What did you do to your Volvo today? Topic is solved

[MrAl]

Interesting - why does high temperature hurt PV performance?

Hi,

One of the things that probably kicks in is the notorious -2.2mv/degreeC voltage decrease. If the voltage decreases, the output voltage will decrease. To get to 21 volts we would need 30 cells in series. That's -66mv/degree C so over 50 degrees that's -3.3 volts. The MPP is typically around 18 volts so we are now below that already.

The interesting thing about charging batteries though is that the IV curve of a solar cell is pretty flat around the short circuit current level current. That means for a 1 amp panel at around 17 volts we would get about 1 amp, and at 12 volts we would get around 1 amp. Either of these, directly wired to a battery, would charge it at the same rate. The only way to get an improvement is to use a buck circuit that can convert the power at 18v to power at 12v and that would raise the charging current. The only problem there is that we would need a very high efficient buck converter to get any practical improvement. With a typical 80 percent efficient converter we would get hardly any improvement. It would take a 90 percent or better to make much of an improvement, and that means the converter has to be designed to fit the application almost perfectly as to input and output voltage and input and output current. This means an off the shelf max power point tracker isnt going to cut it, and neither is one that is rated for a much higher current than it will ever be used for.

There's a lot to consider if we are after the best possible solution. I guess i am after something near that but i wont be too fussy unless i have the time and energy to improve what i've got already, and it's usually simpler and more effective to just get a bigger panel. For now i am going to stick with a 20 watt (rated) panel and see where it takes me. It would be pretty amazing though to have a 200 watt panel that would really beef up the system. It may be too large for the inside of the car though (haha) unless i put it on the roof somehow.

[scot850]

I am no electrical genius (those who have been on here for a long time know that!!). However, you need to measure the drain on your battery as without that we have no idea what to achieve. For your car, I think I stated earlier, should have a drain of around 10-20mA. If you have a 20W solar panel, then by my calculations, with a 12V output that is 1.67A. Even at 14V that would be near 1.5A which at 20% is still be around 30mA. So even at that level you would be filling faster than draining or is my math off or am I missing something here?

Neil.

[brunocerous]

The T5’s previous owner kept decent service records and had an envelope with receipts, one of which shows the timing belt was replaced 10 years and almost 50,000 miles ago. Should I think about redoing the TB/WP or figure it’s still good to go?

362A8A4C-F5A0-434E-BA80-2AF30EBB3114.jpeg (614.98 KiB) Viewed 360 times

Ooof, that’s bad, Bruno. Hard to start, I bet.

I’m hoping that was the cause of the crank, no-starts and the misfire codes. So far, so good!

[bmdubya1198]

10 years is about the service life of the timing belt. I wouldn't consider it extremely urgent, but I would recommend putting it on the list.

[abscate]

I ran Elizabeths timing belt 15 years, 145000 miles. Bad person.

The second one I ran 100,000 miles, and the third barfed up the band new INA idler pulley and killed a new head job with 5k on it.

Your mileage may vary.

[MrAl]

I am no electrical genius (those who have been on here for a long time know that!!). However, you need to measure the drain on your battery as without that we have no idea what to achieve. For your car, I think I stated earlier, should have a drain of around 10-20mA. If you have a 20W solar panel, then by my calculations, with a 12V output that is 1.67A. Even at 14V that would be near 1.5A which at 20% is still be around 30mA. So even at that level you would be filling faster than draining or is my math off or am I missing something here?

Neil.

Hi there Neil and thanks for the reply,

When discussing the power output of a regular power source like a power supply or power line we can usually just multiply the current times the voltage, and with a given power output we can usually just divide by the voltage output to get the current output. solar panels are a little different though, because they have a very non linear behavior that we dont see with regular power sources.

When discussing the power output of a solar panel we have to look at the IV curve (current voltage curve). This tells us how much current we get with a given voltage, or how high the voltage is with a given current. The power is not constant with these things but actually varies from zero to some maximum, and it depends on the current being drawn and/or the voltage at the output terminals.

With a regular 20 watt power source if we have 12 volts output we can just divide the power P by the voltage E and get the current I: I=P/E and in the case of 20 watts and 12 volts output we get 20/12 which as you noted would be 1.67 amps. With a solar panel though, we cant do that. We have to refer to the IV curve. For a solar cell or panel if the current at 17 volts is 1 amps, then the power with a 17 volt output is E*I=17 watts as expected. With a voltage of 12 volts however the current would still be around 1 amp, and that is clearly just 12 watts. The reason for this is because the short circuit current is about the same for a range of voltages from zero to a little less than the max power point voltage. For example, for a 21 volt panel the MPP may be around 18 volts, and that may put out 0.95 amps. At 17 volts it may go up to around 1 amp, but then it flattens out and stays at around 1 amp for voltages that go down from 17 volts to 0 volts. So we can actually, in theory, get no power output at all. For battery charging this works out OK though, because they depend mostly on the charge current.

If you want to take advantage of the max power point that is a little different. In this scenario we force the panel to operate at the maximum power point which for this panel would be 18v at 0.95 amps which is 17.1 watts. If we could somehow use that full power to charge the battery, we could indeed get 17.1/12 amps to charge the battery. That is now 1.43 amps, up from just 1 amp. So we get more than a 40 percent increase in charge current. The other factor that kicks in though is the efficiency of the converter used to convert that 17.1 watts at 18 volts down to 12 volts at 1.43 amps. If we have a regular converter with efficiency of 80 percent we loose a lot of power right there. 17.1 watts times 0.8 comes out to 13.7 watts, and that is just up a little from 12 volts at 1 amp with no converter. That's just 14 percent more charge current. If we can obtain or design a better converter we can of course get more of an advantage.

With no converter however, the charge current remains at about 1 amp for 12v, and even 6v. That's simply because of the way solar cells work which are different than a typical power supply.

I am including a curve of a solar panel similar to mine. The plots shown are the IV curve and the power variation over the entire voltage range of the panel. The blue plot is the IV curve and the red plot is the power variation over the full voltage range of the panel. The green line can be ignored that was just used to verify the max power point, which is the maximum point on the red curve. If you look along the blue line starting from a voltage of about 21 volts (far right, bottom) and going down to 0 volts (far left, near the top) you can see that the current (y axis) increases we follow the voltage down to about 17 volts, then it stays the same all the way to 0 volts. That flat, horizontal part of the curve is nearly constant, meaning the output is mostly that of a constant current source not a contant voltage source. That's why we cant divide the power by the voltage unless we have the curve to guide the calculation.

The solar cell curve is approximately:
i=iL-is*(e^(q*V/(N*K*T))-1)
where 'i' is the output current of the solar cell, 'iL' is the current from the energy conversion, 'is' is the saturation current, 'q' is the charge on one electron, 'V' is the voltage across the cell, 'N' is the ideality factor (different names sometimes), 'K' is the Boltzmann constant, 'T' is the Kelvin temperature.

'N' is probably close to 1 for a solar cell. 'is' is very small, about 1.75e-12 for my panel.

I hope this sheds a little light on these devices.
Also a little interesting is if you can find a regular silicon diode in a clear glass case and shine some light on it, it will produce a voltage.

[scot850]

OK. But for all that, (most of which I follow) it still doesn't say what output you are actually getting from your solar panel charger. I appreciate it changes based on temperature and sun/lighting conditions, but there must be some sort of nominal output or why have them?

Neil.

[erikv11]

The T5’s previous owner kept decent service records and had an envelope with receipts, one of which shows the timing belt was replaced 10 years and almost 50,000 miles ago. Should I think about redoing the TB/WP or figure it’s still good to go?
...

The rubber timing belt yes, but none of the metal parts (WP, pulleys, tensioner) should need replacing.

[MrAl]

OK. But for all that, (most of which I follow) it still doesn't say what output you are actually getting from your solar panel charger. I appreciate it changes based on temperature and sun/lighting conditions, but there must be some sort of nominal output or why have them?

Neil.

Hi Neil,

I am not 100 percent sure what you are asking here.

A nominal value is really just a category, or classification, of a group that behave in a similar manner. There will be different interpretations though. For a couple examples:
NiCd AA cell nominal voltage is about 1.2 volts, but it can be up to around 1.5 volts or of course zero volts. That nominal voltage rating just sets it apart from other cells that might have a nominal voltage of 1.5 volts like an AA alkaline cell or a 1.6 volt rechargeable alkaline cell. It really just helps us understand how it differs from other devices that are similar but have different characteristics.
The car battery most of us have has a nominal 12 volts terminal voltage but it can be as high as 15 volts when charging or 8 volts when starting the engine or 11 volts when just low or even 0 volts when bad.

What this does is helps us match a given device to another given device. For example, AA cells that have voltage 1.5v will work in some radios when you put 3 of them in the battery compartment. AA cells that are rated for 4.2 volts would blow that radio out with 3 of them in the battery compartment. We know the difference by the nominal or characteristic voltage.

For a solar panel, we can have a nominal voltage of just about anything, but usually in graduations of about 0.7 volts (or 0.5 volts). So we can have a panel with approximate nominal voltage of 0.7v, 1.4v, 7v, etc., and this helps us decide if it might work in a given application. We would not choose a 7v panel for charging a 12v battery unless of course we could somehow use a boost circuit with it, and then we could.
So that helps us match the panel to the application, but it in no way specifies the operation completely. The only way to do that is to find out the entire specification of the panel and that would include the open circuit voltage and the max power point, and of course the rated power output with the theoretical max of 1000 watts per square meter insolation level.

You can also think about what happens when the insolation level drops either a little or a lot. At full solar power with 1000 w/m^2 we might get 100 watts out at the max power point, but later in the day we might only have 100 w/m^2 and so we'd be lucky to get 10 watts out. At night we probably get 0 watts out or at least very very low.

The entire specification would be the IV (current voltage) curve as the diagram i posted shows, but even that does not show what happens with lower incident light levels. Of course with lower levels the total available output power goes down.

As i was saying before, if we look at it more like a constant current source it may make a little more sense perhaps. That's only if we have enough voltage also though. For a 1 amp output we would see that 1 amp no matter what kind of battery we connected to it (directly or with a protection diode) unless we combine it with a boost circuit. So if we charge a single AA cell at 1.4v we'd get about 1 amp charge current, and if we were charging two AA cells in series at 2.8v we'd still see about 1 amp charge current, provided the panel could also put out that higher voltage.

It might also be interesting to look at some regular battery voltage and current outputs as we vary the load resistance. Batteries have internal resistance and that means that the more we load it the lower the voltage falls. With a 12v battery with 1 Ohm internal resistance, when we load it with 1000 Ohms we see hardly any voltage drop, but if we load it with 11 Ohms the output will only be 11 volts. If we load it with 1 Ohm the output will only be 6 volts.
Now in the case of 11 Ohms, we have 11 volts output across the battery terminals and 1 amp current out, so we have 11 watts.
In the case of 1 Ohm load resistance, we will have 6 volts output at 6 amps (that's 12/2=6 amps) which is 36 watts out.
If we did not take the internal resistance into consideration we would see 12/11 amps out and 12/1=12 amps out and for this last one that would mean we might think we were getting 144 watts out (12v times 12 amps) when really just 36 watts out (6 volts times 6 amps).

I hope this helps understand these devices but if not maybe you can describe what is still not clear.

[scot850]

What I am asking is simply, how much does your solar panel output? If it outputs more than say 30mA to charge your battery , it should stop or at least match your cars normal drain from the battery. The whole point is as you are like me and don't now drive a lot, we use our cars less so we want the battery to be charged when we do use it.

So 2 simple measurements are needed. What is the drain on your battery in everyday sitting in your driveway? It should be around 10-20mA. If your 20W panel can supply at least that or more your car sitting should always have a charged battery.

Neil.