With the fan running, you may be ok for 3-6 hours (depends on ambient air temp) after fully topped off at 1A.
That's why I did not bump the charge rate on the dual rate charger any higher.
At 5A, even with cooling fans, the cells will get over 120F if the charge is left on for more than 20 minutes after full charge.
I will recommend that a timer is used with the charger, just to be safe.
Roger, from my testing, the 350MA can stay there for hours once the cells are topped off, and that is required for the first full charge just to make sure that all cells get filled.
This is why I recommend having the a fan running.
I could see regular top-offs being useful in winter just before a drive to get the pack to warm up some, and have not read of anyone having problems from frequently topped-off batteries, but wonder if that could lead to oversize crystal formation (it typically takes only 2 - 4 hrs for my charger power to hit peak, even if 350mA is considered a mere trickle charge). Having a sufficiently low voltage setting would help minimize essentially heating of the outdoors, although the more fully the pack is charged and balanced the less fuel gets burned later.
Quote:
Originally Posted by Mike Dabrowski 2000
...
When the charger open circuit voltage is set to say 175 or so, the pack will never reach that voltage, and the AC current draw will stay the same throughout.
That matches what happens, usually a max charged pack indicated voltage of about 172V (I need to check on its open-circuit output and the accuracy of the console-mounted remote meter; I thought it had originally been 167V but that wouldn't lead to 172V pack readings.)
Quote:
Originally Posted by Mike Dabrowski 2000
IMHO A simple timer to stop the whole charge process may be the easier solution if you want to stop the charge if you forget.
Well, yes, that would be much simpler. I need to find/make one that doesn't start the countdown until power is applied, doesn't forget the timer count setting when power is removed, and is of course relay-based so it doesn't weird-out the AC waveform to the charger.
Being able to set a switch in the car to either "simple recharge" or "full balancing top-off" would be nice, having it know when to shut itself off in either case... That actually could be simply done with an additional adjustable voltage supply replacing via a 4PDT (or possibly only DPDT) switch an original one, with one set to max voltage and the other to minimum. The difference might be just enough to bridge the gap, with the power supply charge current and consumption ramping down as the pack dictates in the "simple recharge" mode. With the eBay guy unloading all those little MeanWell supplies, that's easily enough done.
At this point, many other people have had more experience with using the 350MA grid charger than I have. I am basing my opinion on what I see in my cell test.
The 9 hour topping @ 350ma I did here: http://99mpg.com/Projectcars/mimapackwhack/
Rebalancing?
Clearly shows the cells all gradually reaching the same voltage, but it took over 9 hours.
Notice that the cells (fan cooled), are only 7-8 degrees above ambient.
I would recommend one of those lamp timers that have a synchronous AC clock motor. set it at 12 noon, and give it as many hours as you want to run the charge for. They have a set of high current contacts that will easily carry the charger current.
here is an example: Amazon.com: Intermatic TN111C61 2-Pack Heavy Duty Lamp and Appliance Timers: Home Improvement
Can't get much simpler or reliable than that.
I think the difference might be my Insight's lack of MIMA and its resultant generally probably lower typical DoD, possibly by a very wide margin. Today I did let the charging take its time, for a long top-off down to 17W charger power consumption, taking about 8 hours, with a 171.2V final reading (the same as it was at about 6 hours). As usual it took only about 3 hours to reach peak charger power consumption, a very different point from where the sticks equalize at the same voltage as your plots show so clearly. The peak charger power point would be where most of the sticks are reaching full charge, plus many earlier and many not yet, and the balancing point would be long, long after when the sticks have been charged enough to allow only a trickle of a trickle charge through, at essentially the minimum charger power point.
You still have to reset it to the start time or so depending on desired recharge span each time, not that that's difficult of course, but yes that would be about the ultimate in proven, reliable technology addressing that. (My charger circuit is already on a basement dusk to dawn timer aimed more at backyard fountain/birdbath power that extends further to my carport, but also does help, time of day depending, with charger timing.) I guess I'm aiming for the ultimate in connect-and-ignore charging without power waste or possible chronic overcharging.
Chronic overcharging is what leads to nickel dendrites punching through a membrane, requiring metal-melting zapping to eliminate, correct?
I have read several opinions about dendrites. all the way from NIMH does not have that problem,to non cycling or constant trickle charging?
I can vouch for the fact that it happens on our NIMH cells.
I had one cell that dropped in voltage and went into a shorted condition when I selectively applied a charge to it.
I zapped it with a cap, and then the cell charged normally.
Another thing I read said that full cycles would erase the dendrites.
I have a bunch more subpacks to test from Ron at Hybrid battery repair, so I am sure that I will run into more shorted cells.
I still don't understand why your charger current drops at the end of charge?
If the supplies are still in the 350MA CC range, the current AC supply current would be expected to be constant as well?
Am I missing something?
Mike, I might be wrong but I think the current does drop by design for Peters charger once it reaches the top voltage. I know it was part of the thread with his design that there would be resistive losses but that efficiency wasn't a large goal because he didn't intend his to be used on a daily basis but more for pack remediation. I hope I'm remembering what I read properly. Slightly different approaches.. uhtrinity has his dropping towards the end of charge too but his has a higher amperage starting and drops current with voltage in a constant voltage type ending, I believe.
If the top voltage is set below the max that the pack will reach when completely full, yes the current will drop, but I am suggesting that the max voltage should set above what the pack will ever reach, so the 350ma is available to allow the weakest cells to get 100% full. The max voltage that the final settling reaches is a variable that will change based on the condition of the subpacks.
I have seen some cells top off at .1V more than others. You will see this when I put up the tracings of all 20 of my sticks.
.1V X 120 cells, and we have 12V on the whole pack. Most cells are much closer than that, but you get the idea.
The key thing to remember here is that the 350ma will not damage the pack,especially with the fan running, and if the pack is not allowed to fully balance, buy setting the max volts too low, you are not really balancing the cells that need it the most.
The charger power consumption would be cutting itself back by the pack voltage nearing the charger's max output voltage, reducing charger power consumption. I did make a point of not setting the voltage too high to avoid overcharging, but you do have a very good point.
Quote:
Originally Posted by Mike Dabrowski 2000
...
.1V X 120 cells, and we have 12V on the whole pack. ...
The key thing to remember here is that the 350ma will not damage the pack,especially with the fan running, and if the pack is not allowed to fully balance, buy setting the max volts too low, you are not really balancing the cells that need it the most.
So even with regular grid charging, I could still have cells in occasional danger of polarity reversal. Sounds like I need adjust/modify my charger to increase its voltage / give it bi-voltage capability. It does seem odd though, with a battery chemistry that has a flat charging voltage, for it to sit between 155 - 171V and still need me to set the charger to a higher voltage when pre-charger "full" voltage was typically 158V, "144V nominal"... And from _Batteries in a Portable World_, 2nd Ed., 2001, it doesn't expect nickel-based cells to avoid oversize crystal formation without a slow, deep discharge (i.e. a refresh cycle or 3) every several months or so at least, but then it's from when NiMH cells were fairly new... Yes, Mike, MIMA would help usefully getting the average pack voltage down. (For the slow discharge part, I have to wonder how slow the discharge has to be to minimize the number of cells risking polarity reversal when in a 120-cell string.)
I was also planning on swapping its 6V fan power brick for a 12V one and then adding a t'stat and power resistor so it gets enough cooling regardless of weather without adding needlessly to fan wear... This would be a good time for that.
Polarity reversal is prevented by the multiple taps that feed into the BCM. This detection of rapid voltage drop on the groups of 12 cells when a cell runs out of juice is what causes recalibrations.
As I mentioned earlier in this thread, I have seen a subpack that had 1 cell at ~20% SOC, while the rest of the stick was at ~60%, so the 60% ones will always hit near full while the 20% cell never gets charged to even 80% in the car.
The grid charge got the 60% cells topped off at their delta V way before the 20% cell got there.
The temperature graph confirmed what the voltage graph showed, with the 20% cell only rising in temp when I saw it go into the topping delta V zone.
The interesting thing was that after getting the whole stick to 100%, on the subsequent discharge, the cell that was at 20% was actually able to produce the 30A discharge current for quite a bit longer than the cells that were at 60%.
I interpret this to mean so it was a stronger cell, that just found it self behind the others with no mechanism to catch up.
I have seen that on several of the sticks out of the 20 that came out of my car.
How did it get so far behind is the mystery?
I am playing with a spare BCM that I have. I hardwired a connector on the output of the opto isolators, right where the signals go into the Micro-controller, and hope to be able to run those signals into my data acquisition system. This should allow me to discharge a whole pack, and do the same cell drop off detection as the car does.
Polarity reversal is prevented by the multiple taps that feed into the BCM. This detection of rapid voltage drop on the groups of 12 cells when a cell runs out of juice is what causes recalibrations.
But the BCM can only look the cells 12 at a time; I hope it can detect the voltage collapse of a single cell in each of those 12-cell strings.
Quote:
Originally Posted by Mike Dabrowski 2000
As I mentioned earlier in this thread, I have seen a subpack that had 1 cell at ~20% SOC, while the rest of the stick was at ~60%, so the 60% ones will always hit near full while the 20% cell never gets charged to even 80% in the car.
The grid charge got the 60% cells topped off at their delta V way before the 20% cell got there.
The temperature graph confirmed what the voltage graph showed, with the 20% cell only rising in temp when I saw it go into the topping delta V zone.
The interesting thing was that after getting the whole stick to 100%, on the subsequent discharge, the cell that was at 20% was actually able to produce the 30A discharge current for quite a bit longer than the cells that were at 60%.
I interpret this to mean so it was a stronger cell, that just found it self behind the others with no mechanism to catch up.
I have seen that on several of the sticks out of the 20 that came out of my car.
How did it get so far behind is the mystery?
In _Batteries in a Portable World_ there's mention of high capacity cells in series with lower capacity cells leading to the latter getting overcharged while the never-overcharged cell without a "mechanism to catch up" as you put it is eventually able, upon real charging, to out-muscle the higher-charged but lesser cells during discharge. That would be the equivalent to your finding stick voltages swapping places after a prolonged balancing charge. As far as the higher-capacity cell getting behind, the lower-capacity cells would, upon quicker but lesser charging, start blocking the current to the high-capacity cell via their passing the voltage dip point and going into climbing internal resistance while the strong cell hasn't yet reached the voltage dip point. Which would also be why having a high charger voltage setting is so important...
Quote:
Originally Posted by Mike Dabrowski 2000
I am playing with a spare BCM that I have. I hardwired a connector on the output of the opto isolators, right where the signals go into the Micro-controller, and hope to be able to run those signals into my data acquisition system. This should allow me to discharge a whole pack, and do the same cell drop off detection as the car does.
Quick Charger:
Charge C-rate: 0.3 - 0.5C
Charge time: 4h
Temp range: 50 - 113 deg.F (10 - 45deg.C)
Termination method: NdV set to 10mV/cell, uses voltage plateau, absolute temperature and time-out-time. (At 0.3C, dT/dt fails to raise the temperature sufficiently to terminate the charge. [but for 120 cells all packed together???])
Fast Charger:
Charge C-rate: 1C
Charge time: 1h+
Temp range: 50 - 113 deg.F (10 - 45deg.C)
Termination method: NdV responds to higher settings; uses dT/dt, voltage plateau, absolute temperature and time-out-timer.
If the Insight's cells are 6.4Ah rated, 1C = 6.4A, 0.5C = 3.2A, 0.3C = 1.92A, 0.1C = 640mA unless I'm clueless.
Before the table, it mentions "NiMH batteries should be rapid charged rather than slow charged. ... Because a NiMH battery does not absorb overcharge well, ... The recommended trickle charge for the NiMH battery is a low 0.05C [320mA in our case if I have my numbers right]."
That suggests our packs shouldn't even see the 350mA as a real charging current, but that defies observed reality. It also suggests that beefing up the chargers and making them temperature-sensitive would be a good idea. Beefing them up to where the cells overheat even with a full 12V to the fan would however not be a good idea. As weather changes, I suspect acceptable sustained charger power levels should also change.
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