Specific Gravity

[Shadow]

 I was trying to check specific gravity on my lead calcium tel batteries. They all have stickers on that say 1.215, but I got a variety of readings with a cheap Walmart squeeze gauge, from 1.25 to 1.21. First, what is specific gravity? and how crical is it? and how is it raised or lowered? And how accurate of a hygrometer does one need?Do I check every cell? or each battery? Thanks

[bob g]

shadow:

specific gravity is the strength of the electolyte and is the indicator or cell charge and or condition of a cell.

the lower the spec gravity the lower the charge in that cell in an otherwise healthy cell.

you should get the best hygrometer you can afford as the cheap ones arent all that accurate, especially if you care alot about your cells.

the specific gravity will lower as the cell discharges and raise upon charging, that is how you raise and lower it.

you should perodically check each cell to see if they are even, if you have a lagging cell an equalizing charge is used to raise the lower cell as much as possible, at least on flooded lead acid batteries.

hope this helps some, others will give you more in depth explanation i am sure.

bob g

[Shadow]

Thanks Bob, Yes it helps. I guess I wonder how much difference is tolerable? One battery is 1.25 and one was 1.21, that dosent seem like much but maybe on the specific gravity scale it is.As far as equalizing, I read somwhere where some batteries you dont equalize, these are lead calcium telephone batteries. Which I think you can equalize, but that brings up another question, How? I took them up to 56-57 volts yesterday with the windturbine at about 15 amps, I noticed tiny bubbles starting to accumulate in some of the cells, Is this the bubbling I should be looking for? And how long or much do you do this for?How can you tell if they are equalized as much as they are gonna equalize?  and what are the dangers?.It is well ventilated and no people around.

[pwr]

Specific gravity is the relative weight of the electrolyte relative to the weight of water, so an SG of 1.215 means that the electrolyte weighs 1.215 times the same volume of water.  Of course it varies with temperature, so unless you also measure that and compensate you can never know SG accurately.

However, you don't need an accurate measurement.  Think of the hydrometer as just a comparison meter - what you seek in a battery is the same SG in all cells, sort of like compression in engine cylinders.

A common SG for automtive batteries is 1.265.  For batteries that are going to be deep cycled in moderate temperature you may see an SG of 1.3.  For batteries designed for mostly float service with occasional deep discharges you may find 1.2 - one reason telcom batteries don't hold up to deep cycles very well is the fact that in a discharged state the SG is very low and the active material can more easily drop out of the plate grids.

Be aware that the electrolyte stratifies with the heavier acid at the bottom.  What you read at the top is less than the average.  Obviously taller batteries are worse in this regard than others.

To compare cell to cell SG, first charge the batteries fully with 45 to 60 minutes of gassing (rising bubbles in the electrolyte).  Then disconnect them from the charger and  any loads, letting them rest 12 to 24 hours.  Now take the SG measurements and hope they are within 0.03 of each other.

If not you need to equalize the batteries.

[Ungrounded Lightning Rod]

I took them up to 56-57 volts yesterday with the windturbine at about 15 amps, I noticed tiny bubbles starting to accumulate in some of the cells, Is this the bubbling I should be looking for?

Yes.  When the cells are fully charged and you drive extra current through them to continue charging the cells that are low, the fully charged ones electrolyze some water and bubble off hydrogen and oxygen.  This is what you want to see.

And how long or much do you do this for?  How can you tell if they are equalized as much as they are gonna equalize?

I'd say go until they're ALL bubbling (or the voltage is high enough to indicate that all the cells are at the fully-overchaged level).  But stop if you've gone long enough that the charging amphours you've driven in is enough to fully charge a completely empty battery - like maybe 110 to 120% of the amphour rating.  (Somebody more familiar with this can correct me if I'm off, please.)

and what are the dangers?.It is well ventilated and no people around.

That takes care of injury due to explosion from the gasses released.  You should make sure your cells have enough water before you start, so you don't "boil them down" (electrolyze too much water) and expose the plates during the process, and make sure they're full when you're done, too.  Running a cell nearly dry can explode things when the bubbles fully cover the submerged part of the plate and cause a spark in the midst of that explosive mixture.  Overheating if you do it too fast, damaging the cells.  Damage to your electronics on the load side if it's not rated for the higher voltages involved when applying an equalizing charge, or before the surface charge is removed and the battery voltage is down to the normal operating range by ten minutes or so of moderate load on the battery.

I've probably missed something.

Wear safety goggles and have a neutralizing solution spray bottle handy when examining the cells for bubbles, in case they spit at you or pick that moment to blow up and spray you with electrolyte.

All this is very low probability if you made sure your cells were watered before you started, have good venting and no sparks, and don't drive TOO much current in or drive it for too long.

[Flux]

57v is only just above float volts, it should equalise them if they are fairly close but if you have not done it on those batteries before, I would let them go a bit higher to speed the process up.

You need to continue as long as there is any rise in S.G. When there has been no further increase in any cell after 1 hour you are there. You should see all cells gassing freely but no need to make them boil like traction batteries.

When they are fully charged, note all the readings as a reference for future use. If there were variations from new you will still have a variation between batteries. For a new bank it would have been good to correct to the makers figure but if you don't know their history I wouldn't correct them now, you may cause trouble.

As long as the connections are tight, you keep electrolyte level up and there are no sparks there is no real danger.

Once you have got the reference S.G. for each battery you will be able to see how often you need to equalise. If they have been on float for a while and some S.G's are low then it it is time for another equalise.

Flux

[DanG]

A mention on test equipment calibration, the mass produced specific-gravity testors will usually display a range repeatedly correct, but an individual units calibration to the provided scale 'legend' may be off 2-5%.

A smart user check would be familiarizing yourself w/ displayed readings of your best cell when fully charged versus a known discharge, X amph-hour draw against cell capacity... Just one of the trade-offs for low price...