Understanding Electrical Power and Energy in RV and Marine

Understanding electrical power, batteries, inverters and solar is easily compared to water flow, tanks and pumps.

A water tank is like a battery. It fills and depletes to empty.

A tank filler is like power coming in. There are different size water fillers. A low power charging device is like a small/tiny water filler. You are standing there all day trying to fill the tank through a ½ inch hose. A big power charger is like a huge opening to fill the tank quickly.

An inverter is like a super big water pump. It will pump the tank dry if you aren’t careful. If it is dry, trying to pump water can damage things.

A fridge and LED lights is like a smaller pump but always pumping away day and night out of the tank.

The fundamental principle is to always have water in the tank.

That means similar quantity of water has to go in as comes out. Then the tank level will stay at a similar point.

And this is the fundamental principle of well-designed power systems:

Don’t just buy a big tank – ie: a bigger battery. Ask the question: Is the power going in more or equal to the power expected to go out? When the tank is empty, it is useless.

Make sure the tank can accept bigger flows – for batteries that is the continuous charge current is high. Preferably as high as the discharge current. Most batteries have much lower charge current levels than discharge current levels.

Then make sure that the charging sources are adequate. Ask the question: How long will it take to charge these batteries?

Solar at 50% power level. We pick 50% because this is the rating for a beautiful day in July or a cloudy day in summer. 20% of the days are perfect in QLD, 60% are at 50% and 20% are rainy/overcast at 10% output level. We always design for 50% solar yield which is 80% of the days.

DC DC charging. In a conventional analog DC DC the output drops to 50% when hot (users report this for Enerdrive DC DC for example). So again, how long at 50% charge rate? (Scotty doesn’t derate and runs at 96% efficiency)

Then make sure the replenishment energy sources can pour the power in and fill up the tank as quickly as it goes out.

Most couple who are moderate users require around 3,500Wh of replenishment energy a day. For 1,000W on a caravan roof, at 50% yield for 5 hours, that 2500Wh produced. Then the battery is being consumed at 1,000Wh per day. In perfect weather it is adequate. About 1400W at 50% produces 3500Wh a day.

Understanding Electrical power with Solar

The bigger the roof area the better. With solar for an all electric van for 2 people we target 1200-1400W on the roof.

Designing the solar takes the longest time on a job. I consider myself above average at this, having designed/ delivered over 5000 solar jobs, yet it still takes time in the design to squeeze out every possible ounce of solar.

The higher the solar panel voltage, the better. Think of this as the steeper the roof, the sooner you get water in the rain tank. Some 12V solar panels are like having a flat roof. Or a roof with a belly in it and nothing flows out unless there is a torrential rain storm. We like individual solar panels at 33V Voc and higher. The best Sunpower panels are 82Voc.

The most common mistake with jobs coming in is the customer wants to keep or buy solar from a cheaper source. They then end up spending good money on a system but with a dubious power source.

Think of an Inverter/Charger as a very big pump

However, not all inverters or inverter/chargers are the same.

Some inverters are stand alone and cant be connected to the incoming power. That’s like running two pumps. One pumping water into the tank and one pumping out. The water coming in from a connected source is not just passing out.

Understanding Electrical Power with DC DC Charging

DC to DC's are the most mis-understood power conversion device. They are most commonly used to extract power from an alternator.

A DC DC with lithium batteries is two devices in one. It is a a power converter AND a lithium battery charger. The complication is that the power conversion rate is set by the battery charging rate. The two are tightly entwined.

A high power DC DC must respond quickly to the lithium battery voltage or the CAN communication from the battery BMS. BUT at the same time be within the power limits of the alternator. Now you can see the complication. Big DC DC units that don't vary the power, choke or damage alternators at idle.

The best DC DC's are rather complex and need tuning parameters matched to the alternator. This requires some skill. With Safiery's Scotty AI, the AI does auto-tuning to the alternator automatically. It not only protects the alternator with variable power, the temperature sensor on the alternator, sets back the power rate if the temperature rise becomes too fast.

Compliance to Australian Standards

  • Safety is always No 1. Large power systems and inverters have to be safe. The earth wire is the ultimate safety point with RCD’s (Residual Current Detection) tripping if there are faulty appliances. These are compulsory on every installation.

    The Victron inverters Safiery use have an additional safety feature of detecting if the incoming power when you are plugged into a socket at say a farm stay, is safe or not. The incoming power will NOT be activated if it is unsafe. We get 2-3 calls a week from our now nearly 500 monitored systems requesting us to check this as no power is coming in.

    Safiery systems don’t require double pole switches because we activate software to actively check on the polarity of the active wire for incoming power.
    New Standards become mandatory after 19th November 2023. The major change is the location of Lithium batteries which will require either an exterior location or access externally. Safiery's Meteor battery complies with both the internal requiremenst with sodium or the external requirements.

How Good’s That!

New Electrical Standard Apply Soon and impact Lithium Batteries

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