Conference vmszoo::rc

If Gel-Cells are the best answer then
1.  Where do you buy these things?
2.  Do you charge them with car battery charges (that's what I
    use on the motorcycle battery)?
3.  Do they give as many amp hours as the motorcycle batteries?

              _!_
Bye        ----O----
Kay R. Fisher / \
 
==============================================================

    
    	Kay,
    
    		The motorcycle battery on my motorcycle does not have
    	vent holes in the cell caps, the only venting is therefore done
    	through the overflow port. If you drilled a hole and ported
    	this to the outside you should not have an explosive build up
    	of hydrogen when charging. However you now have a problem that
    	tilting your box when carring or transporting will cause acid
    	to leak out on either you or in your car or wherever. Its a
    	no win situation with lead acid batteries.
    		The gell cell type's come in different amp hour ratings
    	just as do the lead acid types. You will have to shop around
    	the find the one that suits your needs. One thing I will say
    	is that I bought the Mafrec (calcium cadmium) battery that
    	Tower Hobbies sells. This battery is a 12v 6 amp hour battery
    	that requires a 500 milliamp charger. They, Tower Hobbies, sells
    	a charger for it for $8 or you can buy one like I did at Radio
    	Shack. I have not had mine that long, but with a full charge
    	I was able to use it to start my plane for 5 runs using a heavy
    	duty starter and could not notice any power decrease. The safe
    	feeling that I have around this battery was well worth the $47
    	I paid for it.
    
    				Michael Gallant
    

My impression from reading the battery article was that there is no
way to safely vent the battery compartment, short of charging in
free air.  And even then only in an area with sufficient passage of
air.  Therefore, if I were to use a lead acid battery, I would make
the top of the battery removable for charging; in fact, I would do
this for a sealed battery too. 

My field box leaves the top of the battery exposed, but I use a dry
cell in that compartment, since I just use the battery for the glow
plug.  And I'll get rid of that when I can afford one of those nicad
glowplug things. 

    I had to do some research on battery's once and I may be able to
    enlighten you a bit. For firing glow plugs you need a battery capable
    of repeated heavy current discharge cycles. Lead acid battery's
    are ideally suited for this but they do have their drawbacks.
    Gel cells will deliver the kind of current you need but they are
    NOT intended for repeated deep cycling applications as what you
    need for your application. It will shorten the life of the battery-
    Gel cells work best under light to medium current draw applications.
    If your looking for high current discharge battery's with ease of
    portability consider Nicads. They can dump lots of current, are
    small and light for their performance,and are rechargable. Plus
    they can take it!! 

Is the approx 3A draw of a glow plug really that much of a load for a gel cell?

    
    	It's simply a matter of amp hour rating. The calcuim cadmium
    rechargable I have is rated at 6 amp hours. I don't know whether
    or not this battery is a quote, gell cell, or more like a nicad,
    however since a glow plug is only run for about 30 seconds at a
    time I could do it (2*60*6)=720 times an outing. I would say that 
    has to be one long afternoon.
    
    				Michael Gallant
    

I don't run the glow plugs off the motorcycle battery.  I run them off 
Ni-Starters or on-board ignitions (portable Ni-Starters).  Tho I have
on occasion charged my Ni-Starter from the motorcycle battery.

Problem is 1. Saftey of batteries, 2. I charge it with a 600ma charger
and forget it on.  Next thing you know I've boiled alot of water out
and got it all over my work bench.  Think I will order a Gell-Cell (for
spinning starters).  Do they have as high a AmpHour rating as the typical
motorcycle batteries?

              _!_
Bye        ----O----
Kay R. Fisher / \
 
==============================================================

        The simple solution would be to replace your 600ma charger for
        your motorcycle battery with a 50ma charger. This should give you
        something that can be left on indefinitely. Switching from the wet
        cell to a gell cell or other sealed battery will not solve your
        overcharging problems and will still destroy the battery. 
        
        Tower Hobbies has a 12V 6AH cell that should be perfect for most
        modelling needs. I just got an electronics catalog that lists 12
        gell cells from 1.2AH to 80AH! You could probably use the latter
        as a car battery. 

>        The simple solution would be to replace your 600ma charger for
>        your motorcycle battery with a 50ma charger. This should give you
>        something that can be left on indefinitely. Switching from the wet
>        cell to a Gell-Cell or other sealed battery will not solve your
>        overcharging problems and will still destroy the battery. 

Sounds like good advice - but it brings up more questions.

1.  I ordered a sealed unit.
2.  How long would you have to trickle charge at 50ma to be sure that
    you have fully charged?
3.  How do I check the charge percentage?  If it was a nicad then the
    voltage tells a lot.  If it was a lead acid then I guess a hydrometer
    (if I had one) might tell.  What about one of these sealed things?
4.  Is this sealed thing a Gell-Cell?
5.  If I did charge at 600ma (like everybody does cause that's what Tower
    and Hobby Shack sell with their batteries) then how long should you
    charge?

Looks like I'll have to stop by Radio Shack and get a resistor to put
in series with my 600ma charger.

              _!_
Bye        ----O----
Kay R. Fisher / \
 
==============================================================

        1) Great, they don't (usually) leak any acid.
        
        2) For any battery, charge_rate x charge_time = 1.6 x Ah. Thus the
        standard charge for a pack is C/10 at 16 hours. If you charge at
        1/10 the 'standard' rate (C/100), it will take 10 times as long
        (6.7 days) to get a full charge. Note that you probably will never
        run your gell cell down all the way, so all you need to do is put
        in about 1.6 times what you used. 
        
        3) Use a voltmeter just like a nicad. Unloaded a gell cell should
        float around 13.5 to 14.0v. Check your cell when new after a full
        charge and use that as a guide. At the C/100 rate, the charger can
        be left on indefinitely with no harm to the gell cell. 
        
        4) Yes. The old technology is sealed lead acid. The Tower battery
        is the newer (more expensive) lead calcium design, but to us it
        shouldn't make any diference. 
        
        5) See (2). The default is still 16 hours. Avoid anything over 2
        days at that rate. 
        
        A neat trick in building a charger for gell cells is to use a
        transformer that is just barely above the cells full charge
        voltage, and then you only need a very small resistor to drop the
        current down. As the cell nears full charge, its voltage
        approaches the supply voltage, and the charge current drops off! A
        self regulating charger! 
        
        Building your own charger is simple. Find a surplus transformer
        that gives sufficient current and voltage. Old calculator power
        supplies are my favorite. Add a bridge rectifier of sufficient
        rating (or 4 diodes), and a capacitor across the bridge output.
        This is a simple DC power supply. Measure the voltage output from
        this unit. The following formula (nothing more than Ohms Law)
        gives the current (in amps) or resistor necessary to charge a
        battery at a given rate: 
        
        	V(supply) - V(battery) 
        	----------------------	= R
        		I

        The wattage of the resistor is at least (Vs - Vb') * I where
        Vb' is the minimum voltage on the cell when you start charging.

        An example of the above. Your chosen supply produces 14v. Your
        gell cell fully charged is 13.5v, and is a 6Ah cell. Thus we want
        to trickle charge at .06A or 60ma. R is 8.33 Ohms. My junk box has
        a 10 Ohm resistor, so lets use that. That lowers our trickle
        charge rate to only 50ma, which is ok. Lets say your cell is
        discharged to 10V. The initial charge rate is now (14-10)/10 or
        400ma. Not quite the 600ma of a fancy charger, but good enough for
        us. As the battery voltage climbs to 13.5v, the charge current
        drops down to the 50ma rate. The peak charge rate times the
        voltage drop is .4A * 4V = 1.6 Watts. Good thing my resistor is a
        10 watt wirewound. As the charge nears completion, the power drops
        to .05A * .5V = .025 W. 
        
        Finally, I think this has been mentioned before, but do not leave
        any batery, car, motorcycle, gell cell, or whatever on a concrete
        floor. This seems to rapidly drain their charge. Also do not leave
        any of these lying around for long periods in an uncharged state,
        or they will suffer permanent damage.