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View Full Version : Disconnect at 11.8V HOW? Please help!

trulka
Jul23-08, 10:52 PM
Hey guys,

I am building an electric racecar for university and I'm stuck!. It has to be run on car batteries and it can only use one at a time... many people are just using an excellent battery with a great amp-hour rating... but I'd like to switch batteries halfway through...

I want to disconnect a car battery when it reaches 11.8V and possibly using an adjustable voltmeter/detector?

Please help, any advice would be great!

Thank you!
-Tom
chroot
Jul23-08, 11:02 PM
You're not going to build much of a race car with only one 12V battery at a time. A single 12V battery, even at 100 amperes of current, is only providing 1,200 watts of power. This is about the same amount of power generated by a lawnmower engine.

- Warren
zeitghost
Jul24-08, 02:24 AM
There are plenty of battery voltage detector chips about.

Maxim make a shedload.

Combined with suitably zonky Power mosfets, then bob's yer uncle.
berkeman
Jul24-08, 11:28 AM
Maxim make a shedload.

Combined with suitably zonky Power mosfets, then bob's yer uncle.

You Brits sure talk funny :rofl:
zeitghost
Jul25-08, 02:42 AM
You Brits sure talk funny :rofl:

We've had centuries to practice... :biggrin:

Zonky, by the by, means "large" as in >100A Id(on).
Bolts
Jul25-08, 05:38 AM
One way for a warning system would be to setup a diode with a resistor (or trim pot) combination that switches at 11.8V.
stewartcs
Jul25-08, 07:38 AM
What the heck does "then bob's your uncle" mean?

CS
mgb_phys
Jul25-08, 08:49 AM
What the heck does "then bob's your uncle" mean?
It means you're problems are solved

Historical note:
Robert Gascoyne-Cecil,was prime minister in the 1890 and was a big fan of nepotism - appointing his nephew as minister, so if Robert (bob) was your uncle you didn't have any problems getting a job.

Interestingly neptosim is from the Italian for nephew and comes from Pope's giving appointments to their (cough) nephews (cough)
stewartcs
Jul25-08, 02:10 PM
It means you're problems are solved

Historical note:
Robert Gascoyne-Cecil,was prime minister in the 1890 and was a big fan of nepotism - appointing his nephew as minister, so if Robert (bob) was your uncle you didn't have any problems getting a job.

Interestingly neptosim is from the Italian for nephew and comes from Pope's giving appointments to their (cough) nephews (cough)

Ah...Thanks.

CS
berkeman
Jul25-08, 04:02 PM
You Brits sure talk funny :rofl:



Couldn't resist. I googled Brittish-ism, and found the correct spelling is Britishism. Anyway, here's a Glossary of Britishisms for Americans:

http://www.sevensymphonies.com/BRITGLOSS/BritGlossaryUSL.pdf
Redbelly98
Jul25-08, 08:02 PM
I want to disconnect a car battery when it reaches 11.8V and possibly using an adjustable voltmeter/detector?


Do I understand that both batteries will be in the car at all times, and you want a circuit that electrically switches over to battery #2 when battery #1 reaches 11.8V?

If this is for a contest, how will you convince the judges that only one battery is being used at a time?