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Nicad chargers
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Ajax
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I have a little issue with my Triton, All the NiMH packs I've been charging have only been taking 1000mAh max When I first spotted it, I thought the battery pack had died... But I tested it today and the pack lasted quite a while. Also charged a good 3300GP pack and the same thing, 1000mAh....


I have had this problem myself.

I found that it was an incorrect setting in the Triton that caused it.

After I changed the settings to the correct value all was fine.
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Post Tue Feb 07, 2006 9:48 pm 
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DumHed
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well I've done some testing with a proper data logging battery analyzer lately and the much hated Omni charger is actually working properly Smile
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Post Tue Feb 07, 2006 9:51 pm 
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Knightrous
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I have had this problem myself.

I found that it was an incorrect setting in the Triton that caused it.

After I changed the settings to the correct value all was fine.


Is it to do with the mV peak detection setting. You can change it from 5mv to 10mv. It's the only thing I can think of at the moment... must go test it.
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Post Tue Feb 07, 2006 10:01 pm 
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DumHed
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generally NiMh cells would be using the 5mv setting and NiCads using the 10mv setting.

NiMh cells drop less voltage when they reach full charge and heat up.
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Post Tue Feb 07, 2006 11:08 pm 
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Valen
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.1c charge? sure everybody has enough batteries to use them once per event ;-P

with a good charger a 1c charge wont be "overly harmfull" if the batteries dont get too hot.
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Post Tue Feb 07, 2006 11:33 pm 
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kkeerroo
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From the label on my 3300mAh 24V NiMH packs:
"Slow charge: 330ma 14-16 hours
Fast Charge: 3.3amps MAXIMUM"

If fast charging NiMH it is highly recomended that thermal probe is used!
Over heating of the batteries will cause permanent damage.

This is from the documentation that came with my Triton (for those who don't read datasheets).
Charging info for 3300mAh packs (what I use):
Charge rate - 0.3A
Max time - 660 min
Quick charge - 5.0A
Peak detection sensitivity 3-5mV

Discharge info:
Discharge current - C/5 (0.6A for my packs)
Discharge cutoff voltage - 0.9V per cell
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Post Wed Feb 08, 2006 3:39 pm 
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dyrodium
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Indeed i know from using my 2/3A NIMH cells that they get really hot in the last 10 min or so of the charge (around 2A). I stick mine in a small battery wind tunnel to cool them during the charge, and it works really well! Smile
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Post Wed Feb 08, 2006 4:30 pm 
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kkeerroo
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From what I researched on NiCAD and NIMH batteries when I first switched to them was that when a NiMH battery is fully charged and still has a charging current flowing through it or is being charged at a high rate (>0.75C) a chemical reaction (I can't remember the specifics. I think hydrogen is produced somwhere.) occurs that degrades the battery and produces heat and causes the so called memory effect. This is what causes the battery to suddenly heat up as it approaches full charge and is why charging should be stopped when a noticable change in battery temp occurs, hence the thermal probes.
The idea is not to keep the batteries cool while charging but to prevent them from getting hot in the first place.
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Post Wed Feb 08, 2006 6:09 pm 
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Glen
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thats why i dont think nimhs have no place in high power robots, itll just end up happening in the robot... the heating and degredation that is. kangs are basically rooted now.

while im on it i wonder what specifically it is inside the batteries that makes them retain so much heat... my nimhs after a hard run just will not cool off to room temperature for hours just letting them sit.
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Post Wed Feb 08, 2006 6:11 pm 
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Valen
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Ni-Cd and Ni-Mh basically just undergo electrolysis when they are overcharged, they produce H2 and O2 out of water, this increases internal cell pressure. There is a Platinum recombination catalist in the cell however which acts to recombine the H2/O2 into water.

NiCd is more robust in terms of surviving overcharge in general.
NiMh is more sensitive.

I believe the cells can get up around 40PSI at full charge.

Temperature rise begins at about 80% charged. Typically when doing a quick fast charge from room temp bats the cutout is about 50c as I recall.
(IE that temp is indicative of 100% charge.)

Angus is right to try to cool the cells however, if you have a good DP charger you will get more capacity into a cell if you force cool it to room temp. But i'm thinking that effect will be in the 5-10% range. Still its best to put cold cells into a bot than warm ones anyway and your better off not temp cycling them twice. (charge, cool , use , cool = bad Vs Charge, use, cool)

The new NiMh's that have lower internal resistance than Ni-Cd are looking good for bots, but thats a recent development, Before nothing could supply the current of the NiCd
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Post Wed Feb 08, 2006 6:23 pm 
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DumHed
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just a bit of related trivia.

The drop in cell voltage at full charge (which is what peak detect chargers use to stop charging) is actually a result of the battery heating up internally.

At higher temperatures the voltage produced by the chemical reaction is lower - and the cells make heat out of the input current when they start not being able to absorb the electrical power.

Faster charging will create more heat due to the cell's internal resistance, which will cause the voltage drop earlier than it should - so the charge will terminate early.
Keeping the pack cool while charging will help keep its temperature constant until it's actually reached full charge.

NiMh cells don't exhibit as much of a voltage drop at full charge, which makes accurate charge termination harder (hence the lower mv per cell detection threshold for NiMhs). It's easier to overcharge them without cutting off, especially if using a NiCad charger.
The NiMh's higher internal resistance will also cause more heating at high charge rates, which is why they generally can't be charged quite as hard as NiCads.
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Post Wed Feb 08, 2006 10:21 pm 
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Spockie-Tech
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There is a *lot* of conflicting Tech Info on battery cells out there..

Everything from the so called "Memory Effect" (that doesnt actually occur in practice) through to people who still market "Burp Chargers" to the gullible as "The latest Tech Secret" years after they've been tested by the battery manufacturers and proven to be bunk.

There is "some" evidence to the effect that NiCd (and possibly NiMh) cells used in High discharge applications (like ours), actually work *better* if you thump power into them at a reasonably fast rate, rather than gradually trickle them up.

Supposedly, its to do with the crystalline structure formed by the contents as they take up charge. Slow gentle charging encourages the formation of larger crystals that have a smaller reactive surface area (meaning less peak current) than a finer more broken-up larger-surface-area crystalline structure that a faster charge is reported to encourage.

I think the notion of "the slower the charge the better" comes from the very-low-current applications (like your RC Transmitter and Receiver batteries) where they are being charged by dumb plug-packs with no delta-peak end-of-charge sensing..

In low-current applications ONLY , a slow charge (.1c or less)

1. prevents serious cell heating (and damage) when someone leaves their pack on charge for 48 hours to "squeeze that little bit extra in" (not knowing that overcharging causes symptoms that are usually known as the dreaded memory effect - correctly called "voltage depression")

2. Allows unevenly discharged cells in a multi-cell-pack to equalise as they all gradually come up to the same voltage/charge state. If you charged them faster, then the cells that reached full charge before the others would start to trend downwards (the negative voltage peak), possibly causing your peak detection charger to false-peak from some cells going down while others are still going up.

This is why when you first construct a pack from cells, it *is* a good idea to give them a slow charge (even better a *individual cell* discharge to 0.5v each prior to pack assembly first, then followed by the slow charge after you assemble them into a pack). This should bring them all close to the same state of charge, and you can then charge them as a pack in future and be reasonably confident they are all at roughly the same discharge/charge level.

*If* you have time to fiddle, inidividually discharging the pack cells every now and then (once every 6 months or so with average bot use) by means of a set of crocodile clips is a good way of bringing them all back into line, or 2nd-best, occasionally do a *slow* (.1c) charge to equalise them, followed by a quick (1C+) discharge and charge to "flex their muscles" a bit and stir up the chemistry.

Liquid Lead-Acid batteries are well known to exhibit this effect as well, since in their case, the fast-charge stirs up the chemicals in the electrolyte, prevent "acid stratification" from the heavier substances settling to the bottom of the cell. Not likely to be a problem in our application where the batteries get the hell shaken out of them.. This effect probably doesnt apply to Gel or Absorbed-Glass-Mat SLA batteries that we use anyway, but it illustrates the point that there are often unexpected effects going on in battery chemistry that arent immediately apparent.

Also, keep in mind that a lot of the advice out there is intended to maximise battery *cycle life*, which isnt really a big factor in combat robot applications. Your batteries are much more likely to get the stuffing pounded out of them, shorted out, overheated from overload and hurried-between-round charging, or sawed by Scoopy-Doo Wink long before they die of old-age. Telling a combat soldier in a battlezone that they really shouldnt smoke because it takes ten years off your life is unlikely to worry them too much.. Power-to-weight Performance is more important to us than a comfy retirement.

Correct Care and Feeding of batteries is a controversial subject.. Its like asking Pet Breeders what food is best, each will have their own preference and reasons for believing that their choice is best. The above is just *my* opinion. Read up and make your own choice (and if you discover something new, let us know..). A couple of good starting points are here

http://www.camlight.com/techinfo/techtips.html
http://en.wikipedia.org/wiki/Nickel-cadmium_battery

*some good info for the Wiki in this thread, I'll go through and compile some of it - thanks for the input everyone*
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Post Wed Feb 08, 2006 10:39 pm 
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Ajax
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@Arron.
It could be a dead cell. That will drop your charging amount.

I had a 1400 mAh pack that only charged to about 800 mAh

I haven't changed the mV peak detection setting.
I should look at then tho
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Post Fri Feb 10, 2006 8:27 pm 
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Knightrous
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Nah, it did it with a brand new 2300mAh NiMH TX battery... Straight out of the packet, I cycled it on auto for 10 cycles and it only showed up as 1000mAh capacity...
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Post Fri Feb 10, 2006 8:28 pm 
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I think Jason had that effect once too, but I forget the reason. Send him an email at jaycowley<at>optusnet.com.au and see if he has any suggestions.. he only watches the forum on occasion.
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Post Fri Feb 10, 2006 8:53 pm 
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