12vdc + 24vdc=smoke?

[MM Farms]

Hi guys,

I have a 40watt 12v panel and a 185 watt 24v panel. Will an lcb regulate the voltage to pump motor or is there something else I need? I haven't bought a pump just yet, but I can find better deals on 12v than 24v. I have to pump to storage 500' at 30' head, 0-1' lift.

Thanks

[ghurd]

Quick and simple advice. Get another ~40W 12V panel to use in series with the other 40W 12V panel, then parallel them with the 185W 24V panel. It will give 265W rated at 24V.

Probably the most efficient use of funds and existing panels.

A 24V system will lose less power in the wires than a 24V system. Cheaper wire could help offset the cost of a 2nd PV and 24V pump.

G-

[wpowokal]

Or check in the junction box of the 24V panel to see if there is a 12V tapping. If there is divide it up there, ensuring each panel has a blocking diode.

Then you will have 3 12V panels and probally around 20amps @ 12v, I assume you are locating the panels at the water sourse.

allan down under

[MM Farms]

Quick and simple-

Never one of my strong points. I should explain better.

 I was thinking of hooking 12v PV direct to pump via LCB, 24v PV to 12v batt bank via Morningstar Sunlight 12v charge/light controller between LCB and batt and run pump during low light (for which I need controller for anyway).

 I didn't figure in peak V of 35v for 24v panel and 30v max for 12v controller though, and I'm now wondering if I could connect controller parallel with 12v, 24v and batt. From what I remember, 12v and 24v paralleled will only add amps and volts would remain 24. The LCB would regulate 12v to pump motor and a diode would protect battery from 24v and panels are already diode protected. I'm hoping pump would create enough Vdrop so I wouldn't have to worry about the 35 Vmax of controller.

           (Diode Protected)    

                BATT 12v_____

                     l      ____l___

                     l      l                   l          

            chrg to batt  Timer                 l

                     l      l                   l             Motor  

   PV24v  PV12v     controller                  l     LCB___12v

     l__l___l_______l____l

A crude drawing, but possibly it might help. So my original question remains about LCB limiting voltage, and new question of what bad would happen in this circuit?

Thanks again folks

[MM Farms]

Man, what happened? My drawing went from crude to crud.

[ghurd]

I think I see a couple glitches.

First, a 24V panel goes to V open when that kind of controller goes into regulation, meaning the 12V controller will see about 43V.  Poof!

Next is the panels are most probably not diode protected. Those are probably by-pass diodes. Protection diodes are usually found in PVs under 20W, but you never know.

Allan down under's suggestion is good, if the 24V PV has the option for 12V.

I guess I just don't see much/any advantage to the SL controller and LCB.

A second 40W PV to series with the first for 24V, or reconfigure the 185W to 12V, and use a proper voltage SS-20 with LVD.

The way I'm thinking about it is with your diagram the 24V 185W panel is effectively reduced to a 12V 92W panel, but it will still soon be into regulation mode, reducing it to as much as 0W.  And the battery is not even full yet due to surface charge.  What power does go into the battery is subject to conversion (elec to chemical to elec) losses some people figure at 10%.

So the 185W panel is doing nothing...

while the 40W panel is working its butt off trying to pump water at a reduced rate, and needing a expensive LCB just to do that.  If you can find a 12V pump that will operate with the requirements of power, flow, and head.

Then the sun goes down, the SL controller turns on the pump to run from an undercharged battery until the LVD shuts off the pump, leaving a dead battery, to be surface charged again the next day.

That will result in a battery being used while in a constant state of undercharge, meaning a very short life.  

(cjdock nearly killed his in a month under arguably better circumstances...

http://www.fieldlines.com/story/2006/5/7/15347/13239  )

Might be a good idea to adjust or trick the LVD into shutting down the pump with the battery at a higher voltage.

I'm working on a variation of a common dump load controller now. The dump load (your pump) will come on in 3 minute (or 5 or whatever) blocks. The battery will remain fully charged. Power made will be used in 3 minute blocks. Then the load is off until the battery voltage catches up. Then on for 3 minutes again.

Ain't this fun?

G-