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Bare with me for a moment  and you might see something interesting

 

so I have a spare 37 voc 250 w panel and was wondering if I could charge a 12v battery with it

 but im concerned that the controller could not accept 37 v or at least 31  while  starting ( yes it pulls the panel down to 12v while charging but still concerned)

so I have  a 110v 1500w geyser element lying around  which I placed in line with the solar panel

this caused a voltdrop  over the element of 18v  which meant that the controller now also got about 18 v  which it should be quite happy with ( 18v x 6 a  = 108 w )

( I also  connected the element straight to the panel and heated a cup of water in 20 minutes while playing around )

 the controller that I used was a TES on off type  not wanting to maybe damage a general type scc

  when the small battery that I used was fully changed then the contact opens and charging stops     also meaning that the power draw by the element stops

 

MY BIG QUESTION

Knowing that a pwm controller throttles down the input as the battery charges up

could we not have all the solar power going to the  element  when the battery is fully charged  . in this case 37 v x 6a = 222 w

 what can I do to make this happen

idealy I need to connect pos to neg  closing the element circuit at the controller allowing all the power to element  but that would probably make the controller go boom!

Im looking at a bigger picture here

solar power from roof in line with element ( axpert scenario 100v ish )

and as batteries get full / load gets less  then more power goes to geyser  

2 for the price of one so to speak

 im sure this could be achieved using a wind turbine controller  with a dump

but that's a slep to get and install

 Kaak idea or not ?

cheers Max

 

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1 hour ago, maxomill said:

 but im concerned that the controller could not accept 37 v or at least 31  while  starting ( yes it pulls the panel down to 12v while charging but still concerned)

Many of them cannot handle it. They use a FET that is PWM pulsed (very similar to a MPPT, except no inductor and no fly-back diode), and hence it has the same open circuit voltage limit.

1 hour ago, maxomill said:

could we not have all the solar power going to the  element  when the battery is fully charged  . in this case 37 v x 6a = 222 w 

 what can I do to make this happen 

You need to impedance-match the load with the PV panel. If you want it to work, then 6A must flow at 37V, so the element you use must have a resistance around 6Ω.

If it doesn't, then you need to buck or boost the voltage to match it, and then you have an MPPT for all practical purposes.

But...

Excuse me while I play Grammar nazi for a bit :-)

It's a hare-brained idea, as in a species of rabbit. I'd let that one slide, but you also bear with someone (as in carrying something), not bare (as in getting naked). Please forgive me for that :-)

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Have you tried to connect the element in parallel with the input of the controller. The amps through the controller should drop and it might take longer to charge the battery, but once the battery is done and the contact opens, you should have the full power available for the element?

Just a guess, dont have everything in front of me to play with and dont know exactly what controller you are using? Please try it?

PS: You will never have the full 37 volts to work with as that is VOC..  (in this case 37 v x 6a = 222 w)

Edited by Jaco de Jongh
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4 hours ago, maxomill said:

so I have a spare 37 voc 250 w panel and was wondering if I could charge a 12v battery with it

Another thought. Get a MPPT.

Smallest is 75/15, 75 volts and 15 amps max from panel to charge 12v battery.
Max panel size for 12v is 220Watt panel, but at 25% oversize, 250watt panel fits, no?

Or 100/30, which is for a 440w panel on 12v, but probably not worth the extra costs.

Edited by Guest
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Ok guys

the spare panel and test was just my way of explaining  how I got the idea

 sorry I didn't make myself clear, my bad

my big picture or question  is

would it be possible to use this idea on a bigger scale like connecting to a house that has a larger solar set up  ?

first of all  could it work ? and if so, what  do we need to do it  so on and so forth

so my theory is  that when the batteries are full or the load is small  then all excess energy goes to heating (geyser) and is not being wasted

like having an inline dump load

my thoughts go along the lines of having  a geyser pre feeding your regular geyser  which has say a 4 kw element  ( 220 v x 18a or 100v x 40 a )  ,  and gets heated by the pv   panels 

I've seen a couple of posts about heating water with the excess power which is where I'm going but just from a different approach or thinking out of the box

im wondering if this way could be easier than monitoring the batteries voltage  and special contactors  and all those things

 

ps I did like the idea of placing the element in parallel and not series  nice

pss  the hair thing was  just a pun as I  have very little left , also used it just to emphasize how hare brained the idea  actually is ,  glad to see someone is awake

psss what do you call 10 rabbits walking backwards     a receding hare line

 

cheers

max

Edited by maxomill
after thoughts
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8 hours ago, maxomill said:

so my theory is  that when the batteries are full or the load is small  then all excess energy goes to heating (geyser) and is not being wasted

The only challenge that I can see on that one is that you wont be able to disconnect the geyser element when the geyser reaches temp. On small scale it should not be a problem to disconnect the dc but when push lager amounts of dc amps I suspect that you will a problem with the arc.  If your element is small enough so that you wont get a geyser that goes boem on a sunny day , it could maybe should possibly work .. 

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1 hour ago, PaulF007 said:

.... able to disconnect the geyser element when the geyser reaches temp.

Reminds me of the time we built on, on top, we lived in a small portion left over. So, geyser was outside on the ground, temporarily connected pipes and a manual on/of switch.

Guess what ... we forgot to switch it off a few times.

You learn that when you hear the water falling on the roof, spewing out of the geyser, boiling hot.

Duratherm geysers with their 10 year warranty, respect. It is still operational.

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