NIMH batteries are tough. Some proof
Take a look at the bottom photo on my battery exposed page.
The 2 pack of prius subpacks started charging at ~85% charged.
The subpacks still work just fine, as I used them today to heat some 25A Nichrome ribbon to incandesence, and they seem to be of normal capacity.
Hope to get some time to test them more thoroughly soon.
I think as a result of this accidental experiment, is clear that the packs are a lot tougher than intuition would lead us to believe.
100A out of a D cell for 100K partial cycles says it all. They are tough.
Instead of the careful computer controlled charging with the Triton,
the pack could be put into a balancing charge with a setup as simple as a variac and a rectifier, using the PTC thermal strip to signal when the pack reaches some average temp that says ok everyone is up to max 100 %. Fans running of course.
This is in practice similar to what Armin used to grid charge his whole pack at 300 MA.
Then we let the pack sit and cool before putting it back into service.
When I get the right IMA pack in my lab, that will be a great and simple reconditioning process to try.
In theory one could even do it in the car with the pack disconnected and fans running.
Lets see D cell spec is 500 full cycles to drop to 80% capacity, that means that one could rebalance their pack with a balancing charge every 6 months and still last for 250 years?
In theory one could just look at the PTC strip resistance which is in physical contact with each cell, to tell when all the cells had reached a maximum temperature, thats where all the electrical watts are turned into thermal watts, or 100% balanced and charged.
should work?
I think 'Armins' low current (300ma) top up/balancing charge is the way to go to boost a failing pack without much effort. You don't really have to do much with this, and it will eventually bring that weak cell up to it's max capacity, so perhaps one top up charge a week would be enough to prolong pack life. Def something I am looking at for my wife's car which only has a short commute that is hard on the pack. It doesn't have a recal/capacity issue, it would just be nice to top it off from the mains.
Sadly I just got outbid on e-bay on a nice 1A Variac for this project
I appreciate accesing the individual subpacks and cycling them with the triton type charge would be better, but that's much more difficult. I'll probably reserve that for when I get the code of death and have to take it out anyway to resurect it
Looking at the pack exposed it is not too difficult to connect a simple charger as had been described with a variac type arrangement.
I was looking at disecting one of those 100w UK240v/US120v converters which are common over here. Add some full wave rectification and decent smoothing to the 120v AC output and voltage is getting upto what we need. 120 x 1.4 = 168v
Meow 77,
Maybe.
There is some evidence that many of the recal issues may be due to a gradual unbalancing of the 120 cell series string, an occasional full charge to equalize them, could improve the condition.
This experiment started us down this path. http://www.99mpg.com/Projectcars/mimapackwhack/
Then IamIan did a subpack by subpack full cycle of a badly unbalanced pack and corrected the condition. http://www.99mpg.com/workshops/mikessaturdayhybri/
The next test will have to be on someone elses pack, since the 4 we have access to are now working normally.
Peter,
The Insight pack with main switch OFF, is pretty safe to handle, as none of the exposed terminals are hot. With the switch ON, one can easily connect to the + pack output on the pack side of the main disconnect and the - directly on the negative terminal. The relays do not need to be on. Care is the key word here.
The stock pack can drop to 120VDC and get up to 177VDC at full charge current.
Hi Mike.. well... I hear you... but how can we apply this in layman's terms.. for practical useage... In other wods how can I do this on my own pack. A. without killing myself B. is there an easy way to do it?
What I've done inadvertently is that I didn't use the car for such a long time that the ima completely drained out. Then I recharged it however with the stock recharging system so that means to only 80% capacity.. how could I get around this to 100% in an easy fashion.
Because of the danger factor, I did not go into too much detail.
A service manual will tell you how to remove the pack, and there are many ways to inject a 300MA charge, if you have any electronics background. If not, it is probably better to not try it than to try it and be dead or sorry.
More experience with the pack rebalancing: I've just finished doing mine, and so far (one trip over the Sierra) it seems to have worked.
For background, I've owned my 2000 MT for a bit over 4 years (115K miles total). For the first 3 years or so, I had about 3/4 use of the battery. That is, in climbing up a mountain road, I could drain the pack down to about 1/4 shown on the gauge, at which point it would go quickly to zero. About a year ago, that shifted fairly quickly to where I could use less than the top 1/4 of the gauge. Further draw, or full assist, would cause a recal and often an IMA code that needed to be reset by pulling the fuse.
After doing the pack balancing per Mike's web site, I seem to be back to original condition: I can draw the pack down to the 1/4 level, where I get a quick drop to 1 bar, and then recharge. I did work it fairly conservatively, using M*MA to limit assist on climbs, so we'll see how it holds up as I drive more.
Caveat for those who want to do it: it's slow. Using a 3A charge & discharge, it took about 4 days to do all the subpacks. I started taking the pack out Monday pm, got it back together & running Saturday pm.
I also ran a temperature pickup (simple indoor/outdoor thermometer) into the pack, and found that it doesn't seem to heat much. (Which was one of my theories for the IMA codes.) Going over Carson Pass I climb from 4800 ft to about 8500, do a couple of ~1000 ft down & up over ridges, then a long downhill to near sea level. This should work the pack pretty hard, but I never saw temps over 80F, with the interior temp about 65.
I also bought one of the Triton chargers and will be doing mine soon. I have been practicing on some 'dead' NiMH laptop batteries. Had one 7.2v string that would only charge to 100 mAh after 1 charge, but after 10 cycles it went up to 600mAh which is still not good, but an improvement.
As a first test of another balancing technique, Ian and I ran an experiment.
We set up a string of 7 prius subpacks, and charged them in series at Armins 300MA.
The idea was to see if that charge level would heat the cells or develop high internal pressures.
The 300MA did not generate any measurable heat with the wide open cooling that we had, and after 20+ hours of charging, I finally saw some pressure buildup.
The subpacks had a small initial voltage difference that disappeared once we reached eight volts per cell, and they remained matched right to the millivolt up to the final 8.47V when I stopped charging.
After settling overnight, each subpack has exactly 8.27V.
The idea would be to use a constant current charger to gently top off the pack and rebalance whenever necessary. A simple two wire charge connection could do it right in the car?
We will follow this development here: http://www.99mpg.com/Projectcars/mimapackwhack/
A little more on the balancing. Discharging, of the 17 subpacks I got accurate numbers from, most had a charge between 3000-3800 mAh. 3 were just under 3000 mAh, and one was only 2000 mAh. (This was with the pack showing near full charge when removed from the car.)
After discharge, all but 3 accepted more than the rated 6500 mAh charge, with about 1/3 accepting 7000 (the Triton's safety cutoff). 2 were just slightly below the 6500 level, while the 2000 mAh one only took about 5700 mAh. (A second cycle took it above 6000, though I don't have the exact figure.) That suggests that the problems were due to the one weak subpack. If the problem comes back, I may hunt for a junked pack and pull out a subpack or two to replace the ones below 6500 mAh.
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Battery pack care and feeding
