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Post by Clinton Cool on Nov 16, 2016 17:38:17 GMT
Electricity. I've picked up a bit of knowledge during the 3 or so years I've lived aboard but so much more to learn!
I'm scratching my head trying to understand voltage loss, whether there's a direct correlation to amp loss. Imagine this scenario:
Boater has 200w of solar panels. It's a brightish day in the summer, his batteries are 50% depleted. The panels could be giving 10 amps to his solar controller, the solar panels are delivering the current at 40 volts. However, the wires that the boater installed between the panels and the batteries are on the thin side. There's no danger of them catching fire, they are rated above the maximum the panels can deliver. When he checks the voltage at the controller it reads 32, a loss of 20%.
Now then, will the potential 10 amps in reality be 8? If not, what will the figure be?
Many thanks!
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Post by Ditchcrawler on Nov 16, 2016 20:44:24 GMT
Electricity. I've picked up a bit of knowledge during the 3 or so years I've lived aboard but so much more to learn! I'm scratching my head trying to understand voltage loss, whether there's a direct correlation to amp loss. Imagine this scenario: Boater has 200w of solar panels. It's a brightish day in the summer, his batteries are 50% depleted. The panels could be giving 10 amps to his solar controller, the solar panels are delivering the current at 40 volts. However, the wires that the boater installed between the panels and the batteries are on the thin side. There's no danger of them catching fire, they are rated above the maximum the panels can deliver. When he checks the voltage at the controller it reads 32, a loss of 20%. Now then, will the potential 10 amps in reality be 8? If not, what will the figure be? Many thanks! OK I thought I had replied but obviously not. What is the voltage on the back of the panels, is it really volt drop in the cable if so there is something seriously wrong to drop almost 25% of the voltage. |
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Post by Telemachus on Nov 16, 2016 21:10:29 GMT
There is no such thing as "current drop" along a wire. Think of it like a hosepipe - at the far end, the pressure (voltage) is lower than it was at the tap, ie voltage drop. Whereas no water is lost and so the water flow rate (current) coming out the end of the hose is the same as the water flow rate coming from the tap. There is no water loss /current drop.
However if you have an MPPT controller then it is a "constant power" device ie the power output to the batteries is the same as the power input from the panels (minus a bit for efficiency). So keeping it simple, let's say the panel can produce 6A at 40V, that is 240 watts. The controller (discounting efficiency) can convert that to 12V at 20A.
But let's say the wires from the panel drop 4v. So instead of the controller getting 240 watts it only gets 6A at 36v = 216 watts, which it can convert to 12v at only 18A.
So the 10% voltage drop in the panel wiring is translated by the MPPT controller to a 10% reduction in current to the batteries.
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Post by tonyqj on Nov 16, 2016 21:43:01 GMT
In case you don't know, power = voltage x current. That should explain Nick's post above if you were confuzzled.
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Post by tonyqj on Nov 16, 2016 21:52:52 GMT
Boater has 200w of solar panels. It's a brightish day in the summer, his batteries are 50% depleted. The panels could be giving 10 amps to his solar controller, the solar panels are delivering the current at 40 volts. However, the wires that the boater installed between the panels and the batteries are on the thin side. There's no danger of them catching fire, they are rated above the maximum the panels can deliver. When he checks the voltage at the controller it reads 32, a loss of 20%. Now then, will the potential 10 amps in reality be 8? If not, what will the figure be? Many thanks! There are a few things wrong with the above. If the panels total 200W then at 40V the maximum current will be 200/40 = 5A not 10A. Secondly, as Nick alluded to, panels don't 'deliver current', they supply power (it doesn't change anything but you might as well get the terminology right). Lastly, maximum power from the panels does not necessarily equate to their maximum voltage. That's the whole point of a Maximum Power Point Tracking controller, it will vary the voltage from the panel to find the 'magic spot' that provides the maximum power at any one instant. |
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Post by Gone on Nov 16, 2016 21:58:39 GMT
It is much easier to think in terms of power (watts) when looking at solar systems as the panel voltage will vary with the sunshine and also by how much loading the MPPT controller applies to get the panels at their max power.
So assuming that you measured the voltage at the panel - rather than reading the spec open circuit voltage on the label on the back of the panel - then the panel voltage times the panel current (this is NOT the same as the current going into the batteries) tells you the power in watts being generated by the panels.
If you then measure the current into the batteries and multiply that by the battery voltage it gives you the power going into the batteries. The difference between the two power figures is losses in the controller and cables.
Note - in your original post you give the figures of 40V for the panel voltage and 10A from the panel into the controller - This is 400W and not possible from a 200W panel. If you meant 10A current going into the batteries and assuming the batter voltage is 14V then you would be getting 140W from your solar system. Depending on the panel orientation, time of day and cloud cover, then 140W from a 200W system may be reasonable.
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Post by Clinton Cool on Nov 16, 2016 22:06:28 GMT
Thanks for the replies. The scenario I gave wasn't a real one, it was one I created just to try to understand things! Trust me to get even that wrong  So, it seems, if I read things right, any loss in voltage due to inadequate wiring will result in a loss of power going to the batteries. Both these losses will be exactly the same, in terms of proportion. So, a device producing 20A of current with top notch wiring will only supply 15A to the batteries if the wires between the device and the batteries allow for a voltage drop of 25%. Is that right? |
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Post by Gone on Nov 16, 2016 22:20:06 GMT
Sort of correct, but your example figures are not good and may confuse.
The MPPT controller takes in power from the panel, say 35V at 5A so 175W this is then converted in the MPPT controller to say 14.4V to charge the battery and let's assume there is zero losses then the current going into the batteries would be 12.2 amps which is 14.4x12.2=175W (approx).
So your 5A panel current becomes 12.2A battery charging current.
So back to your question - if you lose 25% of your power then the power going into the batteries will be 25% lower, which for a given voltage would by a reduction in current of 25%.
Does this help?
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Post by Telemachus on Nov 16, 2016 22:30:32 GMT
Thanks for the replies. The scenario I gave wasn't a real one, it was one I created just to try to understand things! Trust me to get even that wrong So, it seems, if I read things right, any loss in voltage due to inadequate wiring will result in a loss of power going to the batteries. Both these losses will be exactly the same, in terms of proportion. So, a device producing 20A of current with top notch wiring will only supply 15A to the batteries if the wires between the device and the batteries allow for a voltage drop of 25%. Is that right? Yes that is right, but bear in mind that it's only because of the properties of an MPPT controller (its ability to convert power from one voltage to another). If it were a PWM controller it wouldn't be so clear cut and would depend on the panel voltage ratings vs the battery voltage. |
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Post by Clinton Cool on Nov 16, 2016 22:53:33 GMT
Thanks for the replies. The scenario I gave wasn't a real one, it was one I created just to try to understand things! Trust me to get even that wrong So, it seems, if I read things right, any loss in voltage due to inadequate wiring will result in a loss of power going to the batteries. Both these losses will be exactly the same, in terms of proportion. So, a device producing 20A of current with top notch wiring will only supply 15A to the batteries if the wires between the device and the batteries allow for a voltage drop of 25%. Is that right? Yes that is right, but bear in mind that it's only because of the properties of an MPPT controller (its ability to convert power from one voltage to another). If it were a PWM controller it wouldn't be so clear cut and would depend on the panel voltage ratings vs the battery voltage. OK I've read that MPPT is preferable to PWM, without understanding it much. I'm happy with that really, practical expedience is often good enough for me. I'd be interested to know though: with MPPT the loss in power to the batteries is similar in proportion to any voltage drop. Can it be wildly different with PWM? |
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Post by Telemachus on Nov 16, 2016 23:04:19 GMT
Yes that is right, but bear in mind that it's only because of the properties of an MPPT controller (its ability to convert power from one voltage to another). If it were a PWM controller it wouldn't be so clear cut and would depend on the panel voltage ratings vs the battery voltage. OK I've read that MPPT is preferable to PWM, without understanding it much. I'm happy with that really, practical expedience is often good enough for me. I'd be interested to know though: with MPPT the loss in power to the batteries is similar in proportion to any voltage drop. Can it be wildly different with PWM? Well as I said, it depends... the PWM works like a switch. When the switch is on the panel is connected directly to the batteries, so the system voltage is determined by the battery. If the panel's voltage for maximum power is near the battery charging voltage, and then you introduce some voltage drop, the panel will be operating above its max power voltage and thus produce less current. However if the panel's max power voltage is quite a bit above the battery charging voltage then losing a bit of voltage in the wiring isnt going to make a significant difference to the current. This is because when the panel is not well matched to the battery, a PWM controller is fairly inefficient and it matters not whether all the inefficiency lies within the PWM or whether in part it lies in voltage drop in the wiring. |
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Post by loafer on Nov 17, 2016 0:32:56 GMT
OK I've read that MPPT is preferable to PWM, without understanding it much. I'm happy with that really, practical expedience is often good enough for me. I'd be interested to know though: with MPPT the loss in power to the batteries is similar in proportion to any voltage drop. Can it be wildly different with PWM? Well as I said, it depends... the PWM works like a switch. When the switch is on the panel is connected directly to the batteries, so the system voltage is determined by the battery. If the panel's voltage for maximum power is near the battery charging voltage, and then you introduce some voltage drop, the panel will be operating above its max power voltage and thus produce less current. However if the panel's max power voltage is quite a bit above the battery charging voltage then losing a bit of voltage in the wiring isnt going to make a significant difference to the current. This is because when the panel is not well matched to the battery, a PWM controller is fairly inefficient and it matters not whether all the inefficiency lies within the PWM or whether in part it lies in voltage drop in the wiring. In simpler terms, a PWM controller merely switches off its output current as soon as the battery voltage reaches the programmed limit. Then it just keeps switching on and off, to keep the battery voltage more or less constant. The best benefit of an MPPT controller (apart from it being loads more efficient) is that it is also a smart charger, with proper algorithms and charging regimes. IMHO. |
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Post by smileypete on Nov 17, 2016 11:49:16 GMT
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Post by tonyqj on Nov 17, 2016 12:01:26 GMT
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Post by Telemachus on Nov 17, 2016 12:32:07 GMT
I've got something very similar in my caravan, with the MT50 display that allows effortless adjustment of charge voltages. Yes I think it would be quite feasible to use as an equalise function, you'd just need a bit of switchery to change the panels from the normal MPPT to that controller. Of course if the panels were in series you'd have to be careful with the max voltage - probably the switchery could just take 1 panel out of the string to connect to the PWM controller.
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