MPPT <span class="highlight">solar</span> regulators

Gerry
A PWM (Pulse Width Modulation) Solar panel controller does NOT give a continuous flow of electrical current once the charging voltage gets to the regulated voltage (generally around 14 volts), and should the conditions not be capable of attaining a panel voltage that is above battery voltage then there is No flow of charging current. Basically these systems are of the cheaper end of the spectrum and work at their best in fixed installations where the panels are set to obtain maximum harvest. As their name suggests they control the battery (charge) voltage by continuously switching the charge current off and on, so when the switch is ON then the battery voltage is the maximum voltage attained at the panel (with the maximum Amps being whatever the panel can attain at that lower voltage), subsequently when the switch is OFF then the panel attains ITS maximum UNLOADED voltage.

A MPPT (Maximum Power Point Tracking) allows the panels to attain the voltage that gives the maximum power output, it will then correct that input voltage to be suitable for the charging of the battery, and in doing so (where the panel voltage is above charge voltage) provide a greater amperage than the panel is producing. On the other hand when the panel is not getting the best of the sun, and may only be producing say12 volts and 5 amps (60watts) then an MPPT system will still charge the battery @ 14 v and around 4.2 Amps. (60 Watts less any internal losses, as nothing is 100% efficient)

This is the reason as to why a MPPT system is far better for use in a caravan, as you can not always orientate the panels for the best solar harvest, as well as the fact that they can make use of a greater part of the day.

I would still be fitting more that the bare necessity in panels due to the fact that you can not ever get the roof mounted panels to continually be directed straight on to the sun, so any panel fitted to the caravan roof will never reach its full potential.

Like all things electrical it is better to have more power available than what is required/expected to be used, the unit that is forming the load determines the final current flow in the system. As the battery nears its full charge then the internal resistance increases to the point where current flow ceases, regardless of the current (Amps) that is available. A battery in a charging system is no different to the 5 watt light that you connect to a battery that is capable of delivering 400 amps, the current flow at 12 volts would only be 0.4 amp.

Even though you intend to fit your panel(s) to a trailer so you can park your vehicle in the shade are you going to be moving this trailer every hour or so, so that you can obtain the best harvest out of your panel(s).

Hope this is of assistance.

Regards
Athol

Yes a good MPPT is much more efficient than a PWM, keep in mind though your seldom going to see 250W from your panel, best to work on a lower value for average power output over the day, personally I work on around 50%.

I used a victron 75/15 on a 36 volt 180 watt panel and it was perfectly suited. It started to charge in very low light. A 250 watt panel might be a bit too much for it when it hits max load in max sun.
If you run 2 panels in series use a victron 100/50, just make sure you have enough battery to cope or it will just be wasted.

Gerry, what you have to understand is that DC-DC chargers, MPPT controllers and transformers all basically do the same thing. They transform electrical power from one voltage/current relationship to another voltage/current relationship.

Transformers are the easiest to understand. They are simple things that will only work with AC power. The input power input will be roughly the same as the out power. I say roughly the same because there is a little bit of power lost in the transformer.) If you you transform the voltage up the resulting output current will be less than the input current. If you transform the voltage down the resulting output current will be larger than the input current. In every case when you multiply the voltage by the current you will have the same power in both the input and output sides (actually a little less in the output if you take the transformer loss into account.)

When it comes to DC power a transformer will not work. In the good old days we used an electric motor driving a generator to convert DC power. They were often called genemotors. These days we use transistors in circuits to do the job. That's where DC-DC devices come in.

We are familiar with DC-DC chargers. There is always some voltage losses in the cables between the tugs alternators and the vans house batteries. The output of the alternator is also often not high enough to charge the battery quickly. That is why we use them to boost the supply voltage to a higher one that will charge the battery. In doing so we need more current to drive the device than it supplies the battery (plus some extra current to drive the chargers workings.) That is the old voltage times current equalling roughly the same power.

The MPPT controller is just another DC-DC charger. In this case, we are transforming the voltage of the solar panel down to the batteries terminal voltage. Again we have the input power equalling the output power (less some to drive the controllers circuitry.) If the output voltage is lower then the output current will be increased to retain the same power figure (again less than 5 to 10% to drive the controller.)

I hope this clears things up for you.

Also beware of the open circuit voltage of the panels. Your 34.6v MPPT voltage will be lower than the open circuit voltage of the panel. It is probably over 40v. This is no problem with the Victron 75/15, but if you added another panel in series, you have an open circuit voltage of over 80v.
The regulator wont like that. Most likely it wont do any damage. It will just shutdown the regulator. Thats one good reason to go for the 100v unit.

Thanks everyone for your input. One way or another I can see why MPPT Regs are the way to go and more importantly, how they "extract" more power from the sun and deliver it to the battery, which is what I was really asking.