Re: battery theory (NIL Audi content)

["Christopher P. Locke" <d.locke@snet.net>]

Wolff wrote:
> 
> All fine untill the little battery is overcharged and vents explosive
> gas into the boats interior. This could be bad. I have seen like sized
> systems do this. Best solution for a multi-battery setup is separate
> regulators.
> Only my $.02, but I don't want to see any battery foo.
> Wolff

The majority of the battery chargers made today (alternators on engines
included) are the voltage regulated type.  Any type of larger battery
(except Ni-Cad) are self regulating.  As the charge voltage rises, the
internal resistance of the cell rises, limiting it's own current.  If
the charger is worth it's salt, it's a voltage limiting charger, not a
constant current charger.  The problem that would be similar to your
note is a thing called thermal runaway.  As a sealed lead acid battery
heats up (i mean like in high temp enclosures) the internal resistance
lowers, causing the charger (regardless of the type) to allow more
current to enter the battery (unless the charger is "temperature
compensating") charging the battery more.  Charging a battery creates
heat, causing the internal resistnace to go down again, and you can see
where this is going.  Sealed lead acid batteries have a pressure relief
valve that takes a bit of pressure to open (i was just involved in
troubleshooting a large UPS system with sealed lead acid batteries. 
When i arrived on scene, the batteries were already 165 farenheit, and
only 2 of 60 had vented anything)  They are made this way because there
is a limited amount of electrolyte in the jar, and if you take some
away, you're also taking away capacity.  The telephone industry has been
putting different sized batteries in parallel strings for over 125 years
now, with no problems.  When was the last time you heard a news story
about a telephone office blowing up due to explosive gasses?  The
batteries we have in the offices are the size of college refrigerators,
and there are sometimes HUNDREDS of batteries in one room.  It's not as
bad as it used to be, battery and charger technologies as of late (last
25 years) have come a long way, and are much safer than they used to
be.  
chris locke 86 4kcstq