Well at least I think it's solved. The mystery I'm referring to is why did some of the batteries suddenly start accepting more charge than they should have? My first assumption was that they'd moved in their state of charge relative to the others. But really I had no reason to believe that other than I couldn't think of any other reason.
It was then suggested to me that perhaps those particular cells simply have a lower capacity than the others so they reached their full state of charge first. I think that's likely the case, but truthfully it could be a combination of both reasons for all I know. But that really doesn't answer the fundamental question as to why, when they'd been behaving in a very predictable way for 5 months, did they suddenly start showing the tendency to run away?
Well I think I figured it out today, and I'll explain, but I need to give you some background first. I'd mentioned before that the Manzanita Micro charger has a little potentiometer screw that you use to set the cut off voltage for charging. When the pack reaches that voltage, it switches from constant current to constant voltage and starts ramping down the current according to a timer you set. I'd also mentioned that it can be a finicky thing to set.
Some time at the beginning of August, I was charging the car and I happened to stay in the garage for a while. Normally I charge it at night during off peak hours and I just let it finish on it's own. Well this time I hung around, cleaning up a bit and I saw the charger start flashing, warning that it was overheating. When I set the charger up last February, it was nice and cool in the garage. Not so much in August. To make a long story short, I realized I didn't want to shorten the life of the charger by overheating it every time I charged up, so I decided to turn down the current a bit. Turned up all the way and it sends 28 amps to the batteries. I decided I'd turn it down to 15. Problem solved.
Well, little did I know that I'd just introduced different problem. Apparently the point at which the charger changes from constant current to constant voltage is also dependent on the current out. I now have it set to start it's ramp down when the charger reaches 164.7 volts. But that was set when I was pushing 15 amps. Today was a beautiful cool day, so I decided to give it the full 28 amps and low and behold, the charger switched to constant voltage when the pack hit 163.4 volts, leaving the pack about 3.2 kWh's short of it's full capacity. I stood there for a second thinking "what the hell?"
I started experimenting with different charging schemes and trips around the neighborhood to draw some power from the pack. I left the potentiometer screw alone and just tried different charging currents and I found a curve. Send the batteries less current and the cutoff voltage is higher. Send it higher current and the cutoff voltage dropped. Here's what it looked like:
15 amps = 164.7 cutoff
20 amps = 164.4 cutoff
25 amps = 163.9 cutoff
28 amps = 163.4 cutoff
So back in August I simply dropped the current I was pushing through the charger to keep it from over heating, and the result was that the cut off voltage I had so carefully tuned for 28 amps went up. Suddenly it was too high and ultimately ended up pushing more current to the pack than it should have. I don't know the exact date this happened, so I don't know exactly how many times it happened. I think it's more than 3 but less than 7 times.
But there's a lesson to be learned here. Don't change any thing, be it a setting or a process, and expect everything else to remain the same. Always check and double check when making a change to a procedure or setting. Also, for any of you that have a Manzanita charger, or are thinking of getting one, remember this potential issue. Don't get caught by it. I don't consider this to be a major flaw in the charger, it's minor at worst. But forewarned is forearmed.
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5 comments:
Tim:
You're killing me here watching you struggle with these chargers and batteries. You're wearing the spots off of em.
So drop your charge voltage to 160 and leave em alone. They will INCREASE in capacity and then they will DECREASE in capacity and not at all in lock step. But you're dinking around with 150 wH of power playing with it at the top.
The cells will last much longer if you avoid the two ends. T hey are not "sliding around". You did a BAD thing top balancing them. Bottom balancing them now is a LOT of work. So just give up the top end. Lower your voltage and take a smaller pack. If you're driving to the bottom endge anyway, you're not out anything.
You're gradually working your way around in a great big circle right back to where I started you out at.
At least you're measuring and observing. You will eventually learn this way.
Jack Rickard
Hi Jack,
I appreciate your help. I never claimed to be an expert on any of this, so the more advice I get the better. Unfortunately I don't have a learned sage over me smacking my hand when I attempt something stupid.
I had decided to do just what you've said, and essentially give up the top end. I had no intention of playing around with the cut off voltage once it was set, but I discovered this oddity in the charger's behavior when I changed the current input. Now I have that to contend with.
At this point, if I charge to a 164 cut off, by the time the charger has ramped down and shut off, the highest battery is under 3.5 volts. That seems plenty safe to me. Do you disagree?
Since the over charging episodes mentioned in the post, I've made sure they've never gone above 3.6. It took a lot of watching, and adjusting but my goal all along has been to make sure none travel north of 3.5.
I do wish I'd never tried to balance them, but that milk is spilled so I have to live with it. I realize that the best way to maximize their life now is simply stay in the limits.
We've run into this a bit. I've reread and get it now.
The Manzanita, like some chargers, is measuring the voltage at it's output. Let's say this reads 166 volts. Yo u aren't show this voltage, you measure it at your batteries as 163 volts.
Where's the difference? 25 amps through the cables. The batteries of course get this current at the end, and the cables do have some resistance. This voltage drop will be a function of the resistance and the current through them. E=IR. If the difference is 3 volts at 25 amps you have a resistance of 3/25 = 0.12 ohms.
So let's drop this currrent to 15 amps. 15 x 0.12 does not equal 3 volts. It equals 1.8v. So instead of measuring 166 volts which is set and putting 163 to the cells, you are now measuring 166 and putting 164.2 to the cells.
So your pack charge voltage rises to 164.2 by the act of turning down the current to 15 amps.
Hope this helps.
keep up the great work.
Jack Rickard
http://EVTV.me
This is the kind of problem I run into since I'm not an EE. Not only do I not fully understand the problem, but I lack the knowledge needed to put the problem to words so that others can understand it.
I had NO experience with electrical work before this project. Consequently, when something like this happens, not only am I completely caught off guard, but I'm at a loss to explain what caused it. The best I can hope for is to tinker around until I can repeat it and recognize a pattern in the behavior, and then adjust my plan accordingly.
I've understood since this post WHAT was happening, but I really didn't know WHY. Thanks for the explanation Jack! If I'm not mistaken, this sounds like the effects described by Ohm's law. I've heard of Ohm's law before, but I never knew how to apply it or how it would affect me. I've got a lot to learn. It's a good thing I enjoy it.
Tim:
I am a EE. I actually don't do that but have the training and used to. It has taken two years to realize what is happening with these chargers because it happens with SOME of the chargers. What I don't know is how the other chargers get around it.
The Brusa and the Elcon seem to allow me to set a voltage, and in the case of the Elcon, set it by e-mail, and it hits it within 0.6 volts and doesn't seem to matter how long or what kind of wires I use to connect it. It does NOT have separate sense wires.
The Manzanita I had reported as having a 4.5v rise on the end AFTER you set the voltage. it does, but for the same reason. As the CURRENT decreases, the voltage drop across the cable ALSO decreases, and so it LOOKs like the voltage is rising. Not apparent since you can't see anything on the charger end - no meter.
We're testing a new charger prototype right now - exactly the same problem. It cannot sense voltage accurately, and it varies depending on current.
So don't feel mystified. It's taken me two years to figure out what's going on here. And as you have learned, that voltage is important on LiFePo4 cells in a way that just doesn't really matter on lead acid cells.
I THINK this is what has led Rich Rudman to decide he HAS to have a BMS to shut off his charger or it "destroys" cells. He doesn't. But setting the voltage as you have learned is a little tricky. And you have to change it when you change current levels.
Jack Rickard
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