I guess it depends on your definition of cycling ... to me it is not a full cycle unless you get from 0% SoC up to 100% SoC and then back down to 0% SoC... that would be 1 full cycle.
Pulling down to 40% SoC or 20% SoC and then back up to 100% SoC over and over again is not the same thing as going to 0% SoC... which is why NiCd develop a memory effect when you do not occasionally go down to 0% SoC.
Quote:
Originally Posted by ogregev
but is that really necessary? I don't think you'd have to deplete it all the way to accomplish the goal here.
depends on what the goal is... balancing ... no .. you can skip the whole discharge part completely and just do a top off balancing charge.
Reconditioning cycles ... require the discharge part as well... and the closer one can get each cell to 0% SoC without going into voltage reversal the better.
Quote:
Originally Posted by ogregev
Everything else is memory effect. This system will prevent the first two from throwing out the balance and will quickly fix the memory effect.
Not exactly...
NiMH don't have the same chemical reaction that is commonly called a Memory effect like NiCds do... NiMH batteries develop a condition of voltage depression ... which in application is similar ... but they are different things chemically.
Voltage depression can be treated with a few like 3 to 5 full cycles ... but that is full cycles as in down to as close to 0% SoC as you can get without going past... and then back up to 100% full SoC... the less you discharge to the less treatment of the voltage depression you are doing.
Quote:
Originally Posted by ogregev
The last problem that can come up is sticks that overheat or high-resistance ones that cause others to overheat. There is nothing that can be done about them but to replace them, so no charger will help.
Not exactly...
NiMH cells fail toward short circuit or no resistance ... moving toward more resistance is a different issue from a NiMH cell nearing failure ... and they can both be treated ... to varying degrees of success ... but most people don't bother due to the difficulties involved.
Quote:
Originally Posted by ogregev
But isn't it too late then? Can you react fast enough to keep from damaging it? Or has damage already been done?
The damage is done at cell voltage reversal... when the polarity of a single individual cell flips from 0.1V to -0.1V... because the last ~1V of drop during a slow discharge happens right at the end of SoC.. but it does happen over a period of time... as long as you cut off the discharge before the ~1V drop levels off ... and that one cell hasn't flipped it polarity ... you haven't gone into voltage reversal yet.
Now if you leave a NiMH cell in a discharged state for a long time ... it isn't healthy for it ... but that it is a different chemical situation from voltage reversal... extended periods of low SoC can be treated with a few full cycles... voltage reversal does the damage it does ... there is very little that can be done to try and reverse any of the voltage reversal damage... even if it might take many voltage reversal event to finally add up to a death of the NiMH Cell.
depends on what the goal is... balancing ... no .. you can skip the whole discharge part completely and just do a top off balancing charge.
And I think that's what the goal is here. I believe over a series of - let's call them short cycles (20%-100%), the depressed cells will come back up so it will accomplish repair as well as prevention.
Quote:
Originally Posted by IamIan
Not exactly...
NiMH don't have the same chemical reaction that is commonly called a Memory effect like NiCds do... NiMH batteries develop a condition of voltage depression ... which in application is similar ... but they are different things chemically.
Voltage depression can be treated with a few like 3 to 5 full cycles ... but that is full cycles as in down to as close to 0% SoC as you can get without going past... and then back up to 100% full SoC... the less you discharge to the less treatment of the voltage depression you are doing.
I know it's not memory effect, but I had no idea what to call it. The symptoms are significantly reduced capacity that returns to normal with a few cycles (usually less than 4 full cycles).
Quote:
Originally Posted by IamIan
Not exactly...
NiMH cells fail toward short circuit or no resistance ... moving toward more resistance is a different issue from a NiMH cell nearing failure ... and they can both be treated ... to varying degrees of success ... but most people don't bother due to the difficulties involved.
I don't think they could be treated without removing them from the circuit and the housing, so they may as well be replaced with good ones - it's far less work than zapping them.
Quote:
Originally Posted by IamIan
Now if you leave a NiMH cell in a discharged state for a long time ... it isn't healthy for it ... but that it is a different chemical situation from voltage reversal... extended periods of low SoC can be treated with a few full cycles... voltage reversal does the damage it does ... there is very little that can be done to try and reverse any of the voltage reversal damage... even if it might take many voltage reversal event to finally add up to a death of the NiMH Cell.
That's good to hear. I had a charger fail on me. I came back after 8 hours and it had reset itself from Cycle mode to Charge, but it had drained the stick. I had a reading of 0.21V for all six cells in series. I quickly charged the stick up and then ran it through 8 cycles and it performed normally. Hopefully the charger didn't kill it. I'll run some long-term tests to see if it was damaged.
Now here's a new one. I just opened a trade-in pack from a customer that bought an Insight with a spare pack. The previous owner had bought a junkyard pack and installed it. I wound up with the old dead one that had been sitting for six months or more after kicking up error codes.
I always do an initial voltage survey. In testing I got voltages all over the map from 7.7V down to 5.6V and in no discernible pattern.
I use a simple digital VOHM set to 20 volts DC. One of the sticks read 6.86 but it didn't stabilize. It climbed to 6.92, 7.02, 7.27, etc until it finally stabilized at 8.32V (which is WAY higher than anything else in the pack and usually a sign of an internal resistance problem).
Any idea what it could have been? Every subsequent reading showed 8.32V. The VOHM is powered by a 9V battery.
What was the voltage the 6 cell stick rested at after fully charged ... but allowed to rest for ~12 hours?
No, no, no. They were taken about 5 minutes after taking the end plate off the pack (breaking the circuit). The stick was my # 19 which is 2nd from the front on the top row. There had been no charging at all.
I take voltage readings before I do anything to look for abnormally high or low sticks and to look for trends such as heat damage (middle section of the top two rows weaker than the edges and bottom row). That's why I said this one was aall over the map, which I've only seen once before and in someone's pack who was trying to charge his own sticks with a charger.
It's not always indicative of the final results, but it tells me what to pay special attention to.
The problem here is that I have no idea what the changing numbers mean. I was making good contact all along, the voltage just kept rising, and now it's very high for a stick that's been sitting for 6-12 months.
I'll give you the out come of this stick later, but it's in the top row, so I won't touch it for about a week.
I have not been reading this thread and just discovered it so I had to go back and read the entire thread. So some of these comments will refer to earlier posts in the thread.
1) I have added 20 point monitoring to two of my packs so that I can read the voltage on every stick. It has been very useful.
2) I have discovered some "recovery" techniques that can be applied to a failing battery to make it useful.
a) First I have to determine at what level the battery does a recal. recently on my Lazarus battery it started recaling at 13 bars after almost 5 months of trouble free operation after a 5 cycle discharge to 5.4V per stick Charge to full charge (seen when the voltage curve reverses at the top) which is about 8.4V per stick.
b) with that information you then watch the SOC and when it drops near that value, turn on the headlights.
c) If you get an IMA light there is a way to clear it and continue...if the problem is a weak stick....just remember to turn on the headlights BEFORE starting the car and the IMA light will not activate. The reason for this is that, in my case, the one stick was dropping to 0Volts when the IMA battery was used to start the car and then before it could begin getting charge current to bring it back up, the computer detected it and lit the IMA light. BUT by turning on the headlights first, as soon as the engine began to develop power it was also charging the battery so the weak stick came immediately back up before the computer could detect that it was low.
d) If it really gets finicky, just leave the lights on anytime the ignition is on, in fact I was thinking about wiring it like my Caddy where the lights are always on.
I was able to keep going for a month without ANY IMA lights and only one RECAL (when I wasnt watching close enough to turn on the headlights in time) Even though this battery...that I am now testing...has one completely DEAD stick and two that have extremely low capacity, less than 1Ah (0.4Ah, 1.1Ah, and 0.0 Ah) and as you may recall this was given to me with less than 1 volt on every stick. That is why I call it the Lazarus pack. I could have taken it further (yesterday it was still at 19 bars with no IMA light or RECALS), but my refurbished pack was ready so I am bow going to swap them out and rebuild this one. These sticks were verified today while discharging, and suspected for 5 months because I have the ability to read all 20 sticks individually.
I am currently discharging it, and except as noted above on the three sticks, all the rest are perfectly balanced, just as they were when it returned from the dead 6 months ago.
__________________
Jim Isbell
2000, 5 speed, 250,000 miles
"If you are not living on the edge, well then,
you are just taking up too much space."
We still need to source a nice safe male/female connector, and a 1A 200VCD in line fuse and holder.
Anyone want to look into that?
I will have the power supplies in hand first week in May.
a regular household connector (plug and socket) should be perfect as it goes above 166V (Peak AC) and can handle 15 amps.
__________________
Jim Isbell
2000, 5 speed, 250,000 miles
"If you are not living on the edge, well then,
you are just taking up too much space."
Mike discovered the reverse voltage curve of the sticks when they achieve full charge.
I have found that 140 ma never shows the reverse curve of a full stick. But at 240ma that reverse curve of the peak voltage shows itself when the stick is fully charged. That reverse curve is explained by the fact that as the stick gets to full charge it begins to dissipate the excess energy as heat which reduces the internal resistance of the stick causing a voltage dip for the constant current input.
This would lead ME to believe that the current of 240 ma DOES cause heating and that 140 ma does NOT cause heating and that the point where heating is first generated is somewhere between 140ma and 240 ma So I would aim for a lower number than 350ma for down here in the south where our night time temps are often over 80F. It might be safe in a cooler climate, but why risk it?
__________________
Jim Isbell
2000, 5 speed, 250,000 miles
"If you are not living on the edge, well then,
you are just taking up too much space."
I don't know much about high voltage/high amperage connections, but would definitely like a connector with a shroud around the male and female ends to positively keep my fingers out of there when making/breaking any connection with that high of voltage/current potential. And a connector that ensures the proper polarity, with no room for error.
I have been zapped by 120 VAC enough times to know that I really don't want to accidentally find out what it's like with 177 VDC and 100 amps behind it.
The fan will be running so any heat buildup at 350MA will easily be dissipated and will not be an issue. A timer will likely be part of the system.
I did not discover the voltage drop at full charge, this is specified in all NIMH data sheets, and is used by the smart chargers to terminate charge. http://www.panasonic.com/industrial/...MH_HHR650D.pdf
A standard AC plug would work, but the charger end with the prongs sticking out could be dangerous? Worth considering for sure.
I have been zapped by 120 VAC enough times to know that I really don't want to accidentally find out what it's like with 177 VDC and 100 amps behind it.
Jim.
There is no real difference between 120AC and 177DC as far as pain, because 120 AC is really 168V positive to 168V negative. Actually the AC might hurt more. The current potential is also not of any consequence since the human body has a fixed resistance to what ever voltage you apply and the current will not go above a fixed level...until the flesh begins to char and create a carbon path of low resistance.....but by then, you wont care. Usually you wont get the charring unless you are talking about 25,000 volts.....which I HAVE been unfortunate enough to have been hit by. The fact that the IMA is capable of 100 amps doesnt mean anything because it wont go to that level through the resistance of flesh. Probably wouldnt go above an amp or two...but it would make you jump....ask Mike, I think he once got across the output of his solar panel at 177VDC.....
The standard AC receptical would work and in fact I used it on mine to begin with but you have to make sure the power supply is NOT turned on until AFTER the plug is inserted into the socket on the battery. Those two prongs dont want to be HOT. This type of connector is polarity indexed as you cannot stick it in backwards.
BUT what I later changed mine to was a standard automotive trailer connector with a recessed male connector on the battery with a snap closed cover and a female connector on the power supply. The connector and socket are polarity indexed and cannot be put in backwards. While this is designed for 12VDC, the physical clearances are more than sufficient for 10,000 volts and the heavy contacts would handle 100 amps easily as long as the wire you used was capable of handling the current and the insulation resistance was sufficient for your voltage.
BUT....these voltages are NOT considered high voltage. These are relatively low voltages and most wire you buy today, even the cheapest, has insulation resistance capable of handling a minimum of 600VDC
Oh yes, the 100 amp potential DOES become important if you drop a wrench (very low resistance) across the contacts instead of a human body (very HIGH resistance).....dont ask how I know.....#
__________________
Jim Isbell
2000, 5 speed, 250,000 miles
"If you are not living on the edge, well then,
you are just taking up too much space."
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