12V circuit breaker

[tungym]

Dear all,

I used to charge my lipo /A123 with my car battery. However, there are several occasion that I over discharge my car battery and almost cannot return home.

I used to count no# of lipo I charged but sometimes it does not work, especially when I have a great flying day.

Is there any simple available circuit/ buzzer that may warn my car battery has dropped <11.5V. I do not want to carry another high capacity Pb Acid battery as my car is already packed with planes and helis...

Sometime ago I read some post about caravan 's battery switching device? I forgot the keyword. I have no luck to get a solution from google or from rcgroup search.

[tungym]

Will lipo alarm like this work?

http://www.rcgroups.com/forums/showthread.php?t=221018

[orraman]

From the sound of it this device has a relay that disconnects the load and has worked fine for three years.

New SURE START for in-car electronics

http://www.pe.com/wow/offroad/stories/PE_News_Local_D_wow_1226_surestart.4c21d6.html

The MicroScream circuit might well work a relay in place of the Piezo alarm (no R4) with new potential divider resistors to suit your cut-off voltage.

Dave

[phil_g]

A quick google brought this up:
http://www.powerstream.com/battery-protector.htm
Almost any local camping & caravanning shop should have something similar.
Phil

[tungym]

Thanks, you guys are helpful! BTW, my 3s lipo saved my life couple of times (3s 2200 lipo start up my car, or else my wife would kill me!) :p

I have no luck to get a 2N7000 gated FET rated 200mA. Can I use 2N3705 (signal amp, rated 100mA) to replace it?

[Dan Baldwin]

Thanks, you guys are helpful! BTW, my 3s lipo saved my life couple of times (3s 2200 lipo start up my car, or else my wife would kill me!) :p

I have no luck to get a 2N7000 gated FET rated 200mA. Can I use 2N3705 (signal amp, rated 100mA) to replace it?

The circuit is designed to use a FET, and a bipolar transistor such as the 2N3705 cannot be directly substituted. It might be possible to redesign the circuit a bit to make it work.

Dan

[orraman]

This chap sells TEN 2N7000 for £1 and will post world-wide for £1.

Post # 8 on the MicroScream thread confirms that a variable resistor could be used to set the cut-off voltage.

http://cgi.ebay.co.uk/10-x-2N7000-N-channel-Mosfets_W0QQitemZ260307281344QQcmdZViewItemQQptZUK _BOI_Electrical_Components_Supplies_ET?hash=item26 0307281344&_trksid=p3286.c0.m14&_trkparms=72%3A1301%7C66%3A2%7C65%3A12%7C39%3A1%7C 240%3A1318

[jeffs555]

Here is a cheap commercial one.
http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=12542

Here is a circuit for a homebrew one.
http://www3.telus.net/chemelec/Projects/Low-Bat/Low-Bat.htm
I have seen others built around a 555 but can't find the page now.

PS If you are in the UK, Maplin has one for £4.99. http://www.maplin.co.uk/module.aspx?C=SO&U=strat310&T=Galway&ModuleNo=37962&ma=Galway+-+Car+Battery+Protector