Design Battery Cart+Charging system for portable A/C

ashase said:

Photowhit, I apologize if I am posting this in the wrong place but you seem to be doing the closest thing I need to do.  I need to run a portable a/c around 10,000 BTU's.  Since I haven't bought it yet, thats all the info I have.

Welcome to the forum,  Don't worry, if your comment is in the wrong place, the moderators will move it.  It could be its own thread in the off-grid section.

Because you will be recharging with the grid, what you want to do is very straight forward.   You need a battery and charger similar to what a golf cart or forklift truck uses.  You also need an inverter to turn battery voltage into AC for the air conditioner. 

Designing a system is engineering. and engineers use formulas.  Formulas require numbers.  I need two numbers from you... peak load (watts) and energy use between charges (kilowatthours).  Then we can discuss the options.

The MOST important thing for you to do now is research air conditioners.  Efficiency is critical, not just of the air conditioner, but of the whole room that is to be cooled.  Better insulation and efficiency can shave hundreds of pounds and thousands of dollars off the battery bank.  Can you find an air conditioner with a variable speed compressor?  They are more efficient and have virtually no startup surges.  No startup surges means a much smaller inverter is needed because the peak load is smaller.  Smaller inverters have lower tare losses (self consumption losses).  

Your battery is the heart of the system and must be chosen carefully.  How much weight can you handle?  Perhaps LiFePO4 batteries (more energy, less weight, more money) should be considered.

Industrial lead acid batteries (forklifts, etc) are used and charged differently than RE (renewable energy) batteries.  It is cost effective to do much deeper discharges than with RE batteries.  But that assumes that you immediately recharge them with unlimited power and time (in other words: the grid).  If you can do that, you will need only half as much battery as we usually recommend.  It will require 220 volt service to power an industrial battery charger.

To be any more specific to your needs, I need those numbers for peak load and energy use between charges.

--vtMaps

Consider some sort of fan and forget the A/C. If you are outside or in a large hall playing music or whatever, the A/C is giving off more heat than cool.

So the venu will just get hotter.

Speaking from experience as a longtime professional musician.

ashase said:

I am looking at one of these to A/C units.  Thank you so much for your help.  We leave 6/24 and I am in a panic.

I looked at the first unit... it draws 1100 watts.  If you use it for 10 hours, it draws 11000 watthours = 11 kwh.  You will need to draw more than that (about 12 kwh) from the battery because some power is lost in the inverter.

The storage capacity of a battery bank is measured in kwh, and you need to decide how deep to discharge your battery in those 10 hours.  That will depend, in part, on the battery technology that you choose.  If, for example, you intend to draw your battery down to 50% SOC (state of charge), you will need a 24 kwh battery. 

Batteries are rated in volts and ah (amphours).   Volts X amphours = watthours.   If you have a 48 volt battery bank, you need a 500 ah battery bank. ( 48 volts X 500 ah = 24000 wh = 24 kwh).

A deep cycle, flooded lead acid battery bank of that size will weigh about 2000 pounds. 

Does this help?

--vtMaps

ashase said:

I am not going to carry around 2000 lbs of batteries.

My example was based on many assumptions... for example I assumed flooded lead acid and 50% discharge.  Forklift batteries can be discharged down to 20% SOC on a daily basis... as long as they are fully charged before the next day.  That means your battery can be smaller. 

You might consider LiFePO4 type of lithium batteries.  Much less size and weight than lead acid.

One more thing... I assumed that it drew 1100 watts all the time.  That may or may not be true... depends on inside temp, outside temp, humidity, insulation, how many times a minute the door opens, etc.

--vtMaps