Q: I’ve used a solar calculator from elsewhere and it says 80Watts is enough – why does yours over-estimate so much?
The 12Volt Blog uses proper electrical formulas and solar data, not assumptions.
Some websites assume 8 hours sun a day – take a look at the figures in the article on Solar Insolation Data – does that assumption look right to you? Thought not.
Some manufacturers say their fridges use an average of 0.8 Amps. In 24hrs that’s just 19.2Ah per day. I’ve measured many fridges, and never seen one that low (ever!). Let’s just say they’re being optimistic.
Q: My ‘van has a 3-way fridge and while driving along it works on 12 Volt, but by the time I park up the battery in the ‘van is pretty much flat. Shouldn’t the fridge be taking power from the alternator and not from the ‘van’s battery?
The simple answer is yes, it should be taking power from the alternator, but there’s obviously a problem along the way.
It could be that the wiring taking power from the towing vehicle is too skinny – these 3-way fridges are pretty greedy on 12 Volt.
It could also be that the ‘van battery is trying to charge from the alternator as well, and in the tussle with the fridge, the battery is going to come second, every time.
Also, for vehicles after 2010, we need to boost the alternator voltage for battery charging, with a DC-DC charger. If it’s before 2008 the fix is simply bigger wiring, or rewiring the ‘van to keep the fridge and battery apart.
Q: I need an inverter to run my laptop, camera charger and occasionally other stuff too, and maybe a microwave. I can get a 2000 Watt inverter online for a good price – will that do it?
Ok, a few things. First: laptops, and chargers for cameras, phones, etc use very little, so a 150 Watt inverter would do just fine. But a microwave needs a 1500 Watt inverter at least – big difference.
The second thing is that big inverters have a big current draw while they’re just sitting doing nothing, so they are very inefficient at small loads.
And finally, if you’re getting a huge inverter for the price of a small one – well, it’s the quality question, isn’t it?
Q: I have a motorhome with a 24 Volt starting battery for the motor. Can I just tap off that to get 12 Volts for the lights, fridge, etc. in the ‘van?
Yes you can, but you’ll need a separate 12 Volt “house” battery so that you don’t flatten the 24 Volt starting battery.
Also, you’ll need a 24Volt to 12 Volt charger – fortunately these are readily available and even better, they’re Australian made! See post on Charging Batteries.
Never, ever centre-tap the 24Volts – this will wreck both of your batteries in the 24V set – one will over-charge and the other under-charge – it will end in tears.
Q: I have a 120 Watt panel on the roof of my ‘van but I want to add more. Can I just add another panel to this, or must it be the same size?
Size doesn’t matter, as long as all the panels are 12Volt panels, you’re ok. So check on the back for the voltages – for 12Volt panels the Peak Voltage is around 17 to 18 Volts, and 21 to 23 Volts open-circuit.
The panels will be connected in parallel, so positive-to-positive and negative-to-negative, and the total wattage you get is simply all the watts added together.
Just make sure your regulator can handle the total current too. Look for the Short-Circuit current (Isc) on the back of the panels, and add them all up. If you can’t get to the back of the panels, you can guesstimate the current by dividing the Watts by 17V, so a 120 Watt panel will be around 7 Amps (120W/17V = 7.06 Amps).
I can get a 220 Watt panel cheaply from a mate – can I use that on my 12 Volt system?
Panels that size typically have nominal voltages higher than 12Volts – check the back of the panel – for a 12Volt panel the open-circuit voltage will be around 22 Volts, and the peak voltage about 17 to 18 Volts.
If it is not a 12 Volt panel, you’ll need an MPPT regulator to bring the panel voltage down to 12Volts – an MPPT is more expensive than a PWM regulator, so do the sums carefully to make sure that you’re actually winning overall. Also, some cheap regulators claim to be MPPT but are not.
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