utu Senior Member Joined: 14 Jan 2012 Posts: 7 Location: new zealand Expertise: Just starting
Posted Tue Apr 10, 2012, 2:26am Subject: mobile commercial gas powered coffee machines
hi all i have bought a second hand wega airy 3 grp and am having it converted to gas i will be running 1 stand/element to assist with heating my coffee guy has said a 3000 watt inverter will be sufficient to run it ??? well i havent the faintest idea what size battery re amps etc or how many ? any help will be greatly appreciated thanks ...my grinder is a 300 watt machine and other than water pump and small lights will be the only thing running of it thanks pete
kross Senior Member Joined: 19 Feb 2012 Posts: 16 Location: Seattle, WA Expertise: I like coffee
Posted Tue Apr 10, 2012, 4:02pm Subject: Re: mobile commercial gas powered coffee machines
I don't have any experience running a mobile stand, but I might be able to offer some help anyway.
First, you'll want a deep cycle battery. A deep cycle battery can be deeply discharged without ruining the battery. Normal car starting batteries will be ruined when you completely discharge them.
To determine what battery capacity you will need, you will need to add up the power draw, and make some assumptions about what percentage of the time a particular appliance will be running. For example:
60w light, on 100% of the time = 60 watts 100w pump, on 10% of the time = 10 watts 300w grinder, on 10% of the time = 30 watts
60 + 10 + 30 = 100 watts. Assuming an inverter with 100% efficiency (these don't exist, btw), dividing watts by 12 volts will give you 100 / 12 = 8.33 Amps. So you'll be drawing on average 8.33 amps. If you want to run for 8 hours, multiply that by 8, which is 8.33 Amps x 8 hours = 66.7 Amp-hours (Ah) and look for a battery with at least 66.7 Ah of capacity. I would actually double that number, to account for inverter efficiency loss, and just to have some headroom. You would probably need two batteries, since I don't know of any deep cycle batteries with 130 Ah of capacity. For example, Optima's largest battery, the D31M has a capacity of 75 Ah, weighs 60 lbs., and costs about $250.
A few words about inverter efficiency. I noted that inverters aren't 100% efficient. There is some power lost in the conversion. Also, inverters are most efficient when the load on them is close to their maximum rating. A 3000 watt inverter might be over 90% efficient with a 2500 watt load, but only 50% efficient with a 250 watt load. Also, inverters will draw power even when there is no load on them, so it may be useful to get an inverter with a sleep mode, that shuts itself off until it detects you've switched something on, like a light or grinder.
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