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Old 09-23-2003, 04:24 PM   #1
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Default Difference between nicad and nimh peak detection?

What is the difference in the chargers that can charge these types of cells? Is the difference only in how much the voltage drops when the charge reaches its peak?

Techically speaking, can a NiCad charger charge a Nimh cell, but just perhaps not be able to detect the peak accurately enough?

Thanks for the details
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Old 09-23-2003, 05:07 PM   #2
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Yes, the only difference is the value the voltage drops (peak delta). However some NiCd chargers use a "pulse" or similar charge, whereas NiMH's need a linear charge.

A NiCd peak charger will overcharge a NiMH pack.
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Old 09-23-2003, 05:20 PM   #3
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What if I watch the charge as it reaches peak, and pull the charge as soon as voltage increase stops? I have an older Tekin charger, but it was a top of the line 5amp Nicad peak charger, with peak, peak2, and coldstart options.

I am trying to avoid buying a new charger... this one set me back about 160$ a few years ago.
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Old 09-23-2003, 07:17 PM   #4
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Quote:
Originally posted by sosidge
Yes, the only difference is the value the voltage drops (peak delta).
Mmmm...not exactly. The chemistry differences require some more sophisticated controls over current/voltage relationships, and ideally include temperature, but most RC chargers don't seem to worry (know? care?) about that, and use the standard, but less ideal delta-V charge methods.

NiCads are pretty indifferent to charge type as long as it falls inside a certian window of current and voltage requirements. NiMH are a little more picky, and a lot easier to damage. They're better for high discharge, but can't be cycled as hard as often, and repeated force charging (fast charging much more than about 1x the rated capacity (ie 3A for a 3000mAh pack) will reduce pack life and capacity after a short number of cycles. The charge window for NiMH cells is smaller, and if you want to preserve the capacity of a cell for a long time (more than 100 charge/discharge cycles) you've got to be a little more gentle than with NiCads. That means only recharging them after they've been back at room temp for 20 or 30 minutes or so and only doing it at the 1C rate (1A for every 1000mAh of capacity).

My cred for this post? 2 years of designing power management systems (chargers, regulators, etc) for nimh and lion powered portable devices that can't fail for at least a decade.

-dave
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Old 09-23-2003, 08:26 PM   #5
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holycow, your in the same boat as i'm in read this post {Dad needs to buy a charger- please Help} there is a software upgrade for the 112 if thats what you have? I'll look it up, But you will still have to baby sit your pack, at $40.00 and up for 6 cells, I'll just upgrade. My time is also worth money. You can get software upgrades here will help a bit-http://www.rctech.net/forum/showthread.php?s=&postid=519682#post519682
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Old 09-24-2003, 02:25 AM   #6
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dpaton - I'd be interested to hear your suggestion for a NiMNH charging routine, since you have industrial experience.
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Old 09-24-2003, 08:20 AM   #7
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Sisidge-
There are some really wonderful books out on battery management, most of which can be had at a local college or well stocked public library. I'll try and get some names if you're interested. Manufacturers are also very willing to send you tons of information on the ideal conditions for maintaining their cells. For me, Panasonic was particularly helpful. Unfortunately I can't share the algorithms or processes I used, since they're considered proprietary industrial technology, but I'll give you a nice intro.

The basic idea when charging any battery is to apply enough voltage and current to remind the battery chemistry that it used to hold more than it currently does. Depending on the battery, that can mean tons of current at no volts, to tons of volts at no current, and everything in between, with the common thread that Heat Kills Cells. But then again, we all know that already.

Now, in the case of NiMH batteries, a little bit of heat is good. The cell chemistry responds favorably to temperatures between 80 and 90 degrees F. The general range for 'best performance' is usually 65F-100F. More or less and it's efficiency falls off, with the rate dependent on the departure from an ideal state, which is dependent on the particular formulation of the cell chemistry, with the most detrimental changes near freezing and boiling (32 and 212F, 0 and 100C) where the NiMH chemsitry becomes very inefficient. (whew!) By very inefficient I mean the batteries contain less than 1/3 their rated capacity, due primarially to the effects of temperature on the charge reactions. Those of you who took chemistry in high school will remember that cold temperatures cause chemical reactions to slow down, and in the case of batteries, hot ones cause the dissociation of the ions into hydrogen, causing cells to vent, and even a little (as in not immediately noticable) bit will cause permenant damage. That's bad, and can be caused by ambient temperature, but usually is caused by very high discharge or charge rates. Of course, the closer you get to the upper limit, the less forgiving your cells will be of high charge/discharge rates, so keeping them cool is paramount. The newer cells from GP, Panasonic (and others I'm sure) can be purchased with 'extreme duty' chemistry, but that will only prolong the inevitable. If you can keep your batteries under 125F (ideally under 100F) or so during discharge, so much the better. They'll thank you by remaining 'good' for a long time.

In terms of charge methodology, the best way is at the C1 or C10 rates, which are at currents 1x and 1/10x the rated capacity for 1 or 10 hours respectively (plus a fudge factor for charging losses). Unfortunately for the batteries, racers hate waiting, and 1 hour charges. When NiMH's are charged, it can be detected with the same type of deltaV as NiCads, a deltaT, a deltaI, or any combination of the three. There are also gains to be had by tweaking the method by which dT, dV and dI are used, such as the familiar pulse charging used on NiCads. The end goal for all of them is to apply current and voltage in such a way that temperatures remain low and charging efficiency remains high. The other big factor in charging effectiveness, besides the V/I/T stuff, is the actual charge algorithm, which generally includes a setup or discovery cycle (checks the state of the cells), an initial charge cycle (c/3 or so for a few minutes to eliminate false peaking), a fast charge cycle (for bulk charging) and peak detection (duh). There are some variations on that theme (timing, repetition fo stages, thresholds, etc), which is where the secret sauce is. The basic curves (a good starting point) are published in app notes and datasheets for charge controllers and can be had from any of the usual suspects for analog ICs (National, Linear, Maxim, Micrel, etc.)

deltaV is the most common and easiest method to use, since it's the best understood. For NiMH, it's about 0.017V/cell, or 1/3 that of the NiCads most people are used to. I practical terms, it's best to stop charging NiMH cells just when dV/dT is zero, before the cell voltage starts to drop, at the little charge voltage plateau before dV goes negative.

deltaI watches the amount of current going into the battery, and is the only way to charge Li cells (both prismatic Li-P and standard Li-Ion). For NiMH, this is one of the secret sauce charge algorithm ingrediants that gets used, at least for me. It's also a primary ingrediant in intelligent chargers' ability to distinguish between a good and a bad cell or pack, and pre-charged cells/packs.

deltaT watches the temperature of the pack, for something on the order of ~1C per minute rise in temp, signifying the end of the charge process. Also, it can help to protect the packs, by keeping charge current low while the battery is hot, or better yet, delaying charging until the cell is back to a reasonable temperature, both useful for the charging gorillas who throw their packs straight from the car to the charger.

In an ideal world, all the packs used in RC would be equipped with smart electronics, like laptop and cellphone batteries, allowing for intelligent control (dV+dI+dT) of the charging process as well as historical monitoring of pack performance and longevity. Of course, typical RC use is so far outside the realm of today's smart battery circuitry (30-50A of discharge for a few minutes), I think that's a long way off, but it'd be cool if it happened.

This will probably get updated as I think of more things and get some coffee in me (ugh...4 hours of sleep... ).

-dave

Last edited by dpaton; 09-24-2003 at 05:02 PM.
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Old 09-25-2003, 06:57 AM   #8
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Very interesting stuff - perhaps there's room for even higher-tech chargers in RC? - or do the excessive discharge loads mean that cell lifespan will always be compromised?
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Old 09-25-2003, 09:51 AM   #9
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Depending on how mattery chemistry evolves, I'm sure there's a lot of room for improvement. Hell, we got rechargable alkalines, which weren't suposed to be possible.

The high discharge rate on cells used for R/C (not just cars) causes a lot of problems for the cells as they exist right now. People are doing the right thing by only recommending one or two cycles a day, and 2-4 hours 'rests' for the packs, as those things will allow the innards of the cells to recover from the physical effects of the high current discharge (plate biases, bubbles, material discontinuities, etc). Charging, at least at the higher price points (Comp. Elec. and the like) seems to be quite good. I had a look at the BullDog at my LHS a few days ago and it looks like something I'd design. My guess is the other offerings from reputable manufacturers (Novak and LRP come to mind) are equally good.

The biggest place I see room for improvement is come kind of standardization of packs, perhaps to include a temp sensor or preferably full smart battery electronics. It wouldn't be difficult, just a small (.25"x1") PCB with some thin surface mount electronics that could be encapsulated and glued to the middle cells in a SbS pack. The advantage is that if the charger can know things like the temperature of the pack, its recent discharge history (current, final voltage, etc), it can do a much more intelligent job of recharging the pack and maintaining its best possible performance.

Of course, standardization has never come easily to the RC industry, so it's probably a long way off, but don't be suprised if you see a guy at a track with a wierd homebuilt charger and battery packs with attachments.

-dave
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Old 09-25-2003, 10:04 AM   #10
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dpaton-I just learned more about battery chargig in the last 10 minues than in the last 56 months I have been racing RC cars. Thanks!!!

GP told me about the technology of batteries with telemetry!! Thats prety cool stuff.

Can you imagine having a headset on during the race and your radio receiving telemetry from you battery and motor. "Voltage dropping below acceptable levels-conserve batteries." Or your motor sending a signal "Maximum RPM and Torque dropping, need a rebuild"

Too bad we RC racers cannot afford that-but I guess it is possible already.
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Old 09-25-2003, 10:12 AM   #11
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Actually it's not that hard at all. I'm also a hardcore electronics hobbyist, and I bet with a little money and some time, I could come up with a simple wireless telemetry system that would relay the stats of your car's electrical system back to your laptop (for data capture) or to a beltpack (for VOX notification, more expensive). It'd be wireless, pretty light, and should have a minimal impact on the performance of the car. Same thing could be done with Nitro, albeit with some different sensors.

I wonder if Trinity or Novak or somebody big would buy that product. There are data collection dongles for a few ESCs way back when.

Ray, you're a track owner. Would it be out of line to expect track owners to buy the host-end system, if it was only a few hundred $$? Racers could rent transponders, just like for timing.

My business case wheels are turning...

-dave
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Old 09-25-2003, 10:48 AM   #12
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I guess without the datalogger and accelerometer-you could attach some pretty small sensors and sending units to the RC car and it woldnt weight that much.

as far as track buying this technology-I guess it would just come down to how much each track and its racers embrace technology. We have been approached twice now with people trying to sell periphery hardware and software for the AMB system. Its all seems interesting to us.
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Old 09-25-2003, 11:01 AM   #13
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Actually, an accelerometer can be smaller thanyouthink...3x3x1mm, attached to a PCB. I bet with come creative engineering the whole package could be under 2oz without battery.

Of course, this is still at the cocktail napkin stage, so any discussion of an actual product is rather pie-in-the-sky. There is also the issue of how to integrate the system into the car, how to set up for measiring battery and motor voltage and current, etc. Technical hurdles to overcome.

There are a number of similar systems available for the RC airplane folks. I'm suprised someone hasn't come up with something inexpensive for the RC car and boat people yet, especially given the shorter distances that we work with relative to the planes.

Sounds like a fun project for this winter...

-dave
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Old 09-25-2003, 01:02 PM   #14
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ah yes ray, we've all been humbled.

d - given the fact that our chargers have a long way to go, what would you say is the best method of battery maintenance to give our cells the best performance over a moderate period?

should we keep the cells cooled during charging (via forced convection? is there a concern that some cells will be cooler than others if we don't have perfect laminar flow over each cell? will this, over time, cause our packs to unmatch themselves?

most of us are fine waiting for the manufacturers to step it up. well, what should we be doing in the meantime? store with a charge? why or why not? is battery equalization necessary? how often. what is going on in the cell when we dead short? what is the result?

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Old 09-25-2003, 01:34 PM   #15
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Quote:
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ah yes ray, we've all been humbled.
Humbled? I sure hope not...I just have a different background than a lot of RC guys. No ego here

Quote:
d - given the fact that our chargers have a long way to go, what would you say is the best method of battery maintenance to give our cells the best performance over a moderate period?
Most of the more diligent racers seem to be doing it right:
* Only run NiMH once or twice a day
* Use high quality intelligent/computerized chargers
* use well matched packs
* discharge packs properly
* when all else fails, use common sense

I'm actually pleasently suprised by the care taken by some of the manufacturers. The last time I raced, everyone was still using those timer chargers from Hobbico or whoever. Peak charging was still an expensive proposition to us kids. The only advice I have is to charge the cells a little slower, but I'd have to do some research into the new formualtions from GP and Panasonic to say exactly how much, if at all. I think 5A for 3300s is only a little high. I'd use 3.75-4A if it was me. The textbook says use 3.3A (1C rate), but they also say add some due to charging losses. 15% or so seems about right from my experience, so 3.75-4A is a good ballpark. It has the added bonus of not heating the cells up as much, meaning they'll be cooler when you slap them in your car and the initial temp will be closer to normal, and your final temp will be a little lower.

Quote:
should we keep the cells cooled during charging (via forced convection? is there a concern that some cells will be cooler than others if we don't have perfect laminar flow over each cell? will this, over time, cause our packs to unmatch themselves?
I think reasonable airflow should do just fine. On most of the cars I see, the batteries are open to the breeze on most sides. The only ones I worry about are the guys racing 12T doubles on stick packs buried inside plastic chassis. I don't think it's unreasonable to shoot for a 125F battery temperature at the end of a run.

Quote:
most of us are fine waiting for the manufacturers to step it up. well, what should we be doing in the meantime? store with a charge? why or why not? is battery equalization necessary? how often. what is going on in the cell when we dead short? what is the result?
Like I said above, some of the manufacturers are doing just fine. What I'm waiting for is a cheap $79.99 street) computerized 4-8 cell NiMH/NiCad charger that won't overheat my cells during charging. It'd be nice if it had a built in discharger too, so I could afford to have 3 or 4 of them at the track for a day's racing.
NiMHs, according to the chemists, are just fine being stored flat. If you try and store them charged, their internal self discharge rate will kill them quick (it's 2-3x that of NiCads). I'm a big fan of equalization during discharge, since it removes the possibility that a flaky cell will total a whole pack. It also allows you to check each cell if you have one of those fancy indicator dischargers. That will let you know if any of the cells are discharging faster or slower than others during a race, which is an indication that one night be going bad. During a dead short the battery is in full on oh-s#!t mode. The current flowing is causing the internal charge release reactions to run at an alarming rate, causing buildups of certain chemicals in spots and not in others, forming gas bubbles, and damaging the physical structures inside the cell. Basically it's the same set of destructive steps as in an NiCad, but because fo the chemistry it happens a little faster and is more permenant. The result is a cell that doesn't like to take a charge any more, and if it does, will have truly pitifal discharge characteristics.

With 20-30A+ discharge rates, the packs will always have a finite life. I'd say with good care, a good race pack should last a year of races (give or take 6 months ) before being retired to practice or break-in duty.

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i love dpaton.
Wow. I've only been here 2 days and I already have groupies

-dave
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