Solar Panel Q&A: Charging 12V Battery with 24V Solar Panel

[Shawnzyoo]

I have a 24Volt solar panel trickle charging a 12volt Battery
My question is when the sun goes down and the solar panel isn't producing any power. Does the battery then try power the solar panel?
and if so is this bad for the solar panel?
--Shawn

 

[berkeman]

The solar panel will look like a string reverse-biased diodes when the output voltage falls below the battery voltage. This doesn't hurt the panel, but the reverse leakage current of the solar panel array will slightly discharge the battery. You can measure this leakage current with a DVM, and use the measurement to tell you if you want to bother physically disconnecting the array from the battery at night.


Oops, Edit -- was that a typo, or did you really mean to say that you have a 24Vdc solar panel charging a 12V battery? What are you using in the middle to drop the panel output voltage?

 

[Shawnzyoo]

i thought that to charge a battery
the voltage potential just had to be higher than the batteries
cause this was a solar panel designed to charge car batteries and the like
and the battery is getting charged each day...
could you help clear up my misunderstanding to what actually is going on
thanks

 

[berkeman]

Well, how you charge a battery depends on the battery chemistry. You recharge a NiCd different from how you recharge a lead-acid battery. The discharge and recharge curves and info are available for each type of battery, so look at the battery info to see how it is best charged. Guaranteed if you use a stiff 24Vdc power supply to try to charge some 12V batteries, you will damage the battery. If the solar panel that you have was specifically designed to recharge car lead-acid batteries, then it probably has a soft output voltage characteristic, and some series resistance to drop the voltage down to 13-14V, where a car battery will recharge well. Don't *ever* hook a stiff 24Vdc power supply up to a car battery to try to recharge it. Bad things will happen.

Also remember that the solar array output voltage and current will depend on the incoming irradiation, so the output voltage will only make it up to 24Vdc in bright sunlight.

 

[Shawnzyoo]

oh no...i am in wisconsin and we haven't had a sunny day for like 4 months
i just measured the voltage this morning and it was at 14volts
thanks for all the information
-shawn

 

[Ouabache]

Sulfation with solar charging

Another thing to bear in mind. If you use a lead acid battery (e.g. automotive, deepcycle, gel cel) and then trickle charge using solar panels; over time sulfation (precipitation of lead-sulphate crystals onto the plates of the cells) occurs, which weaken the batteries until they no longer hold a charge. This actually happens using any other trickle charging not just solar.

You can put this salt back into solution by using a http://www.dallas.net/~jvpoll/Battery/aaDesulfatorSurvey.html in parallel with your charger. You can build your own or use a commercially made desulfator.

 

[pallidin]

Solar panels generate DC current. A "blocking diode" is used to prevent reverse flow. The diode effect of the individual cells is not sufficient to prevent this, so, a separate(but built-in) blocking diode is used.