Quote:
Originally Posted by neiloliver
If people are charging at >4A then there is a name for that.. its called natural selection 
I too remember the days of the N-1300SCR and KR-1800SCE etc... and yes we managed five minutes.. that last lap was somewhat fingers crossed though 
Technical bit.. You know I mentioned the 1:1 match.. do you know what that is? I will explain if anyone is interested... All NiCd and NiMH cells have what is called an electrode match, it is the ratio between positive and negative electrodes. The negative electrode is ALWAYS larger than the positive. This is because a charging cell gives off oxygen gas from the positive electrode and this is recombined in the negative and keeps the cell in equilibrium (this is why you can trickle charge batteries in cordless screwdrivers, emergency lighting etc).. The problem with the match is that the capacity of the cell is driver by the shorter electrode, so a cell with long life that you can trickle charge needs a much longer negative electrode and this means low capacity. You can increase the capacity by having the electrodes similar lengths but this reduces the life and the cells ability to withstand overcharge abuse.. it also reduces the cycle life as the longer negative electrode is used as a reserve as it oxidises due to the oxygen in the system.
The faster the charge the more gas is generated, and this has to be combined.. if gas is generated faster than it can be combined then the cell pressure builds up and the cell has to vent. Venting is a safety measure and I am sure we have all heard it on old cells while charging. (a little fizzing sound) the problem with venting is that some electrolyte is lost and this dries out the cell, so a cell that has vented will quickly deteriorate. Of course if you are charging a fully charged cell at high rate then the gas cannot recombine fast enough and the cell vents... or explodes if there is an issue with the vent (sealed shut with solder, bent by abuse, blocked etc etc..).
Lastly, our hobby chargers terminate on what is called 'negative delta V' which is the roll over of voltage when the cell is fully charged, this phenomenon is caused by the recombination reaction and is handy because we can use it to stop our charging.. except that it does not happen instantaneously when repeaking a fully charged cell so you are pushing a fully charged cell further into overcharge by re-peaking. Couple this with the fact that the charger is not looking at each cell, it is looking at a six cell string and you have the problem of matching, where one lower capacity cell can be well into overcharge with it's pressure building up but your charger cannot detect it because the other five cells are masking the fall in voltage...
so..
only charge at 4A max
use a low -dV threshold (3mV/cell)
use a Max T temperature backup (45°C)
do not repeak
Always allow cells to cool before recharging
balance cells as often as you can on a discharger with a 0.9V cut-off |
For anyone wanting good advice on charging, Neil seems to speak a lot of sense. I've used this regime and have IB cells still going strong (lost a bit of volts, but little capacity) after 18 months Club and National racing. This is from the IB site:
Charge the battery with quality charger, and max. current 3~4A.
Charging cut-off setting : Peak voltage = 5mV per cell, Temperature <55ºC,
and best with timer.
[Example on Peak voltage calculation : 6-cell pack = 5mV x 6 = 0.03V]
The battery can offer high current discharge, and be good for the level of
10A+ (with Tamiya plug).
If you expect to leave the battery idle for >2 weeks, charge it for 1/3 capacity
(say, 3A for 20 minutes)
Avoid to store the battery in hot & humid environment. Best temperature
would be around 20~25ºC
What's interesting is the recommended charge rate, and anticipated discharge rate!
Quote:
Originally Posted by Ole C
If that thing had blown up in my face I'm sure I would have been blind today. Fingers would have been lost if it had gone of when I held it.
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Good to get your direct experience, Ole. I think we know what can happen, but the truth is that in all these incidents with cells over the many years, I know of only one where there was an injury. Part of the problem (we leave cells to charge on their own) is also part of the reason that injuries are mercifully few.
Would anyone here give up their IBs if asked by their Club, or BRCA Section?