Faster Discharge?

Is it possible to make a simple apparatus to discharge a module at a faster rate? I understand why the small hobby chargers can only discharge at 5w or so but what challenges would one face if they were to simply use a 20watt light bulb (just as an example). Obviously monitor voltage and stop at 6v (or 6v-Vdrop?). Would you average the current using starting v and final v / watts to figure out mah discharged? I suppose on the third discharge you could use the hobby charger to get a more precise capacity. It just seems like a massive waste of time being limited to 5w discharge on 84 total module discharge cycles when even a 50w discharge should be plenty safe and sufficient for at least the first two cycles of an individual module.
I apologize if this has been discussed before or if my understanding of the topic is way off, just brainstorming here in the event I need to rehab my hv battery someday (I love learning and researching ahead of time).



iPhone ?

 

Thank you. My thinking was to make the discharge cycle manual - as in I sit down in front of the pack, hook up some light bulbs to a few modules and carefully watch voltage, disconnect each when they get to 6v or slightly below to account for voltage drop. Then use the charger for the charge phase only. It seems too simple to work (other than having to sit and watch). But say I only do it for two cycles and let the charger do it at the slow rate on the third for the sake of precision. That could save 6 hours per module per cycle.. (300 hours if using one charger?! 75 if using four?)
I guess my mission is to buy a module on ebay and test it out.. I agree that a method to boost the discharge rate of the charger would be awesome and optimal but I wasn't even considering anything that ambitious


iPhone ?

 

Good info, thank you. One hour vs almost ten using a charger that maxes out at 5w for the discharge sounds like a good trade off to build a little rig.


iPhone ?

 

If I'm discharging a module to 6v and one cell drops to zero then that module would be bad anyway, right? What is the discharge rate of any one module under heavy driving conditions? It's got to be at least 5c or so, probably higher? if it can survive that during the (admittedly narrow) charge cycle in the car then it should be ok at 1c on the bench?
I promise I'm not trying to be difficult, this is just how I try to work through things. I was raised to immediately doubt any shortcuts (my father built homes from the foundation up and shortcuts meant quality cuts) so I'm skeptical myself, but I see this as a limit of the hardware - hobby chargers not built to dissipate heat quickly..

Edit: I think my in-my-head math is off on the max discharge rate.

 

Does anyone here have a module they would be willing to loan or sell so I can test things out? I want to build a simple affordable rig that anyone here could duplicate. I want to test it using my thunder ac6 charger to accurately measure capacity then compare that to the rig to come up with a simple time/mah graph (this should be linear right?) so capacity can be estimated.
Hopefully I'll never need to rebuild one but others will and anything that could safely help people get back on the road sooner is good. Maybe figure out a way to add in a 6v auto-off so it wouldn't have to be watched constantly...
I just tested my traction battery and my delta V never went over 0.14v through a full stress test (and the low bank kept changing through that test). I hope that's a good sign, it's a 2006 with 148k so I'm in the "start thinking about options phase".


iPhone ?

 

You'd still be way way ahead time wise. Especially considering the module will be at least 40% charged to begin with.


iPhone ?

 

That's a nice unit, I love the idea of the regen discharge, but $250 for the unit and another $150 or so for the power supply isn't practical. And is that just to do one module at a time? If so that's $1600 to do four modules at a time. At that price it wouldn't make sense for an individual who isn't planning on rehabbing batteries for a living. If I'm wrong and it'll do four modules at once then it's not bad.
I found a schematic of an auto-shutoff circuit with a variable cut off voltage, cost of parts is $5, add another $5 for a resistor to handle the high current discharge. Two of those and a four port hitec charger with power supply built in for around $250 total or $150 if you already have a power supply.


iPhone ?

 

That's an awesome charger, I want! if it does 20 cells then we could do three modules at once helping out the cost factor. I'm envisioning a setup where someone can recondition an entire hv pack in a day.. Hmmmm....


iPhone ?

 

Regarding my post about the $10 discharger, if I were to build that discharger I'd target the discharge at 0.66C and then charge at 0.33C using the hobby charger (I'm using those numbers assuming I would build two dischargers and use a four port hobby charger). Assuming 0.1C discharge from a hobby charger this would cut the theoretical time to balance one module from 39hours to 16.5hours. 28 modules later you've cut 630(!!!) hours from the project (obviously divide that by the number of chargers if using more than one). And that time difference includes the extra initial charge mentioned previously on the faster method. I still honestly believe there is no harm in bumping those rates up even higher for three cycles.
Now here is where I start thinking again... Dangerous territory according to my wife..
First: I no longer see a need to do a slower 0.1C discharge so the hobby charger can tell me capacity. As long as I use the same method for all modules and the same calculations time will be the important measure. On the final cycle starting V and ending V(minus v drop if significant) can be the same, the load is obviously the same so the average watts should be the same meaning time to discharge is all that matters. Higher discharge rates will reduce a measured mah rating somewhat (very little at only 1C), but that's just a measurement at a given discharge rate. We could change the measured capacity several ways depending on the method on the same module, but it doesn't change the condition of the module. We should be less interested in a theoretical maximum capacity as we are in a balanced and matched set of modules. Keep the method the same.
Second: I still say discharge first. If three discharges to 1v per cell at 1C were dangerous there wouldn't be a hobby nimh pack left after a week of use (they have discharge rates much much higher than 1C). Then trickle charge at 0.05C give or take (perhaps a two stage charge to save time if you have the capability). Without some way of individually controlling the discharge of individual cells they won't balance from a discharge anyway. The balancing comes from a trickle charge at the end of the charge cycle. I think we would see a higher and more consistent benefit of balancing using that method. Fill up those lagging cells with no threat of heat build up in the higher charged ones. We wouldn't have to worry about any mah safety cutoff or guessing what that number should be; these aren't lipos. Boosting the discharge would buy us more than enough time to make up for a good trickle charge at the end of a fast charge cycle. 0.1C discharge is just not logical, it's just the limit of the hobby charger to keep it cheaper. Standard practice for conditioning nickel based packs is to trickle charge unless you have means to manage each cell individually (with a tray charger for instance). We can't do that.
Third, an unbalanced cell is like a runaway train. It will charge to higher voltage degrading it's capacity even more resulting in higher charge voltage next charge degrading it even more.. On and on and on. The narrow charge cycle of the traction battery is brilliant in slowing that process but it will still happen.. Best thing would be a way to trickle charge that whole darn pack to balance it all once in a while. Why wait for one or two modules to get so far out of balance that we have a failure?


iPhone ?

 

For a second I read that as "we used over 1100 light bulbs" and I imagined a glorious set up I agree, keep the method the same and the variables (other than time) will even out for finding modules that are shot.
What is your opinion on a trickle charge at the end of a rapid charge? If you do three cycles would you trickle on all three? Only the first? Last? Is three cycles still the norm for reconditioning?
These nimh cells in the prius are going to be fine with rapid charging and discharging. Be gentle with the balance portion for obvious reasons and I can see a four charger setup doing a whole pack on a weekend easily, assuming a two stage charge.


iPhone ?

 

Okay next question.. What would be necessary to trickle charge this pack as one unit while still in the car? I havent looked around here enough but I bet someone here (lots of great minds) has already done it. I'm almost certain a once or twice a year balance would help everyone get the most out of these packs. Yes any rechargeable will eventually degrade, but these failures all seem to be caused by one or two severely unbalanced cells that went into a death spiral and could have been avoided.


iPhone ?