Re: (ET) Electrolyte circulation

["David Roden (Akron OH USA)" <roden ald net>]

I remember quite a few experiments with electrolyte circulation in the 70s 
and 80s.  It helps because at high discharge currents the chemical 
reaction 
takes place right at the surface of the battery plates and tends to 
deplete 
the electrolyte concentration there.  Forced circulation keeps fresh 
reactants where they need to be.  

Most attempts used some kind of mechanical pump.  A couple of intruiging 
variations were tried.  One scheme bubbled air into the bottom of the 
cells. 

Another had the cell interconnects in the bottom; they were deliberately 
undersized so as to generate heat, which was supposed to provide some 
electrolyte circulation through convection currents.  This was in one of 
Bob 
Aronson's versions of his so-called "lead cobalt" batteries.

I think the problems (with mechanical circulation) were mainly the 
complexity and cost.  Also, today many of the problems of diffusion of 
reactants have been greatly ameliorated by AGM (absorbed glass mat) 
batteries, which don't have any free electrolyte, and which usually have 
their plates very close together and under pressure.  

> Advantages
> of recirculation could include keeping all batteries at the same
> temperature by heating or cooling the electrolyte and
> filtering out any precipitate from the solution and
> probably reducing the frequency of equalizing charges.

Temperature regulation is probably more easily carried out with liquid 
jackets, as is done on some of the high-performance nicad and nickel metal 
hydride modules used in prototype EVs.   

Filtering precipitate is an interesting idea and possibly a useful one.  

I don't think recirculation would help much in reducing equalization 
frequency - - you can't really share electrolyte among cells, and if you 
did 
(say when the battery was idle) that wouldn't equalize the state of charge 
on the plates, only in the electrolyte.  

> In an emergency, a quick "charge" could
> be had by replacing the electrolyte (shortening battery
> life, though).

Same problem as immediately above.  It's worse than just shortening the 
life; it ruins the battery.   

The electrolyte concentration of a battery is in balance with the amount 
of 
active material in the plates.  When you over-concentrate electrolyte in a 
discharged battery, you force the plates to try to match the new 
"capacity." 

They over-discharge and are permantly damaged.  Not good.

BTW, you make a battery that lasts a long time by doing the opposite -- 
you 
deliberately make the electrolyte weak ("starved electrolyte").  This 
reduces the usable capacity, so that when the user runs it all the way 
down 
it's like only discharging a normal battery to 80%.   

> A low tech solution would
> be letting the batteries charge and warm up in a warm
> garage or using battery heaters (or heat from the 
> elec-trac charger) to warm the batteries
> while charging. 

An excellent solution!  Batteries can hold their heat for a long time 
because of their mass, so insulation helps if you operate them daily or 
nearly so. Their own internal resistance keeps them warm when charging and 
discharging.  

However, if they sit for several days between cycles, battery heaters can 
be 
very helpful.   

A warm garage is obviously helpful but it takes a fair bit of energy.  
It's 
more efficient to deliver the heat right to where it's needed, using 
battery 
heaters.  Several types have been used, the most common being pads under 
the 
batteries.  I rather expect that some of the electrolyte circulation 
experiments included some temperature regulation, though I don't recall 
for 
sure.   

A liquid jacket might again be very useful, as it is for cooling nickel-
based batteries (above), which love cold but don't like heat.   

I heard of an interesting scheme using coconut oil poured into EV battery 
boxes -- it changes state from solid to liquid at just over 80 deg F, 
IIRC, 
which happens to be almost the ideal temperature for lead batteries.  

> A very high rate of charge might be accepted
> by the batteries if the electrolyte is kept at an
> ideal temperature. 

Actually, batteries can be charged about as fast as they can be 
discharged, 
until they reach 75-80% charged (then you have to cut back).  If they 
don't 
need cooling during discharge, they don't need it while bulk-charging.   

For extremely high rate charging (in the thousands of amperes), 
electrolyte 
cooling might be useful.  But most people who are experimenting with high- 
rate charging are using AGM batteries (as above), which can accept and 
deliver enormous currents and don't have any free electrolyte to 
circulate.  


> The specific gravity could be monitored
> for all the batteries on a continuous basis for very
> precise state of charge monitoring.

Very interesting!  Measuring state of charge during use is a long-standing 
problem with batteries.  




David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1970 GE Elec-trak E15 36vdc
1974 Avco New Idea 36vdc
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