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LiPo Saving Op Amp In-Flight Blinker.

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Patrick

Sun Feb 09 2014, 07:13PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Purpose: need safety device "fuel guage" for battery.

3s1p LiPo battery. Two are used for my current Tricopter.

3 cell, with 3.7 to 4.2 V per cell, 3.3Ah per battery, so 12ish V at 6.6Ah for about 8 minutes in flight.

Schemiti-fied version of the above two physical batteries.

Background: ive already got a really good (expensive) battery killed by excessive discharge in a 9 minute flight, 300 feet up. got my drone back, but one of the cells went high resistance. Im wanting to measure the in-flight voltage and then continuously blink my bottom running lights when i get to 30% or less useful capacity remaining.

Possible solution: first, there are commercial products that do almost what im describing, but so far im not finding one that A) measures each cells voltage simultaneously and indepedently, then B) has a pin for external action to be taken.
Second, most just measure the total series voltage, but if you have 3 cells, two reading high, one reading low, you can have a false "continue to fly" signal with the low cell being permanently damaged.

So, Im thinking of using a 339 or similar opamp/comparator to measeure each cell off the balance tap connector. Which would have a variable sensitivity pot or dip. Now my cells top out at 4.2V each for 100% SOC, and 3.7V is nearly dead for a long cycle life. 3.85V/cell is considered 50% SOC...

Shrad

Sun Feb 09 2014, 07:54PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

LM3914?

Patrick

Sun Feb 09 2014, 08:31PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im thinking about the specs needed to make all this work, i need 0.01 or 0.015 v resolution....it would appear 40% of the batteries usefull power spans the lower 3.85 to 3.65 volts/cell range....

Proud Mary

Sun Feb 09 2014, 08:45PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Have you checked out Maxim's battery fuel gauge ICs?

Patrick

Sun Feb 09 2014, 09:19PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Proud Mary wrote ...

Have you checked out Maxim's battery fuel gauge ICs?

Well those might work, so long as there's no significant difference between li-ion and li-poly...

And 0.01 v = 5%...at the shallower derivative end...

Dr. Slack

Sun Feb 09 2014, 10:51PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I'm thinking something conceptually simple, though it may run to quite a few components.

Put a uP supply supervisor across each cell, Maxim have several types and will supply a few to you free as samples, or something like the Torex XC6126.

Then wire-or their outputs. Unfortunately even those with open drain outputs have an insufficnent Vmax on the outptus to take them all to the most positive rail, but their outputs can be cascoded with an NPN on each, like this ...

Patrick

Mon Feb 10 2014, 06:54AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im thinking, Dr. Slack, your very close to the circuit i originally came up with. i wanted the inputs to be a cell's V and a precison V setting. then the output would be on or off, then any one dropping to low, cuased the entire device to report from a high to a low on a signal pin.

from there i can beep, blink, puke, panic or whatever.

im looking at your cascode idea...for logical or'ing.

Patrick

Tue Feb 11 2014, 05:54PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im thinking of using a triple array of electrical isolated opamps, powered by 3 secondaries at 4.5 volts each, from a transformer with a single primary. then signal would escape these islands via opto-coupler to the common electrical circuit of the rest of the machine.

Patrick

Wed Feb 12 2014, 02:12AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im wondering what to do with the UA741's pins 1 and 5, if im not wanting to use those pins? im also hoping the input of 2M ohms isnt a problem in my app.

it would take 3 of these analog "channels" to monitor 3 cells of a battery. this schematic is incomplete.
the power enters through the the isolation transformer, then the signals under monitor are kept floating with the op amp, with the output signal coupled back to the whole machine ground through a opto-coupler.

Shrad

Wed Feb 12 2014, 08:36AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

there are other op-amps more suited for this task

why not use a jfet op-amp in a 5 pin SOT package?

you could also use a simple R/2R ladder with high R values and read the resulting voltage at the output using one said jfet op-amp, maybe?

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