12V Guru – calculate your caravan power needs

Who is 12V Guru?

Q&A inspired by readers of Caravan World Magazine. BMPRO Brand Ambassador David Bayliss answers a range of questions related to your 12V battery and caravan power needs.

Question to a 12V Guru:

We own a Jayco Starcraft Outback (18.6). We are largely self-sufficient in regards to power management, with 2 x 100Ah deep cycle parallel batteries hooked up to an integrated Setec power management system.

We have two 150w solar panels fixed to the van that charge the batteries in conjunction with the tug vehicle and 240v charging when hooked up. As a backup and for some appliance operations off the grid, we have a 3 kva generator. We have plenty of battery capacity to suit our off-the-grid needs, however, I am exploring installing a 3000w pure sine wave inverter running off the batteries to operate appliances such as the microwave or coffee machine, on the odd occasions we are not hooked up to power (rather than disturbing people with the gennie).

I have received differing advice; some of which is that the two 100Ah batteries will not be sufficient to run such appliances for the 10-15 minutes a day we would be using them, taking into account the low battery safety  cutouts and significant depletion of the batteries, and the difficulty of fully charging them during daylight. I am advised that there is a formula that would determine if the batteries are sufficient.

Are you able to provide advice on whether the two 100Ah batteries, in parallel, would be sufficient to run such appliances for a 10-15 minute period, and if so, could we recharge the batteries sufficiently within daylight hours?

For extra information, the most demanding appliance is the coffee machine which draws a combined total of 2200 watts if all the components are used in tandem.

Kind Regards, Rob Bonde, TAS

12V Guru answers:

Hi Rob,
Glad to hear that you are thinking of other campers and not firing up the old generator – too easy to kill the “serenity”! You are correct that there is a simple formula for calculating how much your appliances will use if drawing energy from batteries through an inverter. From this we can determine the battery requirements.
Whilst you have given us the combined total power required at 2200 watts, hopefully you will have factored in that for the microwave, you would need to have multiplied the specified power of the microwave by 1.5 to allow for inefficiencies. For example, an 800 watt output microwave needs 1200 watts of power to operate.

Will your batteries handle this load?

With the 2200 watts, the formula is to divide by 10 and this will cover the worst-case scenario. Therefore, 2200 watts / 10 is 220 amps. This is substantial, and your batteries would not be able to handle this.

Whilst the rated capacity of your batteries is 200Ah in parallel, you really should only be drawing a max on 50 Amps at any one time from the setup. To maintain the longevity of your batteries, most  manufacturers would only recommend discharging at C/4 – in this case 200/4 = 50 Amps max.

Whilst the batteries could handle a higher load for a short period of time, it will greatly diminish the capacity and longevity of the batteries very quickly. With high current draw from the batteries, the voltage will consequently fall away, and it may be too low to sustain that load. This may happen even with a battery that’s still well charged. A couple of suggestions may help you to decide upon an appropriate setup.

Reconsider your appliances

However, first I would reconsider your appliances. A standard 800 watts output power microwave should only draw around 1200 watts, so your coffee machine seems very heavy on power. Or use the devices in isolation if that is the combined power. At 1200 watts / 10, this gives 120 amp which your batteries could handle for a short period and recover okay.

Opt for Lithium batteries

Otherwise, my suggestions would be to explore Lithium batteries which don’t mind being discharged at high rates and can also be discharged to lower capacity than lead acid.
Drawing 120 amps for 15 minutes from a fully charged 200Ah will certainly put a good dent in your available energy but the two batteries
should be fine depending upon their age. As you will be pulling out the energy very quickly, the available energy is less than if you were to
draw it out slowly.

Consider solar panels and monitor energy requirements

It’s difficult to ascertain if your solar panels will be able to keep up with a full recharge each day, as this will depend heavily upon the sun and other appliances being used. I’m not sure if your van has a BMPRO Drifter already installed, but you could consider one of these to help better manage your caravan power needs. Happy camping!

The article first appeared in Caravan World Magazine

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