What effect does adding water have on SG?

jeffkrusejeffkruse Solar Expert Posts: 205 ✭✭✭
How does adding water to a battery change the SG? I added a cup to a cup and a half to each cell. My batteries are 12”x7.25”x17”.

The drawings says to keep the electrolyte level ¼” from the bottom of the Vent Well/Chimney. How practical is that? I added water to the point where the electrolyte touched the bottom of the Vent Well. How does this effect what my SG readings will be? I measured around 1.250 before and 1.235 after. The batteries weren’t fully charged but they were slowly starting to bubble.


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What effect does adding water have on SG?

    Adding water dilutes the electrolyte and lowers SG. Water has an SG of 1.000 - it is the 'baseline of comparison'.

    Why did you add water? So long as the electrolyte is 1/4" or more above the plates and below the vent of the fill tube it's fine. You only add water if the level is getting low, and then it should be only to a fully-charged battery (unless the fluid doesn't cover the plates). When the battery is charged, some water bubbles off in the form of hydrogen and oxygen. It should be a very small amount. If you're losing significant amounts of water during charging then your charge Voltage and/or time is too high. It's a bit of a balancing act because during charging is also when the sulphur that gets attracted to the plates is re-combined with the water, fortifying the sulphuric acid mix.

    You should always take an initial reading with new, fully-charged and equalized (yes, right away) batteries to compare future readings to. Variations in conditions lead to differing SG readings. The only variation you want to count is the one that indicates battery state, so the others (hydrometer accuracy, altitude, the way you read the scale) need to be controlled.
  • jeffkrusejeffkruse Solar Expert Posts: 205 ✭✭✭
    Re: What effect does adding water have on SG?

    I added about a cup of water when I first got the batteries. This was so they would all be at the same level, a level I could predictably stay at. I thought a ¼” below the Vent well would be impossible to maintain accurately so I filled until the level touched the bottom of the vent well. I figured ¼” wouldn’t matter much.

    Now I can’t remember if I took my baseline/initial readings with the water ¼ below or touching the vent well. :cry:

    I charge to a higher than recommended voltage because I can’t stay at the recommended aborb time for the recommended 8 Hours! Charging to the higher voltage gets my SG close(1.260) to full every day, full being >1.275

    In a month I added a cup to each cell.

    Is a cup a month to much?

    Should I try to keep my level exactly ¼” below the vent well?
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: What effect does adding water have on SG?

    A cup per battery, or a cup total for all 8?

    A cup for all 8 is nothing, a cup per battery may be normal. (I don't know how big physically these batteries are off hand)

  • jeffkrusejeffkruse Solar Expert Posts: 205 ✭✭✭
    Re: What effect does adding water have on SG?

    A cup per cell or 3 cups per battery, I have 8 batteries. They are 12"x7"x17"tall.
  • RCinFLARCinFLA Solar Expert Posts: 1,394 ✭✭✭✭
    Re: What effect does adding water have on SG?

    If your cup is 8 oz. per cell per month, for that sized battery, is a bit much.

    What voltage are you running your bulk at and for how long? When bulk current drops to about 1-3% of A-H rating you should be close to full charge.

    How old were the batteries from date of manufacture when you got them? Usually assume they had not received a recharge from manf. date to date you got them.

    If you are able to get up to 1.275 SG then you probably don't have sulphation. As long as you get back to 1.275 SG within a period of month or two you will not build up sulphation. Maybe back off on bulk voltage a bit and do a longer bulk up once a month to get back full SG.
  • VicVic Solar Expert Posts: 3,112 ✭✭✭✭
    Re: What effect does adding water have on SG?

    Hi Jeff,

    Well did not find the dry vs wet weight of UR Deka 8L16's (I assume that these are the batts in question), but using the mid capacity Surrette L16, there is about 135 oz of electrolyte per cell. Eight oz added per cell is about 6% of the total per cell. But, you were not going from the recommended level to the point of miniscus deformation (my term), but probably from a point below the ideal level to the point of deformation, so you prob have not overfilled by 8 oz.

    I, too, choose to overfill my batteries to that point, as it is much easier to see when that level is reached. But, I choose to fill my batteries about mid-way through Asorb. My theory is that this is where the cells are creating many bubbles, which displace some electrolyte, thus raising the level. I believe that most batteries have the ability bring a small amount of extra water back to the correct SG when charging and EQing.

    For my bank, do maintenance about every two months, and 24 cells require about one to 1.25 Gal of distilled water, but there is about 22 pounds of electrolyte per cell, so this is a small percentage.

    An 8-hour Asorb sure seems LONG. The higher Asorb voltage you mentioned should cause more water useage than the recommended V. You might want to check /add water more often. I do not know the exact battery that you are using, but, the higher capacity L16s appear to have a bit less electrolyte than standard ones, and you do not want to run the risk of exposing the plates. YMMV and so on. Good Luck, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • jeffkrusejeffkruse Solar Expert Posts: 205 ✭✭✭
    Re: What effect does adding water have on SG?

    I have the absorb set to max (8hours) but that doesn’t mean it’s getting an 8 hour absorb. It goes into absorb mode around 11AM this time of year. Around 4 pm the output of the panels is equal to my usage. That gives around 5 hours for absorb BUT the clouds come and go during this time.
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