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

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Discussion Overview

The discussion centers around the operation of a 24V solar panel charging a 12V battery, specifically addressing concerns about reverse current flow when the solar panel is not producing power and the implications for battery health and charging methods.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Shawn questions whether the 12V battery tries to power the 24V solar panel when sunlight is absent and if this could damage the panel.
  • One participant explains that the solar panel behaves like a reverse-biased diode when its output voltage is lower than the battery voltage, which does not harm the panel but can discharge the battery slightly.
  • Another participant emphasizes the importance of understanding battery chemistry, noting that different types of batteries require different charging methods and that using a stiff 24V power supply could damage a 12V battery.
  • Concerns are raised about sulfation in lead-acid batteries when trickle charging with solar panels, which can weaken the batteries over time.
  • A participant mentions the use of a blocking diode to prevent reverse current flow, indicating that the built-in diode effect of solar cells may not be sufficient.

Areas of Agreement / Disagreement

Participants express differing views on the implications of using a 24V solar panel with a 12V battery, particularly regarding the potential for battery damage and the effectiveness of charging methods. The discussion remains unresolved with multiple competing perspectives on the best practices for charging.

Contextual Notes

Participants highlight the need for specific knowledge about battery types and their charging requirements, as well as the effects of environmental conditions on solar panel output. There is also mention of the limitations of solar charging systems, particularly in low sunlight conditions.

Shawnzyoo
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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
 
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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?
 
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
 
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.
 
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
 
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.
 
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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.
 

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