Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


How long will a battery charge last?

How long will a battery charge last?

How long will a battery charge last?

I am trying to work out how long a 12 volt car battery will last before recharging is required? My basic understanding is that a 150 amp/hour battery will supply 150 amps for one hour, or 15 amps for ten hours. But what i need to know is how much charge can be taken from a battery safely and still be able to start a car; that is before it goes flat. Example: I would like to run a fridge which draws 3 amps and I have a 300 A/H battery.
Thanks in advance for any help or links.

RE: How long will a battery charge last?

If you are referring to 300 cold cranking amps, the battery is probably only a 33 amp hour battery (approximately).  At 3 amps (.09C rate) a new battery would be dead in about 10 hours.  I probably wouldn’t want to go more then 2 hours if I wanted to start my car.  A lot depends on how much current your car needs to start and what the temperature is.  See the Power Sonic “Sealed Lead-Acid Batteries Technical Handbook” available on the Web.

RE: How long will a battery charge last?

Suggestion: It appears to be too risky to use the car battery for such a big load like fridge (3A). Perhaps, it may be better to have a dedicated battery for your fridge and use the common charger from your car to charge the fridge battery. This way the fridge will have much more time to run and your car will always start safely. It could ultimately also pay off for you handsomely.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


White Paper - Considerations for choosing a 3D printing technology
The adoption of 3D printing into major companies’ product development life cycles is a testament to the technology’s incredible benefits to consumers, designers, engineers and manufacturers. While traditional production methods have limitations in manufacturability, 3D printing provides unparalleled design freedom due to the additive method of building parts layer by layer. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close