Solar Charger for Drill

[Junkie]

I have a cordless drill charger and some spare solar panels (40W) that aren't being used,
so I was thinking why not connect the panels to this charger and make use of them.
The charger is an iron cored transformer rated about 40W, it says on the label that the output is
21.5V 1.42A and the batteries it charges are 18V.  So could I connect these panels after the bridge
rectifier to charge these batteries ? The charger isn't a smart charger loaded with electronics but it
has a small board in side which seems to have a transistor and a few components. It has an indicator
LED which shows when the battery is charged.

I am not bothered if this doesn't charge in cloudy conditions, full sun only is fine.

Any way, I thought I'd as here as I'm prone to letting the magic smoke out of electronics 

[TomW]

Might be easiest to just direct charge the battery. If the battery does not have electronics in the pack. Some newer ones do.

The electronics in the charger may or may not mind low voltage. Again it varies. I suspect you need the 21.5 volts to get a full charge. 40 watt solar panels means nothing. What voltage are they rated for? What is their output voltage in full sun? The voltage is what matters as far as getting charged up. Even a kilowatt at 17 volts will not charge them fully to 18 volts. Most battery voltage specs are "nominal" which means it will be higher when fully charged and maybe lower when discharged some amount. See where I am going here?


Tom

[ghurd]

Problem #1.  A single 12V panel will not do it.
18V nominal means 15 cells, which need charged to about 21V.  A single panel does not have enough voltage under load.

Problem #12.  Two 12V panels in series will smoke most common electronic parts I would expect to find in the charger.

The easiest way would be to use a small battery like a 7AH, charged with a 40W panel and decent controller, 200W-class inverter.

Direct solar would be cool, but it would be a lot easier with three 6V panels.
G-

[Norm]

Here is how I do it 3 VW panels in series for my 18 volt.....hooked directly to
battery figure how fast it's charging and closely monitor it accordingly....a timer
with automatic timer cut- off is nice like (("What's that beeping?"), wife).

Another method I like is real small solar chargers that will take all day to charge
......bunch of scavenged solar cells.

[zap]

Most of us here are big on experimentation.  Go for it and let us know how it works.  Or...

...ditch the battery and charger all together, hook the panel to the drill and strap the solar panel to your back.
That way you only have to work when you're out in the sunshine

Tom and G- both said it.
In theory, a typical NiCd 18v pack would hold around 45 watt hours.  Your panel, in theory, would charge the battery in 1:07:30 hr/min/sec if your panel was an 18v panel.  Nominal voltage for your panel is probably somewhere around 14.5v and open circuit somewhere around 20v.
The panel might be able to charge the pack in a few days... the problem is the panel doesn't produce enough voltage to push the pack's voltage much past 14.5v.

[Junkie]

Ooops, I guess I should have included the panel specs in my OP... Here they are

1 of these: Open circuit voltage : 21.9V , Short circuit current 1.27A, 36 cells, Max power Vmp 17.5V, Max Current (Imp) 1.14A,
This is a no name Chinese panel, but they do but out good power, close to their name tag from what I can remember from
my tests last summer. And one amorphous panel rated 15W (thought it was 20), this doesn't haven't any specifications on it though..

I doubt the batteries have any electronics in them as they aren't particularly expensive, the drill is Ryobi if anyone is familiar with those.

I guess judging from your comments that it may work, so....

I think I'll try it and let you know what happens! I'll measure the voltage of a fully charged battery and see what that is

[zap]

A healthy pack fresh off a charge should read somewhere around 21v.

Fully discharged to 1v/cell (tempting cell reversal with that low of a discharge) is 15v which is probably already higher than the nominal voltage of you panel.  I hope you can see why your panel will take a long time to fully charge that battery.

I have a Ryobi 18v charger a friend gave me.  If I get the time I'll crack it open and have a look at it.

[Bruce S]

1 of these: Open circuit voltage : 21.9V , Short circuit current 1.27A, 36 cells, Max power Vmp 17.5V, Max Current (Imp) 1.14A,
This is a no name Chinese panel, but they do but out good power, close to their name tag from what I can remember from
my tests last summer. And one amorphous panel rated 15W (thought it was 20), this doesn't haven't any specifications on it though..

With these specs, it's probably easiest to use the Ghurd method of charging a smallish 12V battery and use an inverter to let the electronics charge the drill.
Might be a whole bunch of inefficient but the quickest and simplest route.
Bruce S


 

[ghurd]

"Might be a whole bunch of inefficient but the quickest and simplest route."

With the right inverter, the stated load, and a 40W panel, it's really not all that bad.

I imagine for the first few minutes the inverter will pull more from the 12V battery than the panel is putting in.
As the drill battery voltage increases, the drain on the 12V battery will be less than panel is putting in.

The transformer rating is Max.  It will be less than that.

It does depend on the drill charger too, but most transformer based chargers use a Lot less amps after a few minutes.
G-

[Bruce S]

G-
Agreed, thought I'd put that in there for the purests

B

[Junkie]

The batteries are about 21V fully charged. So I guess it wont work.. The reason I wanted to do this is because I don't have any 12V batteries (not working ones anyway), but I'll just have to wait until I save up for my new battery bank 

Thanks for all the help anyway!

[Norm]

If there are 16 batteries in the pack ....you could solder a lead wire mid way
and charge half with 2 seperate equal rating solar panels.

at your own discretion of course....(possibly will void your warantee....or whatever)

[Bruce S]

The batteries are about 21V fully charged. So I guess it wont work.. The reason I wanted to do this is because I don't have any 12V batteries (not working ones anyway), but I'll just have to wait until I save up for my new battery bank 

Thanks for all the help anyway!

Junkie;
 Do as your moniker states, go find an old car or motorcycle battery that reads at 11.80Vdc. fill it with fresh distilled water and throw it on the charger for a day or two, OR grab a UPS battery (USED) but not rattling and due same. You'll be all set.

Cheers
Bruce S

[tecker]

Wind a coil of welding wire wrap with teflon tape then masking tap . Maybe 40 turns and 2" diameter ( wide enough to get a spool through . wind a toroid with 20 turns bifilar 21 awg magnet wire . You might be abl to use the secondary of the charger transformer for the coil.
On the drill pull ot a connection to the motor neatly make a switched connection to the motor leads with the positive motor lead to the one end of the coil.Connect in the solar to the standard battery leads (poach an old batter for connections) . The other lead of the coil  thru a 4 amp diode to the positive of the 18 volt batt ( maybe use the connections of the charger )motor lead to ground common to the panel and the drill batt . use a tie Wrap or fashion a hold for the trigger . wala a boost charger .
 the drill trigger is a PWM around 4 k should work .IF you wna try it post back and I'll gin one up for test.I got room  on the bench and solar coming in there .might be fun I use the 24 volt battery bank now to drill chargers. You can charge two old car batts  for this chore

[tecker]

Ok the wording crashed when I looked at the circuit.thos will work there's enough of a short to build back EMF in the coil .the secondary of the drill charger is has a few too many turns but will work . winding the toroid seems like a better plan because you can just use the socket for the battery.