(ET) Why lead acid batteries need high current charge.

[Markus Lorch <ev lorch net>]

Hi all,

David has mentioned that lead acid batteries (occassionally) need a high 
current
charge. I have come accross an explanation on why this is the case that 
I find
reasonable and from a trustworthy source. The original author is Dr. E. 
Meissner of
Varta Battery Research and Development (part of the Exide group). The 
original
article is referenced here 
http://cat.inist.fr/?aModele=afficheN&cpsidt=1835178
.
I will try to put what I understood into simple terms here:

The lead oxide electrode used in (flooded?) lead acid batteries is must 
have a large
surface area to provide a high electrical potential. The outside parts 
of the electrode
can be seen as balls. These balls are connected to eachother. The 
stronger these
connections are the lower the internal resistance of the battery and the 
higher current
it can deliver.

Now when a battery is discharged with a low current the lead from the 
outer balls is
dissolved and when charged with a low current then the lead settles on 
the outer balls
again. The connections remain the way they were originally and all is fine.

When discharged with a high current the connections between the balls 
are also dissolved
or weakened. When charged again with a low current the connections are 
not rebuild but
only the balls. Hence we end up with a heap of (larger) balls that is 
connected to each
other with higher resistance connections than they were originally. Also 
if the connections
were dissolved completely then they may not be rebuilt leading to less 
(but larger) balls which
in turn results in a smaller surface area of the electrode.

When charged with a large current however the connections are also 
rebuilt. Hence lead
acid batteries should be charged with about the same current as they are 
discharged.
The ET discharges the batteries with typically 25 to 125A, the built in 
charger provides
about 25A max. .  In part because of this fork lift batteries have 
chargers that pump the
electrodes with 50 - 200A into the batteries (another reason is the 
reduced time).

The good thing about this is that the condition of a deformed electrode 
can be reversed with
appropriate charging. Other people say they had very good results with 
discharging their
batteries at a low rate to 5.25V per 6V block and then charging them 
with a high current.
Doing this may bring back some of the capacity.

I had done this on my tractor to some old batteries I got for free by 
letting the motor run
with no gear engaged for quite some time. (cruise control). And then 
charge it with
the built in charge plus and additional 10A charger to pump as much 
current in there as
I could. I cant say how much it helped and how much just the simple 
exercise of discharging
and charging helped.  For my electric car I have a 40A charger, which is 
also not as much as I
would like it to be.. Some day I will get my hands on an old 100A 
charger from a fork lift or so.

I also don't know and havent found literature that discusses if and how 
this applies to valve
regulated and gel cells.

Take care and happy trakking.

Markus