T&T: Deep Cycle Battery Woes Deep Cycle Battery Woes

[Candy Chapman and Gary Bell]

Greg Bradley wrote:

>> I have 6 group 31 batteries cabled together as my house bank.  The
>> stickers on the batteries show they were new around Jan-Mar 2003.  I
>> heard that you could get at least a good 7 years from deep cycle
>> batteries, so I've been reluctant to replace them, but I am thinking I
>> need to now.
>> 
>> I have one battery that is hot to the touch and boils water out of it
>> in less than a week.  It's in the middle of the bank and is the only
>> battery that shows a little corrosion around the terminals.  I realize
>> this is not a good situation but I'm trying to understand it and I'm
>> curious to know if it is an immediate safety issue.  The other
>> batteries are fine.  
>  
>
Richard Cook replied:

Might a reasonable strategy be to remove the bad battery and see how 
things work with the remaining five, before replacing all six?  (And 
check/fix condition of wiring and connections while you're at it)


Gary Bell also replies:

I presume this is a 12 volt bank, with all six group 31's in parallel.  

I am more concerned than Richard appears to be, because I feel that your 
middle battery is hot and boils off its water because it is drawing a 
LOT of current from the remainder of the bank (and your charging system, 
when it is active).  Seems to me most likely it is a low resistance load 
'shorted' across the remaining batteries, because it has warped one or 
more internal plates or spalled off a chunk of lead which fell and 
shorted at least one internal cell, so now it is something like a 10 
volt battery shorted across the remaining 12 volt bank. The remaining 
five batteries have a LOT of current capacity to continuously drive that 
2 volt difference through the ailing battery like a resistance heater 
--  driving off water and causing more plates to deform and/or spall off 
chunks of lead sponge, creating even more of a short.  If I'm right you 
have a runaway meltdown of a battery in your immediate future, 
potentially starting a nasty fire and/or at least rupturing the case and 
testing your battery tray's ability to contain an acid disaster.  In 
this scenario the corroded terminals merely indicate that lots of acid 
fumes spit out of the vents while it was boiling off water, and happened 
to corrode the exposed hot metal terminals nearby.  A much less likely 
cause of your situation (in my never humble view), is a high resistance 
at those terminals from a loose connection, causing resistive heating.  
If that were the case though, I can't see enough current going to the 
higher resistance battery to make the case hot and the electrolyte boil 
off...

RIGHT NOW, NOT LATER, REMOVE THAT TROUBLED BATTERY FROM YOUR BANK!  
Confirm my suspicion with a volt meter on the now isolated battery, it 
would most likely have compromised an entire cell -- or 2 volts worth.  
If you waited for several of the six cells to short the heating would be 
much higher, and just imagine how spectacular a disaster would result 
from a 'crowbar short' of five group 31 batteries!