How To: Low Ambient Voltage Alarm

So, you have a 36V pack of sealed lead/acid batteries powering your favorite scoot. It's been a few years since you bought it, and the batteries are now spent.

This is not really a bad thing.. you've been thinking about fitting a pack of one of the new (NiMH, Lipo, A123) battery chemistries, and now there is every reason to step-up. Have you considered that the stock controller may have a high/low-voltage setting that would shut the system down too soon?

Maybe you are ready to push your voltage to 96V of 18650s, and too-low voltage is simply NOT an option.

The Low Ambient Voltage Alarm (LAVA tm) is a very small peripheral device that velcros to your handlebar. It's user adjustable flash memory alerts you (by L.E.D. and/or chirp alarm) that the high/low voltage parameters for your new high teck pack have been met.

Sorry. I just made that up. :wink:
I have no idea how to design such a thing, but i would buy one.. if it worked.

A few folks here might know how to do it, though.

A V2 DrainBrain could benefit from that function. Hmmm..
Dear Justin, ...


-S

Seems easy enough to do the voltage gets divided by two resistors. Then you have two reference voltages also from resistor dividers or a zener diode or a ten turn pot even, these would correspond to the high and low voltages. For the actual comparison use two comparators, for the overvoltage alert you would have the reference going to the inverting input and the divided battery voltage goes to noninverting, then for undervoltage the reference goes to noninverting and the divided battery voltage goes to noninverting. The two signals goes to a logical or gate. The output of the gate then goes to a transistor used to switch whatever you want, a led or a buzzer.
Joe

Steve
do u really need an alarm?
i use a DVM, $2.99 from harbor freight.

No problemo Stevil. The Vego had a lame version of this. The light on the throttle turned red when the voltage dropped below a certain level. Any decent controller should have some kind of low voltage cutout to prevent overdischarge as well. The set point of the LV cutout can be tweaked. A simple voltage threshold alarm would be pretty easy.

The real problem with these things is the battery voltage sags under load which can give you a false low reading during acceleration. If you had a really smart circuit (microprocessor), you could possibly take the current draw into account to compensate for the sag.

Things get a bit more complicated when you're running lithiums, because you need to monitor every cell (this is actually a real good idea for any battery chemistry). Ideally, the LAVA should be a part of the BMS.

Not a very elegant solution, but many RC equipment vendors sell low voltage detectors like these (scroll down a little):
http://www.maxamps.com/index.html?lang=en-us&target=d6.html&lmd=38519.529109

Suppose they could go in series to make detectors of different voltages in 3 volt increments....

Well yer royal emperorness, I guess LAVA gets added to the list of claims in yer next patent:

"Retinal Implant for Wireless Realtime Monitoring and Display of Electric Vehicle Powertrain Status and GPS Telemetry, with Embedded Satellite Radio"