Thread: Simple solar setup

On Straddie for a week at Christmas a 160 watt folding panels kept 2 x 120 amp batteries powering a 65 litre Primus fridge- freezer charged indefinitely in our camper.

I have a 120w flexible solar panel running through a PWM to an 80ah battery that runs my 38l Engel on the boat 24/7 and it has been doing this for months, The Engel is set at just before "2" on the dial.
I also have a fixed panel 160w on the roof of my Navara running through another PWM to 2x 125ah batteries that run a Primus 40l fridge 24/7 without any issues even in 40c+ temps, I do however have to give the batteries a bit of a chance to catch up with this setup by plugging into 240v if i also run the 80l Waco as a freezer as well.
If I added my 160w foldout panel to the Navara setup I wouldn't have to use 240v at all

I have a couple of spare MPPT solar controllers wired up with anderson plugs (just in case) in my toolbox.
Are these MPPT controllers more efficiant/better for the batteries than the PWM controllers I have on my boat and truck?

As I understand it, PWMs just limit the maximum charge to a set amount, but MPPTs may allow more in some circumstances. But I'm sure an expert will correct that if necessary.

Originally Posted by DiscoMick

As I understand it, PWMs just limit the maximum charge to a set amount, but MPPTs may allow more in some circumstances. But I'm sure an expert will correct that if necessary.

Of course you're right, that's quite wrong.

Basically, a PWM regulator is a fast electronic switch between the solar panel and the battery. If the battery charge level is low enough, the regulator stays on 100% of the time and allows the battery to be essentially direct connected to the panel. The panel output voltage is dragged down to just above the battery voltage, reducing its power output. Once the regulator sees the battery voltage reach it's set limit it switches off the panel for a percentage of each pulse that the regulator lets through, keeping the voltage to that level. This is what Pulse Width Modulation means, the length of each short pulse's on time can be expressed as a percentage value. This is far better than a simple analogue controller, which would get quite hot dissipating the extra power.

Now a MPPT controller does even better, it allows the panel voltage to be much higher than battery voltage, keeping the solar output near the most efficient output level. Then the magic beans inside the regulator turn the DC output into high frequency AC and transform it to battery charging voltage with a small transformer. Then it is rectified again to DC to charge the battery. This "switch mode" operation is similar to that used by modern power supplies and is quite efficient. Of course as the battery gets fuller the efficiency is irrelevant as you need to reduce the current to prevent over voltage. It is therefore only more effective at charging a battery that is not too full to accept the maximum current the regulator can provide.

As a simple example, a 120W panel will produce say 6 amps at 20 volts under ideal conditions. Connect that to a discharged battery with your PWM regulator and you'll likely get only 6.5 amps at 13 volts, or about 84W of power. Use a MPPT regulator on the other hand and you'll have 120W less say 10% efficiency loss equals 110W. Divide 110W by 13V and you get around 8.5A available for charging. That's where your extra 30% output current comes from, reading the output power of the panel and converting it with smart electronics instead of dumb ones.

Hope that's clear.

Excellent explanation - thanks. I knew someone would say it better than I could.

Have a cheap 100w panel on the disco with a cheap pair controller, 100ah battery. Keeps up with Engel ok, but I do 100km most days too. Usually drops to 12.6ish over nite. In the habit of keeping frozen 2L coke bottles in fridge if there is not much in there as well. Seems to help

Originally Posted by bee utey

Hope that's clear.

Well thats the theory at least - like all things real life experience may be different.