Mobile Barista at wits end!

[Seb217]

All I want to do is sell coffee!!! ... yet I have found myself stressing about power supply!

I have a mobile coffee van. I'm done with using generators and so are many event/festival organisers who are now banning petrol/diesel from their sites. In an ideal world I would like a battery fed inverter to power all the equipment that I can't run on LPG.

I would like to achieve 1.5kW of power - I actually currently need 1.2kW of power (not all continuous) but want the extra to future proof against any further equipment that I may want to put on the van in the future.

I need to be able to have power for approx. 10hrs per day. I have sufficient space (but not endless amounts of) to house the batteries and inverter.

As all the equipment that requires power is plugged in to sockets on the walls of the van, can I plug the ‘hook up’ plug into the inverter?

I understand everything there is to know about coffee (and it's more of a science than many people may think) but I know nothing about power. Is there anyone that can please please help.

 

[shaun1]

Remember as well that you will need a charging system capable of charging all of the batteries, and you will probably need to have the van plugged in all night to get a full charge.

You could buy a chesp home plug in energy monitor and use that next time you are running from generators/event power. You might find you are using lesd than you think.

 

[Lucien Nunes]

I agree with James the Spark that from your table of requirements, the average running load is around 700W. This assumes the fridge compressor runs about 50% of the time. It might be more on a hot day if you keep opening it.

What has been overlooked in all the above calculations is that the batteries have a rated capacity that represents their absolute maximum capability when new. Using that capacity cyclically, i.e. fully discharging and then fully recharging, is like working a horse until it collapses, reviving it and doing the same day after day and expecting it never to lose stamina. Ordinary leisure 'deep cycle' batteries will not last like this, their capacity will soon drop off. After a few hundred cycles use it might be down 25% or more, i.e. you needed to start off with 25% more battery to account for their decline, or your working day will end 25% early. The more deeply you discharge, the more rapidly the battery wears out. I.e. to give good working life, you need extra capacity discharged less deeply, but this means extra weight and volume to carry about.

Next you have to consider the discharge rate. The rated capacity of most lead acid batteries is given at the 20 hour discharge rate. If you discharge them faster, e.g. use all of their capacity in 10 hours, their usable capacity will be lower. Equivalent to the horse being able to make 10 round trips in a day if it trots, but only 8 if you make it gallop. Then, as Shaun points out, you have the minor detail of recharging. To fully recharge a lead acid in less than 12 hours is stressful for it, as the last part of the charge is absorbed only slowly. But if it does not get fully charged, it starts to lose capacity through sulphation. You can solve these problems using full spec traction batteries as used on electric vehicles. These are rated for discharge over 5 hours and for rapid recharging, so although they are more expensive, larger and heavier per Ah than leisure batteries, you need add less margin of capacity and will last longer.

What size battery do you need?

700 watts for 10 hours with an inverter efficiency of 85%
700 x 10 / 0.85 = 8.2kWh energy taken from the battery

I'd use 24V, not 12V, to reduce the currents involved and hence cable size and losses. 24V inverters may be a teeny bit more efficient for the same reason.

Taking the average usable voltage at the inverter terminals as 23.5V:
8200/23.5 = 349Ah at 24V actual capacity utilised.

With a full traction battery rated at the 5 hour rate, you will get 110% of rated capacity over 10 hours. If you want a 25% margin, then you need:
349 / 1.1 x 1.25 = 396Ah rated capacity.

Yuasa reckon on 1200 cycles / 4 years use at 70% depth of discharge to about 70% of rated capacity. If you want this kind of lifespan, you will need 396/0.7 = 565Ah rated capacity or more. At that capacity you would best have a battery made up from 2V cells. If you wanted to use series-parallel 6V blocks, we could use their GC200 as an example, wired as 3P2S (three parallel strings of two in series). These would need a battery box about 550mm square and 300mm high, total weight about 200kg. Hydrogen will need to be vented and not allowed into the vehicle.

I'll let you do the sums for the equivalent in leisure batteries, personally I'd stick with traction. But first, see what you can do to get that consumption figure down, and more accurately known, e.g. a more efficient fridge will save ££ in batteries. Or, switch to Li-ion, but make sure you're sitting down when you check the pricing!

NB. this is why small generators are popular!

 

[Silly Sausage]

It's a farce really all the weight you take there uses energy that has to be generated by some means . I bet there is a massive car park too.

There's a big 'Organic (green)' type place near me. It always amuses me looking at the long queue of cars waiting to get into the car park, most of them with their engines idling away, belching crap into the air ...

 

[telectrix]

grind the coffee at home prior to setting out. thats 0.35kW saved.

 

[Silly Sausage]

My exact requirements are:

APPLIANCE​	kW​	USAGE​
Coffee machine pump	0.350​	Constant​
Coffee grinder	0.350​	15min/hour​
Flo-Jet (water pump)	0.046​	10min/hour​
Fridge	0.300​	Constant but fluctuating​
Till	0.002​	Constant'ish​
Main strip light	0.072​	Constant​
LED blue lights	0.075​	Constant​
TOTAL	1.195​

Couldn't you just sell iced coffee?
Brew up a big vat of juice at home, a great big ice box and a load of candles for lighting ... well cool, dude!

 

[Lister1987]

How about an FLT battery? Granted thrycsn be quite large but I'd imagine they pack a punch

 

[Lucien Nunes]

Yes, a FLT battery is a full-spec traction battery as per my recommendation above. If one happened to find a 24V example in good condition, or half a large 48V one, it would probably fit the bill.

 

[flangey]

You can get LPG conversion kits for Honda Eu20i etc, might be a good solution?

 

[Seb217]

I would run far far away from a 6v system. A 24 volt would be much better

I wonder if there is any way that you could make me understand why? The explanation will need to be very very basic

 

[Seb217]

watts = volts x amps
therefore amps = watts / volts
1200 watts / 12 volts = 100A
1 single 100Ah 12v battery in peak performance will provide you with 1200 watts for 1 hour.
this is the calculation, it is not negotiable!
the only thing to add is that that is the pure mathematical calculation and is not including any losses that you will get in the distribution and conversion of electrical energy.
I would estimate this to be 15% to 20%

Thank you soooo much. I actually understood the maths (I really am mathematically brain dead as I mentioned before)! This has helped me understand the whole situation/problem.

 

[Seb217]

I'd just take a giant bottle of gas with a camping stove...

But that's not going to power my coffee machine and instant coffee is NOT an option - the stuff should be banned!

 

[Seb217]

You can get LPG conversion kits for Honda Eu20i etc, might be a good solution?

I've been looking at the EU22i LPG inverter generators today but I really really don't want to have to have one.

 

[Seb217]

I agree with James the Spark that from your table of requirements, the average running load is around 700W. This assumes the fridge compressor runs about 50% of the time. It might be more on a hot day if you keep opening it.

What has been overlooked in all the above calculations is that the batteries have a rated capacity that represents their absolute maximum capability when new. Using that capacity cyclically, i.e. fully discharging and then fully recharging, is like working a horse until it collapses, reviving it and doing the same day after day and expecting it never to lose stamina. Ordinary leisure 'deep cycle' batteries will not last like this, their capacity will soon drop off. After a few hundred cycles use it might be down 25% or more, i.e. you needed to start off with 25% more battery to account for their decline, or your working day will end 25% early. The more deeply you discharge, the more rapidly the battery wears out. I.e. to give good working life, you need extra capacity discharged less deeply, but this means extra weight and volume to carry about.

Next you have to consider the discharge rate. The rated capacity of most lead acid batteries is given at the 20 hour discharge rate. If you discharge them faster, e.g. use all of their capacity in 10 hours, their usable capacity will be lower. Equivalent to the horse being able to make 10 round trips in a day if it trots, but only 8 if you make it gallop. Then, as Shaun points out, you have the minor detail of recharging. To fully recharge a lead acid in less than 12 hours is stressful for it, as the last part of the charge is absorbed only slowly. But if it does not get fully charged, it starts to lose capacity through sulphation. You can solve these problems using full spec traction batteries as used on electric vehicles. These are rated for discharge over 5 hours and for rapid recharging, so although they are more expensive, larger and heavier per Ah than leisure batteries, you need add less margin of capacity and will last longer.

What size battery do you need?

700 watts for 10 hours with an inverter efficiency of 85%
700 x 10 / 0.85 = 8.2kWh energy taken from the battery

I'd use 24V, not 12V, to reduce the currents involved and hence cable size and losses. 24V inverters may be a teeny bit more efficient for the same reason.

Taking the average usable voltage at the inverter terminals as 23.5V:
8200/23.5 = 349Ah at 24V actual capacity utilised.

With a full traction battery rated at the 5 hour rate, you will get 110% of rated capacity over 10 hours. If you want a 25% margin, then you need:
349 / 1.1 x 1.25 = 396Ah rated capacity.

Yuasa reckon on 1200 cycles / 4 years use at 70% depth of discharge to about 70% of rated capacity. If you want this kind of lifespan, you will need 396/0.7 = 565Ah rated capacity or more. At that capacity you would best have a battery made up from 2V cells. If you wanted to use series-parallel 6V blocks, we could use their GC200 as an example, wired as 3P2S (three parallel strings of two in series). These would need a battery box about 550mm square and 300mm high, total weight about 200kg. Hydrogen will need to be vented and not allowed into the vehicle.

I'll let you do the sums for the equivalent in leisure batteries, personally I'd stick with traction. But first, see what you can do to get that consumption figure down, and more accurately known, e.g. a more efficient fridge will save ££ in batteries. Or, switch to Li-ion, but make sure you're sitting down when you check the pricing!

NB. this is why small generators are popular!

WOW! Thank you so much for your extensive and very interesting and informative reply! It really is very very much appreciated!! Whilst I couldn't fathom most of the maths (without a lot more coffee), I'm coming to the conclusion that batteries aren't feasible.

 

[Edtwozeronine]

But that's not going to power my coffee machine and instant coffee is NOT an option - the stuff should be banned!

You've not heard of a cafetiere then?

Personally, I'd rather have a nice cup of tea!

 

[Seb217]

grind the coffee at home prior to setting out. thats 0.35kW saved.

Great idea powerwise, but pre-ground coffee = stale coffee. Oh yes, I'm a coffee snob