Re: (ET) More on gel cell charging

["David Roden (Akron OH USA)" <etpost drmm net>]

On 29 Nov 2006 at 8:26, tbamc wrote:
> 
> 1.  A  < 14.3v limit on charging gel cells was mentioned.  I gather this 
> applies to ALL 3 phases?

To answer this we need to look at what happens as a battery charges. 

As SOC rises from 0% to 80%, on-charge voltage rises.  For much of this 
period, virtually all of the energy goes into charging.  

Around 2.4 volts per cell (depends on temperature and battery chemistry) 
is 
the gassing voltage.  From that point on, more and more of the input 
energy 
goes into heat and into disassociating the battery's water into H2 and O2. 
 A 
small amount of both is inevitable, but in excess they are potentially 
injurious 
for VRR batteries.  Thus current has to be limited carefully during that 
last 
20% of the charge.  

It turns out that if you limit available voltage to that magical 2.4vpc 
(often a bit 
lower for gel batteries), the battery naturally accepts about as much 
current 
as it can handle.  

But typically the chargers provided with rechargeable household 
appliances, 
Hot Wheels, lawn mowers, and the like are neither smart nor regulated.  
They're more or less current limited devices.  The current limiting is 
mainly 
enforced by the impedance of the transformer and the resistance of the 
wiring.  

Usually they can't produce more than an amp or two.  The manufacturer 
counts on this to minimize the damage to the battery.  But battery life is 
generally pretty dismal anyway, because these chargers almost always 
overcharge.

If you have a regulated power supply, limiting its voltage to 14.3v for a 
12v 
module, or 7.15v for a 6v module, will safely charge the battery.  If it 
has no 
charge controller, you can monitor charging current (not voltage) and stop 
the 
charge when it falls to 1-2% of the battery's C20 amp-hour rating 
expressed 
in amps.  For your 4ah batteries, this would be 40-80ma.


> 
> The practical reason I ask, is the attempt to charge some 6v 4Ah gels 
> with a wall wart.   One I have has a rated 9v 200mA output.  

Most wall warts are unregulated and their output varies widely with the 
applied load.  Even if yours is regulated, 9 volts is really too high for 
constant 
voltage charging of a 6 volt battery.  

What you have is about what I described above - a dumb charger, with no 
voltage control and its current limited only by the supply's internal 
resistance. 
 

An easy way to improve it is to add an inexpensive regulator IC to its 
output, 
and adjust the voltage to 7.15 volts.

> Charging 
> voltage on a fairly dead battery starts out at 6.7 or so and gradually 
> rises.  Could I simply keep an eye on this and stop once it reaches 7.1v 
> or so?  

Reaching the gassing voltage means that the battery is about 80% charged.  
So, if you just stop charging when the OCV reaches 7.1 volts, the battery 
will 
be undercharged.  

If you don't want to add a regulator IC, you might try regulating the 
charge 
manually.  Add a bit of series resistance, such as a rheostat.  If your 
wall 
wart really can't produce more than 200ma, you could probably use a 5 watt 
control.  

When the on-charge voltage exceeds 7.15v, just trim the control to hold 
the 
measured voltage at that level.  Carry on doing this, perhaps every 15 
minutes or so.  As above, stop the charge when the current falls to 1-2% 
of 
C20.

Of course there's the danger that you'll get busy watching the game or 
cruising the web, and forget to watch the voltage.  Not that I've ever 
done 
anything like that.  ;-)

> Or, would it work to make a voltage divider--say, 10 and 35 ohms, which 
> would do the whole charge, never exceeding 7.2v?

You could try it, but I'd use a regulator.  See above.

> 
> And last, would a 'smart' charger for 6 and 12 v batteries be 'too much' 
> for these little 4Ah guys, or is it likely smart enough treat them 
> gently?

Most batteries can accept a bulk charge (to 80% SOC) at as high a current 
as they can comfortably produce.  This might be 3-6 times the C20 rating, 
expressed as amps.  For your little battery, 12 amps would probably be 
safe 
up to 80% SOC.  But, you will have to watch (feel) the battery 
periodically for 
excessive heating during the charge.  

Ironically, undercharging might be more of a problem than overcharging.  A 
charger programmed for larger batteries might shut off at too high of a 
current, instead of waiting for it to fall to that 40-80ma that signifies 
"full" for 
this battery.