high voltage but low SG

DGSmooth

good day all, new here but been reading for days, so much good info here lol.   but i have a question of my own now, hoping somebody can shed some light.  is SG and voltage directly related?   as in go up and down together very predictably?  i have a 48 volt system, Rolls 6V lead acid batteries, 2 packs of 8 batteries, in parallel together to make 48V.  sun is blazing right now on the panels, batteries and charging like crazy, 59.9V, but my SG is down at 1.22.... what does that mean?   couple days ago i topped the batteries up with distilled water, could that by why?

Graham Parkinson

Yes, having added distilled water will reduce SG until charging proceeds to recover the acidity.   Give it a few days and will likely resolve!

Voltage and SG only correlate if the electrolyte isn't stratified (denser acid sits on the bottom of cells till charging bubbles stir the electrolyte) and if the plates aren't sulphated.  An impervious layer of hardened sulphate on the plates can result in high cell voltage having no charging effect and no raising of SG (because the sulphur can't be driven off the plates into solution to make the electrolyte acidic)

BB.

Just to be clear, specifically used for flooded cell lead acid chemistry batteries that you can draw electrolyte from (not "sealed" or AGM or GEL type lead acid batteries).

SG also needs to be temperature corrected (77F is the "standard temperature").

https://support.rollsbattery.com/en/support/solutions/articles/208145-specific-gravity-temperature-correction

Besides temperature adjustments, the maximum SG value differs between battery design (telecom, forklift, car batteries, tropical use batteries, etc.).

So, the typical method is to fully charge the battery (EQ--Or equalization charging--Which is controlled "over charging" which brings up "weak or low SG value" cells to the fully charged value). Once EQ/full charge is completed, that is the "new 100%" fully charged value.

https://support.rollsbattery.com/en/support/solutions/articles/428-state-of-charge-flooded-lead-acid-batteries

EQ charging also generates gases (hydrogen and oxygen) which mix the electrolyte too--To prevent stratification like Graham said above.

If you have not done EQ charging of your bank (typically done every one to for months), then you can have low and high SG cells.

Rolls has a nice user manual--Here is the section on Corrective Charging:

https://support.rollsbattery.com/en/support/solutions/articles/430-corrective-equalization-instructions

Note that you need to raise the charging voltage to upwards of 62.4-63.6 volts:

https://support.rollsbattery.com/en/support/solutions/articles/654-battery-user-manual-flooded-agm-gel


More or less, you want to raise the charging  current to 5% (or at least 2.5%) of the bank AH capacity (20 Hour capacity). For example a 500 AH battery bank * 5% = 25 Amperes of charging current... Note that EQ charging will cause the bank to get warm, and can overheat the bank if done for many hours at a time. You will need to monitor the bank temperature and stop if the batteries overheat and restart the next day.

If you are having issues with "low SG", it is possible that you will need to monitor the SG of the cells (typically you pick a single "pilot cell" to measure when doing your maintenance checks) and when the SG stops rising, then the battery is fully charged.

Go ahead and cycle the battery bank (draw down below 75% or so State of Charge), then a week later go through the EQ charging cycle again. you may need to do this several times to bring back the SG to expected levels.

Also--Make sure that all plates are covered with electrolyte before charging--Exposed plates to air can quickly damage them. Also, do not fill the cells to full on a "cold" battery bank... Only fill the battery 1/2 full or so... At the end of the EQ cycle (battery warm/hot, gasses from bubbling present, etc.), then you can fill the battery cells to the normal fill point.

If you fill to "full" when cold, it is very likely the batteries will get hot/gassing, and spit electrolyte all over the top and sides of the batteries. Not a good thing to lose electrolyte. Normally only distilled water is added--Never add sulfuric acid to a battery bank in use.

Note that charging in the 62-63 volt range can cause some Inverters to high voltage fault... Check your brand/model of inverter and see how high of DC input voltage they support. In some cases, you may need to turn off your inverter during "high voltage EQ charging".

-Bill