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'[EE] Signalling low battery voltage'
2005\08\11@065513 by Vic Fraenckel

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I am in need of a simple (read low cost) way to signal that a lead/acid 12 volt gelcell is in need of recharging. I have in mind a device or whatever that will turn on an LED when the recharge point is reached. Is there such a critter?

Any enlightenment will be appreciated.

Vic Fraenckel
________________________________________________________

Victor Fraenckel - The Windman         victorf ATSIGN windreader DOTcom
KC2GUI

2005\08\11@081518 by Russell McMahon

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I am in need of a simple (read low cost) way to signal that a
lead/acid 12 volt gelcell is in need of recharging. I have in mind a
device or whatever that will turn on an LED when the recharge point is
reached. Is there such a critter?

Any enlightenment will be appreciated.


Circuit here

       http://homepages.which.net/~paul.hills/Circuits/BatteryMonitor/BatteryMonitor.html

Something even simpler that works moderately well could be designed.
Lead Acid voltage is fairly flat across most of capacity and drops
more sharply towards end of capacity. How close to end you get for a
given voltage depends on discharge rate and temperature (and other
factors).


       RM


2005\08\11@161725 by csb

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Easy :
1 op-amp (or comparator), a few resistors, a NPN if the IC
is weak, and a diode (whatever kind) + LED. The diode serves as a 0.6V
(or whatever, it could be a zener) reference. You have a divider
network formed by 2 resistors arranged so that the voltage is 0.6V
when discharge point is reached. Connect a LED+resistor on the output.
I made one mounted on a LM741 body:
rsc-mirror3.netfirms.com/1/elec/batmon.htm
The LED is hidden behind the potentiometer.

Good luck
Christian

2005\08\11@171836 by Michael Rigby-Jones

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>-----Original Message-----
>From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu]
>Sent: 11 August 2005 13:09
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Signalling low battery voltage
>
>
>I am in need of a simple (read low cost) way to signal that a
>lead/acid 12 volt gelcell is in need of recharging. I have in mind a
>device or whatever that will turn on an LED when the recharge point is
>reached. Is there such a critter?
>
>Any enlightenment will be appreciated.
>
>
>Circuit here
>
>        
>homepages.which.net/~paul.hills/Circuits/BatteryMonitor/
BatteryMonitor.html

Something even simpler that works moderately well could be designed. Lead Acid voltage is fairly flat across most of capacity and drops
more sharply towards end of capacity. How close to end you get for a
given voltage depends on discharge rate and temperature (and other
factors).

Does this need to be connected permanently to the battery, or will it be a "press to test" type arrangement?

If it has to be connected all the time, consider (as I have!) a nanowatt PIC arrange to wake up from sleep every few seconds or so.  To minimise current consumption, power the voltage reference from a PIC pin so you can easily turn it off when you go back to sleep.  Temperature compensation or terminal voltage is then just a cheap thermistor away.

Regards

Mike

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2005\08\11@171842 by Michael Rigby-Jones

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>-----Original Message-----
>From: piclist-bouncesspamKILLspammit.edu [.....piclist-bouncesKILLspamspam.....mit.edu]
>Sent: 11 August 2005 21:17
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Signalling low battery voltage
>
>
>Easy :
>1 op-amp (or comparator), a few resistors, a NPN if the IC
>is weak, and a diode (whatever kind) + LED. The diode serves
>as a 0.6V (or whatever, it could be a zener) reference. You
>have a divider network formed by 2 resistors arranged so that
>the voltage is 0.6V when discharge point is reached. Connect a
>LED+resistor on the output. I made one mounted on a LM741
>body: rsc-mirror3.netfirms.com/1/elec/batmon.htm
>The LED is hidden behind the potentiometer.

Very cheap, but the tempco of a silicon junction could mean your discharge indication voltage varying significantly with temperature.  A proper voltage reference would be much better.

Regards

Mike


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2005\08\11@185628 by csb

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> Very cheap, but the tempco of a silicon junction could mean your discharge
> indication voltage varying significantly with temperature.  A proper
> voltage reference would be much better.
>
> Regards
>
> Mike

Right... And, you also need to make sure the diode gets enough current to
give 0.6V. I calibrate the potentiometer now and then (it's constantly
plugged into a NiMH battery pack), so that the LED lights up when I'm approaching
1V/cell . This solution is of course far from ideal, but it requires very few
inexpensive parts, all easily found in dead circuit boards.

How small are voltage refs made? I suppose they could be found in SOT-23
package... then it would make the whole thing more reliable, as well as reducing
current consuption. Sounds good...

Christian


2005\08\11@214123 by Jinx

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> How small are voltage refs made? I suppose they could
> be found in SOT-23 package

They are. I use an MCP1525 (2.5V) SOT-23 and 12F675
together in a configuration similar to what MR-J suggested.
MCP1525 is powered by a 12F pin and the whole thing has
an extremely low, a few nano-amps, standby current. There
is also the MCP1541 (4.096V, for the application above
that's too high as the battery is a 3V lithium) which I've used
in other ADC circuits, including a 6V SLA monitor

In one low-voltage circuit I have the MCP1525 divided
down with (1k8 + 8.2R) and 8k2 to produce 2.048V for
easier ADC maths. A cheaper option for me is to buy the
2.5V and divide rather than the more expensive 2.048V

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