in series for 24 volts) 265 Ah batteries.My calculations used are 24 hrs x 100 watt = 2400, used 2 days of autonomy = 4800 Wh, 40% DoD = 12000 Wh, 60 Degrees multiplier = 13,320 and divided by 24 volts to come up with 555 Ah needed on the batteries.
The wind is limited, the solar has 40 - 36 volts to my controller and 24-26 volts output to the batteries.
My problem is the wind/solar does not keep my batteries charged and failure on the system.

Do I have to much battery for the charging system?

Can I regulate the DC power usage without wasted power?
Thank you

220 Posts
Aug 15, 2009 03:58 pm
Re: Battery failure on DC powered exhaust fans

hello ron,

looking at the battery calculations. it seems you did a nice job at trying to figure out your needed power for your loads. two days is a bit of a close margin though.

things get a little sketchy with your pv numbers?

the first thing we need to find out is do you have a 600 watt pv array? could be even more depending on your location! we need to get at least 2.4kwhs back into that bank every day.

kind regards, dave

Aug 16, 2009 11:33 am
Re: Battery failure on DC powered exhaust fans

To make a long story short, think in terms of averages. If you calculate an alternative energy source such as wind or solar to exactly what it appears you will need, chances are it will fall short from time to time. As well there will be a surplus from time to time. Better to have a surplus more often. Espcially if off grid and the load is a critical one.
You state that your wind resources are minimal although its averages are governed by the same factors, I will leave it out and stick primarily to PV.
As I am sure you already know, the output of your power supply, in this case; PV and wind, is an estimate based on averages depending on several factors but suffice it to say, the weather? Each season has its own extremes as far as weather goes. Summer brings more hours of intense solar radiation and heat as Winter brings less with cold. The geographical location and local terrain as well as local flora can have an impact on those averages.
All of these work hand in hand to create what is known as, "The number of hours of equivalent full rated power from a PV module." Its not an uncommon practice to have to, increase the size of a PV array by a certain factor after its size is calculated. Even doubling it in some extremes.
The link below provides, in my opinion, a fair estimate of those number of hours of equivalent full rated power from a PV module when the lowest denominator is used. But its still not an absolute. The exact, intended site most also be taken into consideration as well. http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/

Myself, I like to use no less than a factor of 5 when sizing a battery bank. My figures would have been -
100 watts / 24 volts = 4.17 amps
4.17 amps x 24 hours hrs. = 100 amp hrs.
100 amp hrs. x 5 = 500 amp hrs. (No less!)
(Not that much different than yours so far.)
To replace that with PV -
I would go to that site and choose; "Average", "Annual" and, "Flat Plate Tilted South at Latitude." (Even if I were contemplating the use of a tracker and mppt controller I would use this.)
In my area the lowest number of hours would be 3. So -
2.4 kWhrs. / 3 = 800 PV array (No less!)
Again, since you stated that wind power is minimal I left those figures out. Think of that as an unexpected "wind fall" when the wind does blow.
Site specifics such as but not limited to; mountains, hills, trees, buildings may cause one to increase this by a certain factor.
So, no less than 500 amp hours of battery capacity and 800 watts of PV (in my area) all at 24 volts nominal.

As for wind - http://rredc.nrel.gov/wind/pubs/atlas/maps.html
Unfortunately, I live in one of those areas that show, on a yearly average, a big fat 0. Even though we do, from time to time see a signifcant amount of wind, because of; tall trees, local terrain, soil conditions and, the general rule of thumb that states, "the bottom of the swept area of the rotor should be no less than 30 foot above anything within a 500 foot radius' keeps wind from being a cost effective alternatinve for me. I spent those thousands of dollars on more PV instead of a guyed steel tower and tons of concrete.

3 Posts
Aug 18, 2009 09:50 am
Re: Battery failure on DC powered exhaust fans

Dave,
Thanks for the reply. I have 175 watt solar panel output that I see is probably to low.
I think I will put a 24 volt photocell with relay to remove the load during the night hours and will reduce my needed charging load by half.
I would like to reduce the fan RPM without wasting power (resistor) but that gets a little more complex on DC.

3 Posts
Aug 18, 2009 10:04 am
Re: Battery failure on DC powered exhaust fans

Steve,
Thanks for repling and the sites to visit. As you can see on my reply to Dave that my solar output to the charger is way to low. The budget will not allow more panels but I can reduce the load. The fans are on the Ann arbor parks "outhouse" tank vent stacks. The original application had small solar panels (12"x12") mounted on top of the vent stacks and wired directly to the fans and this allow them to run only on direct sun and at a reduced RPM. When we hooked up the new system the fans run at the max RPM but we don't need this as only a lower RPM will do an adequate job of venting.
Do you have any ideas of how I can reduce the RPM without wasting power?
Any help is appreciated.

220 Posts
Aug 18, 2009 08:08 pm
Re: Battery failure on DC powered exhaust fans

ron,

these solid state PWM controllers may be of some help here.

http://www.bakatronics.com/shop/item.aspx?itemid=444

shutting down at night and running at a %50 duty cycle may just get you by.

good luck, dave

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