?? How to charge 12v batteries with 24v panels???

[Dualfuel]

I was given three 24volt panels...new...its a great boon, except, I am running 12volt components. So I was wondering if there was a device I could make that converted the 24vdc into 12vdc. What about a joule thief in reverse? thoughts? Links?
The most practical option I have right now is to reconfigure some batts, and a Xantrex 60amp charge controller for 24v, then powering two 1kw UPS's to power some 1/3hp motors to turn 12v SI10 alternators to charge the main bank....waiting....for....the ....laughter to die down. I never said this was efficient or simple, just that all the components are here and ready to plug and play.
So...anybody got something simple to build that could just change the 24v to 12v....

[Dualfuel]

http://www.solar-electric.com/inverters-controllers-accessories/chco/ouchco/outback-power-smartharvest-sccm10-100-solar-charge-controller.html

looks like MPPT tech is the answer...

[analogman]

I may not be understanding what you are saying but I had the same situation where my system is 12 volts as built years ago then when the price of panels fell a couple years back I just bought another xantrex c60 (non mppt) and hooked up 8 24volt panels (225 watt panels). Its been working perfect for the last couple years. Am I getting the most out of my panels?...no, but I keep the most (money) in my pocket because a mppt that would handle 1800watts of panels would have cost me more than all the panels did. Works for me. The batts will clamp your 24 volt panels to 12 volt you don't need anything else (other than a c60 or a tristar works too)

[BruceM]

'' The batts will clamp your 24 volt panels to 12 volt you don't need anything else (other than a c60 or a tristar works too)''

Only a pv charge controller that is chosen for a higher voltage input and 12 v battery output will do this- lead acid batteries don't ''clamp'' anything, they would just go over voltage and start generating hydrogen and oxygen gas like crazy before self destructing.

The unit dualfuel selected is limited to 10 amps maximum to the battery, but takes a wide range of higher voltage inputs.  For that unit more than a rated 200 watts of pv input would be wasted on a sunny day at optimum panel angle, though the extra capacity would help greatly for off axis 9early and late in the day) or haze/clouds.

[analogman]

I used the wrong word technically, should have said load the panels voltage down to  battery voltage + (drop through fets in the controller) .3 from memory in a the c60. I stand  corrected

"Only a pv charge controller that is chosen for a higher voltage input and 12 v battery output will do this- lead acid batteries don't ''clamp'' anything, they would just go over voltage and start generating hydrogen and oxygen gas like crazy before self destructing"

Both those units have specs that greatly exceed the voltage from any 24 volt panel I have ever seen, those are the only two I mentioned as while I am sure there are many others, those are models in existing systems charging 12 volt banks with 24 volt panels,that I know work. I never recommended doing this without a controller so "generating hydrogen and oxygen like crazy before self destructing" is not a issue. Duel fuel mentioned the C60 so I thought maybe he has one already, and with free panels thought he could be up and charging without further cost. Without further cost is always good

[mike90045]

The answer is a MPPT charge controller.  Morningstar, Rogue, Midnight all make good reliable ones at various wattages

Your batteries charge at about 14.5V (deep cycle styles with solar power)  So you need about 18v on the panels.

If you have 24V panels, you will loose some wattage due to mismatch (no matching "transformer" like 300ohm twinlead and 75 ohm coax)
say the panels are 24V, 200W, they would produce 8.33A..   That is all the amps they can produce (under normal lighting conditions)
If you "load them down" to a 12V battery charging at 14V @ 8.33A = 116.66 watts using a PWM controller

So, depending on how many panels you have, how much power you need, you have the simple math (above) to see if a $100 PWM controller will work, or if you need to shell out the $$ for the MPPT controller.

Where it really matters is not the long bright summer days, but the cloudy, marginal winter days.  Yesterday, my 5Kw of pv produced all of 2430 wh. with 2 top of the line MPPT controllers. The generator will be in great use for the next 10 cloudy days

[BruceM]

Mike, I've found that I get very effective charging on overcast days without MPPT, with my own linear charge regulation system on my 120VDC battery bank.

Where panels match the battery voltage, MPPT is likely of modest benefit... but where you want more flexibility for panel voltage vs battery voltage, they are technically marvelous, assuming you don't care about EMI, which can be equally bad for just the PWM types.

Analogman, sorry for the tactlessness; I was just concerned that someone might take you literally regarding batteries clamping voltage and do something destructive.  You did clearly mention the controllers being there.

[buickanddeere]

As previously stated. Just use a charge controller which will limit terminal voltage to 13.8Volts or so. A 24V PV will still charge a 12V battery during cloudy, mornings and evenings when a 12V PV is not charging .

[BruceM]

MPPT chargers with higher voltages specified for PV input can also save you some cost on wiring and reduce wiring losses from the PV array to the charge controller.  This can be significant for larger arrays, especially for 12 and 24V battery banks.  Wire voltage loss depends on current alone, so increasing the voltage of the PV array to reduce current helps greatly.  Unlike a basic PWM controller, the MPPT will do a buck inductive conversion and convert watts in to watts out with only a very small loss.

[veggie]

My panels are 24 volt and my battery bank is 12 volt.

The MPPT charge controller takes care of everything.
You just have to tell the charge controller what voltage you battery bank is.
In my case, that is done with a DIP switch.
In other cases, the MMPT charge controller is self sensing for battery voltage.
So, just make sure that your Charge Controller is MMPT and you should be ok.

veggie

[buickanddeere]

As previously stated. Just use a charge controller to limit  outage to the batteries at 13.8V.
The 24V PV's will charge mornings, evenings and while overcast . When a 12V PV would not charge .