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Thermal Runaway  
User currently offlineDiego From Italy, joined Apr 2001, 135 posts, RR: 0
Posted (14 years 6 months 2 weeks 3 days 19 hours ago) and read 2075 times:

Hi folks, is there any body out there among those mech wizards willing to explain to me what is a thermal runaway, what action are you expected to take if your battery temperature gets to high, if any other than to isolate the affected battery and what in the world the reset button of a starter generator does, (I read it resets an overvoltage circuit but I don't have any clue when that tripped)?

3 replies: All unread, jump to last
User currently offlineOld Virgin From , joined Dec 1969, posts, RR:
Reply 1, posted (14 years 6 months 2 weeks 1 day 7 hours ago) and read 1983 times:

Hi Diego

Thermal runaway is not nice!! It is caused, primarily, by heat build up in a battery during charging - usually due to over-charging. The effect of the temperature is to increase the internal resistance of the battery to the charging current which in turn leads to even more heat, then more resistance, then more heat etc., etc. and voila ...thermal runaway - very nasty, exploding batteries, nasty smells and so on. Avoid, if possible, by isolating electrically which will cut off the charging current. Allow to cool down and FIND OUT WHY!!  Smile

User currently offlineL-188 From United States of America, joined Jul 1999, 30408 posts, RR: 57
Reply 2, posted (14 years 6 months 2 weeks 1 day 6 hours ago) and read 1976 times:

Lets see if I remember this right.

Correct me if I am wrong, it has been a year and a half since I had the DC electronics class.

On a ni-cad battery, what has happened is that the Celophane that seprates the plates in the battery have litterally melted, for the reasons that Old Virgin mentioned. That celophane acts as a barrier between the plates in the battery and the "electrolyte" in the batteries. Because that barrier is gone, the plates are free to oxdize(I know that isn't the right term. It is 2AM here OK) uncontrollably.

What do you get when you allow something to oxidize uncontrollably. Sometimes rust, but in this case your battery is in the scientific venacular......ON FIRE!!!

Like I said, I am trying to see if I remember what they where teaching a year and half ago, so if I am out in left field, don't torpedo me too harshly.

User currently offlineFr8tdog From United States of America, joined Feb 2000, 120 posts, RR: 0
Reply 3, posted (14 years 6 months 1 week 4 days 14 hours ago) and read 1932 times:

NiCads will have a thermal runaway with a rapid discharge also, I.E. during a start, with a turbo prop.
The Sf340 will draw over 2000amps during initial rotation then down to 800amps once the starter starts to spool up and gain momentum.

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