Re: A couple of questions...

[Subscuba@aol.com]

Hello Sean,

Here is my thoughts and a little background on the infamous battery problem.

1) I do not have a completly satisfactory solution to the battery problem. I
think ultimately fuel cells will be the answer. Very high energy density.
Third world countries who buy military submarines use diesel/electric, nuclear
or fuel cells as the power source. More attention is being given to fuel cells
because of long term costs, safety and reliability. Even the nukes carry
batteries. I would keep my eye on fuel cell development, especially for the
electric car.

2) As has been mentioned elsewhere you will probably end up with the lead acid
because of cost considerations. I don't like the discharge characteristics of
lead acid especially when compared with Ni-Cads but the lead acid battery
still offers the biggest bang for the buck. They are also easier to reliably
charge. They do not have a flat discharge voltage and the lead acid cell's
internal impedance and terminal voltage both degrade with discharge and
temperature. That is why I designed my motor controller with a PWM controller
to remove as much capacity as possible from the batteries I have initially
selected. This enables me to completely discharge the batteries and degrade
the life due to potential reverse charging and 100% depth of discharge. Both
good and bad. I can make the choice when I operate the sub.

If you crawl the web you can find information about the various lead acid
batteries. I have the following suggestions.

a) Do not buy starting or deep cycle/starting batteries. Buy deep cycle
batteries specified for deep cycle operation. There is two reasons for this,
the first is that the deep cycle will withstand many more cycles of deep
discharge and because of plate constrution has more capacity. Don't expect to
get anywhere near the capacity the battery vendor states. There is a marketing
influence on the capacity ratings which are also a function of temperature and
discharge rate as well as the vendors dreams. Avoid the so called maintanence
free batteries, they are simply conventional batteries with excess
electrolyte. Also avoid the "Gel Cells". The material used to stabilize the
electrolyte degrades the battery performance.

b) Spend money on your charger, do not buy a cheap charger, you will spend a
lot of money on the batteries and the battery life will be influenced by the
charger. The newer chargers on the market have "Step Charging" which are
preferred to the cheaper chargers and extend your battery life. The best thing
is to find what vendor makes your batteries and follow his recommedations in
detail. Do not charge your batteries in a closed compartment. Ventilate during
charging and use catalyst caps during charging and discharging. If you use
"Gel Cells" or AGM batteries stay away from "Equalization Charge".
Equalization charge attempts to solve electrolyte stratification with flooded
cells and is simply an overcharge which stirs up the electrolyte. Again you
can find these chargers on the Web.

That leaves the question of what batteries to use. The answers I have depend
on your application. The use of oil  and hydrogen/oxygen evolution ( read that
as "BOOM!") remains the two biggest issues.

I was involved in developing an underwater light at about a 1KW level and we
used wet cell Ni-Cads which worked quite well. I designed the charger and you
normally had to remove the batteries from the housing to charge. The end user
found a way to charge the batteries without taking them out of the housing. At
a depth of about 110 feet he switched the unit on and it promptly blew up,
fortunately without causing injury to this clever fellow. We then added
catalytic converters and made sure the unit was charged with ventilation.
After that we didn't hear of any more problems. I hope because the unit was OK
and not a missing diver. This points out the problem with a hydrogen/air
mixture is real. Also the bigger the battery box the bigger the "BOOM". 

A lot of people have been using oil insulated batteries for some time. Harbor
Branch Oceanographic Institution (HBOI) uses oil to cover and insulate
batteries on several of their subs and uses vented cells on one in which the
batteries are at 1 atmosphere. The oil  that covers the batteries transmit the
hydrostatic pressures and insulates. It also gets contaminated with battery
acid during charging and becomes , in a word, yucky and messy. It would look
like this may be a solution for small submersibles like my wet sub except for
the following problem. What happens when the sub is inverted? Assuming you
have solved the problem of what to do with oil and electrolyte in the inverted
position, the oil gets on the battery plates and essentially removes these
plate areas from active service and the battery capacity goes down. If you
have a dry sub and do not have excessive angles of roll or pitch, the oil
covered batteries work. The lead acid battery chemistry will perform under a
great deal of pressure as evidenced by the ALVIN which has used this system.
You can find on the web a discussion of the battery investigations for the
ALVIN at Woods Hole. At last look they still use lead acid.

What has worked for some people is to use a conventional deep cycle battery
with catalytic caps on the vent holes. These are devices which convert
hydrogen and oxygen back into water and greatly reduce the evolution of
hydrogen during charging and discharging. They don't completely solve
electrolyte spilliage and hydrogen evolution but a little acid and hydrogen
around doesn't hurt anything does it? My battery box is located between my
legs so I am avoiding this technique. If your battery compartment is pressure
compensated you could use nitrogen as the gas source and reduce the explosive
problem futher. Actually use of nitrogen and the catalytic caps may be safe
but I would worry about the air spaces internal to the battery.

After all this pontificating, I recommend the newer Absorbed Glass Mat (AGM)
batteries. They are availiable from at least two vendors , Hawker Energy and
Concord. I think Powersonics is also a vendor. They are basically the same
technology as the common Gates cells and are sealed lead acid batteries that
are advertised as solving the hydrogen evolution problem, maybe. They are
availiable in group 27 size and larger and can be operated in any position. As
I understand it there is no free electrolyte internal to the battery
construction so operation upside down is fine. You might also consider calling
Hawker Energy at 816-429-2165 for their application manual.

Now the down side of the AGM batteries. One they are more expensive. The ones
I am planning to use, group 27, 105 Amp-Hr, are $138 plus shipping which is
more than the conventional vented cells. 

Two, and more important, is that the batteries have to withstand the ambient
pressure if you have a pressure compensated system as I do. The battery
information I have been able to get indicates that the physical constrution of
these batteries can take 3 or 4 atmospheres of external pressure. The recently
advertised "Hydrospeeder" has a  clever pressure compensation system with
Hawker Genesis batteries, I think. This would limit use to about 100 feet as
stated on page 32 of the 1996 Hawker Genesis Application Manual. I haven't
confirmed that the Hydrospeeder actually uses these batteries so take the
statement above with a grain of salt. I am not intending to use the Genesis
batteries since I don't think they have a group 27 battery. I found on the
internet a source called "Battery Specialists" or something like that located
in Miami that I intend to buy my batteries from at $138 each. I require 3
each.

I hope this helps and is the result of my investigations in batteries for use
in my sub. I have to go to a Maya symposium so this is all for now. I hope I
didn't make too many grammer and spelling errors. I can answer more questions
if you have any.

Any comments or suggestions from anybody else? 

Ken Martindale

PS  If you write back please don't include the original messge, sometimes I
remember the text and I keep a record. 
 Also, I wear Nikes, they taste better!