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12-volt Battery Capacity

ATPMSD

Active Member
Mar 12, 2021
1,999
1,922
Atlanta, GA
I posted this question elsewhere but apparently not in an active section, so please excuse the repost:

One of the things I love about my Tesla is learning new things, and so a question for you 12-battery gurus!

  1. My car has a 12V 30A (20HR) battery. I think this means the battery capacity is 600 amps (30 * 20), which can also be stated as 7,200 watts (600 * 12). Is this correct?
  2. Assuming the above is correct, if my car draws 1 kWh / day when sleeping, and the BMS tops off the battery when it reaches 50% capacity, this means the car will charge the 12-volt battery roughly once every 3.5 days?
  3. And if the above still holds, if I arrive at a location with a fully charged 12-volt battery and stay for just two days, the HV battery will remain at the same SOC as when I arrived (except perhaps a minor drain due to natural discharge)?
Thanks!
 

ucmndd

Well-Known Member
Mar 10, 2016
10,106
19,840
California
I believe a Model S 12v battery is somewhere around 33AH (amp hours) of capacity. Converting that to watt hours (33AH x 12v) gives a rough capacity of about 400wh - 0.4kwh.

So the 12v charging cycle is much more than once every 2-3 days. More like 2-3x/day depending on how long the car sleeps.
 

ATPMSD

Active Member
Mar 12, 2021
1,999
1,922
Atlanta, GA
I believe a Model S 12v battery is somewhere around 33AH (amp hours) of capacity. Converting that to watt hours (33AH x 12v) gives a rough capacity of about 400wh - 0.4kwh.

So the 12v charging cycle is much more than once every 2-3 days. More like 2-3x/day depending on how long the car sleeps.
That is not what my battery says, it reads as I stated. This is from the battery cover:

  • AGM U1LS
  • 12V
  • 30A (20HR)
 

ucmndd

Well-Known Member
Mar 10, 2016
10,106
19,840
California
That is not what my battery says, it reads as I stated. This is from the battery cover:

  • AGM U1LS
  • 12V
  • 30A (20HR)
Read here to understand discharge rate for lead acid batteries:


That rating - 30A (20H) - means that the battery will provide 30 amp hours of energy when completely discharged over a period of 20 hours. With lead acid batteries faster rates of discharge result in less energy being available. Most deep cycle AGM batteries are measured on a 20 hour rate as a standard.

So again, 30 amp hours x 12 volts = 360 watt hours = 0.36 kWh assuming the battery is discharged from 100 to zero over 20 hours.

I assure you, you don’t have a multi-kWh lead acid battery. If we could store 5-7 kWh of energy in a 27 pound lead acid battery like you’re suggesting, All EVs would use lead acid batteries instead of lithium ion.
 
Last edited:

ATPMSD

Active Member
Mar 12, 2021
1,999
1,922
Atlanta, GA
Read here to understand discharge rate for lead acid batteries:


That rating - 30A (20H) - means that the battery will provide 30 amp hours of energy when completely discharged over a period of 20 hours. With lead acid batteries faster rates of discharge result in less energy being available. Most deep cycle AGM batteries are measured on a 20 hour rate as a standard.

So again, 30 amp hours x 12 volts = 360 watt hours = 0.36 kWh assuming the battery is discharged from 100 to zero over 20 hours.

I assure you, you don’t have a multi-kWh lead acid battery. If we could store 5-7 kWh of energy in a 27 pound lead acid battery like you’re suggesting, All EVs would use lead acid batteries instead of lithium ion.

Thank you. This was most helpful.
 

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