Still trying to figure out what my issues are with these Batt-Bridges
I have done many, many tests
but, I keep getting inconsistent readings
sometimes it signaled an imbalance @ ~1V imbalance
other times it took like a ~4V imbalance to signal an imbalance.
*If I am going to rely on this to monitor an 2kwH lithium battery pack between my legs, I want consistency
So, I have double & even triple checked
...that the LED's are positioned & wired correctly
...I ordered & tried ultra bright red LED's
Still getting inconsistency's
So, "back to the drawing board"
...when I went back to the Batt-Bridge web site, I noticed that the description & diagram had been changed a little bit.
http://www.sunrise-ev.com/LeesEVs.htm#battbridge
The description used to read:
"The Batt-Bridge is about as simple as you can get; that's why it is so inexpensive. If all you want is an 'idiot' light to say, "Stop driving, your batteries are dead," I can't imagine anything any simpler. You really don't need dozens of ICs and hundreds of components just to light a light.
The Batt-Bridge divides the pack in half, and compares the voltage of each half. It lights an LED when one of them is 1v less than the other.
If a cell dies somewhere in the pack, it typically causes a 2 volt change. So the Batt-Bridge warns you that a cell went dead. There are two LEDs, so they indicate which half-pack contains the bad cell.
R1 and R2 are chosen to draw about 10-20ma from the pack. For example, if you have a 120v pack, R1 and R2 each have about 60v across them. At 15ma, they would be R = 60v / 0.015a = 4k ohms. They need to be identical values (1% or hand picked or trimmed). And they must be power resistors; 60v x 0.015a = 0.9 watts, so use at least a 2 watt resistor.
Use an ordinary low brightness green LED. Its purpose is just to indicate that power is on, and to act as a low-voltage 2.4v "zener" diode. However, the red LEDs should be high brightness types -- the brighter the better, so you can see them even in daylight.
Here's how it works. All voltages are relative to the pack center tap. If +pack == -(-pack), then the green LED lights. The green LED's anode is at +1.2v, and its cathode is at -1.4v. The red LEDs don't light because they only have 1.2 volts across them (they need over 1.5v to light).
Now, suppose you have a dead cell in the upper half of the pack. Then +pack is 2v less than -pack. R1 and R2 form a voltage divider, so both ends of the green LED are 1v more negative; its anode is at +0.2v, and its cathode is at -2.4v. This means there is now 2.4v across the lower red LED; so it lights! Likewise, if the dead cell is in the lower half, then the upper red LED lights.
The total resistance of R1 and R2 sets the sensitivity, and the ratio of these resistors sets the desired center-tap voltage of the pack. If both LEDs light, then the resistors are too low a value; increase the resistance of both of them proportionately. Ten milliamps through the resistors is low sensitivity (over 2v difference to light an LED); 20ma is normal sensitivity; 40ma gives you high sensitivity (less than 1v difference to light an LED).
If one LED lights when the half-pack voltages are correct, then adjust the value of one of the resistors. This is also how you deal with packs with an odd number of batteries, where the "center tap" is off by one."
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Here is the updated version:
"An EV's pack consists of many cells or batteries. In theory, they are all identical. In practice, they aren't. There will always be a "weak link" somewhere in the pack. That's the cell that limits your range, and limits how much you can charge before damage begins.
But it is difficult to know if you have a weak cell. Total pack voltage won't tell you until too late. The amount of circuitry needed to monitor every single cell can get very complicated and expensive!
The Batt-Bridge is a quick-n-dirty "idiot light" to give you a good/bad warning when any cell in the pack goes undervoltage (dead) or overvoltage (overcharged). It works with all types of batteries; lead-acid, lithium, nicad, or nimh. If it lights up red, back off on the current until it goes out. If it stays on even at zero current, stop driving or charging until you find the problem and fix it!
How it works: The Batt-Bridge divides the pack in half, and compares the voltage of each half. When the two halves are equal (within 1 volt or less), a green LED is lit. If a cell goes dead or begins overcharging somewhere in the pack, its voltage typically changes by more than a volt. This imbalance lights one of two bright red LEDs to tell you which half of the pack is low.
D1 is a standard brightness green LED. D2 and D3 should be ultrabright red LEDs for best visibility. R1 and R2 should be identical resistors, chosen to provide about 10ma at your pack voltage. The current sets the sensitivity and brightness of the LEDs."
It looks like the math used to determine the proper resistors needed, depending on the pack voltage, has been changed a bit too.
Here is some testing I have done, gathering data
https://www.youtube.com/watch?v=OnWLNoCSE5I
that stupid "idiot light"
