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Author Topic: Charging Gell Cells  (Read 230 times)
N4NQM
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Posts: 36




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« on: April 06, 2003, 09:13:13 PM »

I have two Yuasa 12v gell cells which have been sitting unused for about five years.  They measure about 10.2 v.  I put the (automotive) battery charger on them: it said they where almost fully charged.  No change in the meter on the charger or output voltage over several hours.

Is it ok to charge gell cells with the car battery charger?

Should I ignore the meter and let it trickle charge overnight?

Can the cells be rejuvinated if need be?  

Can I be rejuvinated?  I thought of posting this query to the Elmers net about a dozen times but by the time I got the computer booted up I forgot what I was doing.  Old age ain't what it's cracked up to be.
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AA4PB
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« Reply #1 on: April 06, 2003, 09:47:39 PM »

Letting SLA (Sealed Lead Acid) batteries (commonly called Gel-Cells)sit unused for 5 years without keeping the charge topped off is not a good thing. You may not be able to bring them back to maintain a full charge.

Whether the car battery charger is safe depends on the size of the SLA batteries. If they are the typical 7AH or so, the trickle current from the car battery charger (usually several amps) will probably be way too much. You must limit the maximum charge current to the battery mfg specifications. 1/10th the rated AH capacity is a safe value. The actual specs are usually a bit higher, for example a PowerSonic 3.9AH is spec'd at 600mA maximum bulk charge rate.

A good SLA battery that is discharged to 10.5 volts will draw much too much current from a 13.8 volt charger if the current is not limited in some fashion. If the battery is bad (one or more high impedance cells} then the voltage will immediatly go to 13.8 when the charger is connected so it will not draw any charging current (or very little). It will also drop back to 10.5 shortly after the charger is removed.

Once a good SLA is charged, it can be maintained across a 13.8 volt supply or charger.
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AC5UP
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« Reply #2 on: April 06, 2003, 10:50:00 PM »

A lead-acid battery (and that's essentially what a gel cell is) should be charged at least twice a year while in storage. Every three months is mo'better and a monthly charging is recommended. Assuming your cells are similar to the common 7 amp-hour alarm panel batteries, charging should be limited to no more than 2 amps of current at less than 14.5 volts, max. Float charge would be 13.8 vdc around 100 mils (1/10 amp).

Anything above that will encourage 'gassing', where gas bubbles form on the plates and break the plate-to-electrolyte contact. This is not good. In service these cells start to lose charge depth at around the two year mark and by 3+ years they're ready for replacement. With the exception of a very healthy (wet) battery in its prime, a lead-acid battery that's allowed to fully discharge will rarely accept more than a very light surface charge at maybe 1% of its rated capacity.

There's a very good chance your cells pull almost no current from the charger and discharge almost instantly as the plates have sulfated so deeply that only the top surface is active... In other words, you have a pair of door stops. All gel-cells that I've seen have a date code marked on the case and anything over a year or so generally isn't worth carrying home, no matter how good the price... And I speak from experience on this.

Try running the batteries through a few charge / discharge cycles (an 1157 automotive brake light lamp makes a dandy discharger) just to see if there's any life in them, but don't be surprised if they're beyond salvaging.

- AC5UP

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K5LXP
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« Reply #3 on: April 06, 2003, 11:10:35 PM »

When you have a battery in this state, you've got nothing to lose.  I would connect it to a current limited supply set to about 250mA, with no voltage limit.  Let it sit on there for 36 hours, then do a discharge test using a load such as the aforementioned 1157 bulb.  Make a note how many minutes the battery stays above 10.5V.  Repeat the cycle a few times until there is no increase in capacity.  The final determining factor whether you keep it or not is how much of the original capacity you require for your application (50, 80, 90%, etc).  In my experience, once they hit 50% they are very near their demise.

Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net
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N3NYC
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Posts: 63




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« Reply #4 on: April 07, 2003, 03:29:01 PM »

N4NQM,

Mark hits it right on the nose-- the voltage across the battery really doesn't matter as long as the current through the battery is at a safe value.  In fact, you may have to put what would be an "unsafe" voltage across the battery to force any charging current through it at all-- the insoluble lead sulfate on the plates makes a pretty good insulator.

If you've got some time, you may want to follow Mark's suggestion-- use a small charging current, with no voltage limitation-- and let the charger work for several weeks, the better to let the excessive voltage "encourage" the lead sulfate to dissolve. (The chemical reactions which take place in a cell are voltage-dependent.  It may not be too much to say that the voltage present on a cell "drive" the reactions to take place in one direction [charging] or the other [discharging].)

Good luck and 73,

Hale, N3NYC
(November Three New York Central)
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