Equalisation with a Difference
I think mixing Prius and Vectrix results is confusing to many.
I'm no expert on Vectrix, but I am one of the primary contributors on keeping older Prius running (on Prius Mk1 user group). I also keep my 14yo NiMH battery going in an 1998 Prius, for fun... though at this age, I am about to do a LiPo conversion (to my own plans) on the Prius.
Laird said "Certainly in the case of the Vectrix charge process 'overcharge' is the norm and it occurs every 12 or so hours of use and the damage can be seen in the failed batteries and/or the bulging cells which are contained within most (probably all) Vectrix batteries."
But regarding the Panasonic HHR650D NiMH used in the Mk1 Prius (NHW10), the Panasonic patent on the matter makes it clear that the long cycle life is achieved through 'shallow cycling' of the cells, and indeed the Prius controller keeps State of Charge (SOC) between 20% and 80%. Also, the HHR650D cells will deliver 300A on shorting, yet the Prius in practice draws only 55A upon electric use, and never charges above 45A (regenerative braking). So the current is kept under 8C, so within specs. I've read on this site that Vectrix draw 250A at top speed, which seems way over-spec, so no wonder cells fail so quickly in comparison.
The Panasonic patent regarding the HHR650D and 'vehicle use' of NiMH makes it clear that some rejuvenation of the NiMH cells is achieved by holding at 45-degC for 3-4 days.. but in practice, in car equalisation is achieved by periodically uniformly charging above 80% SOC, but still to 'knee' and then keeping low current running for a while. This is so that already-charged cells can easily lose that extra energy as heat, while the not-quite-full cells get that little bit more (slowly) to all end up equally charged (provided they are not already defective).
That same patent notes that the best periodic rejuvenation is achieved if you do three-deep-cycles of charge-discharge, using a computerised charger, set to NiMH mode. I've done this and the computerised charger (eg Bantamtek) does report larger storage (mAh) on the later cycles.
But as 'shallow cycles' is the secret to long life in NiMH, you don't want to do deep-cycling too frequently... and arguably deep discharge is one of the quick ways to kill NiMh cells. I still feel that slow/slight over-charging is better to discharge to achieve equalisation - and this is what the PEVE JV between Panasonic and Toyota decided to implement in Prius and have kept with for 15 years. So doing deep discharges to achieve equalisation and then using high-current draws in Vectrix, you might be employing BOTH known ways to quickly kill NiMH cells.
Graeme Harrison
prof at-symbol post.harvard.edu
Just a little thought.
I like the equalization idea.
It might be possible to add a fixed voltage at about 92V, then after a while all cells will end up with a voltage of 0,9V (+/- the tolerance of the resistor).
My personal experience after discharging Vectrix cells says it is only a few mAh left when voltage goes under 1,1V/cell, so a 0,9 discharge can be seen as completely empty cell. A benefit is that the built in charger can power up since Voltage is kept over 80V (I think 80V was the magic limit), and it might be more gentle to the cells.
Just a little thought.
I like the equalization idea.
It might be possible to add a fixed voltage at about 92V, then after a while all cells will end up with a voltage of 0,9V (+/- the tolerance of the resistor).
My personal experience after discharging Vectrix cells says it is only a few mAh left when voltage goes under 1,1V/cell, so a 0,9 discharge can be seen as completely empty cell. A benefit is that the built in charger can power up since Voltage is kept over 80V (I think 80V was the magic limit), and it might be more gentle to the cells.
What an interesting idea! Much easier than adding a diode at every cell level, it seems to fit in with The Lairds suggestion to use the KISS principle.
What would you use for the constant voltage source?
Could "Special Freddy" be modified for the purpose? The MR capacitor would need to be chosen so that Freddy produces 30mA at 92V. Or am I barking up the wrong tree here?
0.9V / 39.0 Ohm = 23mA; add to that the 7mA from the constant-on motor controller and you get a total requirement of around 30mA constant current to keep the battery at 91.8V.
Personally if I would go for a fixed Voltage, I would buy a "Lazy Frolle" :- ) from Ebay. Search on Ebay for "90V 300mA".
I have ordered two to put in series, to do a EQ-top-up-charge without heating.
One would keep the Voltage above 90V if the specifications are correct.
Hello Folks,
Frolle's idea and Mik's variation are part of that 'Eureka' moment which I had some time back. However, I didn't pursue that because what has happened is that the equalising resistors (those fitted to each cell) are doing the job without the battery voltage dropping below 110 volts.
Whenever any cell gets to be lower in voltage than the remainder then that cell, the low voltage one, loses less current through the equalising resistor. Over a period of time all cells are equalised, provided that they are not faulty. By faulty, I mean lower than 90% capacity when compared to the others or leaking internally like the proverbial sieve.
But, considering the idea posed, my original 'Eureka' was to provide a constant current into the battery sufficient to maintain the battery voltage at 120 Volts. Of course a constant voltage would do the same thing but I was hoping to use a simple 'Freddy' style supply rather than go for a constant voltage design.
If anyone wants to develop this one then I suggest that you go for a constant voltage (with current limited to 250mA) plus the equalising resistors (39 ohms 1% 0.5watts) and hey presto, you can claim the glory for the success.
Must go, some air bags to deal with.
The Laird.