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Gel cell battery questions

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flag-d:
Hello all

Background: I'm running a pair of Yuasa NP7-6 (or is it ...6-7?) gel cells, 7Ah 6v, in series to give me 12v to a pair of mTronik Marine 15 ESC's, each of which runs a pair of Graupner 500E motors.  I use a Robbe Lader 6 battery charger and charge the batteries as a pair at 500mA.  Hope all that makes sense.

Questions and problems: here goes...
1. Around Christmas time, I snagged weed around a prop and put a very heavy drain on 1 motor which badly affected 1 cell in one of the batteries.  Badly affected in that it would no longer take a charge.  Took it back to the shop and clever man put affected battery on whizzy charger (gave it lots(?) of amps) and said it was now OK.  Please explain (short words, please, not too many syllabubs).

2. I now appear to have a similar problem on 2 more of the same type of battery.  I don't think they're taking a charge, or at least, 1 of them isn't as model appears very under-powered all of a sudden (yes, checked everything else).  How does this happen and can it be cured.

3. The Lader 6 charger is handy because I can charge the Tx, Rx and drive batteries all from the 1 unit and all at the same time.  However, I don't like the fact that there is no automatic cut-out once a set of batteries have charged, nicad or lead acid.  Can anyone recommend an alternative that won't over-charge nicads or lead acids?  Is it best to have separate chargers; nicad and LA?  Should I charge the batteries separately instead of in series?

4. I've heard that a lead acid which is fully charged will hiss slightly and a gel cell which is just about to be killed by over-charging will do the same!  Thoughts on this?  How does one tell, for certain, that a gel cell is fully charged?  They're not cheap items and I really don't want to trash any more than I really have to! (or burn the house down!)

5. What is the max current I can draw on those Yuasa batteries without destroying them?  I'd like to increase prop size and possibly motor size (to a 600) but don't want to fry the cells.

Thanks for taking the time to read.  Hope someone can help.

Mike

Jankers:
Hi Flag D,

I'm not familiar with your Lader charger but Nicads are normally charged at a constant current, whilst LA cells are charged at a constant voltage. This usually means two seperate chargers ( though modern equipment may offer both in one package?)

1. It is possible to 'repair' a damaged cell by giving it a huge charge of amps for a very short spell (seconds). It is normally a risky operation but is cost effective as a damaged LA battery usually ceases to operate if a cell goes down, so nothing lost really.
2 & 4. you will need to check with a multi-meter for both voltage and amps (not at the same time) to see if the battery is taking the charge. It may be too old? Wet LA batteries do deteriorate (think car battery, old style 'top-up') and I assume gels do likewise. Wet LA's do hiss at full charge, normally a bubbling of the gasses and this indicates a fully charged battery. Not sure about Gel cells for bubbling but if Nicads hiss this indicates the seals are emiting gass; usually a forerunner to failure. Nicad failure from overcharging is evident from the hissing and white deposits around the positive terminal. Cells may continue to work but at reduced max voltage.

5. See if you can pick up some info from yuasa/google.

Sorry I can't be more specific, I have very little experience of Gell cells. I understand these should be charged at 14.4volts (constant) and full charge is evident from the amp meter reducing to near zero (as per a car battery). Most chargers made for models seem to include a "float" option which switches in at the near full-charge point, generally indicated by some sort of LED sequence.

If modern eqipment allows both types of charging no doubt others will post, otherwise I should only ever charge a single battery at a time.

J.

Doc:
Mike,
If a gell-cell makes any noise, or if it appears to have swelled in some way, or if it gets warmer than you can hold, you have just broken a battery, junk it.  Gell-cells really are sealed, and need to stay that way.
 - 'Doc

PS - The only time you should hear a gell-cell is when it is humming a catchy country western or Celtic tune...

flag-d:
Jankers and Doc, thanks for the prompt replies.

Jankers: the Lader 6 is, on the face of it, a jolly good thing, as things go, as it has 6 charging outlets from 60mA up to 500mA.  The 500mA o/p is also the one recommended to use to charge lead acids.  The gel cell's are pretty new: only used/charged about half a dozen times so far, so I hope that doesn't make them 'old' as otherwise this hobby just got a lot more expensive!  wet LA's generate hydrogen durng charging, which causes the bubbling/hissing, which is normal.  Nicads hissing=good time to evacuate the building as far as I'm aware.  I've heard that a gel cell hissing in an obvious manner (stage whisper type of thing) is also a bad thing.

Doc:  Surely a gel cell must make some sort of noise as it charges?  I know that they're not liquid filled, as in the classic car battery type, but I believe the chemistry is the same and hence hydrogen will be evolved during charging and this has to vent somewhere.  If I ever hear one of my cells humming anything approaching a C+W tune, then it will be time for it to go!

Thanks fellas.  I think I need to do a bit more digging around on this subject.

Mike

bluesy:
Just googled "gel cell charging" and found that the gasses ( hydrogen and oxygen ) created during the charging and discharging cycles are recombined inside the cells to form H2O which keeps the gelled electrolyte from drying out during repeated uses.

hope this helps......

doug

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