Re: (ET) Hydrometer caps

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

On 18 Jun 2003 at 22:57, Bill wrote:

>     I am wondering if there is such a thing available as screw-in
> hydrometer caps having  the green eye indicator. I've never seen one.

Me neither.

> Also, what's the secret on those batteries never needing water added?
> Are they sealed airtight? 

They are sort-of sealed.  The term for this type of battery is "valve-
regulated recombinant" (VRR).  They have a small amount of platinum 
catalyst 
in each cell, which helps recombine the hydrogen and oxygen into water at 
relatively low temperature.  

This process works best under some pressure, so they have a pressure valve 
that allows a bit of gas pressure to build up in the cells.  You have to 
be 
more careful with charging so you don't generate too much gas though; 
otherwise the vent will release the gases to prevent damage.  When that 
happens, the recombination is spoiled and you lose water from the battery 
(which can't easily be replaced).

A little trivia.  The above accounts for "valve" and "recombinant," but 
what 
about "regulated"?  It turns out that the VRR design evolved from a 
prototype battery which used the gas pressure to control (regulate) the 
charger.  This apparently proved to be too expensive to be practical, but 
the other elements of the design live on in sealed-gel and AGM (absorbed 
glass mat) batteries.

> Can our regular 6 volt batteries be sealed so
> they would never need water added?

Not exactly, but ...

You can get gadgets called Hydrocaps which perform some recombination and 
significantly reduce the need for watering.  They don't fully seal the 
battery though.  They have a finite lifetime (sorry, I don't know how long 
that is).  They're used now and then in alt energy systems 
(solar/wind/water).  The problem with them for our tractors is that they 
add 
something like 2-3 inches to the height of the batteries, so you'd need to 
make appreciable modifications to the tractor.


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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