Inverter calculation

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nizak

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Looking for some help on trying to figure battery life operating a 12V DC to 120V AC inverter.

Info I have is the following:

12V auto battery with 950 CCA. Free standing with no charging capability.

I will be looking to operate a 250 watt submersible fish tank heater once utility power is lost.

The inverter is 750 watts continuous output/ 6.25 continuous output amps.

Even an approximate running time would be helpful.

Thank you.
 
By battery life, do you want to know how long the inverter will operate on a single charge, or how many charge / discharge cycles before the battery craps out?

To answer the first, you need to know the amp-hour rating, not the cca.
They answer to the second will be "not many" using an automotive engine starting battery.
 
Automotive batteries are terrible for back-up systems. They're designed to be capable of providing most of their stored energy quickly, thus getting the most to start an engine.

Deep-cycle batteries work far better for long, even current draws. That's what they're designed for.

That said, there's too many variables to say your system will last x number of hours. Charge state of the battery, it's age, the number of charge/discharge cycles it's experienced (if discharged at all), wire size, efficiency of the inverter at al.
 
And hopefully the heater will cycle in and off rather than running continuously.

Sent from my XT1585 using Tapatalk
A 50% versus 10% duty cycle will make a factor of 5 difference in battery power duration.
 
A dual purpose battery, both for cranking and deep cycle is assumed. CCA divided by 7.25 gives a rough approximation AH. So in OP case, 950/7.25=130AH. Discharge at 2A for heater 250W, 120V. The system may operate more than 65 hours before recharging.
 
Sahib said:
A dual purpose battery, both for cranking and deep cycle is assumed. CCA divided by 7.25 gives a rough approximation AH. So in OP case, 950/7.25=130AH. Discharge at 2A for heater 250W, 120V. The system may operate more than 65 hours before recharging.
Click to expand...

Major OOPS there Sahib.

250W at 120V is indeed about 2A.
But to supply that same power from the battery, even with a 100% efficient inverter will take 20+A at 12V.
That is a high enough drain on a 130AH battery that you will be getting some Peukert losses and taking the battery to 0% State Of Charge (SOC) which will be very hard on the battery (very few cycles possible) you will be lucky to get 6 hours of operation at 100% duty cycle.
A realistic regular depth of discharge of 50% (still not a long cycle life) will give you only 3 hours of operation.
 
Computer UPS

Computer UPS

There is an alternative to what you are trying to accomplish. The 250 W / 2.1 Amps of the Tank Heater is about the same power consumption of a Home PC. A Computer UPS would take the guesswork out of trying to calculate Inverter Efficiency. Most are rated by estimated Computer up time while on the UPS.

A UPS is relatively small in size and comes with automatic switching as you would plug the Tank Heater into the UPS and the UPS into the Wall Socket. The Manufacturer would be taking all the factors on battery life into account and giving you 'about' times of operation.

When power is restored to your house, the UPS puts the load back to the utility company and charges the battery to await the next failure.

JimO
 
100AH will run just shy of 8hr(footnote below) on a ~90% efficient inverter that has roughly 150w(rms) continuous on its output.

definitely need a "AGM Deep Cycle" or a GEL batt, otherwise a std AGM or FLA batt will die rather fast.

example (w/ footnote)
1AHV = 3600J
100AH*12V = 4.32x106 J

150W for 8hrs = 200J/s * = 4.32x106 J

However, you typically run AGM Deep Cycle to about 50% DoD to keep cycle life high. these batts can go as far as 80% DoD, but that shortens their life. so at 50% DoD you get approx 4hr run time at 150w(rms) continuous.

the best ratio for inverter is 48vdc to 240vac, but sometimes you cant always use that combo, etc.
 
Sahib said:
A dual purpose battery, both for cranking and deep cycle is assumed. CCA divided by 7.25 gives a rough approximation AH. So in OP case, 950/7.25=130AH. Discharge at 2A for heater 250W, 120V. The system may operate more than 65 hours before recharging.
Click to expand...

Your "rough estimate" is very rough indeed! Batteries will have a reserve capacity equal to CCA/5, according to the estimates as studied by a Hawaiian university (LINK). Remember that the reserve capacity is the time the battery can supply 25A until the terminal voltage becomes 10.5V at 80degF. Conversion to ampere-hours will give you--> AH = RC/2.4. Inthis case your AH = (950/5)/2.4 = 79AH!
 
You'll be lucky to get 3 hours full power. The inverter will probably shut down on low voltage to protect the equipment, even tho a purely resistive load as you have needs no protection, it would simply put out less heat as voltage dropped.

Leave your high beams on for 3 hours then try to start your car. Have jumper cables handy...

eta: you can get a Honda Eu1000i generator off of Amazon right now for $800, and it'll run 8 hours on 0.6 gallons of gas at a quarter load.
 
JFletcher said:
You'll be lucky to get 3 hours full power. The inverter will probably shut down on low voltage to protect the equipment, even tho a purely resistive load as you have needs no protection, it would simply put out less heat as voltage dropped.

Leave your high beams on for 3 hours then try to start your car. Have jumper cables handy...

eta: you can get a Honda Eu1000i generator off of Amazon right now for $800, and it'll run 8 hours on 0.6 gallons of gas at a quarter load.
Click to expand...

Verified!
It would take 2 hours, 16 minutes and 6 seconds! See it here:
AH = 79
Discharge rate of battery (at 12V) = 250W/ 90% inverter efficiency/12= 23.15A
Assumed Peukert's constant = 1.13
time = AH/(discharge current)^1.13 = 79/(23.15)1.13 = 2.268 hours!
 
FionaZuppa said:
example (w/ footnote)
1AHV = 3600J
100AH*12V = 4.32x106 J

150W for 8hrs = 200J/s * = 4.32x106 J
Click to expand...

sorry, some numbers got goofed here

150w * 8hrs = 150J/s * 3600 * 8 = 4.32x106 J


@topgone post #13
a batt will not be at 12v when charged or discharging. a AGM deep cycle will be mostly done at 12v if you keep to a 50% DoD cycle.

@ post #12, thats because a car batt is not a AGM or Gel deep cycle.
 
FionaZuppa said:
@topgone post #13
a batt will not be at 12v when charged or discharging. a AGM deep cycle will be mostly done at 12v if you keep to a 50% DoD cycle.
Click to expand...

It's a given. Battery voltage drops when discharging. Reserve capacity is defined as the time measured from start of discharging a constant 25A until the voltage drops to 10.5 V. It was mentioned only to arrive at an ampere-hour estimate. Estimation on the time it takes for the battery delivering a certain amount of current was studied in detail by Peukert which he demonstrated to follow his equation: Battery discharge time = (capacity AH)/(discharge current)(n).
 
topgone said:
... Batteries will have a reserve capacity equal to CCA/5 ... reserve capacity is the time the battery can supply 25A until the terminal voltage becomes 10.5V at 80degF. Conversion to ampere-hours will give you--> AH = RC/2.4. ...
Click to expand...
Um, no. There is no direct correlation between capacity (usually designated "C") and cold cranking amps, ("CCA") which is primarily a measure of a battery's internal impedance at low temperatures. The number calculated is completely bogus.
And the calculation deriving capacity (often expressed in amp·hours) from reserve capacity is valid ONLY if the load is a steady 25 amps.

Is backing up this heater necessary at all? What duration of power failure are you planning for? How much will the fish tank cool off in that length of time?
Water has a substantial thermal capacitance and the temperature difference between a fish tank and room temperature is small. I would guess that it would ride through several days' outage without endangering the fish.
 
nizak said:
I will be looking to operate a 250 watt submersible fish tank heater once utility power is lost.
Click to expand...
these heaters cycle on/off.

there are many variables that affect the on/off cycle. and what about the water circulation/filtering pump?

probably best to start with a 100AH AGM deep cycle batt and test that with the heater. fully charge the batt, then run heater, take voltage measurements every 1hr until the batt reaches 11.8v (approx 50% DoD).

the inverter also has idle amps (waste), so in this case i would seek out a DC version of the heater (or make a setup to work, simple Arduino + thermocouple + mosfet + DC heater).
 
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