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Old 3rd July 2013, 10:32 AM   #1
teleman is offline teleman  Norway
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I am trying to add a battery level monitor to 4 x Li-ion batteries in series to monitor the levels. I found this seemingly simple circuit on an another site & the LED indicators are set at the following voltages.

Above 11.6v..............Green "on"
Between 10.5v -11.6..... Yellow "on"
@ 10.5v..............Red "on"

However, the above settings are a bit low for Li-ions & I need to change these to the following;

Above 11.9v ..... green LED "on" ( 2.975v/battery )
Between 11.9-11.5 ..... Yellow "on" ( 2.975-2.875v/batt )
Below "on" ( 2.875v/batt)

Can someone please show me how to calculate the component values to obtain the above levels?

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Old 3rd July 2013, 10:52 AM   #2
DUG is offline DUG  Canada
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The thresholds are based on the zener diode ratings.

To change them you have to get different zeners.

Or a more complicated circuit that is adjustable.

Hope that helps
Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."
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Old 3rd July 2013, 11:01 AM   #3
teleman is offline teleman  Norway
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Thanks DUG!
I guessed it too! Would you care to explain briefly how these 2 zener values control/change the voltages? What formula or math is needed to do this?

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Old 3rd July 2013, 11:53 AM   #4
Mooly is offline Mooly  United Kingdom
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Such a simple circuit has very imprecise levels and I would doubt you would get it exact enough for a Li-On battery for the results to be useful.

Its easy to work out how it all works. Remember a zener won't conduct until the voltage across it exceeds its marked value. So the 8.2 volt zener does nothing until the supply exceeds 8.2 volts plus the forward voltage of the LED (say 2 volts). So at around 10.2 volts the zener starts to conduct and the LED dimly comes to life getting brighter as the supply increases. So the problems there are a poorly defined operating point and an LED that varies in brightness. The other LED is "lit all the time" via the 1K but relies on a transistor to short out the LED when the other zener begins to conduct at around 11 volts plus the Vbe of the transistor. Again a poorly defined set up and also wasteful of battery power.

I once designed a circuit using a dual bi-colour LED to monitor a car battery. It used a comparator set up as an oscillator and the LED changed gradually from green through yellow and orange to red as the voltage altered.

For Li-On voltage indications I would look at using comparators and a stable reference voltage to set the "trip" points exactly... sadly that becomes a much more complex project. I would also design for "micro power" use too so that current consumption is at a minimum.
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Old 3rd July 2013, 11:54 AM   #5
spwalek is offline spwalek  United States
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I use a variation of this circuit for projects and it is adjustable.

I'll see if I have the link for the one I use at home. It uses fewer parts and has a zener instead of a regulator for the voltage reference.
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Old 3rd July 2013, 12:36 PM   #6
teleman is offline teleman  Norway
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Thanks Mooly for your timely warning! Li-ion battery charge/discharge voltages are indeed very critical & such need very precise monitoring. So I will try the one Spwalek has linked.

Spwalek, thanks for the link. I do have this schematic & actually wanted to try this,but was a little hesitant due it's rather high current draw. So I wonder if I could substitute either LM2940 or LM1117 LDOs than LM7805 regulator as used in the original circuit?

I have a TA7805S which is supposed to be somewhat better than other xx7805s. Would you guys recommend 7805 reg? Noise is on of my prime concerns other than high current draw!

Could you link your mod to this circuit please?


Last edited by teleman; 3rd July 2013 at 12:41 PM.
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Old 3rd July 2013, 04:55 PM   #7
teleman is offline teleman  Norway
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Just found out that the TA7805S has a max input voltage-Vin-of only 10V! This means I can't use this device!
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Old 3rd July 2013, 05:21 PM   #8
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If you don't need constant monitoring, you could easily add a momentary pushbutton to your battery gauge circuit power. This would considerably reduce the needed battery draw.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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Old 3rd July 2013, 06:10 PM   #9
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I do not know If you have access to a micro but attached it is an schematic showing how easy it is done using a micro.

The micro program will read the battery voltage input and depending of the different setting in the program it will turn on the led light. Simple and the programming can be done using a Picaxe or a Basic Micro NANO8 micro.
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Old 3rd July 2013, 06:20 PM   #10
spwalek is offline spwalek  United States
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As long as the chips are pin compatible I don't think it will matter. I've subbed tl082 and tl072s with no problem. I usually set this up as a power on indicator LED with the voltage monitor as a bonus. I did not see a noticeable power use increase, but I'm using an SLA. You could also set it up with a momentary contact switch on the power lead for an "as needed" check.
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