Can I Plug a 18v 10w solar panel intoa 12v NiMh charger??

[Owl]

Hi,

I want to charge NiMh batteries with a 10 watt solar panel. Its about 21 volts open circuit and the specs on the back says it drops to 17 V and 0.6 A on load.

I've got a commercial charger that runs off a 12 V plug-in mains adapter. Is it ok to plug the solar panel directly into that or will it fry it from having too high a voltage (17V instead of 12V)?

I've read that solar panels are "current devices" and that the voltage will drop harmlessly to whatever you plug it into, but I don't really understand how this works.....

At first I was thinking I would need to use a voltage regulator like a 7812 or something, but after reading the above and I think also a few people on this forum said they are plugging the panels straight into a 12V charger and it works ok, so now I'm thinking maybe it will be ok to plug it straight in (with no regulator or anything else in between)??

[Mike90250]

If your 12V charger is cheaper to replace, than to buy a 12V charge controller, sure - try it. I think most 12V devices will not survive 17-21v.

If you connect a panel directly to the battery, the BATTERY, acting as a load, will pull the PV voltage down to battery voltage, and the solar will charge the battery. The problem arises when the battery is full, but still connected to the solar, and still charging..... Boom or at least it damages the battery.

[Sunking]

What size in mah is the NiMh?

Solar panels are current sources, and you rpanel can only supply .6 amps or 600 ma. If your battery is say 100 mah or larger you can connect the panel directly to the battery. The problem is terminating the charge. If you leave it on too long you will likely destroy the battery causing to operate the vent valve and release the electrolyte. When that happens game over.

[PNjunction]

I've got a commercial charger that runs off a 12 V plug-in mains adapter. Is it ok to plug the solar panel directly into that or will it fry it from having too high a voltage (17V instead of 12V)?

Charging AA's directly via solar is bad unless like Sunking says, you have an accurate way to detect the EOC (end of charge). Let your existing charger do this job.

The easiest solution would be to actually use a 12v battery to power the charger, and let the panel charge the 12v battery. I don't know how many AA's you are trying to charge, but I think a small 5ah agm battery might do nicely if you are only charging 2 of the typical 2000mah nimh's.

Ideally, you want a charge controller between the panel and the battery. Using a regulator like a 7812 is wasteful of energy, and kind of defeats the purpose of doing the whole solar bit.

Depending on how big your project is, you may be looking at a much larger panel, 12v battery, and the addition of a charge controller, unless you don't mind burning things up.

[Owl]

Thanks everyone.

So it looks likely to fry it then..... If I don't want to buy a whole other battery and maintain it, and a charge regulator for the new 12V battery, a 7812 or similar is the only way to go then? So it will lose about 1/3 of the volts and watts, thats not so bad. I would imagine the transfer losses in charging a completely different battery and then using that battery to charge the AAs and AAAs would be at least that much anyway.....

Even if i did connect the batteries directly to the panel (which im not intending to) is it correct that the current doesnt magically increase past the panel's maximum just because the battery is only 12V vs 17V for the panel. In which case it seems to me that directly connecting them would be just as bad as the 7812 in terms of losses, and using an intermediate 12V battery would suffer the same losses to charge it and then more losses to charge the AAs from it...

[PNjunction]

In which case it seems to me that directly connecting them would be just as bad as the 7812 in terms of losses, and using an intermediate 12V battery would suffer the same losses to charge it and then more losses to charge the AAs from it...

Ah yes, but the convenience of being able to charge your AA's anytime, and not just in daylight makes up for it in practicality. Also, your charger may do a reset every time a shadow crosses the panel, subjecting the AA's to repetetive EOC detection, which is not healthy for them, especially if the transition during the shadow period is not clean and a detection is missed and the charger just keeps pumping away overcharging them.

If your charger is a simple trickle charger, typically running at the C/20 rate (ie, only about 150-200ma per battery) then there is no detection, just a time limit which is usually about 16 hours for a depleted 2000mah battery. That's about 4 to 5 days depending on solar-insolation.

There's no stopping you from doing this - but at the very least I'd make sure the panel / linear regulator you choose to do this with meets the same power requirements as the oem wall-charger. Does the 12v charger supply more than 600 milliamps dc? If so, your existing panel is too small. Keep the charger itself out of the direct sunlight too.

[Owl]

Ah yes, but the convenience of being able to charge your AA's anytime, and not just in daylight makes up for it in practicality. Also, your charger may do a reset every time a shadow crosses the panel, subjecting the AA's to repetetive EOC detection, which is not healthy for them, especially if the transition during the shadow period is not clean and a detection is missed and the charger just keeps pumping away overcharging them.

If your charger is a simple trickle charger, typically running at the C/20 rate (ie, only about 150-200ma per battery) then there is no detection, just a time limit which is usually about 16 hours for a depleted 2000mah battery. That's about 4 to 5 days depending on solar-insolation.

There's no stopping you from doing this - but at the very least I'd make sure the panel / linear regulator you choose to do this with meets the same power requirements as the oem wall-charger. Does the 12v charger supply more than 600 milliamps dc? If so, your existing panel is too small. Keep the charger itself out of the direct sunlight too.

Hmmmm it seems like a big inconvenience having two sets of completely different batteries to maintain and keep charged (though there may be other uses for the 12V one, but in terms of using it just for charging the AAs).

I hadnt thought of that about the EOC detection.

the charger i was thinking of using wants more current than the panel will give. it wants 800mA for AAAs and ive got 600 so thats not much less. it wants 2A for AAs though. i thought that just meant it wouldnt charge as fast.... do you think that would be a problem? it charges ridiculously fast anyway, too fast i think, the batteries are really hot afterwards. it takes about an hour to charge them. which i guess makes sense if the AAs are about 2000mAh. if it took three hours in the sun instead of one hour in the house that would be fine.

i use it all the time because its the only charger i have that can charge odd numbers of cells and i have several things that use three batteries or one battery.

i was just going to buy a 7812 voltage regulator which converts any voltage up to 20something volts into 12 volts, and costs about $2, and wire something up quickly. the ones i was looking at in Jaycar are 1A which should be enough if the panel can only put out 600mA

[Owl]

Another question - do Lenovo laptops have the charging circuitry in the laptop itself or in the power pack / "charger" that plugs in. they take about 18V . if the charging circuitry is in the laptop and the "charger" is just an 18V power supply, can i plug the 17V panel straight into the laptop?

[Sunking]

Even if i did connect the batteries directly to the panel (which im not intending to) is it correct that the current doesnt magically increase past the panel's maximum just because the battery is only 12V vs 17V for the panel. In which case it seems to me that directly connecting them would be just as bad as the 7812 in terms of losses, and using an intermediate 12V battery would suffer the same losses to charge it and then more losses to charge the AAs from it...

No solar panels are current sources. With any series or shunt type voltage regulator Current Input = Currant Output. Now a MPPT controller is a DC Power Converter. It can take the 17 volts @ .6 amps and convert it to 12 volts @ .85 amps.

An easier regulator to work with is a LM714

[Robert1234]

Another question - do Lenovo laptops have the charging circuitry in the laptop itself or in the power pack / "charger" that plugs in. they take about 18V . if the charging circuitry is in the laptop and the "charger" is just an 18V power supply, can i plug the 17V panel straight into the laptop?

Look at your charger carefully. Is it putting out voltage as AC or DC? Surprisingly, some laptop chargers are simply reduced AC voltage. Not sure what would happen if you decide to give it DC instead through that port.

EDIT: Lenova's do appear to be DC.

[Owl]

The 7812 looks pretty easy to wire, its just an in and an out and a couple of caps if i want it really clean

http://www.researchcell.com/electronics/7812-pin-and-circuit-diagram/

How much easier than that is the op amp?

The MMPT looks pretty complicated - i was hoping for something very simple, i got a very cheap panel (it was like 80% off the price), its not like a whole house roof or anything

Yeah good point about the AC vs DC i will have to check that.... ive never thought of this before but what would happen if you put DC into something that was wanting AC but the first thing the device does after the AC input is rectify it?? if you put DC into a rectifier wouldnt you just get DC out so no different to putting in AC of the same RMS voltage??? (apart from being cleaner maybe)

[Robert1234]

Really dunno on the AC vs DC effect question. You'd think it have to be made DC somewhere . It had never even occurred to me till I was fixing a broken wire on my daughter's laptop charger and was surprised by the voltage reading when testing it. Thought at first the charger was actually bad when I tested it's power with the meter. When I actually looked at the specs printed on it, I had to do a double take. Sure enough switching the meter to AC gave me the proper value.

Just thought I'd through that out there - just in case. Sounds like you got a pretty solid handle on this.

[PNjunction]

the charger i was thinking of using wants more current than the panel will give. it wants 800mA for AAAs and ive got 600 so thats not much less. it wants 2A for AAs though. i thought that just meant it wouldnt charge as fast.... do you think that would be a problem? it charges ridiculously fast anyway, too fast i think, the batteries are really hot afterwards. it takes about an hour to charge them. which i guess makes sense if the AAs are about 2000mAh. if it took three hours in the sun instead of one hour in the house that would be fine.

Most high-quality LSD nimh's can stand a 1-hour charge at 1C like Sanyo Eneloops. Most enthusiasts will drop that to .5C max (1A) to help ensure that the EOC signal is a delta-V instead of an overtemp. High-quality chargers also don't always use a constant current. For example, a Maha/Powerex MHC9000 always uses 2A to charge, but does the regulation via PWM like many solar controllers do. But feeding it less current than required isn't just a simple matter of taking longer time. The charger is no longer operating efficiently, the pwm controlling gets confused etc, things run hot and so forth.

Essentially, to get an equivalent 2A to supply your charger under the best of conditions, you'd need at least a 40-watt panel. About 9-10 times what you have now. BUT will your 7812 or 317T handle that much current? Looks like you need a beefier linear regulator and plenty of heatsinking. Still, the 12v battery and charge controller in between is more appropriate to me.

i use it all the time because its the only charger i have that can charge odd numbers of cells and i have several things that use three batteries or one battery.

That's GREAT. Most consumers don't understand the importance of charging cells individually, and although things looked brighter a decade ago about this, many manufacturers just returned to charging in pairs to save money. I guess it doesn't matter since most consumers use crap or old-stock rechargeables anyway with poor performance - hence resorting to dumping zillions of AA's into landfills. I swear it is a manufacturer conspiracy at times. Either they have great batteries with crappy chargers, or good chargers with crappy batteries. It's a no-win situation for the unenlightened, or those who rely on Grandpa's advice from the 70's.

This site might be interesting to you and provide possible solutions from those who dabble in candlepower, and not necessarily full-blown solar setups:

http://www.candlepowerforums.com/vb/forumdisplay.php?9-Flashlight-Electronics-Batteries-Included&

You can see that to do this right means to stop using the wrong product for the wrong application. You can force-fit it, burn things up in the process, and surely have fun, but the scope of the variables is just too large to help out much. I think you'll have to be the pioneer for the project and just see what works and what doesn't for you.