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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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Batteries and Packs
Original by Spockie-Tech here
http://www.robowars.org/forum/viewtopic.php?p=1948#1948
According to all Nicd/NiMh cell manufactuers, *any* soldering of cells is too hot. They strongly reccomend against any soldering to cells. the top part (+) of the cells have a plastic pressure-seal around the edge that allows the cells to vent if the internal pressure gets too much under heavy charging or discharging. (Thats why they make a mess when abused). Any temperature high enough for soldering is higher than the melting point of this plastic seal according to the specs, thats why all factory-packs have spot-welded tabs, the split-second tiny pulse of heat dissipates before it can warm up the whole cap enough to hurt the seal.
Having said that, a lot of people (and pack manufacturers) do go and solder their packs anyway, and if done by a skilled soldering person with a high-heat-capacity (not temperature, heat storage) iron so it can be done quickly, the seal *should* survive a second or so of heat. then you get the advantages of the lower resistance, mechanically strong solder joint over the small-contact patch spot welds.
If you are getting the whole cells near soldering-hot, chances are good that the seal wont be be very happy with you. it maybe ok, it might have totally melted, most likely it will have deformed somewhat. If it has melted "open", then the gases produced during the cells normal operation will be able to escape (rather than recombining) and the cell will lose capacity after a much shorter period of time than it would have otherwise, so the damage will not be immediately evident.
If the seal has melted-shut, then as long as it doesnt short out the top to the case (which will kill the cell instantly), then the cells will likely be ok for a while, but if worked hard and the gas-pressure builds up in them, they will go bang, rather than just vent as an undamaged cell would.
So how much heat is OK ? Ideally, None, in practise, the less the better.
To solder up battery packs successfully, get a powerful, high wattage, temperature-controlled (so it doesnt get too hot if you leave it without using it for a few minutes) soldering iron, with a good heavy tip (for heat storage, like a heat capcitor), and practise on a bunch of old useless cells until you can make a good solder joint without cooking the whole cell. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Wed Sep 01, 2004 11:01 pm |
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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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Original by Kkeerroo in the same thread as above
Aocording the bloke who wrote this site bellow you may have damaged your cells by charging them at too high a current.
[url]http://www.angelfire.com/electronic/hayles/charge1.html [/url]
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. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Wed Sep 01, 2004 11:03 pm |
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