Simple single cell low voltage indicator

Batteries, Chargers, and Battery Management Systems.
oofnik   100 W

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Simple single cell low voltage indicator

Post by oofnik » Nov 30 2008 12:48am

I want to build a circuit that will monitor the voltage of all 12 cells in my pack and tell me to stop when any one of them drops below the cutoff. Here's what I had in mind.

Each cell will be connected to an independent comparator's inverting input using the balance plugs. A 12V regulator will power each device, and a voltage divider will provide the reference cutoff voltage for all comparators (somewhere between 2.0-2.5v). Using the values here 2.5v should be around 2.6kohm on the pot. A little hysteresis will be added by putting a 100k or so resistor from the outputs to the + inputs. The outputs will be connected directly through small indicator LEDs, lighting up independently whenever the cell voltage drops below the reference. In addition, if any LED lights, a buzzer will sound to annoy me enough to disconnect the batteries.

Being a very inexperienced circuit designer, I've got some concerns. Firstly, each LM339 chip has four comparators built in. If I use 3 LM339's pulling from the same reference voltage, will I have to worry about it dropping? Should I use a more stable reference voltage like an LM317 regulator or something? Also, if all of the outputs are routed through the LEDs before being wired to the buzzer, can I count on the LEDs to prevent current flowing from one output to the next, or should I design this differently? My intent is to have 12 separate LEDs but just one buzzer. Might have to wire the outputs to a small transistor to drive the buzzer. But anyway, that's the general idea. I plan to mount the comparators on/near the pack and run some wires from the comparator board to the LED board on my handlebars. Please let me know what you think!
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fechter   100 GW

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Re: Simple single cell low voltage indicator

Post by fechter » Nov 30 2008 11:33am

The basic layout would work for a single cell but you need to have a regulated 12v from somewhere. This cannot be shared for multiple cells in series.

How many cells are you planning on monitoring?

The LED has a peak reverse voltage rating that is probably around 6v, so that could be a problem too, but you could add a regular diode in series. Cells higher on the chain will have more voltage, so would drive the buzzer with more current.

I would be much easier to use TC54's to monitor the cell voltages. These have a precision voltage reference and a comparator built in. You still have to combine the outputs to trigger the buzzer and since they are all at different voltages, you need some kind of logic to combine them

Here's the datasheet for the TC54:
TC54 voltage monitor.pdf
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oofnik   100 W

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Re: Simple single cell low voltage indicator

Post by oofnik » Nov 30 2008 5:12pm

Thanks fetcher. I'm going to be monitoring 12 cells in 12s1p.
I've redone the schematic(s) showing more clearly what I intend to do. The comparator board (schematic top left) will be on/near the battery, and a cable will attach it to the LED/buzzer board (top right) on the handlebars. I've added series diodes in such a way that they act as a 12 channel OR gate. If any one of the outputs are high, current will flow through the LED of that output as well as the transistor that turns on the buzzer. Not sure how right this schematic is as I've never wired anything like this before but I don't see any obvious problems..

I don't know what kind of diodes I'd need, and I'm also not sure about the resistor driving the 2N2222 - might be too high. I have no idea how much current those piezo buzzers draw, but the 2N2222 should be more than adequate.

The DC-DC circuit I will be using is the same one Karen is using in her EV Piaggio moped. It uses a LM2576HV IC which can handle inputs up to 60V.

I looked at the TC54, but I'd feel more comfortable being able to adjust the reference voltage directly for uber tweakability (is that a word?). Oh, and LM339 quad comparators are everywhere.
lowvolt.png
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Last edited by oofnik on Nov 30 2008 10:29pm, edited 1 time in total.

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fechter   100 GW

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Re: Simple single cell low voltage indicator

Post by fechter » Nov 30 2008 6:55pm

With 12 cells, you're going to have something like 40v on the highest cell.

You could use a divider on the higher cells to get the input to the comparators below 12v.

One issue will be standby current drain, but if you use high value resistors for the dividers, it won't be a problem (like over 1M). The LM339s could be turned off when you aren't doing anything.

I'd go with 10k into the 2N2222. Most piezo buzzers don't draw much current. Around 100ma on ones I've used. The diode logic looks OK.
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oofnik   100 W

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Re: Simple single cell low voltage indicator

Post by oofnik » Nov 30 2008 11:06pm

The voltage will be taken care of by the DC-DC circuit. I haven't decided between 5v and 12v, but the LM2576 comes in both flavors. I need 5v anyway for the servo tester (throttle), but with 12v I could add all kinds of cool gadgets (GPS? DVD head unit? :D).. I'll have to think about it. Not a big deal.
As far as standby current, note that the comparator board is wired to the +12 from the DC-DC converter, not the batteries. The LM2576 has an on/off feature and a quiescent current draw of 200 microamps max. If it's off, the comparator board won't be drawing any current. And I can't imagine the comparators drawing anywhere near that from the balance taps, powered or not.

I've attached a snapshot of the circuit in the LM2576 datasheet (pdf). Pretty neat IC. Thanks to Karen for the idea!
lm2576hv.png
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Re: Simple single cell low voltage indicator

Post by pgt400 » Dec 01 2008 7:59am

I'm doing the same thing but with the TC54 and opto (like the BMS here). Very simple and mounts right to battery. The collectors from the optos will OR into my Kelly controller.
03' Ninja EX250, Agni #95 reinforced, 25S1P 40ah CALB Lifepo4, Mini-BMS.
05' Mongoose 20" BMX, Golden Hub Motor, 48 volts @ 9ah, A123's, Signalab BMS.
http://www.evalbum.com/1955
http://www.evalbum.com/3337

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fechter   100 GW

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Re: Simple single cell low voltage indicator

Post by fechter » Dec 01 2008 9:39am

The regulator looks good. You could use a 12v one and a LM7805 running off that for the 5v. A small 78L05 should be enough to run the throttle.

You could use the 5v regulator for the voltage reference also. All the op amps could share a common reference and have separate dividers for the inputs.
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oofnik   100 W

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Re: Simple single cell low voltage indicator

Post by oofnik » Dec 01 2008 5:41pm

Alright, I'm really dumb.

I need some sort of way to give each cell voltage relative to the preceding cell, NOT system ground. I don't know why I didn't catch that when you said that the highest cell would produce 40v... Duh! :oops:

Back to the drawing board. I'm looking at LiPo cell balancer schematics for ideas. Obviously they must have some similar functions.
I don't like this circuit stuff very much.

Optoisolators maybe?

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fechter   100 GW

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Re: Simple single cell low voltage indicator

Post by fechter » Dec 02 2008 12:02am

Oh yeah, that's right. I recall going through the same exercise when trying to get a PIC to do it.

Hard to beat those TC54's and optocouplers. If it makes you feel better, each one has a precision reference and an op amp. It looks much like your circuit, but it's all crammed into a TO92.
You can use a resistor in series with the input pin on them to make it adjustable (upward). I don't trust the 1.4v ones though, that's too close to where it goes "undefined".
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oofnik   100 W

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Re: Simple single cell low voltage indicator

Post by oofnik » Dec 02 2008 12:36am

Yeah..
After reading about shunt/series references and doing some searching, the TC54 does look like it could fit the bill. I like the idea of getting the 2.1V one and having the ability to tweak it up if I'm feeling particularly cautious. But then the issue of calibration arises, so I'd probably just leave them at 2.1v. They will hit that under load, so once they start sticking under 2.1v at no load I'll call it quits.

Looks like the easiest way to do this is just to build a separate circuit for each cell. Otherwise I get in to all kinds of nasty resistor network circuitry and that's just no fun. In today's digital age, even analog circuitry has been made practically obsolete!

Think each one can drive a little LED plus the base of a 2N3904? Datasheet says up to 50mA...

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fechter   100 GW

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Re: Simple single cell low voltage indicator

Post by fechter » Dec 02 2008 9:39am

It will turn on at 2.1v, which means that's all you have to drive stuff. It is enough to light a red LED. I'd put a 100 ohm resistor in series.

Here's the basic circuit:
Single cell voltage monitor_LED.jpg
Single cell voltage monitor_LED.jpg (5.04 KiB) Viewed 3288 times
The output of the TC54 pulls down and you don't want it to exceed 10v when it's off.

An optocoupler emitter would look just like the LED in the drawing above.

If you really want to avoid optocouplers, you could use a chain of transistors like Randomly's circuit or use separate PNP transistors to pull up the base on a NPN that switches the buzzer.

If you look at the transistors on the right side of this drawing, you can get an idea for how this works. This circuit has a different voltage monitor chip with both high and low voltage outputs (you would only need one set for low voltage).
AAYA BMS.jpg
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momo   10 W

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Re: Simple single cell low voltage indicator

Post by momo » Dec 02 2008 9:14pm

Question, Is it easier to use GGoodrum's new circuit. and only utilize the LVC portion of the circuit to kill the throttle signal or light up an LED?



I'd like to see a small LCD screen displaying all cell voltages at the same time, And have the cell or cells with 2.1v blink while power is
cut off from the throttle signal. :)

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fechter   100 GW

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Re: Simple single cell low voltage indicator

Post by fechter » Dec 03 2008 8:03pm

momo wrote:Question, Is it easier to use GGoodrum's new circuit. and only utilize the LVC portion of the circuit to kill the throttle signal or light up an LED?

I'd like to see a small LCD screen displaying all cell voltages at the same time, And have the cell or cells with 2.1v blink while power is
cut off from the throttle signal. :)
Our BMS circuit could be used with just the LVC components. It would work except you won't know which cell tripped. With some additional circuitry, you could possibly latch on a LED for the low cell, but then that would drain the cell even more. Perhaps a LED with a timer on each cell could work. The LED would stay lit long enough for you to check which one it was, then go out to conserve power. Normally you don't want to be hitting the LVC on every cycle. It's better for the batteries if you don't discharge them that much.

To monitor individual cell voltages, a microcontroller based system would be needed I think. That's the next step and others are working on that. There are a few commercially available one now $$$.
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oofnik   100 W

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Re: Simple single cell low voltage indicator

Post by oofnik » Dec 04 2008 1:40am

OK, here's the current incarnation, fully isolated.

The next version was so dumb that I had to edit this post and post the next next version.
Funky blue thing is the connector between the monitor board and LED board.
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momo   10 W

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Re: Simple single cell low voltage indicator

Post by momo » Dec 05 2008 12:44am

OK..

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dontsendbubbamail   10 kW

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Re: Simple single cell low voltage indicator

Post by dontsendbubbamail » Dec 05 2008 12:11pm

Do you think something like this uses a special chip or just a small pic?
http://www.unitedhobbies.com/UNITEDHOBB ... Version_2)





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oofnik   100 W

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Re: Simple single cell low voltage indicator

Post by oofnik » Dec 05 2008 2:54pm

Most likely a microcontroller, since it's got the LCD. I've been looking for schematics for those things but they're nowhere to be found.

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fechter   100 GW

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Re: Simple single cell low voltage indicator

Post by fechter » Dec 05 2008 3:53pm

oofnik wrote:OK, here's the current incarnation, fully isolated.

The next version was so dumb that I had to edit this post and post the next next version.
Funky blue thing is the connector between the monitor board and LED board.
Yes, that would work. If your V+reg is not too high, the diodes could be LEDs :wink:
It would beep and if you look carefully you could see which one was lighting up. Most LEDs can handle 6v reverse.

That United Hobbies thing looks easy to use too. Not bad for the price.
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Re: Simple single cell low voltage indicator

Post by graevus » Dec 05 2008 7:16pm

You guys ever use multiplexors? (mux's). I really think you should look into it, would greatly simplify your circuit, i think. Without getting too deep, you could drive the mux using a repeating binary counter to switch it, and just have it cycle a single low alarm comparitor to check the cells one at a time every few milliseconds, stopping with the error condition . I'd have to think about how you could get it to tell you which cell was low....wait, when the error condition hit, the counter would be stopped on a particular binary number which would corespond to the cell location...hmmm...It would just be a matter of figuring out the logic to output to some sort of human type readout like a 7 seg display or a led array from the binary counter, either that or get used to reading binary numbers from leds, 6 bits would be enough for 64 cells . This would work without having to use a micro at all i believe, and you would only need one tc54 instead of 12. Lot less wiring. You might also think about using a relay to trigger the buzzer if you want it loud enough to hear over traffic, that way you could wire in a bullhorn if you wanted. You would need some hysteresis on the comparitor to avoid relay chatter tho.

Just a thought, hope you like it. Sounds like it would work.

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fechter   100 GW

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Re: Simple single cell low voltage indicator

Post by fechter » Dec 05 2008 7:25pm

I looked at MUX's before. The problem was the voltage range on the inputs was not enough to do many cells. Putting a divider before the inputs casuses increasing accuracy degradation as the voltage increases. You also have the standby drain of all the divider resistors. I think it would be OK for 8 cells or so max.
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Re: Simple single cell low voltage indicator

Post by graevus » Dec 05 2008 8:15pm

you would definately not want a big divider on the input, try something like this:

http://search.digikey.com/scripts/DkSea ... 4052BCN-ND

3-15V inputs should do fine for individual cells, and you could calculate the 80 ohm on resistace into the comparitor formula.You can daisy chain these together to make all the cells you need at 50 cents a chip.

Granted, there would be a learning curve involved, probably a bunch of crap im not thinking about. Ah the joys of prototyping....kinda fun if it works, nightmarish if it doesn't and you dont know why. Not that the 12 comparitor thing wouldn't work, it probably would just fine; its the soldering of the thing on a cramped protoboard that would be an issue IMO.
Soldering is an art form of sorts.

Anyway, good luck to you fechter

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Re: Simple single cell low voltage indicator

Post by tailwind » Dec 06 2008 5:08am

This product has a neat feature. Unfortunately its currently setup for LiPoly packs.

Assan 6S Lipo Battery monitor
http://www.hobbycity.com/hobbycity/stor ... oduct=7367

It displays the individual cell voltages and total pack voltage much like the Battery Monitor 2-6S
Supposedly it has better accuracy than the Battery Monitor 2-6S
http://www.hobbycity.com/hobbycity/stor ... nitor_2-6S

The neat part is that it has a sleep mode that wakes it up when a cell drops below the low voltage limit.
Power-saving: Push the power-saving mode switch to “ON” to start up the power-saving mode. In the power-saving mode, the total voltage as well as the voltage of each cell will be displayed 3 times only, and then the display will be shut off to save the power. But the monitoring of cells voltage still continues. When the voltage of any cell is lower than 3.4V, the display will be back again and flash to prompt the low voltage. This mode is very suitable for real-time monitoring the using status of cells when the monitor is fixed on the model.
There is a mini review over at rcgroups

http://www.rcgroups.com/forums/showthread.php?t=906404

and a discussion at myhobbycity on cell monitors.

http://myhobbycity.com/showthread.php?t=3933

If this device had a wake up cell voltage in the range 2.0 - 2.5V, ie A123 LiFePO4 cutoff,
I would buy a couple and put them on my bike.

Greg

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myzter   1 kW

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Re: Simple single cell low voltage indicator

Post by myzter » Dec 06 2008 12:37pm

2.0 - 2.5V, ie A123 LiFePO4 cutoff,
Im not super sure about going that low on the discharge. Sure the batts could be drained down that far.

I think staying with 2.8 - 2.6v cutoff keeps a safe amount of power still inside the A123's as to not cause any loss of current capacity.

Try these 3s-4s each:
http://www.hobbycity.com/hobbycity/stor ... oduct=6260
3 - 2.8v the red light flashes
Sure they are for LiPo, but should work to show me a low voltage indication.
I use 2, for only 8 cells for 24v, from that point of a flashing red... I can chart how much more run time or distance, I can ride before really having to let go of the throttle. It is not an exact science, however @ $3.89 per 4s - works for me!

oofnik   100 W

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Re: Simple single cell low voltage indicator

Post by oofnik » Dec 06 2008 11:40pm

Something to keep in mind is that the cells will hit 2.1 or 2.0v under load, meaning they will still be at 2.4 or 2.5 or more when idle. So if the indicators are showing that one or more cells are dropping to 2.1 under load, there's still a safe amount of charge left I think.

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dontsendbubbamail   10 kW

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Re: Simple single cell low voltage indicator

Post by dontsendbubbamail » Dec 08 2008 10:44am

I decided to try two of the Dimension Engineering CellShields for my 6S packs. The good thing about these is that the LVC is adjustable so that it will work with my Lifepo4 and my Lipo batteries. This may be a good solution for people that are using RC technology.

http://www.rcdude.com/servlet/the-9/Dim ... sh-/Detail

Bubba
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