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Re: (ET) Min. battery voltage

Elie, Larry (L.D.) wrote:
Cells only reverse under load. The load may be other cells, but it is still under load. That said, I have never seen a battery under load at 5.2 V (for a "6V", half a "12V") battery reverse. A battery at only 5 V unloaded, and then seeing a LARGE load can be reversed, but the load will pull it way under 5V. That's only anecdotal; not a guarantee. Think load.
Once you go below 1.66 volts per cell, odd things start to happen in the battery. Current is still flowing through the battery, but that one cell that is "empty" doesn't have anything to give. Since it really is just two lead plates seperated by water, it will drop to 0 volts, then start "charging" backwards. This is cell reversal.
It can happen rather quickly, and when it does your battery pack is now working to charge the reversed cell. Rate of charge depends on the current draw from the pack: If you're drawing 90amps from the pack then that cell will be charging at 90amps.
This will cause a lot of heat to be generated in this cell. The electrolyte will bubble and vent, and things will get fun.
You can reverse a cell easily; just draw a current thru a dead battery. The problem isn't reversing the cell so much as the charge rate. Also what you wind up with is a cell that will need to be "discharged" during charge. Thus while it's neighbors are "charging", it's "discharging" till it hits zero, then it charges and is now behind the other cells in the battery. Which leads to having the other cells at full charge, and this cell being "below full". Which will make it the first cell to die next time you drive, causing it to reverse earlier, etc. 

Down the drain it goes.

Chris
(just dealt with a 50 battery pack of this stuff)



Larry Elie


-----Original Message-----
From: elec-trak-bounces cosmos phy tufts edu
[mailto:elec-trak-bounces cosmos phy tufts edu]On Behalf Of Christopher
Zach
Sent: Monday, June 21, 2004 11:57 AM
To: Markus Lorch
Cc: elec-trak cosmos phy tufts edu
Subject: Re: (ET) Min. battery voltage




I have read several posts and online documentation on the lowest
save voltage to which a lead acid deep cycle battery may be discharged
and found values around 10.5V / 80% (for a 12V bat).
I think this is consistent with what Chris' recent battery monitor
looks out for. 


Yep.



The question I have is: Is this the lowest voltage under load or the
voltage immediately after load is removed? I read somewhere that to measure battery voltage one should let the battery sit for 2 days without 
charging/discharging to get an accurate reading.
Always check voltage while under load. That's when you can get a reasonable reading. If you're dropping below 10.5 volts under load, you are running the risk of reversing a cell. 



Anyhow, I have one (12V) battery that seems to be not so good, it reaches
low
voltages before the others do and when I keep mowing after I see it go to
10V it is getting very hot and drops to as low as 6V when you start the
mower


That is a battery that is reversing. Consider it "toast"



or start driving. If I stop the current flow it returns to about 11V or so
almost immediately. So when do I reach 80% depth of discharge, when the
battery first dips to 10.5V or when it holds 10.5V after load is removed?
When it dips to 10.5, though my guess is it will fall thru to 6 really fast after that. 



Also at what voltage do I have to fear cell reversal? the other two 12V
sections
in my pack do not dip below 12V when this troublemaker is already below 
10V,
so I
am concerned about them forcing current through these weak/discharged 
cells.



10.6 volts.

Chris





Thanks
Markus


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