Precharging new 12V lead acid gel cells for use in series

[5065AGuru]

Hi,

I just bought two new lead acid gel cells 12V 26AH to install into an HP 5089A backup power source.
The manual states " each battery should be pre-charged for 24 hours from a DC source set at 15.30 +- .01V and current limited to 2.0A +-.1A. "
Not that I don't trust HP but isn't that a bit high??

Any comments welcome.

Cheers,

Corby

[thm_w]

It is high. It seems they are just worried about balancing the cells.
If you have a decent 12V battery charger I would hook them up to that in parallel and charge as normal.

If you want the thing to last forever you could replace with lifepo4.

[mtwieg]

I'm guessing this is meant to be an equalization charge. But 24 hours is a very long time for an equalization charge (6-8 hours is more typical). I would not leave them unsupervised for so long, especially with sealed batteries.

[edpalmer42]

I just bought two new lead acid gel cells 12V 26AH to install into an HP 5089A backup power source.
The manual states " each battery should be pre-charged for 24 hours from a DC source set at 15.30 +- .01V and current limited to 2.0A +-.1A. "
Not that I don't trust HP but isn't that a bit high??

I wouldn't say that's a bit high, I'd say it's ridiculously high!  A voltage that high will damage and/or reduce the life of any modern battery.

I had to check the manual to be sure and they do specify sealed lead-acid batteries.  I was expecting to see flooded wet cells since 15V3 could be considered an equalize charge.  It's almost like they converted the unit to sealed batteries and forgot to update the voltage.

Anyway, you should completely ignore that voltage.  Most sealed batteries these days are AGM batteries that specify a cycle-charge voltage in the range of 14V5 and a float voltage range of 13V6 to 13V8.  Most batteries actually print their desired voltages on the side.  I'm not sure it's necessary, but if you want, put the batteries in parallel and give them an overnight charge at 14V5.

You should also check all the other adjustments.  If I read the manual correctly, it specified an output voltage of 28V2 or 14V1 per battery.  That will age the batteries unnecessarily.  This is a float voltage application so the voltage should be reduced to an appropriate voltage, typically around 27V6.  Higher temperatures require a lower voltage setting so you should tend toward the low end of the acceptable voltage range.  You might want to adjust any alarm or cutout voltages to match the new float voltage.

The lower voltage may reduce the backup time.  If that's important, you can adjust the voltages higher, but at the expense of battery life.  You may also find that the batteries outgas more which might cause corrosion in the circuitry and could require more ventilation to clear out the hydrogen gas.  I've seen discussion on Time-Nuts about traces being eaten off the board by battery outgassing, but that was from batteries internal to the unit.

Ed

[ConKbot]

If it's actually a gel cell and not AGM, then yeah, a "gassing" charge to balance the cells is generally a bad idea. AGM at least has a chance to get the gas out of the electrolyte. A proper gel cell doesn't and you can end up with gas bubbles holding the gelled electrolyte anyway from the plates.

[Gyro]

An alternative method of balancing is a low constant current charge (a few 10s of mA maximum) with the batteries already in series. This encourages all cells within to equalize, not just the two batteries. Keep going only until both batteries reach the same voltage level. Perform on nominally charged batteries otherwise it will take for ever.

[SeanB]

Same batch of cells, do not bother, just put them in service, and set the float voltage to give 13V5 per cell. Slightly lower than full charge float voltage at 20C, but in a warm enclosure it will be correct for batteries at 40C, letting them sit at 95% charge indefinitely. They will last longer then, not being cooked to death in the warm enclosure.

[5065AGuru]

Thanks everyone! I charged them 12 Hours in parallel at 14.5V and then installed them and did the quick alignment.
Looking good! 

Cheers,

Corby

[Siwastaja]

Also with lead acid batteries, any charge voltage instructed without related temperature specification is meaningless. Charge termination / float / equalization voltages are all functions of temperature, and steep enough functions that it really matters to the point of any half-decent charger having a temperature sensor for voltage compensation. Car alternators have this (you can see how they output up to 16V on a really cold winter day, and 14V on hot summer day), decent UPSes do as well. If you use any random lab supply, you need to be aware of your lab temperature to maybe +/- 3..5 degC accuracy and find an actual voltage table from the battery manufacturer you can trust.

And generally, the fewer cells in series, the better chances of successful equalization without damage of "drying out". 12V pack already is internally 6s, but at least the cells are (possibly, maybe) better matched than two separate 6s packs (12V bricks) put in series afterwards. If stars align properly, you can series charge even higher voltage packs just fine, it's just trickier and more prone to errors causing premature failure. Cell-level (2V) battery management would be optimal, like in the li-ion world, but AFAIK literally no one ever does that on lead acid.

[edpalmer42]

Cell-level (2V) battery management would be optimal, like in the li-ion world, but AFAIK literally no one ever does that on lead acid.

A lifetime ago I worked for a telco.  The power guys told me about a presentation they went to where a company was trying to sell exactly such a device.  It kind of made sense since large telco power plants are made up of strings of 2V cells and they have a 20 or 25 year pro-rated warranty.  But the cells are monitored, tracked, tweaked, watered, charged, adjusted and generally babied throughout their lives.  I don't know if the product gained any popularity.  AFAIK, our telco didn't purchase them.

Ed