Proper Discharging
 

So I have a Pulsar Comp Charger and a Novak Smart Tray.
I discharge with the pulsar (10a)and then equalize them with the Novak.
What is the advantage of having a dedicated discharger? Do they discharge each cell individually rather than the puslar discharging the whole pack?
Would you still need to equalize them if you had a good discharger, or is the Novak needed when you discharge the whole pack like me?
I think 20a discharge is better for stock and 19 turn? What is 10a or 30a good for?
Basically, Should I invest in a good discharger and get rid of my Novak? Or is the Novak still good to have?

20 amp should be good for stock and 19 turn and 30 amp would be better for modified. The higher amp discharge is just a little closer to the current the lower wind motors pull and keeps the pack trained the same way it was matched. Discharging the pack at the higher amperage will get the whole pack down to the cutoff voltage, but the equalizer takes each cell to the cut off. You should always use a tray on your cells before you charge to get them as close to equal voltage before the charge as you can.

So even if I get a good discharger you would still recommend having the discharge tray?
I read somewhere that with a dedicated discharger you didn't need it
because it discharged at the rate you needed it to as well as monitering each cell.
Am I wrong?

So what do you set your trays cut off voltage too and why? .9 or lower?

A dedicated discharger offers you one rather large advantage... you can leave your charger to just charge packs.

I don't like to charge packs the night before, so I keep the charger running pretty steady on race night, the separate discharger (have a 30A integy reactor) discharges whats left in my packs while I can keep charging for the next heat.

I am going to pick up a Smart Tray, which I will use primarily just before I charge a pack, though I am running Intellect cells which don't like to go below .9v/cell

Ok, so I should keep my Novak.
So basically the two major advantages are:
You discharge at a higher rate and it keeps your charger free.

Yes, you still need a tray. When you hook up a regular discharger like the Indi Reactor or discharge the pack on your charger, it reads the voltage of the whole pack, not each cell. The problem with that is each cell is inherently going to have a slightly different internal resistance and therefor will discharge at a slightly different rate. So when you discharge the pack, you may have four cells at .9v, one at 1.0v and one at .8v. You still cut off at 5.4v (six cells at .9 volts) but the cells are not all discharged equally. By equalizing the cells before you charge them, you're starting all the cells at the same voltage so they charge as evenly as possible. But because they have different internal resistance, they will all charge at different rates. Keeping them equalized will keep them closer matched longer so the pack stays strong longer. The closer matched they are, the more evenly they will charge and discharge.

I set the tray to .9 volts. If you go farther than that, you can start to loose run time. I've been experimenting with going a little lower but haven't been doing it long enough to have any results.

I'll 2nd this, it's exactly right...

- JB

Jon, that was a good explaination, and I'm one to believe a seperate discharger is best, but I'm still rather confused on equalizing cells. Is the point to have them charge equally?

I know that if unmatched, a pack will have some weaker cells reach full charge before others, and this creates excessive heat and overcharging while other cells are still working on fully charging.

But this is where I'm confused. Consider a two-cell pack, with cell A and B.
If B is a weak cell, it will have discharged more than A after a run. A will have 1.1V
because it's more charged, B will have 1.0V, it's empty. When you equalize, both now have .9V. But now when we charge, B still reaches full charge sooner than A.

Wouldn't we want A to be left with a little more charge than B, so they will both peak at the same time ??

I think so. But consider NICAD cells, you must fully discharge them to keep up their capacity. If you don't equalize, then cell A isn't getting fully discharged, and will "remember" the same discharge level as B. Thus, the pack will eventually perform only as well as the weakest cell.
It will still charge unevenly.

If NiMH cells don't degrade like NiCADs from lack of full discharge, equalization would not be needed. In fact, wouldn't it cause uneven charge peaks, and thus overcharge weak cells, which would over time, make them even weaker?

What you need to remember is that equalizing them from the time you buy them will help to keep them matched closer longer and not have weak cells. You are right about the cells not charging equally, but the point is, equalizing them from the beginning will keep them charging and discharging as closely as possible longer. You build internal resistance by not discharging the cells low enough. So if you don't equalize them from the start, you will have certain cells that will discharge slightly less. As they get older and never get equalized, the IR will continue to get higher in those cells and the gap between the good cells and the weaker cells becomes greater and you really start noticing some cells getting hotter during charge and discharge cycles. What you're trying to do with a tray is keep your matched cells matched.

This happens the same with NiMH cells also...

- JB

when you guys race do you discharge your cells automatically after each heat or do you charge them again?

I always discharge my cells down to 5.4v per pack right after I take the car off the track. You don't have to equalize them after the run (although I do just because I'm anal ;) ) but you should at least discharge the pack.

well the problem is that i have only two packs and i'm not sure if i'll have time for practice,qualifying, and the main.

Axel skip one qualifier or buy more packs

run qualifier 1 with pack A while pack B charges for qualifier 2

Discharge pack A

run qualifier 2 while pack A discharging/resting

skip qualifier 3

get pack A charging in time for main

2 packs is really about 1 too few, I think you could get by with 3, 4 and you would be all set.

I discharge @ 30A then puttem on a 20A tray after each heat. Once discharged I won't charge again, for at least a couple of hours or more, and only then on a race day. The second charge of the day can be better than the first. On practice days I will only run a pack once, unless its a real long session.