# Solar Battery Charger Questions

Discussion in 'General Electronics Chat' started by Andy Gee, Jun 28, 2014.

1. ### Andy Gee Thread Starter New Member

Jan 17, 2013
5
1
Hello, I am new to solar and am designing a solar battery charger. At the moment, I would like to use a 7.2V solar panel with 100mA output, a 1N5817 Schottky diode, and a 5V voltage regulator and resistor for limiting current to 100mA. I would like to charge 3 x 1.5V batteries. I have a few questions about things which Im struggling to understand:

1. Assuming the panel is rated at 7.2V and 100mA output, does this mean that the maximum possible output of the panel can never exceed these values?

2. I have seen some battery charging circuits which use a voltage regulator in series with a resistor to limit current. Assuming the panel cannot output more than 100mA, is there a need to use a voltage regulator in series with a resistor at all?

3. Assuming a constant panel output of 7.2V and 100mA, is this an appropriate panel with which to charge 3 x 1.5V batteries? And does the remaining power 0.1*(7.2-4.5) dissipate as heat?

4. Will there be negative effects if there are fluctuations in the level of solar irradiance? I suppose that if the panel outputs less than 5V (due to clouds or shading), the batteries would simply stop charging, but would this result in battery damage if the charger was repeatedly turning on and off due to changing light?

5. Will the batteries all be charged by an equal amount, assuming they are connected in series? I mean will I ever find that one battery is charged significantly more than the others, assuming they are all in the same condition?

Thank you kindly,
Andy

2. ### Bernard AAC Fanatic!

Aug 7, 2008
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What type of cells? A h capacity?
The SP, solar panel, specs are prob. at max. power point under ideal conditions, and will not be seen in practice.
Resistor and V reg. prob. not needed, but a charge controller might depending on cell chemistry.

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3. ### NorthGuy Active Member

Jun 28, 2014
611
122
If 100 mA is Isc, it is unlikely to be exceeded, although it is possible that a panel can produce slightly more than its rating because of effects such as snow reflection or cloud edge effects. So, you do not need to limit current.

If the panel is connected to a battery, an empty battery will consume current and drag the voltage down to its level. Therefore, the panel produce less - 4.5 x 0.1 = 4.5W instead of rated 7.2W. The extra power is simply not generated, so there's nothing to dissopate as heat.

When fully charged, a battery cannot take much current, so voltage will increase and you need to disconnect the battery at this point.

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4. ### wmodavis Well-Known Member

Oct 23, 2010
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5. ### Andy Gee Thread Starter New Member

Jan 17, 2013
5
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Thank you for your responses so far!

I would like to use 1200mAh NiCad batteries because they can overcharge at 100mA without damage.

I have a few more questions:

If I wanted a "fast charge" mode, would a 1A panel be suitable if it automatically disconnects once the battery voltage is, say, 0.2V below their maximum rating?

I've looked into MPPT but I am trying to build a low-cost system - I have read that MPPT increases efficiency by up to 35% in the summer - but with a panel voltage as low as 7.2V, is this worth it? I realise that there may be no right answer to this question, but simply an analysis of cost vs outcome.

6. ### NorthGuy Active Member

Jun 28, 2014
611
122
MPPT is good for lead-acid batteries because they need a range of voltages to charge (from 11 to 15 for 12V battery). Therefore you would need a panel with Vmp of around 18V to charge 12V batteries. With your batteries, you could closely match panel voltage to battery voltage, but you didn't.

MPPT converts Vmp x Imp x efficiency into Vbatt x Ibatt, so you get Ibatt = Vmp x Imp x efficiency / Vbatt. If Vbatt is 4.5 and Vmp is 7.2, you're likely to get 7.2V x 0.1A x 0.9/4.5 = 0.15A = 150mA

Without MPPT, you get about Imp, that is 100mA

7. ### ErnieM AAC Fanatic!

Apr 24, 2011
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1,789
When a solar cell gives you a voltage & current rating it is a shorthand for open circuit voltage and short curcuit current. These are the points giving you the outer bounds of the supply envelope for a given condition.

Make it brighter and you can get more current. Make it colder and you get more voltage, but it's still a good shorthand.

With a whopping 100mA to deal with forget any advanced charging scheme. The losses due to conversion will outweigh your gains. The money would be better spent on better solar cells.

Finally, since the short circuit current is on the same order as the max Ni Cad charge current (I/10 or 120mA) it is safe to just connect the batteries thru a (Schottky) diode and have a complete charging system: that's all that garden solar lights do.