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kkeerroo
Experienced Roboteer
Joined: 17 Jun 2004
Posts: 1459
Location: Brisbane
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Aocording the bloke who wrote this site bellow you may have damaged your cells by charging them at too high a current.
http://www.angelfire.com/electronic/hayles/charge1.html
for those who don't want to skim through the site to find the relative info here it is:
"Rechargeable battery capacity is rated in mAH (milliampere-hours). The total capacity of a battery is defined as "C", that is it can supply C mA for 1 hour, or 2C for 30 minutes etc. Charge rates can vary from trickle charges to keep the battery 'topped up' of 3.3% of C to 5% of C, a slow current charge of 10% of C to 20% of C or a fast charge of 50% of C to 100% of C. Slow charges are not meant to be continually applied, and since NiCd/NiMH batteries are about 66% efficient, they normally last about 8-15 hours. Fast charges such as 100% of C should be terminated after about 1.5 hours, providing the battery is flat to begin with. Once a battery is fully charged, the battery produces gas creating a high internal pressure, and a sudden rise in temperature. The charge should be switched to a trickle charge at this point or the battery will begin to vent and release its electrolyte. My old battery was rated at C=1300mAH and my old charger was rated 400mA (30% of C) so the charger should have been switched off after about 4 hours, provided that they were almost flat to begin with. However there is no way of knowing if C was actually 1300maH or if it had decreased a bit, and once the a battery starts to deteriorate, I suspect this becomes a vicious cycle and the battery deteriorates rapidly due to more and more overcharging. The manufacturer suggests these cells should be good for 500 to 1000 cycles if properly treated!"
I am sure cutting and pasting like that will annoy many people but it is all good info. The above site also has good stuff on "memory effects" as well. _________________ Get Some!!!
Secretary of the Queensland Robotics Sports Club inc.
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Sun Aug 15, 2004 6:38 pm |
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ffej
Experienced Roboteer
Joined: 22 Jun 2004
Posts: 595
Location: Kurrajong, NSW
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Well, having charged drill batteries at 5A, I really doubt you killed them with 2A. The GWS charger is a peak detection charger, meaning it senses the slight drop in voltage that occours in NiCd and NiMh batteries when at the end of the charge of the cycle due to gas build up in the cell. Unless the charger was set up wrongly, its pretty hard to kill batteries.
Toward the end of a charge cycle, the pack will get pretty warm, but not too hot to touch(55^oC or so, depends on the batteries). If the packs arent warm at all then the charger is stopping to early, either detecting a false peak, or for some other reason. If they were hot, then most likely they have overheated and lost gasses, reducing the capacity.
Have you tried to charge the bot with the 400mA charger? If they do still charge up fine when using it, then your charges set up wrongly, if they dont, then either your charger was set up wrongly and didnt detect the peak, hence overcharging and killing the batteries, or for some reason one or more cells have collapsed, screwing up the packs capacity( cheap batteries will do that too you, I mean, the whole drill is only worth $17, you cant expect too much ).
Also, the drill batteries seem to take a lot more power when being charged more slowly. At about 2.5C, ive had them only take 600mAh, and remember you have to put 140% in to get 100% out. _________________ Jeff Ferrara
fb@ffej.net
ffej.net
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Sun Aug 15, 2004 10:49 pm |
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JohnMuchow
Joined: 25 Aug 2004
Posts: 21
Location: New York City, USA
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quote:
Originally posted by andrew:
I bought a gws charger for 75 and charged them with it and for some reason it does really bad. lasts about 30 secods then slows down to dragging point.
I agree with nightshade....
Peak-detecting chargers can often "false-peak" and shut off too soon leaving your packs only partially charged....especially with certain NiMH packs.
If you have a watt-hour meter (like the Astroflight Whattmeter), you can double-check the pack's charge and if the charger false-peaked, just restart the charge (noting the accumulated AH being put into the pack). For new cells, this can often occur several times during each charge. With our Astro 112D's we sometimes have to restart the charge half a dozen times with NiMH packs....very frustrating. But, with proper forming/conditioning of the cells when new, and regular conditioning of the cells over the years, there should be very little (if any) false-peaking occuring again.
If you don't have a whatt-hour meter, just test the packs after charging with a known current source. You can try a 1/10C rate charge to 110%-120% of capacity, or charge to about 100% at higher current levels. But, monitor the temperature of the cells. The outside of the pack might be just warm, but the cell could be way too hot due to localized heating and the insulation prpvided by wrapping on the cell, pack shrink wrapping, etc. Check the temperature of the cells a couple of minutes (up to 5 miutes for larger cells) after the charge is over. It's a better indicator of what heat is being generated inside the cell during charging. Our cells get abused enough during use, we should be kind to them during charging.
We often find that, after initial low-current forming/conditioning of the cells, that a higher current charge is better at preventing false peaking with problematic packs. Unfortunately, lots of experimentation is the best way to see what works for your packs.
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Wed Aug 25, 2004 3:46 pm |
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