Car Battery

AlexisV

I'd only worry if you were leaving the car for a month. 5 days will be fine.

Strider590

mikey72 wrote: »

If you do want to check it, just use a meter on milliamps as Joe Horner says.

I try to steer people away from using a multimeter to measure Ampere's.... It's when they grab the meter a week later to measure a mains socket voltage and only realise they've forgotten to swap the leads back when they blow every the fuse or circuit breaker in the CU :rotfl:

mikey72

Strider590 wrote: »

I try to steer people away from using a multimeter to measure Ampere's.... It's when they grab the meter a week later to measure a mains socket voltage and only realise they've forgotten to swap the leads back when they blow every the fuse or circuit breaker in the CU :rotfl:

I'm assuming you don't know a lot about solar cells, even in dull conditions, the voltage will be high, but low current. It doesn't take much to excite them up to the junction voltage, but to get any current to flow, you'll need more light.
So doing as you say, there is a good chance you'll always get a higher voltage at the panel, even when the current is negligible.

Strider590

mikey72 wrote: »

I'm assuming you don't know a lot about solar cells, even in dull conditions, the voltage will be high, but low current. It doesn't take much to excite them up to the junction voltage, but to get any current to flow, you'll need more light.
So doing as you say, there is a good chance you'll always get a higher voltage at the panel, even when the current is negligible.

As soon as current starts to flow, the voltage will start to drop..... That's what ohms law is all about.

The higher the rated output current, the more current it can supply before voltage drop.

Take a 7.5v 150mA solar cell and apply a 150mA (50ohm) load.... At (or just beyond) 150mA the cell will effectively be short circuit, it will read zero volts in any weather conditions.

mikey72

Strider590 wrote: »

As soon as current starts to flow, the voltage will start to drop..... That's what ohms law is all about.

The higher the rated output current, the more current it can supply before voltage drop.

Take a 7.5v 150mA solar cell and apply a 150mA (50ohm) load.... At (or just beyond) 150mA the cell will effectively be short circuit, it will read zero volts in any weather conditions.

You're getting mixed up because you're thinking it's a resistor.
It's a junction device, because it's a semiconductor.
To use an example for you, it's like the diode you mentioned earlier. The forward voltage is 0.7v, it doesn't go up for a bigger current, and doesn't go down if you lower the current. Once it's on, it's on.
A solar cell has a potential of about 0.5v across it, then after that more excitation = more current.

Have a look at

http://www.solarserver.com/knowledge/basic-knowledge/photovoltaics.html

for a quick explanation.

Strider590

mikey72 wrote: »

You're getting mixed up because you're thinking it's a resistor.
It's a junction device, because it's a semiconductor.
To use an example for you, it's like the diode you mentioned earlier. The forward voltage is 0.7v, it doesn't go up for a bigger current, and doesn't go down if you lower the current. Once it's on, it's on.
A solar cell has a potential of about 0.5v across it, then after that more excitation = more current.

Have a look at

http://www.solarserver.com/knowledge/basic-knowledge/photovoltaics.html

for a quick explanation.

I'm not talking theory, I don't even need to read any of that (not that I haven't read up on the subject in the past) im talking actual hands on measurements, if you overload a solar cell and the output voltage drops to zero.... It just does.

I have a box full of the things, from various hobby projects over the years.

podcake

Ah Dear,

Seems people are all arguing the same things here and it is slowly coming down to semantics...

1) Solar panel chargers are a great idea, they keep the battery topped up and conditioned, they are cheap, and don't kill the world every time you use them. They put a few milliamps into the thing just to keep it going. (ps: solar panels have a shelf life of 30 years before somebody tells me about the energy used in manufacture).

2) if you are worried about the battery you COULD disconnect it ( check with the manufacturer before doing this as SOME cars have personality issues when it comes to reconnection - see Lexus as an example)

3) I'm sorry to say that Strider is almost right, but slightly wrong in a critical way. As you change the load on a solar panel the IV relationship DOES change in that respect he is correct. HOWEVER, it does not change as you might expect (like a resistor) but in a very non-linear way (hence such terms as fill factor). (if anybody would like to read some texts on this I'm more than happy to help find them)


I hope I have helped

mikey72

At £13 a panel, it's not going to be a high fill factor.
The load, the battery, is near enough constant, so the output voltage isn't going to change much.
And without measuring the light intensity, all you'll see is more light = more current.

podcake

This is very true, I actually have a pair of panels very much like these which I tested extensively for part of a postgraduate degree project. The fill factors (and as such the peak power point) are actually a lot better than you might expect. But I agree, you will basically see more light = more current (although not generally linearly). One other point to note, the battery will itself cause the efficiency of the panel to drop significantly. This is why people use things like PPT methods when designing charge controllers worth having.

edited: I measured approximately 0.58 as the fill factor

mikey72

That's not bad for a cheap panel.
But I don't think a MPPT charge controller would be in for the same price.