Re: Battery charging using solar cells


"Chris Newport" <crn.news@xxxxxxxxxxxxxxxxxx> wrote in message
news:drk0on$j46$1$8300dec7@xxxxxxxxxxxxxxxxxxx
> Brian wrote:
>> A friend and I, both electricians at one time we discussing the merits
>> of fitting solar cells to our Narrowboats to keep our batteries topped
>> up while the boat sits on the mooring.
>>
>> We are having problems deciding what size of cells we should use and how
>> they are rated.
>>
>> For instance a cell rated at 15 volts 3 amp I assume means it will power
>> a 45 watt lamp rated at 12 volts approximately.

Not quite because the solar panel is not a linear device, if you see its
characteristic curve you will find that the current doesn't rise as the
voltage falls, in fact it will probably have quite a flat characteristic
with a knee at 15 volts, which is optimised for battery charging.

>> But how does this relate to battery charging where the battery may be
>> showing a voltage of 12.5 volts against the 15 volts of the cell so
>> only 2.5 volts to drive in the charge.

Same way a cheapo car battery charger works - when connected the charger's
open circuit output voltage (about 18v typically - just like a solar panel)
is held down by the the battery to about 14v, and current flows into the
battery. The battery terminals are of course also now sitting at approx14
volts, unless as described below there is an excessive volt drop in the
connecting cables. (If your solar panel is remote from the batteries cable
size might need beefing up.)

>> Does it mean that the maximum current will be approximately 0.5 amps or
>> will it still rise to 3 amps

My neighbour has a solar panel rated at 65 watts, he angles it at about 45
degrees aiming south, and for most of a clear summer day it outputs just
under 5A into the battery bank, measured with a reasonable meter, and the Ah
counter in the boat. So yes, your panel should produce about 3A into the
batteries.

> Short answer - If there is no load you only need a small solar array and a
> diode to keep your battery "topped up". You just need enough to replace
> the internal discharge of the battery. Some (but not all) solar arrays
> have a built-in charging diode.

> Long answer - A 12 volt battery is not 12 volts, the voltage at the
> battery terminals will vary between about 11 volts (discharged), 13.8
> volts (float charge) and 15 volts (gassing). The terminal voltage of your
> solar array will vary with current and with illumination. The charging
> diode will drop about 0.7 volts and you will also have voltage drop in the
> wiring. This is a complex application of Kirchoff's equation with 2
> sources, both variable.

Agree - none of my electrical textbooks bother to describe what is going on
in sufficient detail, and I think we have to assume that solar panel (and
battery charger) manufacturers have done the maths!

> In practice the voltage drops in the diode and the wiring will ensure that
> your battery is nicely charged at around 14 volts.

Charles Wing's 'Boatowners Wiring Manua'l has a reasonable section on solar
panels, lots of figures and suggestions for positioning. In fact he
suggests that a single panel is best positioned flat, in our latitudes, and
that aiming at the midday sun reduces the overall daily power input. His
graphs seem to support this.

Paul Scott



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