1. Mar 16, 2015

beany_bot

Hi, I recently bought a smallish 12v DC solar panel for trickle charging my jetski battery. I've had a look at the panel with a multimeter and in good sunlight it makes easy 24V but very very low current. I was wondering if I stick a 99pence buck converter on in, forcing the voltage to a max of 14V. will this increase the current and in turn charge the battery quicker/better?

Don't know too much about these things so came here to ask the experts. Just I had a thought pop into my head that if I reduce the voltage I would get more current and charge the battery more? Is this the case?

2. Mar 16, 2015

Pierce610

I'm not an expert but I would try to do an aswer.
First, the convertion from radiant energy to electrical one is function of the efficience factor which is not equal to one, and this limits the maximum of obtained power by the solar device.
Second, if this device has a power P, say for example 2.4 watt, that is P = VI = 24 x 0.1, then the current converter get in input this power and it can't amplify it, because it must keep constant.
So since P = VI and V= 12 for example in the second device, the current must be about 0.2 A, that is the current is double.
But this in the case that the efficience of the current converter is 1.

3. Mar 16, 2015

beany_bot

Thank you for your reply and forgive my ignorance. So I'm right in thinking, by halving the voltage i'm doubling the current? Now, would this be better at charging the battery? Or because "P" is still "P" will it not make a difference? Thanks.

4. Mar 16, 2015

Staff: Mentor

Hi beany_bot. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

In principle, the idea sounds feasible. Does the panel's documentation indicate its rated current output?

Last edited by a moderator: May 7, 2017
5. Mar 16, 2015

beany_bot

It doesnt state the current output, it "claims" 1.5W but as we know it will be very variable depending on sunlight. This is it http://www.amazon.co.uk/Solar-Batte...F8&qid=1426503627&sr=8-1&keywords=solar+panel

The amps are very very low, just a trickle charge. Basically wondering if increasing the amperage with lower voltage will charge faster or because the power is the same it wont make a difference.

6. Mar 16, 2015

Staff: Mentor

Look at some data sheets, see whether you can find an efficient buck converter that demands low current for itself. You may be able to boost the current capability so the battery charges a bit faster.

7. Mar 16, 2015

beany_bot

8. Mar 16, 2015

Staff: Mentor

How did you measure the current? Shouldn't the voltage and current come into equilibrium depending on the load?

9. Mar 16, 2015

beany_bot

I havent measured the current. I just assumed it is very low because...well, its a small solar panel operating in Scotland on a cloudy day lol.
A wont lie, a lot of this is going over my head, (I dont really have much knowledge about electrical physics).
Was just essential wondering if more amps / less volts will charge the battery better. And indeed will a buck converter do just that? will decreasing the voltage increase the amps? I was led to believe it will. Alas, doesnt more amps less volts just mean its getting more current, but slower, making the overall "energy" delivered to the battery in any given time frame the same? Argh my head hurts...

10. Mar 16, 2015

Staff: Mentor

Well, I'm a mechanical engineer, not an electrical engineer, so I'll want someone else to verify, but typically the open circuit voltage isn't the real operating voltage on an unregulated source like a solar cell or battery. It naturally finds its own equilibrium with the circuit. If it is rated for 12V but produces 24V open circuit and you hook it up to a 12V battery (13.xV?) I'm pretty sure it will charge the battery at around 14V.

I'd just try it and see.

11. Mar 16, 2015

beany_bot

The more I think about it, the more I think about Einsteins conservation of energy law. I think the energy is going to be the same, no matter what I do. Doesn't matter if I give "x" amps at "x" volts or "y" amps at "y" volts. The energy im putting into the battery will be the same. So Einstein says.

12. Mar 16, 2015

sophiecentaur

That is by no means a general principle. It only works if the Power remains the same and there is no reason to suspect that it applies when a non-linear device like a solar cell is involved.

13. Mar 16, 2015

sophiecentaur

There will always be an optimum condition for running a PV charger and you can buy devices (MPPT) that will try to get the best Power transfer into a load under all lighting conditions. For a small installation, the cheaper solution is just to use more area of cells - and the cheaper MPPT units are pretty poor, I've heard.

14. Mar 16, 2015

Staff: Mentor

Sopiecentaur had the right answer. But if your panels are small enough, you do not need the expense. Charge controllers extract maximum energy, but their primary purpose is to prevent damage to the battery by overcharging.

If the charging current is 1 amp or less, it is considered a trickle charge. You can charge at that rate forever without damaging the battery. (You can stretch that to 1.5 or 2 amps). Just connect the panel to the battery directly, and don't worry.

If the current is in the range 2-7 amps, then an inexpensive pulse width modulator controller is best. They simply limit current to prevent battery voltage from ever exceeding 13.8 volts. Find some here.

For currents higher than 7 amps, buy a charge controller big enough for your needs.

The most expensive MPPT controlers not only protect, but optimize. But the biggest gains are with large installations and with voltages 48v or higher. For smaller or lower voltage applications, the extra expense of MPPT is not justified.

Bottom line, read the nameplate on your panel. Divide the watt rating by 13 to get currrent.