Battery state of charge: specific gravity vs. voltage

Cariboocoot wrote: »

This is Westbranch'es point: SG ranging from 1.240 to 1.275 is too wide a scope; a difference of 0.035 between any two cells is definitely indicative of a need for equalization.

BB. wrote: »

Eric,

My two cents... I have not seen the batteries and I am not a battery engineer--So take my comments with a grain of salt.



Cells may fail with high resistance (open) or shorted... Open cells would prevent charging current from fully charging the rest of the cells and could interfere with equalization too (they take all of the voltage drop and limit current flow).

Shorted cell(s) would cause excessive current to flow during charging even with normal charge voltage set points and could "over charge/over equalize" the other cells. The Battery Bank would appear to discharge very quickly (voltage drop by "x" number of cells discharging, then hold that lower voltage while the rest of the cells do their job.

Over charging/lots of equalization is bad too.... Can cause plates to shed material, use excessive water, and cause battery positive grid corrosion (O₂ forms at the positive plate during charging).

Did you notice how much water you need to add to each cell? Where all cells using water equally? Very roughly, if you add water every two months (always prevent plates from being exposed to air)--you probably are charging about right... If you never add water or need to add water every month or less--You probably are under/over charging the cells (although--I think--Surrette 4000 series batteries have calcium added to reduce water usage and need higher charging voltages for proper recharging).



Note it probably takes more than 3 hours for batteries to diffuse the electrolyte concentrations through the cell to measure proper resting voltage. The partially discharged readings, however, tell the story of the dead cells.

Battery #10 probably needs replacement too... The rest of the cells discharged by an average of ~0.020-0.025 SG units. That cell discharged by ~0.038 -- Not good.

When looking at your cells, most vendors recommend equalization if the cells are around 0.015 to 0.030 or more SG units apart. At this point, spending money on generator fuel to equalize before you get the new cells is probably not the best use of fuel. When you get the new cells equalize the battery bank (high voltage charging, a maximum of ~5% rate of charge, measure measure/log the temperature compensated SG for each cell--When all of the cells stop SG rising between readings--Log the final values and stop equalization).

Have a container of distilled water you can rinse the hydrometer out after each set of readings--If the hydrometer dries out with electrolyte/gray dust from the battery inside--the float can become sticky and give inaccurate readings.



I am not the guy filling out the RMA's... You have 5 bad batteries--Not three (in my humble opinion). The four that show very low/zero voltage after nominal discharging, and the one with the debries inside (the 0.038 specific gravity drop and the very high starting SG does not look right (unless there was an inaccurate reading on your part).



Check the batteries during charging. Very likely, the "bad cells" will limit charging current. You want to measure the voltage on the "good cells" and make sure you are not damaging them further. As long as they are charging--I guess I would just continue to run the bank "as normal".

And on the failed batteries, as long as they are "normal" to high voltage, you probably are not over charging/damaging the rest of the bank.



Many of the "Surrette" problems we have read about here seem to have very low SG in failing/poorly operating cells (1.100 or even lower SG).

Your "bad cells" all have normal full charge to higher SG readings when reading zero/low voltage. There is probably nothing you can do for them to even temporarily get them functional again. Although--If they are highly sulfated--Charging/Equalizing the cells A BUNCH may cause the insulating sulfates to shed and expose fresh material--However, because your SG is already high--It would not seem you have a lot of sulfation (which locks up the sulfur in the electrolyte--leading to low SG readings over time, not normal to high readings).

If you try anything--talk with Surrette first--You don't want your extraordinary efforts to cause the Mfg. to deny warranty claims by your attempt to get the cells functional again.



It is all too easy to "murder batteries" with under charging / over discharging (guest, kids, etc. using power like there is no tomorrow during bad weather, something left on during a trip, just not quite fully recharging every time--but slowly declining state of charge over time--"deficiet charging", etc...

But we have a few reports here of Surrettes going bad soon after installation, or even in once case, appeared to have a cell/battery filled with distilled water instead of electrolyte.

Normally, when installing a new bank, a mfg. will have a very specific set of instructions on charging to put the bank in service--obviously, follow those recommendations.

We also suggest that you inspect/number the batteries on the pallet and log any lot numbers/etc., measure the resting voltage of each battery/cell, and log the temperature corrected specific gravity of each cell. And reject anything that does not look "right" (you have at least 24 cells--If you find any high/low voltage or SG readings, cells with low electrolyte/wet batteries or pallet, mixed date codes, "old date code batteries", batteries delivered with less than 75% SOC, cracked/split/leaking cases, etc. are all indications of "issues" that need to be addressed.



Batteries are very sensitive to charging voltage... Nominally ~59 volts is a "vigorous" charging voltage and 62-64 volts volts is "equalization" (and equalization is very hard on a battery bank--so you want to do the minimum required for good battery life). The Surrette 4000 series require something like 63 volts for Absorb charging--Which is already a very high voltage--but within their specifications.

With potentially shorted cells in a battery bank, they will "subtract" voltage and raise the charging voltage on the rest of the "good cells" which can cause them to be over charged/damaged. If you did not notice the "bad cells" in time--it is possible that the other cells were over charged. Checking/logging each cell voltage during charging (once a week?) will give you an early warning of issues.



Failing/unreliable batteries has been a complaint we have seen in the last few years about Surrette batteries... Of course, none of us here on the forum work for Surrette or sell batteries--So we have no idea if this is a wide spread issue or just bad news traveling quickly while huge numbers of installations are working well with nothing to post here.

Best of luck.

-Bill