We were asked to design a system which would allow the operation of two rooftop air-conditioners while the vehicle was not plugged into a 240v mains supply and while the vehicle is in motion.

We decided to use two Dometic Harrier air-conditioners, these are an Australian unit and use inverter compressor technology.  Inverter technology allows the compressor to operate at different power levels, this means it is not simply on or off. On the highest power mode each unit will draw 6.5 amps or about 1500w, so we had to design the system for a constant draw of 3KW.  The reason we chose the Dometic Harrier is over the cooling period of the habitation space they would use far less power than a conventional aircon.  Once the required temperature has been reached the aircon does not simply turn off, it reduces the power of the compressor, this means that to maintain the temperature far less power is required.

To handle this type of power we immediately dismissed using any type of lead battery as they simply could not have coped.  The decision was taken to use four 100AH Transporter Energy Lithium batteries. These batteries will happily withstand a constant draw of 100amps each giving us a total potential continuous draw of 400 amps DC.  Transporter Energy batteries are warranted for 5000 discharges at 80% or 10 years, making them one of the best batteries currently on the market.

To allow for reliable operation we installed a Victron Quattro 5KVA.  This unit is capable of supplying about 4200w continuously, this will equal about 400 amps DC allowing the full capacity of the batteries to be used.  This unit was installed along side a Victron Colour control allowing the whole system to be monitored and easy adjustment of the grid assist feature.  If you are plugged into a mains supply, ideally you would like not to have to worry about what you have turned on at the same time. For example if you were running both aircons and the microwave you could potentially have a site power requirement of 14.5 amps, if you add the TV, fridge and appliance charging you could easily require 16 amps.  Most sites in the UK offer 10 amps and some European sites only offer 5 amps, so if you are in Italy on an extremely hot day but only on a 5amp supply you couldn’t even run 1 airconditioner. This is of course unacceptable and is where grid assist helps.  On the colour control you tell the Quattro it only has 5 amps available; it will then automatically use those 5 amps and invert any additional requirements.  This will insure you will not trip the site electrics and allow you to continue to use the equipment with no worries.  As soon as your power requirement drops it will use all available power to recharge the batteries.

To allow for the use of the aircons when not being plugged into the mains we needed to recharge and assist the batteries.  We were limited on roof space as we had already installed two aircons, a 90cm satellite internet system and an additional fan rooflight.  Ideally we also wanted to install 1kw of solar however there was simply not the room to do this but we did manage to fit 830w. We achieved this by using 15 x 40w Victron solar panels and a 230w flexible panel glued to the curved luton space.  This solar array was controlled by using two Victron MPPT solar regulators and the generation was displayed on the colour control.  The solar array is capable to of supplying 70 amps in perfect conditions, as the aircons have inverter compressors. This will allow the aircons to be run for a considerable amount of time.

With every system like this we always like to have an alternative way of charging the batteries when off grid. Due to the size of the system we increased the alternator power from the standard 150amps to the larger 200amp alternator.  This will mean that while the engine is running, we can pull between 150 and 170 amps from the alternator to charge the lithium batteries.  This alternator will also be capable of continually running one of the aircons while the vehicle is in motion.

To allow for longer periods off grid we also installed two under slung gas tanks, these had a total capacity 85 litres allowing for extended use between fill ups.