How to correctly discharge a battery?

[timie1]

I have just built a bulb discharger that I can hook up 3 different ways. It can discharge at 20amps, or if I connect it a different way I have a 6amp discharger and also a 14amp discharger.

My question is........what is the best way to discharge a decent nimh pack? Like for example....should I discharge it to 5.4V at 20amps or 5.4V at 6amps etc?
If I discharge at 20amps to 5.4V, take it off discharge and measure the voltage with no load, the voltage is well above 5.4V. So then I put it on the 6amp discharge circuit and take it down to 5.4V but I end up getting a couple extra minutes out of the pack before the 5.4V threshold.

People say discharging at 20amps is best because it matches closest to the rate at which the battery discharges in the car. But the car discharges at a variable rate depending on going full throttle, half throttle, slowing down, tight windy track and so on. So how can we say 20amps steady is best to discharge it at.

What I'm really trying to say is, assuming that we never let the voltage get below 5.4V, if we discharged the pack at say 6amps (or 10amps or 14amps etc) down to 5.4V, wouldn't that give a better discharge without overdischarging it, than a 20amp rate would? A lot less wear on the cells and in reality, it may be closer to what the car actually discharges it at?

[kschauwe]

Lower discharge rate has less heat build up. 6a is fine after a run, since the pack is almost empty anyways.

[guver]

I think I see what you're asking. A lower current (6 amp) will discharge deeper (more capacity) at the same volt-cut-off (than 20 amps) and can actually hurt the battery more since cells in a series will be closer to reversal. It will be less heat though as above.

My solution is to not discharge series batteries.

[timie1]

I think I see what you're asking. A lower current (6 amp) will discharge deeper (more capacity) at the same volt-cut-off (than 20 amps) and can actually hurt the battery more since cells in a series will be closer to reversal. It will be less heat though as above.

My solution is to not discharge series batteries.

Thankyou. That's more what I wanted to know. But say if it did get slightly below 5.4V cutoff at 6 amps, that's hardly close to cell reversal point isn't it? Wouldn't cell reversal happen at the point when the battery under NO LOAD is at or below 5.4V? Hypothetically, if you discharged to 5.4V @ 6 amps, then decided that you would then go to 5.4V @ 3 amps, then 5.4V @ 1 amp, still under NO LOAD the nominal voltage of the pack would be above 5.4V and in theory cell reversal wouldn't have occured.
Correct me if I'm wrong!!!!!
I just want to get a decent charge in the battery without damaging it by over discharging it. When you look at the high end chargers out there, a lot of them have a discharger built in, and/or cycle functions. To obtain a decent charge these high end chargers first discharge the packs using a low rate. Then some of them condition the battery before charging.

So, on that logic, if you manually discharged it to 5.4V at a low rate and trickle charged the pack, it would in theory bring each cell up to a full charge without overcharging it. Much like a battery conditioner does.

Once again, someone correct me if I'm wrong!

[guver]

You've got it down pat.

Cell reversal will depend on the imbalance of the cells (or capacity difference) as the voltage gets down to low levels. It also increases as the number of cells increases.


2 cells in series discharged to 1 volt may never see reversal, yet 30 cells in series discharged to 30 volts has a great chance for 1 or more to be reversed.

.8 or .9 or 1 volt is usually the spec for a happy medium between emptying the cells and chancing reversal for 6 cell packs.

[guver]

Oh , I forgot. If the cells are identical then they could be brought down to 0 volts for the whole pack and never get reversed.

[timie1]

Oh , I forgot. If the cells are identical then they could be brought down to 0 volts for the whole pack and never get reversed.

Or if you manually discharged each cell in an unmatched pack to 0 volts it would in theory be alright?

[ta_man]

Oh , I forgot. If the cells are identical then they could be brought down to 0 volts for the whole pack and never get reversed.
The cells in a pack will never be "identical". Even the cells in a "matched" pack develop variance after one or two cycles. That's why there is such a market for equalizers for side-by-side packs for racers.

[timie1]

The cells in a pack will never be "identical". Even the cells in a "matched" pack develop variance after one or two cycles. That's why there is such a market for equalizers for side-by-side packs for racers.

Yeah I suppose you're right. Which leads to one more question, can you use an eqaliser on unmatched side-by-side packs? Like if I pulled my stick packs apart and soldered them side-by-side.

[guver]

Yes, you can discharge each cell by itself to 0 and even dead short it. Some cells might not like that though. You can also do what you propose with a discharge tray on unmatched pack. The result though is going to be a balanced or equalised pack at the bottom and a unbalanced pack at the top (when charged)

[timie1]

Hmmmm, you have certainly made me think.
Thanks for all your help.

[GSMnow]

When I see my packs getting out of balance (one cell hotter after charge or run), I charge the whole pack to peak at just 2 amps or so. Of course Ihave to monitor the temp of each cell so I don't cook the one starting to overcharge. Sometimes I have to stop it short when it gets too hot. Then I just re-peak each cell separately at 4.2 amps (or whatever C rate is). And that usually runs fine for 10 cycles or so again. For discharging after a run I just use 10 amps on my Checkpoint with the threshold at 0.9 volts per cell (5.4 total) when it stops the pack rebounds to over 7.2 volts every time, but that is fine. Then I charge at either 3.6 amps (easy on the pack) or 4.2 amps (a litle faster, a tick more punch) on the 4200's. Going more than 4.2 has not shown my any increase in performance, just much hotter when it peaks.

On the checkpoint, the 20 and 30 amp discharge settings are a few second on, and then a few seconds off, it only times when it is pulling current, so you get true mAH drawn, but it allows the battery and charger to cool off a bit, and is more like driving a car. I use the 30 amp mode to test my packs once in a while, but for taking them down after a race, the straight linear 10 amps is just as fast. If your car can run for 10 minutes (I get 12 to 14) you are AVERAGING 6C discharge current. 4.200 x 6 = 25.2 amps. The peaks are much higher. With my Mamba Max system, it could be well over 80 amps for short spikes. WHEN I go LiPo, I will make sure my packs can take 100 amp surge and at least 80 amps constant to be safe. The IB4200 cells are rated at either 40 or 45 amps constant discharge depending on who's page you check.

[GSMnow]

Oh, a discharge tray that stops pulling any current on each cell as that cell goes under 0.9 volt is a very good idea to use, it should really help keep a good matched pack running strong. But as noted, if you have a weak cell, then that cell will peak out first and either cause a false early peak and the rest of the pack does not get a full charge, or that cell will overcharge and possibly vent and lose even more capacity. I want to make a tray that individually peak charges each cell, but that is much tougher as it needs 6 floating power soures to work well. If I build it, it will be fairly low current, like 2 amps per cell so I can get away with cheap linear regulator chips and not create too much heat. The tough part is the floating supplies. I have a few ideas I have drawn up, but short of a multi tap transformer or switcher it seems to still leak some current to the other cells. Is there a market for this if I can keep the price reasonable?

[Sparrow338]

@ GSMnow: When you say your using a floating power source do you just mean the trans doesn't have a ground/grounded center tap? I've seen the word floating used for a lot of different things in eletricity, what do you mean when you say it? If you mean you just want multiple outputs that are isolated from eachother why not just get several (one for each cell) cheap low voltage trans and wire all their primaries up?

Also I have some questions concerning discharging batteries. I've been doing a lot of research trying to figure out what is the best way to discharge NiCad as well as NiMH batteries. I've heard that with NiCad you should completely discharge them until the read 0 volts, is this true? Or wil this hurt my batteries. With NiMH i've heard that you should never discharge them below .9 volts per cell, in any cell configeration, even just a single cell. But i've seen discharges that are made to do just that, discharge a NiMH battery down to 0 volts per cell. So now i'm confused, what do you guys think is the best for my batteries?

Thanks.

-Sparrow©

[guver]

I like to put it real simple. Never discharge any battery unless doing a test.

[Sparrow338]

So don't discharge the NiMH's, how about the NiCads? I've always been told that it is important, due to their memory problem, that you discharge them, to what extent is what I want to know.

[GSMnow]

In normal use, NiMh cells really don't require being discharged. I like to do it after races to see how much power I had left, and then on the charge up I can see how much capacity the cells have. It is a good diagnosis for me personally. Pulling the latest hi cap NiMh cells below 0.9 volts per cell (some say 1.0 for the IB's) seems to really shorten their life. NiCd memory effect is not what everyone makes it out to be. It did show up in comunication satelites with the very steady solar charge during the day and run down at night. Evidentally the plate area that was not exercized just stopped working. In real world use, it is more likely that the loss of capacity is from cell venting, normal aging, or overheating. It is still a good idea to exercise the full capacity at least once in a while and the guys trying to get every last bit of punch out of the cells find that pulling them to 0.0 volts and bashing them with a high charge current will get a hair more kick out of the hole and pull hard futher into the run. They seem to "remember" how much current they were charged at, so the harder they are hit when charging the harder they hit discharging, but it is a point of diminishing returns. If you leave some charge in the NiCd, the area that is already charged is not cycled to give that hit form the high charge current. All that eing said, I have never been able to really see enough of an effect to blast my packs. Back in the 1200 mah cell days, we did EVERYTHING to get any kind of edge. With 4200 mah cells that punch out 45 amps continuous, it is not as much of a concern. Sure we ask alot more than 45 amos, but not for long. The latest NiMh cells hold much more voltage at these high currents than any NiCd's ever could.

[Sparrow338]

Thank you very much, that really helps clear that confusion up for me.

-Sparrow©

[timie1]

Yeah me too.
Thanks for that:)

[highroller]

When it comes to NiMH cells it kinda depends on the brand and which generation of cell it is. Panasonics had 3-4 (3000) generations, Sanyo had two(3000), GP had three generations of 3300, IB has two generation of 3600 and now two of the IB4200, EP are on their second generation.
The cutoff voltage is generally at .90 volts per cell, with the IB4200 the recommended cutoff is 1.00 volt per cell.
The loads used in charging or discharging will have a effect on performance and also life of cell. Higher loads are used in high level competitive racing with charge rates in 5-7amp range and discharge in 20 to 35amp range but for long life 3.50 to 5amp charge rates and 5-20amp discharge works.
Dsicharging is used to gain consistancy and ensure cells are charged completely, equalizing aids in this process by discharging each cell individually at a lower load so that cells start out the charging process more closer to the same voltage levels. For stick packs are discharge resistor, or pack equalizer accomplish somewhat the same process though cells may not be as close as individual disharging.
For competitve racing I discharge the pack at 25amp for offroad, 35amp for oval/TC down to .90 volt cell level, 1 volt for IB4200. The GP3300 are the only NiMh cells that had some form of performance from dead shorting, but the pack lost 20-30seconds of runtime but did seem to be more punchier which was great for stock and 19turn racing - runtime was iffy on where the pack would fall off for modified oval racing sometimes it would make a 4.01 other times it would be 3.48-3.53 minutes.

[timie1]

Now that's the sort of information I was after. Thankyou highroller.
How can you tell what generation of batteries each are? Are there any special markings?

[highroller]

Panasonics went through a couple of name changes.
Sanyo you had the 3000 and 3000HV
GP 3300 you have three improvements first cells had very low voltage, runtime and higher international resistance, 2nd had higher runtime while voltage and IR remained about the same, third generation had the high voltage.
IB 3600 the first had about the same as you GP3300 and didn't seem to hold their numbers as well. The second was more stable and in the ream of the 3800 IB cells very few of these were used as everyone seemed to jump for the hi voltage 3800s.
IB4200 first generation are the ones that tend to loose voltage while stored, many matchers recommend storage at 1. volt or putting at 15 minute to full charge back in pack for long inactive periods. The new (2nd generation) are suppose to be less prone to this cell self discharge as much- basic numbers seem to be the same.
EP4200 the first generation didn't hold their numbers, after 2-8 cycles cell became inbalanced, the newer cells remain more stable close to the IB4200 except voltage average .02 points lower average 1.223.
Very few matchers are matching the GP or Sanyo cells the few 3700 I tried are basically used for bashing now or to power lathe.
Some of the new packs I got this week were matched at 40amp with runtime in the 375 to 381, voltage 1.240 - 1.244, IR 1.3 to 1.5 from TQ Cells.