solar powered circuit

[overmind]

hello, here's what i want to do, i have a load that operates for 24 hrs and consumes 10W (24v). i connect two 12v 15Ah lead acid batteries in series and connect these batteries in parallel with a solar panel (30W 17V with a solar shunt regulator). my question is, is my setup correct? will this be sufficient for 24hrs of operation? or is this an overkill? will it fully charge the batteries during daylight? any comment would be much appreciated.. thanks..

 

[mneary]

You didn't fill in your location.

Most locations only have 8 hours of useful sunlight each day, with a lot of that being less than full intensity. 30W x 8h would be 240 Watt hours. The sky must always be absolutely clear, the sun directly against the panel, and the panel must be kept at its rated temperature (probably 25c), so deduct power when the sun heats the panel. If your panel isn't moved to continuously face the sun, deduct some more. If the panel collects dust, deduct more. If you're not within a few degrees of the equator, deduct more for the shorter days in winter.

Your battery will have losses in the charging process. Expect only 80% of the energy back from charging a lead acid. If you are using a 17V solar panel on a 24V battery, you'll need a boost converter to get the 28V that the battery needs to accept energy. If your design is good, it might be 90% efficient.

You need 240 Watt hours each day. At a minimum, you need twice as much panel, maybe 3x.

Your battery is 360 Watt hours, so it will run the fans for about 36 hours. If the solar panel was big enough to keep the battery charged, it would still be dead at the end of two rainy days.

 

[overmind]

hi thanks for your very informative explanation, what i want to do to get 24v is connect the two batteries in series and connect these batteries in parallel with the solar panel. with this setup, will it efficiently charge the two batteries since i only use one panel for the two batteries?

 

[helpmonkey]

your are correct in your wiring but you cannot charge the batteries in paralell with only a 17 volt panel ... you need to charge tha batteries in series but as mneary said it appears you do not have the pv panel capacity to keep the batteries charged.

 

[mneary]

hi thanks for your very informative explanation, what i want to do to get 24v is connect the two batteries in series and connect these batteries in parallel with the solar panel. with this setup, will it efficiently charge the two batteries since i only use one panel for the two batteries?

The 17V solar panel will DISCHARGE the 24V battery. But not for long, since it will quickly destroy the solar panel, the wiring, and possibly the battery as well. If it fails to destroy the battery immediately, the resulting fire will take care of that.

If you mean connect the batteries in parallel and series at the same time, don't. This too will burn everything up.

 

[helpmonkey]

overmind ... a solar panel is a power source like a battery but it is also a semiconductor... as mneary said if you hook up a more powerful battery to the solar panel its like hooking up a diode where the battery voltage is higher than the diodes breakdown voltage ... heat then maybe fire and it then possibly shorts the battery... bad bad bad... if you are measuring your solar panels open circuit voltage and it is 17volts then it is a 12volt panel ... think of your PV panel as a battery but a battery that has its own charging system ... so never try to charge the panel with a battery or you will destroy it ... the answer to your origional question is NO ... you cant charge a 24volt battery with a 17volt PV panel.. connect two 12volt PV panels in series and connect and have them charge the two batteries which are in series but the PV-> batteries would be parallel...

 

[helpmonkey]

you must also consider capacity or the system will completely discharge the batteries which will sharten their life span... if you need mare capacity just add more PV Panels in parrallel with the first two

 

[overmind]

hi.. thank you so much for all your replies.. i really appreciate it.. this is my new circuit.. will this be able to sustain my project? i welcome any comments.. thanks again

**broken link removed**

 

[helpmonkey]

ok est your batteries should be able to power the load for 24V x .5A=12W so 70aH / .5aH = 140 hrs (fully drained) ... your panels when at peak performance supply 50W / 24V = 2.08aH ... your load consumes aprox .5aH so for every peak hour you panels can only supply enough energy to power you load for 4 hours 2.08aH / .5aH = 4hrs .... so you would need 24 / 4 = 6 peak hours of solar production on a daily basis to keep up with your load ... that is cutting it a little thin and also depends on your location... in a sunny location say US Arizona you might get away with it ... at a more polar location Canada, Ontario you would probably not be able to maintain the load... of course a solar tracker would help. )

 

[mneary]

Each solar panel is 50W (about 4A, not 2.08aH). So the break even point is nearer to 3 hours of sunlight per day. Your reserve is 140 hours, or almost 6 days. If you drain your reserve completely, it will take 8 or more sunny days to replenish. Over the long term, the sunny days must be more numerous than the cloudy days, although you might get some power during a light overcast.

The fan should have a low voltage cutout so it doesn't damage the batteries by discharging them too deeply. If your fan is "24V" you might save a little power by making sure it doesn't use more power when the battery is charging and its terminals are 28V.

The PV array was probably rated at the Equator, and should be adjusted accordingly by latitude, climate, and season.

 

[Gary B]

Charging specifications call for a float voltage of 2.25 volts per cell for a led acid battery. Since a 12 volt led acid battery has 6 cells, the float voltage should be 13.5 volts. To charge a series stack of 12 cells, the charging voltage has to be 27 volts minimum to obtain a full charge. To get a quick charge when the sun is available, you will have to have an even higher voltage available to get to the 90% charge point. Then you can switch to float operation. As long as your solar controller can monitor the charge state of the batteries to prevent over charging, I don’t see a problem that hasn’t already been beaten to death by the other replies.

 

[mneary]

I did not intend to define a proper solar controller. I had hoped that mentioning some voltages would not confuse a careful reader. When charging or float potentials are being applied to the batteries, those voltages are greater than 24V and would also appear at the load.

If a voltage at the load greater than 24V causes waste, then it might be worthwhile to regulate the load.
And it would be wise to equip the load with a low voltage cutoff to prevent discharging the battery beyond recovery.