# Question about charging voltage

Discussion in 'Electronic Basics' started by [email protected], Sep 29, 2004.

1. ### Guest

I would like to charge my portable DVD player using a portable solar
charger rated at 20 watts with 16.5 volts (23 volts open circuit) but
am concerned that I may damage the player with too high a voltage.

Is this a valid concern?? If so, is there a device to step down the
voltage from the solar charger that would not significantly affect the
charging rate??

Thanx,

Jake

2. ### Brian OakleyGuest

What does the charger that the DVD player uses now put out? It depends on
the voltage of the batteries primarily as to the likelihood that the solar
charger you want to use can in fact be used. As far as something to step
down the voltage, if the voltage difference between the required charge
voltage for the DVD and the 16.5 volts output that the solar charger will
put out is minimal (within +- 5 volts), a step down resistor would work just
fine. If the voltage difference is much more than that, then your charge
rate will increase accordingly.
A few more specifics about what youre trying to charge would help more.
Brian

3. ### Guest

12 volts. It could also be charged using cigarette lighter in car
meaning it should be able to handle a bit more than 12 volts.

Where can I get a "step down resistor" and how would I hook it up??
Would it require soldering and a permanent change to the solar
charger?? Sorry if the question is silly as I am rather ignorant in
matters of electronics (if that already was not obvious).

Actually I would like to be able to charge a variety of devices while
away from AC including GPS, PDA, laptop computer, portable DVD, etc.

Thanx,

Jake

4. ### RylosGuest

Normally step down resistors are considered a wasteful way to dissipate
excessive power but in this case, it's the sun's energy which we don't use a
fraction of anyway so who cares. Just thought I'd blurb that little
anecdote, ;-).

-Dave

5. ### RylosGuest

Jake,

What I would do is measure the current draw into the battery when the player
is hooked up to it's regular charger either from the wall or car. You've
already measured the voltage. Plan on stepping down the voltage on your
solar charger by an amount to equal your regular charger and assume (here's
where things get a little gray) that your solar charger under ideal
conditions can deliver an equal amount of current at the same voltage. Ohm's
law, R=V/I should get you in the ballpark for how much dropping resistor
you'll need. But you'll also have to make sure it can dissipate the amount
of power it will be wasting as heat, another formula, P=(I^2)*R or
P=(V^2)/R. Measure current again. If it's off it will most likely be on the
low side (better than high) and you can adjust your dropping resistor
accordingly to bring it up to a reasonable charge rate. Resistors of all
shapes, sizes, values and power capacities can be found everywhere,
Radioshack might even one. You're probably going to have to do a little
fiddling to get it just right.

One other thing. Would it not be possible to just simply cover some of the
solar panels to reduce the output voltage of the charger down to a safer
level? You could try that too, might be simpler. Best regards,

-Dave

6. ### John FieldsGuest

Forget the resistor.

If the load your device puts on the photovoltaic (PV) array changes
while it's charging, then the voltage dropped across the resistor will
vary with the current through it, with the result that under light
loads and full illumination of the PV array the voltage out of the PV
array could rise high enough to damage the batteries or the charging
circuitry in the device.

What you need is something like an LM317 wired like this:

PV
ARRAY LM317
+-----+ +---------+
| +|-[DIODE>]---|VIN VOUT|--[Rs]--+---->BATTERY+
| | | ADJ | |
| -|--+ +----+----+ [240R]
+-----+ | | |
| +-------------+
| |
| [2.4K]
| |
+----------------------------+---->BATTERY-

Go to

http://cache.national.com/ds/LM/LM117.pdf

and particularly to "12V Battery Charger" for more detail.

As long as they all take a steady 12V in to charge them, the circuit
above should work as long as the input voltage to the lm317 is >=
14.5V. If they need different voltages, read the data sheet to find
out how to change the LM317's output voltage. One caveat, you may
have to put a heat sink on the LM317 if the battery charging current
causes it to heat up too much.

You could also use a 7812...

7. ### Guest

Would this charge controller work?

http://www.icpsolar.com/specifications.php3?id_article=340

8. ### RylosGuest

What John is saying about the voltage drop across the charge resistor
changing is true, however as the batteries come up to charge they will take
less current and would probably be okay. Still if you can regulate the
voltage as he suggested that's a better idea since you don't really know if
there's any sensitive circuitry in between the battery and the charger. And
as I mentioned previously you could try just covering some of the panels to
reduce the voltage down to a safe level in full sunlight. Not the most
technically glamorous solution but certainly simpler. Good luck.

Dave

9. ### John FieldsGuest

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