Charging batteries

[patty3]

I recently bought a solar panel designed to charge 12 volt lead acid batteries. Can this solar panel also be used for charging 6 volt lead acid batteries of approximately the same amphr rating?  Thanks  Pat!

[tecker]

Series em up

[wpowokal]

To expand a little a 12 volt charger can not be used directly on 6 volt batteries, if you had two 6 volt batteries placing them in series ie negative of first connected to positive of second with your sola panel connected to the positive of the first and negative of the second.

Amp hours are a measure of how much the batterie holds (capacity).

You would also need to monitor the voltage of the batterie(s) if no regulator is used.

regards Allan

[jacquesm]

if you don't have more of them for a series setup (or if you have to charge them
while they're 'online' so you can not series them) then you could put a 'regulator' at the output for a nice solution. A regulator is a little device that will clip the input voltage using a zener diode to the desired output voltage.
It will still have to have a higher output voltage than the nominal battery voltage, otherwise they will never get charged. Another option
(a little more expensive, but it would be more efficient) is to use a
switching regulator, also called dc-to-dc convertor. The best way to compare
these two is to think of it as a butcher that gives you a 'flat' steak from
a thicker one. In the case of the normal (linear) regulator the butcher will 'slice'
the top part of the steak and toss it in the garbage (heatloss), the switching
regulator is more like pounding the steak until it reaches the desired overall
thickness. Same thickness steak, but you get much more of it

For a quick and dirty solution, just hook it up and check with an amp meter that
in full sun you are not exceeding the panels maximum current rating (check
it briefly, not for hours !). Some kind of charge controller is probably needed anyway if you don't want to damage your cells due to overcharging when you are not monitoring battery charge status, most good ones are 'switched mode' and will take care of higher input voltages automatically. That's because solar panels that are nominally rated 12 V output anything between 0 and 18 V when they are not connected, after all the sun doesn't always shine equally bright. They will also taper off the current as the battery reaches full charge.

solar panels have an ingternal resistance far higher than the batteries so all you'd really need is a way to limit the current to whatever the panel can provide without being damaged. Because the battery has an internal resistance much lower than
the solar panel the battery will drag the solar panel down to it's own level, which most decent panels will be able to cope with. If there is an 'Imax' rating on the panels that's the one to set your current limiter to, possibly with a bit of a safety margin. You will have to keep a sharp eye on the batteries charge state when you do this, as soon as the batteries are full you should disconnect the panel.

If the panel is really small then you'd just be trickle charging it against the
self-discharge of the battery, and no regulator will  be needed. This situation can come in handy if you have an old 6V car or tractor that you don't use very often. It would take forever to actually charge a battery with a panel like that when it is really empty though.

[patty3]

I thank you very much for all the good information. I do have a tractor with a 6 volt battery that I don't use very often, and usually when I do the battery is very low or dead. I bought a 1.8 watt solar panel from Canadian Tire which was advertised that it would keep any idle 12 volt battery fully charged and ready to use without over charging it. My main concern was when hooking it up to a 6 volt battery that it would not damage the solar panel. Also I was wondering if a solar panel would be damaged if the - and + wires were unknowingly left touching each other causing a dead short? And if so, how long could they be shorted out before damage would start to occur? Thanks again  Pat!  

[jacquesm]

The Canadian tire 'trickle charge' panels will not get damaged if you short out the leads (they simply don't have enough power), however be VERY careful not to short out the leads on your battery because that could get nasty in a hurry.

<P>

Also, you probably should wear gloves when working with old batteries (and new ones, but old ones usually have acid all over them because they sweat a bit), the acid can cause skin irritation or ruin your clothes (they just sort of fall apart the next day, like they got burned real slow).

<P>

If your battery loses charge really quick (like say in a few days) then possibly the battery is defective, or you could have a wiring problem (not uncommon on older tractors, and if yours is a 6 V unconverted one it is probably way over 40 years old).

<P>

In that case the solar panel will not do much good. You can check this with a milliamp meter in series with the battery with the solar panel hooked up, if there is flow into the battery with the ignition (and whatever else there is on there, such as lights and so on) off, then the battery is receiving the charge and should at least stay at that level and / or slowly improve.

<P>

If there is flow the other way you probably have a wiring problem. Another way to check this is to measure the resistance of the + contact (lead slug) that goes on the battery to ground, this should be very high, preferably an 'open' circuit. If that's not the case check the wiring for any shorts - or partial shorts - to ground. take care !

[erne]

get yourself a volt meter and some duck tape. in direct sun set your open circut voltage to 8 volts using the tape to block off some of the panel. then you wont need too worry about any of the above. that is how I do mine...erne

[Saw Dust]

Erne,

I mostly just read here and don't post a lot but you really caught my attention. I just wanted to thank you for the great idea.  I am a techno geek and like many I get so wound up in the technology that I make things far to complicated.  

It just goes to say that even though Red Green gets a lot of laughs sometimes he even gets the last one.

Ian

[patty3]

and they told me only black tape was for electrical use!

[jacquesm]

not that easy ! It seems like it works because the open circuit voltage does drop (it would have to wouldn't it), but I just tried your scheme because I liked the low tech approach. What happens is that the open circuit voltage drops as you describe, but when connected to the battery the current drops to next to nothing compared to what the panel should be producing. It seems that having one partially shaded cell in the string screws up the whole string... This panel I've got here can be 'hacked' though, you could easily open it up and parallel the elements (cut the wire connecting the two centre cells, then parallel the two resulting strings).
That would give you the most bang for your buck as well.

[ghurd]

Just hook up the panel!

Hate to be the bearer of bad news, but a 1.8w 12v panel only puts out about 0.110 amps at peak power, pretty much making it a novelty item. Most trickle chargers for 24/7 use put out 0.750 to 1.5 amps (often too much for 24/7 in smaller batteries)

Basic rule of thumb is you lose 1% (Battery's Amp Hours) a day to self discharge. Also, de-rate a panel by at least 10% for loses in heat, wires, etc.

If your battery (6 or 12v) has 100 amp hours, it self discharges 1 amp hour per day, and a 0.110 amp panel gets de-rated to 0.100 amps, then you need 10 hours (0.100A X 10H = 1AH)of high noon sun at the equator, with low humidity and no clouds... Just to BREAK EVEN!

We don't get that much sun anywhere on earth!

In real life, in Canada, I would bet this panel would have a hard time just keeping up with the self discharge of a 30 amp hour battery.

So- you don't have ANY worries about over charging. And don't cover half the cells, you'll need all you can get for cloudy days.

(the 'brown' panels test about 15% higher current than rated for the first 6 weeks, the 'blue' panels test about right the whole time. If it's brown and tested at 0.140 amps- don't expect it to last.)

Hope this helps.