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hello all im asking this question on behalf of a farmer i know without internet access'
"G'day everyone, i have 300 watt windmill on my farm house roof, and i have
my battery bank is set up for 24 volts,
I would like to add a small 5-20 watt 12volt solar panel to keep my 24volt batteries
trickle charging on days when theres not much wind around.
my question is can the 12 volts from the solar panel go directly into
my battery bank which is 24 volts,
thankyou for any help you might be able to give, arthur."
I hope someone can answer the question for him,
cheers
bob
VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539
Posted: 10:43am 29 Aug 2010
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Hi Bobt
The short answer is no, not 1 12 volt panel but if he gets 2nd and connects them in series the voltage is high enough to charge 24 volts, with such low power panels it will only apply a float charge, and shouldn't need a regulator.
If he expects to get any usable power he should go for 2 80 watt panels and a regulator. Make sure the panels have a series diode to prevent back feed during non sun time which will flatten the batteries.
All the best
BobEdited by VK4AYQ 2010-08-30Foolin Around
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904
Posted: 02:22pm 29 Aug 2010
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Bob, I actually measured the reverse (discharge) current of a 80W solar panel connected to a battery without a series diode overnight.
It is minuscule and nothing like the "flatten the battery" that often gets quoted.
And, "non sun time" is only one night before the batteries are charged again.
Most solar panels do have a diode already fitted and I think the manufacturers do that for a good reason but, IME, leaving it out is not quite as catastrophic to the battery as many seem to think.Klaus
VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539
Posted: 04:22am 30 Aug 2010
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Hi Klaus
You are right about the reverse current being very low without a diode, but it is a good protection for a couple of dollars, I have a small 20 watt panel that I use to keep batteries up in the farm machines, I measured it without the diode at 48 Ma not a lot but it does flatten the battery a bit over a few cloudy days.
I have purchased some of the cheap panels being imported from China and found several didn't have a diode included in the terminal box, so I am working on the premise that its better to be sure than sorry. The other thing is that I have been known to connect the panels wrong polarity on occasion when having a bad day, so a diode helps there to.
Thanks for your comments.
All the best
BobFoolin Around
imsmooth Senior Member Joined: 07/02/2008 Location: United StatesPosts: 214
Posted: 12:29pm 30 Aug 2010
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I agree. A diode is cheap. A connection mistake without one isn't.
VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539
Posted: 04:55pm 31 Aug 2010
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Hi Arklan
The voltage doubler circuit only works on AC not DC as in a solar panel, it would require a switchmode power boost converter which is a bit lossy and beyond the capability of a person at this level of electrical knowledge to construct, but are available on ebay.
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