Author Topic: still new to solar and need some help  (Read 147 times)

Offline Zahkar201

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still new to solar and need some help
« on: October 12, 2018, 11:34:58 AM »
need suggestions on a cheap but decent charge controller.
 i have a small 12v system that runs a couple small 12v pumps, 12v solenoid, and sme 12v led light strips. i have two batteries for it as well.
  i found some solar panels on a craigslist like form. they were 300w but what i didn’t realize is that the output was 24v and i smoked the charge controller. granted, it was a amazon cheapy.
 i need a suggestion for a charge controller that will take the 24v input yet run everything at 12v, or am i looking at this all wrong?

Offline Alan Georges

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Re: still new to solar and need some help
« Reply #1 on: October 13, 2018, 07:19:13 AM »
It sounds like you need an mmpt charge controller.  Many of those can step the voltage down to 12v, and all of your batteries and devices will work fine on the low side of the controller.  These people have a lot of good FAQ pages and can set you up:
https://www.solar-electric.com/solar-charge-controller-basics.html
https://www.solar-electric.com/mppt-solar-charge-controllers.html

300 watt panels are big.  Have you sized out how much battery you're going to need, and how much power you really want to generate and use each day?  The reason I ask is that the rest of the system to keep up with a 300 watt panel is not cheap.

Offline fratermus

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Re: still new to solar and need some help
« Reply #2 on: October 13, 2018, 11:20:31 AM »
Yes, MPPT will do what you want.  A 30A MPPT charge controller will probably be something like $160 if you shop aggressively and are ok with Chinese gear

Unless you are curious, ignore everything that follows.  :-)


The 3x price difference between PWM and MPPT is significant but it can handle high voltage panels and nominal 12v banks.  MPPT also , generally speaking, makes more power available to your batteries and loads even with both panels and battery bank are the same nominal voltage (e.g. 12v).  Particularly when the batteries are cycled deeply by morning.  Or when running loads after your batteries are in Float mode.   There are exceptions involving scenarios like 12v poly panels and high ambient temps in which PWM can make slightly more power than MPPT in Absorption mode.

Not trying to sell you on MPPT, btw, although they are a good fit for my uses. Technically the PWM/shunt/whatever cheapie probably would too, but you only get about ~150w of power out of the panels due to running them at about half their normal voltage. The trick is the controller has to know what voltage the battery bank is before connecting to the panel. 

This is one reason why we:
  • connect the controller to the battery bank; then
  • connect the panels to the controller

Backstory:  when PWM (or shunt) controllers are working they run the panels at battery bank voltage (Vpanel == Vbatt).  So the 24v panel would be running at 12.8v, or 13.8v, or 14.8v, or whatever the bank is at that moment. If the battery/loads aren't accepting current at that time the PWM  knows to go to 100% OFF (effectively disconnecting panel from controller) and the panel can do whatever it likes with no ill effect.   

If the controller is hooked to the panel before the battery, the controller is not moored to a sane voltage by the battery bank.  This can allow the panel to soar to Voc (highest possible voltage), around 40v with the controller's front door wide open.  40v in a 12v controller is Not Good, as the currect example likely demonstrates.