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MPPT on solar pump panels


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Can someone please confirm to me if this will work, I am going to put the solar panels back on my pump after they were stolen by my neighbours disgruntled employees. I would like to put some lights and an alarm, or electric fence maybe on a 12v battery. The solar pump panels will be 500W 24v, can I just attach a suitable mppt to charge the 12v battery, or will it interfere with the pump operation?

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Without going into much detail...

The way an MPPT works is that it constantly seeks for the best voltage / current point (maximum power point) where it can get the most (power) from the panels connected to it. That simply means that the voltage measured at the panels will alter constantly. If your solar pump also contains a MPPT on the input, it will do the same and the two MPPTs might "fight" constantly as each one of them is trying to get the most out of the panels. PWM controllers on the other hand are not "intelligent" and will take what they can get and put that through to the load, depending on what the load requires - so technically two PWM controllers will not necessarily "fight", each one will take what it can get to supply it's load.  Depending on if the solar pump makes use of MPPT or PWM technology (or neither) it might or might not work if you add a MPPT. I would suggest the following:

  • If the solar pump does not contain any electronics and is directly connected to the panels, you can add a PWM or MPPT charger for the battery
  • If the solar pump contains a MPPT controller, I would suggest to add a PWM controller to charge the battery
  • If the solar pump contains a PWM controller, you should be able to add a PWM or MPPT controller to charge the battery

Maybe @plonkster has a different opinion and can make a better suggestion.

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I'm not sure to agree or disagree :-)

Depends on the kind of pump. I know some pumps have a DC motor and a fairly simple controller. Pump simply runs at the speed corresponding to the power available from the panels. Having a small charger connected to pilfer some power away to charge a battery might be okay with such a setup.

With the other setup, you usually have VSD (variable speed drive) which is essentially an MPPT combined with an adjustable inverter. The MPPT will find the optimum power point and then select a corresponding voltage and frequency to drive the pump at so that the power is used efficiently to drive the motor. Attaching extra loads here would be a bad idea. The MPPT tries to find the optimum power point mostly by making small adjustments and measuring the effect, and when you have another device also drawing power, they will always end up fighting each other.

I'm also not sure if two PWMs will live in harmony. The PWM controller essentially tries to find the mark/space ratio to operate the switch at so that the battery sits at a certain voltage. There will be a control loop that adjusts that ratio. When the battery voltage fails to rise (because another controller on the same line has increased it's mark/space ratio in order to charge another battery), this controller might overcompensate and I can easily see the two controllers get into a nice oscillation trying to get more current for their battery.

In other words, rather use a separate panel for this.

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On 4/25/2017 at 9:30 AM, DeepBass9 said:

OK thanks, makes sense, but I just realised that I have a spare 12V 50W panel that is not being used so its probably better to dedicate that to the job of charging the 12V battery. 

yup. And it gives you a backup panel in case something goes wrong. 

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