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Topic: Expected voltage when charging batteries? (Read 720 times) previous topic - next topic


Hi all,

Forgive me if this is an obvious question but I cannot figure out why the following happens for the life of me.  I am working on a solar battery charger and for testing purposes, I hooked up the battery to the solar panel with the positives connected to each other and the negatives connected to each other like this:  Assuming the it's sunny outside and the solar panel is putting out 9V, shouldn't node A be at 9V?  But when measuring node with my multimeter, the voltage is at 6.3V, which is the voltage of the battery.  I thought that the voltage between two power sources connected like that would be the higher voltage.  Can anyone help explain what is happening?  The solar panel is rated for 8V open circuit voltage and 310mA short circuit current but it can actually go up to 9.5V/340mA.  The battery is a 6V 4.5Ah SLA.

Thanks in advance for any help!


I thought that the voltage between two power sources connected like that would be the higher voltage.

Naw-uh. Why on earth would you ever expect that? Associated with this, if you
connect a 9V battery to the 120VAC mains, the battery won't read 120VAC, it
will explode.

Actually what happens is that the voltage at the middle node will be the weighted
sum of the 2 voltages, as weighted by the output impedances [ie, resistances] of
the two sources.

V = Vlo + (Rlo / (Rlo + Rhi)) * (Vhi - Vlo)

In this case, Rlo << Rhi, so V = Vlo approximately.


Solar panels are current sources, which means that the current they can produce is directly
proportional to the sunlight falling on them.
The voltage rating of a Solar Panel is the open circuit voltage, and thats determined by the number
of cells in the panel which are usually wired in series.
The open circuit voltage doesnt change much with variation of light falling on the panel.
If you want to be able to extract the most power from a Solar Panel to charge a battery
then an additional piece of electronics called a charge controller is needed between the Solar Panel and the
The charge controller essentially optimises the load thats presented to the Solar Panel so that depending on the
amount of light on the panel, it will provide the maximum amount of power into the battery that the panel is capable of producing.
Small charge controllers are devices that can be easily built with small micro controllers like Arduinos or similar.


When your charging a battery you use constant current not constant voltage at first, so watever voltage it needs to be a set current is applied, for charging my 12v 4.5a battery say its at 12.5, with 1amp constant current the terminals will read 12.7-8 at first and slowly rise, once it reaches the proper voltage I set it to constant voltage of 13.8 volts and the current slowly tapers down with the voltage constant
so if you don't read a voltage raise when charging its probably just charginf real slow, the difference in voltage when charging is how fast it is charging, you can say in my case the .2v difference is all the voltage needed for 1amp to flow with the resistances of the charger and battery involved


Thank you all for your helpful replies, this is really helping me understand what's happening!

@oric_dan: That equation is perfect!  While I don't have the most electronic experience, I'm surprised that I have not seen that equation before.  Do you know of any resources where I can learn more about it?

@mauried: I was looking for one but I couldn't find one that was cheap and I wasn't able to find an schematics or instructions to build one either.  Any recommendations?

@winner10920: What you just said actually explains the phenomena I was witnessing when trying the same thing with a DC power supply, it's all starting to make sense


Ive made my own MPPT Solar chargers using a 16F886 PIC and a few other components.
They arnt hard to make , basically a controllable buck converter feeding the battery.
I can post the circuit if you are interested.
I built these long before I got interested in Arduinos, so I dont have an Arduino version yet.


Hi mauried: I'm very interested in the circuit, I might try to port it to Arduino if it's within my capabilities.


Real world voltage sources (such as a battery) are not perfect - we model this as an ideal voltage source in series with a resistance (for a battery we call this the internal resistance).   So drawing a current from a battery (or feeding a charging current into it) will change the voltage at its terminals - for a good battery this is a small change as the internal resistance is low.

As said earlier a photo-voltaic cell is a real-world current source, but a rather interesting one - it can be modelled as an ideal current source in parallel with a diode.  Without an external connection the voltage rises until this diode conducts all of the current from the source (about 0.6V per cell in direct sunlight).  Your 9V panel is probably about 15 cells in series.  When the battery is connected across it then most of the current is diverted into charging the battery (its 6.3V will thus rise a little because of the battery's internal resistance).  With only 6.3/15 = 0.42 V across each solar cell the diode is taking almost no current and the battery gets the majority.

The physics of a photo-voltaic cell is just that of a semiconductor diode - optimized to allow light-generated electron-hole pairs to diffuse to the electrodes rather than just recombine.  So light generates holes and electrons, these mostly diffuse to the relevant electrodes.  As the (external) voltage at the electrodes builds up then the diode can start to conduct just like any other diode.

[ I won't respond to messages, use the forum please ]


That equation is perfect!  While I don't have the most electronic experience, I'm surprised that I have not seen that equation before.  Do you know of any resources where I can learn more about it?

It's just basic circuit theory derived from Ohm's Law. The equation is really just a
voltage divider eqn applied where the lo-end voltage is about ground. Any book or
website on basic circuit theory.


oric_dan: Figured it out, thank you for your help!

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