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Shunt Equalizer Test.
waitatian:
It would make the two it is connected to the same but the combination might not match the others. A simple test is to mimic the equalizer by putting a variable resistor across the high battery and keep its voltage equal to the low one while checking the volts on the others. That should show if only one will work.Of course after a while the equalizing current should go down as the low battery gets a full charge.(Doing that was what I based my design on)
Watching my 48V equalizer working, it appears that different rates of charge will make a different order of batteries that are bypassed, but when the regulator fixes the voltage the same ones will eventually be bypassed.
SparWeb:
The others are rather "average". There just happens to be a coincidence of a star right beside the slug, in both cases. All the rest are middle ground, which is very surprising given the age of the entire pack of them.
I could have cell #6 steal a bit from #5 and that would even out all the cells in string 1 to within a few %.
In string 2, having cell #8 steal from cell #7 would also help, even if it might not be enough.
Here's the last reading:
Cell # Cell V
1 2.287
2 2.283
3 2.279
4 2.278
5 2.285 (above average)
6 2.276 (2nd lowest)
----------------------
7 2.291 (3rd strongest)
8 2.254 (lowest)
9 2.282
10 2.277
11 2.281
12 2.288
----------------------
13 2.285
14 2.278
15 2.275
16 2.273
17 2.292
18 2.300
----------------------
19 2.291
20 2.279
21 2.289
22 2.281
23 2.278
24 2.280
----------------------
Average 2.282
waitatian:
It would appear no.7 would be limiting the total charge and no.8 is suffering, or, no.8 is pulling more charge, making no.7 get overcharged.
I haven't tested it, but a zener diode and resistor in series across any high cell might work to bleed off excess charge without using any power when the batteries are being used. It should be possible to work out where on the charge curve to select the zener voltage and the resistor value could be periodically changed if the low cell charges up. If that wasn't enough, adding a single zener/resistor from cell 9 to cell 12 would add more charge to no.8
SparWeb:
Hah. Well this gets simpler day by day.
You mean a zener with a cut-off about 2.20 volts?
We are working with currents much less than an Amp, too, aren't we?
I have some 10W/10ohm resistors that might be just right.
Float V is between 27.0 and 27.5 therefore 2.25 to 2.29
Resting V is between 25.0 and 25.8 therefore 2.08 to 2.15
The right zener voltage seems to be 2.20 V.
The whole bank can be kept at float with <2Amps. With 2 strings then each string needs about 1 Amp to maintain float. Obviously bulk charge is higher but variable, up to the capability of my combined sources, which is > 20A.
If I bypass one cell with a resistor, I'd start at about 25% current and see what happens first. For a cell at 2.25V to bypass 250mA then:
2.25V / 250 mA = 9.0 Ohm
Which means I can use one of my 10-ohm resistors in series for my starting point with a zener to shut off the bypass overnight.
Another point of interest: only as I was writing yesterday, did I start to wonder if there's some inter-play between cells 7 and 8. You suggest a possibility, too. Perhaps there is some detriment to cell 8 having it in series with a strong cell like #7. The cells are mounted in a rigid steel casing, and have swelled a bit, so I can't pull them out any more.
waitatian:
You might need more bypass power initially to bring up 8 while stopping 7 overcharging.
The most bypassing current will be at full charge.
If this is say 2.55v, minus the zener volts of 2.2 = 0.35V across the resistor.
W=V*A, so a 1W zener at 2.2v will pass 1/2.2 = 450mA
R=V/I so to drop .35V at .45A = about .78 ohms
Power = I*I*R so it would need .45*.45*.78 = .16W resistor.
At float, the voltage across the resistor would be about .07V.
I=V/R so there would be 0.07/.78 about 90mA bypassed at float.
These calculations imply a sharp zener cutoff voltage.I don't know what zeners are available but they can be paralleled for more wattage and the .6 volt drop of a power diode in series can adjust the voltage.
The conditions at different places in the string seem to vary. It probable has to do with inductance or capacitance effects. I used to rotate my batteries before the equalizer was installed. If the intercell connectors are removable is it possible to rewire 7 or 8 to a different position?
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