Help me plan my battery system

Re: Help me plan my battery system

For a typical Energy Star Fridge, you are probably looking at needing a 1,500 watt minimum AC inverter for running one of these guys.

Somebody that can get into the thermostats, you can probably set up a relay or two to have the power go to the freezer first, then if the fridge starts up, switch the AC power to the fridge.

Note, a lot of this will depend on your appliances. True home style energy star fridges (many times) use electronic defrost timers, so that every time the AC power turns off, it resets the defrost cycle the next time the AC power is on (the switching between to units may cause extra defrost cycles)--Don't know what you have.

The power load sounds low--but you are probably going to be limited by starting surge for sizing your battery bank. I would suggest a 24 volt system--But lets start with 12 volts because you probably have a good feel for what two golf cart batteries look like (2x 6volt at 220AH in series)...

Anyway, lets start with a 2,000 Watt AC inverter to power your loads (a "good" 2kW AC inverter can support 4kW of starting surge). And assume that C/2.5 for starting surge support (a few seconds of heavy load to start the compressors and run defrost heaters--if you have those, plus lights, a few fans, etc.)... So, some worst case numbers (conservative battery bank numbers):

• 4,000 watt surge * 1/12 volts * 2.5 surge current = 833 AH @ 12 volts (or 1/2 that--416 AH @ 24 volts)

That would be 8x 220 AH @ 6 volt golf cart batteries. You could start with 4x G.C. batteries if you want to try (assuming you don't need 4kW surge).

Next, C/8 is a good load for flooded cell batteries (that would support your loads for ~4 hours to 50% discharge level):

• 833 AH * 12 volts * 0.85 inverter eff * 1/8 hour discharge rate = 2,124 Watt AC Load

Note that the batteries are rated at C/20 discharge rate capacity (~220 AH)--And will be closer to 190 AH--Which will give you less than 4 hours at 2kW continuous load.

Anyway--if your loads are not too great (closer to 2kW surge and 1kW average maximum continuous load) you could look at 4 instead of 8 G.C. batteries.

I would also suggest a 24 volt inverter to keep the wiring battery wiring a "reasonable" size.

If you are back at home base and can recharge at night/use a genset part of the day--It should be doable.

-Bill

Re: Help me plan my battery system

Everything above is "right"--Just fixed the units (just like your chemistry/physics teachers kept telling us -- watch the units).

Yes--Something could be done if you are willing to go into the wiring of one of the coolers.... Just wire up the thermostat of one to a Normally Open/Normally Closed and Common contact.

For example, Wire this up to the freezer. When the freezer is "off", the relay is "off" and the NC to Common passes power to the Fridge. When the Freezer turns "on", the relay switches to NO/Common which then supplies power to the freezer motor directly.

You could probably do it another way--Use a timer/programmable relay/setup of some sort... 30 minutes the outlet to the freezer is on and another 30 minutes the outlet to the fridge is on. A timer like this may work for you.

I am a bit surprised that both units turning on at the same time tripped a 15 amp breaker--Did you possibly have other loads on at the time that may have added to the current draw?

The inverter should go into shutdown (either just while both loads are present, or possibly you would have to kill DC power to reset--depending on brand/model). While it is not good to over current an inverter--It should not kill a good quality one to have that happen a couple times a year (waiting for all the folks to post that their inverters smoked on the first over current event :roll:).

-Bill

Re: Help me plan my battery system

I understand the 15 amp was an extension cord plugged into utility service...

I am still not sure why two chest freezers/fridges would pop a 15 breaker... The only reason I can think of are a) other loads or b) undersized wiring (too much voltage drop from two motors starting preventing sufficient current for starting).

-Bill

Re: Help me plan my battery system

800 AH @ 12 volts is a lot of battery... 400 AH @ 24 volts will make wiring up a 2kW inverter much easier and cheaper--But does not elimiate the need for a hefty charge controller.

I assume you will want to recharge the battery bank over night (call it 10-12 hours)--So you don't want a "smaller charger" that takes the better part of a day to do that.

The Honda eu2000i is probably enough to keep the bank breaking even--But you would want a larger charging source (my guess) for overnight use.

If you want the most efficient charge controller--We kind of beat that issue to death in this thread (if your eyes glaze over 1/2 way in--I understand:

Question about battery charger selection with EU2000 generator.

I am all for a balanced system design--And it is very easy to go overboard on one element or another (charging, battery bank, inverter, etc..)... Without hard measurements and experience, it is difficult to figure out if an 800 AH @ 12 volts (or same thing at 400 AH 24 volts) is overkill or not. It is very possible that you could "get away" with 1/2 the bank size--But it is an unknown at this point.

What you don't want is the system to work great for 6 months and then start chasing problems for the next year trying to figure out if it was an undersized battery bank, bad connections, too small of inverter, etc. down the road (so to speak)...

If you do implement this system--You will be the expert here. 8)

-Bill