For this instalment of The mechanic, we welcome a submission from the club’s chairman;
SR PWM MPPT – A question of control
If you have or thinking of getting a PV (photo voltaic)
solar panel, then these terms may concern you.
There have been several articles about the use of
solar panels to provide power in vans when there
is no hook up available. The panel(s) will almost
certainly be used to charge a battery for use when
there is insufficient power available from the sun. The
maximum power available from any panel is in a very
clear set of circumstances, the sun needs to have an
energy at the panel of 1000 watts per square meter,
the sun’s rays must strike the panel perpendicularly,
the air temperature should be 23 deg C. So, if you set
up your panel at noon on a cloud free midsummer’s
day carefully angled so the sun strikes it at 90 deg and
there is a gentle breeze, a 100w rated panel will give
100w of electrical power. In any other circumstances
the power will be substantially less. So, in reality it
is better to estimate the average power to be 30 to
60w from a 100w panel.
The next thing is how to make the most of the power
we do get. If you examine the “rating plate” fitted to
almost all solar panels you will see some numbers.
Ok you see 100w max power but look at the “ipmax”
this is the current at maximum power, ”vpmax” this is
the voltage at maximum power. A typical example
of a 100w panel ip max =5.55a vp max =18v 185.55 =100w. So we need a control unit to regulate the power sent to the leisure battery. Small panels less than about 30cm square sold as “trickle chargers” to maintain a battery while laying on the dashboard have so little power they are self regulating (SR) as the current is so small as never to damage the vehicle battery. Those for phone or device charging rely on the internal battery controller in the device to regulate the power and prevent overcharging of the internal battery. This leaves the choice of the two types of actual control unit PWM (pulse width modulation) or MPPT (maximum power point tracking). At first the generally available controllers were all PWM and cost from £8 up to around £35. These work by monitoring the battery voltage and sending pulses of power to provide an average voltage to the battery. Initially when the battery is low, the power pulses are very wide, but as the battery voltage rises then the pulse width is reduced. It is important then for the controller to “know” when the battery is at full charge so the pulses can be reduced. Different (lead acid) batteries fall into at least 3 types; Flooded, AGM and GEL. Each has a different charging requirement. So, any controller needs to be set to the correct type. Cheaper controllers may have no settings at all or be described as “automatic detection” and are probably best avoided! When you look at the typical full power voltage and current from a solar panel you will notice the voltage is too high as the maximum needed for the battery is 14v so the best this controller can do is to give 145.5=77w. The rest of the power is wasted due to
the effective internal panel resistance.
So around 25% of the power we do get is just
wasted, to overcome this a MPPT controller can be
used. This is often a combination of PWM control (for
trickle charging when full power is not needed) and
an inverter which is controlled by a microprocessor.
This changes the 18v 5.55a into 14v 7a, this is an
example as the controller constantly measures both
panel output (change in sun intensity) and battery
condition (low, charging, full) and adjusts the inverter
to maximise the power to the battery. This results in
an efficiency of better than 95%.
SOLAR PANEL CHARGE CONTROLLERS
Transporter Talk Issue 169 | 23
I have tested this and can confirm that just changing
the controller increased the current from 5a to 7a
. If as I have, you have more than one solar panel
(I use 3) and they are all slightly different outputs,
the MPPT sorts out the balance even when one is in
shade and 2 are in sun.
The MPPT controller is as you would expect, more
complex and expensive up to around £70. This may
mean that some suppliers may claim to be MPPT
when they are not. I was fooled by this but claimed
back from the seller as the description was clearly
false. I have some photographs of the various types;
PWM 10 amp, fake MPPT (plenty of usb points on it!)
and a real MPPT 10 amp unit. So check that you get
the correct item!
I have 2 panels on the roof of my Westy and when
raised the angle is quite close to optimum. I also have
one on the front luggage rack so I can get power
even as the sun passes over. I have this arrangement
to support not only lights and water pump, but the
compressor fridge that is of course run 24/7. I would
not want to run out of ice for our G&T’s after all!