Need Some Circuit Help

[Roy Von Rogers]

Hi all.

This is not related to ebikes but is related to battery power control.

Several years back I had optained two A123 developers pack. I originally bought those very expensive batteries looking for a better alternative to nicads, which brought me to this awsome site with some awsome technical geniuses. Also after lurking here for some time I was smitten with the idea of ebikes, and are thinking about getting one. But since I had a bad back and just recently had back surgery which only partially helped and maybe needing another one, I held off getting one. But seriously looking at the Utha Quad and powering it. But the help I need is in another area.

I have many battery operated devices that either use 3 volts or 6 volts. I have a 3v led headlamp that I use a lot for my appliance repair business, also have a weather station 3v, which I'm getting tired of buying batteries for. I want to take a A123 battery and hook it to the led light and weather station, I have a FMA charger which does an exellent job of charging, but the problem I have is how to prevent the battery from going below 2 or better 2.5 volts. Keeping an eye towards dimming is problamatic, since those batteries have a sharp discharge curve after 2v.

Can anyone think of a simple circut that would cut off the discharge in case I forget to check, that is practical ?? Something like a low cut off that turns an fet off ?? Some advice of a device and how to circuit, would be apreciated. I have done a lot of electronic work in the past, mostly rf type, and if given a circut, would have no problems building it.

Roy

 

[amberwolf]

If you don't need high current, one of the nicest ways to do it is to take the little BMS board off a celphone battery pack (as long as it has the cutoff voltage you want, which you can check by doing a discharge test on it), and put it on the A123 cell.

If you have a lot of room, you can build a simple relay and zener diode circuit, so that as long as the zener (picked for whatever LVC you want) has current flowing thru it, the relay coil remains energized, and the relay stays closed (on) to keep power going to your device. Once the voltage drops below the zener's spec, then no current will flow thru the coil and the relay will turn off, opening up the circuit and disconnecting your device, preventing further discharge.

You can do the same thing with a MOSFET or other transistor for a much smaller circuit, but it will be more complicated.

 

[Toorbough ULL-Zeveigh]

maybe something here for u that could be adapted.
http://www.rcgroups.com/forums/showthread.php?t=570473
http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/95028-very-basic-low-voltage-detector.html
http://www.discovercircuits.com/DJ-Circuits/undervol.htm
http://home.cogeco.ca/~rpaisley4/xLowBattery.html

 

[fechter]

That's a fairly difficult thing to do with a single cell. It gets much easier with 2 cells. Most FETs take at least 5v to turn on fully. With only 2.5 volts available, a FET won't work well. It may work better to use a low saturation bipolar transistor, but this will give a bit of voltage drop during normal operation.

For two or more cells, I think you could find an off-the-shelf chip that does that or just use a TC54 and a logic level FET.

If your load is extremely low current, you could possibly drive the load directly from the output of a TC54 with complimentary output. I'll have to check the TC54 datasheet to see how much they can take.

Here's one possibility for a single cell. Q1 is a high gain darlington. When the voltage drops enough, the transistor will no longer be biased and turns off. Adding another diode in series with D1 will increase the cutoff voltage. As shown, it should turn off somewhere around 2v. Once off, there will be essentially zero drain on the cell. A low saturation version of a darlington would minimize the voltage drop when on.

 

[reagle]

Seiko has lots of chips designed to do exactly that, and they come in various voltage settings laser trimmed. Something like S-8261, which coincidentally is often used in cell phone batteries. You'd use it's UVL feature and use the signal to control relay/FET
http://www.sii-ic.com/en/product1.jsp?subcatID=5&productID=379

 

[Toorbough ULL-Zeveigh]

$1.79
* Keep 3.7V Li-Ion Battery pack from overcharge beyond 4.35V
* Keep 3.7V Li-Ion Battery pack from over-discharge below 2.4V