[SOLVED] battery duration calculation

pls I need an explanation on the run time of a battery. for example with 500W load, how lond will it take a 12v 7ah battry to discharge. not that the battery is used to power inverter of output 220volts.

Hi Adajo,

First there si few things to clear, there is lots of amperage on 12V.
Given battery capacity of 7Ah applies only if you taking /20 parts of current (7/20=350mA).
Also the same situation is with car battery 55Ah means only if your load is /10 or /20 of Ah. (According manufacturers product data).

We have (theoreticaly) :
500W@ 12V = 41,66A 7Ah / 41,66A = 0,16h (its just theory!)
This works on lead acid only if you have proper discharging current /10 /20.
Regular 7Ah/10=0,7A, but we there pull 42A from it. Should I say its little battery for that job.

But must consider manufacturer data about battery discharging current for short and long period.

Battery internal resistance and state also are calculated. Its new or its old, sulfated,....

Efficiency of inverter is important thing. Often efficiency of small desktop PC UPS is poor.
Switching dedicated inverter have much better efficiency say 87% (but should read manufacturer data).

But battery is chemistry, look diagrams in this datasheet for minutes, and discharging current, and time.

On 500W you will get minute two, on 7Ah as new battery, maybe less.

For 55Ah car starter battery on 500W you will get 10-15-18minutes (from expirience) - note: with new battery.
Calc say 55Ah / 41,66A = 1,32h but this is not /10 or /20 this is much harder job for battery, this is almost /1 and capacity of 55Ah is only true for /10 or /20 current.

Also all inverters have standby current. That mean that they in standby consupt some current in range 0,3 to 0,9A depend from manufacturer and size (power) of inverter, this is data in device specification given by manufacturer.

Lots of inverters deep discharge batteries to 10,5V this can kill starter lead acid battery. Starter batteries have 11,9V lower limit or 1,99V per cell. Starter have ability about 300 cycles in range 11,99V to full charge, after that goes to heaven. Deep cycle batteries are different storry.



Take in mind Peukert's law :
Peukert's law - Wikipedia, the free encyclopedia

What is an Amp Hour and How to Calculate Battery Capacity | OverlandResource.com – Overland expedition travel info.

7Ah implies that the battery can draw current of 7 Ampere for 1 hour...
so, for a 500W of load,
Watt=Volt*Ampere
Which implies, A=W/V => A=500/12 => A=41.66
That is for power consumption of 500W, the battery will draw a current of 41.66 A...
To calculate the time it can withstand:
Since 7A lasts for 1 hr,
then 41.66A will last for x hr
therefor x= (7/41.66)*60....... | 1hr = 60 minutes
=> 0.16 hr or approximately 10 mins...
Since there is an obvious power loss by the inverter and also some other effect, the time shortens to 7-8 mins...

https://www.batteryweb.com/pdf/inverter_battery_sizing_faq.pdf

Genovator said:

7Ah implies that the battery can draw current of 7 Ampere for 1 hour...
so, for a 500W of load,
Watt=Volt*Ampere
Which implies, A=W/V => A=500/12 => A=41.66
That is for power consumption of 500W, the battery will draw a current of 41.66 A...
To calculate the time it can withstand:
Since 7A lasts for 1 hr,
then 41.66A will last for x hr
therefor x= (7/41.66)*60....... | 1hr = 60 minutes
=> 0.16 hr or approximately 10 mins...
Since there is an obvious power loss by the inverter and also some other effect, the time shortens to 7-8 mins...

Click to expand...

No. No my friend there is no 7A for 1h!!!!

7Ah is only for /20 current, I attach in post #2 datasheet from manufacturer (this is just one datasheet as example. I have lots of battery datasheets). Lead acid batteries are /10 or /20 manufacturer define this and capacity is for that current.


Battery is chemistry, my friend. In datasheet you can see exact curve for what current and duration.

People think that car battery 55Ah is 55A for 1h is theory this is not reality. Given capacity of 55Ah is for /10. 55/10=5,5A 5,5A x 10h = 55Ah if you go under that this capacity is shorter.


It is standard how manufacturer declare their battery capacities, they say its for /10 or /20 current.

Every battery is chemistry, even small 2032, AAA, AA,.... capacity is declared for planned consuption of current /x (given capacity/dividing factor).



Smaller battery size, capacity and typical draining current:
Battery Capacity



Take in mind Peukert's law :
Peukert's law - Wikipedia, the free encyclopedia

What is an Amp Hour and How to Calculate Battery Capacity | OverlandResource.com – Overland expedition travel info.

May be my concept is wrong... But I learnt this fact in some websites...
Any way, thanx for the correction...

Genovator said:

May be my concept is wrong... But I learnt this fact in some websites...
Any way, thanx for the correction...

Click to expand...

No problem my friend, always can experiment to see what happen, this is purpose of experiments.


7Ah is for /20 that means that 7Ah/20 = 350mA. (you can pull 350mA 20h from 7Ah battery, and capacity will be 7Ah )


Often for small batteries manufacturers give resistance of load (this determine current), or just say maximum continious current or pulsed current.

Also keep in mind that ups small batteries (gell/glass) are designed for life of 3 to 5 years, but this is often very shorten, because higher temperature, discharging with higher amperage then permited,.... of course people buying new, money goes,..... very often this is good bussines for manufacturers.

Here is two example for datasheets to see how manufacturer give curve and data about discharging current and capacity (in post #2 I give datasheet with curve for 7Ah) :