Re: [PSUBS-MAILIST] Battery charging

[Alex <spm2@nomad.ignorelist.com>]

Lead acid cells put out a little over 2 volts per cell. Don't get that
confused! A 12v battery has 6 cells, and is fully charged when it sits at 12.6
to 12.8 volts, for flooded lead acid. Other sealed types put out a bit lower
voltage, like 12.5v when charged, which is some kinds need a specific charger.

Redundancy is a good idea but using 4 sets of batteries separately is going to
be difficult to keep track of one sets gets used more and goes dead first etc.
isn't it?

I would make two main separate battery sets (A and B) with the power wires
coming inside the hull in to two separate switch boxes (A and B), and then a
 small C set of batteries for emergency power, which isn't normally used and is
kept fully charged. Your 4 motors (motor A, B, C and D) will half run off the A
switch box and the other half on B. Your A and C motors run off switch box A and
your B and D motors will run off switch box B. Then you can have a FUSED and
switched circuit that connects your A and B battery set together in PARALLEL so
that they discharge at the same rate, and can be charged together.

For example, in box A you would wire it like this: The wires come from battery
bank A to a switch which connects to the power rail in the box A. The power
rail is connected to a switch and fuse which connects it to the switch and fuse
in the power rail on switch B, which is wired the same way. Two switches and
smaller fuses connect motor controllers A and C. Switch box B and its power
rail are connected the same way, but to box A, and motors B and D. The motor
controller switches are all double poll switches. Each motor controller should
have a light to show when it is getting power.

Now, the 3rd emergency battery has its own 4 fuses and switches which are kept
turned off at all times. The first two switches connect to the off positions
connectors on the 2 motor controller switches on power box A, which connect
motor controllers A and C. The second switch connects to box B, and connects
motor controllers B and D. Because it is wired to the off positions on the
motor controller switches, the emergency battery can only be used to run a
motor controller when its switch is in the off position, disconnected from its
main power rail.

Now you have a fail safe system, here are some things that are prevented from
going wrong:

1) Each motor controller can be turned on and off by its switch on the power
box, as the switches or emergency bank C are kept off, or from main to
emergency power if the corresponding switch on bank C is turned on.

2) If a motor controller shorts out, it's fuse will blow, or it can be turned
off by the switch.

3) If a switch box shorts out (which is very rare!), the fuse on the connecting
wire will blow and the other switch box will continue to operate the two motors
connected to it. The remaining two motors will probably still be able to
be switched to the emergency bank C.

4) If a battery bank fails, it can be turned off with the main switch. The
circuit will still get power from the connecting wire from the other bank. If a
battery bank shorts out, blowing the fuse on the connecting wire, the two
motors on the other side will continue to work. The battery switch on the
shorted bank can be turned off, the fuse replaced, and all 4 motors will work
again. The motors can run on emergency power while the fuse is being replaced.

5) If the operator lets the batteries run dead, the emergency bank C can be
used.

6) All circuits are isolated from each other by fuses, no single short in any
part of the system can interrupt power to all 4 motors.

7) In any panic situation any motor controller can switch to emergency bank C
in a few seconds by flipping two switches, by turning it's main switch to
OFF/emergency and turning on the corresponding switch on the emergency bank C.

The emergency bank C is isolated from the main battery banks and can never be
switched to connect to the main power bus A or B.

I've always wondered how airplane power buses worked. You know they have an
emergency battery and turbine in case of power loss.

Alex

On 17-Sep-2009 ShellyDalg@aol.com wrote:
> Hi Dan. Yes, The trojans used for golf carts are regular lead/acid wet  
> batteries. A step up is the AGM batteries which are still wet batteries but
> the 
>  acid is held in place within an Absorbed Glass Mat.
> Mostly done so they don't spill.
> The trojans have thicker lead plates so they can accept more cycles without 
>  the plates going to hell. Typically a golf cart type battery can be 
> drained  80% max. 
> Bang for the buck makes them pretty economical. Because I need 14  
> batteries, the cost is going to be pretty high. 
> It seems that the 6 volt batteries have more available amps. I think that's 
>  due to the plate size as well.
> A typical battery cell puts out 1-1/2 volts. Everything from an AA size  
> battery to a big Semi-truck battery are like that. 
> The amps available are determined by the size of the cell.
> A 12 volt battery has smaller cells than a comparable sized 6 volt battery. 
> I need to use 12 volt batteries because I'm using 4 motors and want to keep 
>  each motor and set of 3 batteries separate. This is just a safety issue so 
> I can  have a measure of redundancy available.
> If a motor or controller or whatever blows, the breakdown only affects that 
>  one motor circuit.
> I have the batteries inside the sub so as a last resort, I can jump a  
> battery bank over to another circuit.
> The individual banks of 3 batteries each are contained in a stainless box  
> with a lid and gasket. Not "water proof" but sealed up pretty good. 
> Charging after a dive requires opening the lids and using a fan to vent the 
>  space during charging.
> I'll need a gas detector in the sub too, to monitor any build-up  during 
> use of the sub as well as during charging. Batteries off-gas during  use as 
> well as during charging. The stainless boxes are a snug fit so the volume  of
> space available inside is minimal, hoping to reduce the danger of an  
> explosion. 
> The golf cart chargers I'm looking at charge at 36 volts/20 amps for the  
> 110 volt model but a 220 volt charger will do the job a lot quicker. The  
> chargers also have the circuits to adjust the charge rate automatically as
> the  
> batteries reach full charge.
> My worry there is that not all harbors have 220 available at the dock. My  
> local harbors have 220 but not sure if other smaller harbors do. Maybe will 
> need  more than one charger. 
> I've seen some info on the li-ion batteries in the "automotive" size but  
> they run $1200 each and are prone to exploding if the draw down gets too 
> high.  With the small space in a psub, that would be deadly!
> Frank D. 

-- 
Mailed with XFmail on 17-Sep-2009.
God saved Noah, but Noah had to build an ark!



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