Originally Posted by sailinglegend
This is another controversial subject in the “electrickery game”, and is another good reason why SmartGauge is much simpler and much easy to use.
Measuring SG of an open wet lead acid battery accurately may be a difficult and time consuming procedure, but unless the battery is healthy they may not be the best method because the SG reading gets lower as the battery ages.
The SG of each cell at a 100% SOC should be 1.265 or higher. If the cells in the battery are not equal then an average of all cells has to be made. If individual cells are lower it means their Ah capacity is reduced due to sulfation. A value of 30 points between cells indicates that the battery should be equalized.
For accurate results the SG must only be taken when the cells have been topped up with distilled water
and then only after the batteries have been charged, this stirs up the electrolyte to give a true SG reading for each cell. The SG reading must only be taken when the charged battery has been allowed to rest with no charge or discharge for several hours, the temperature of the battery must be measured and compensation has to be applied to all readings. All this is not very practical on a typical cruising boat, so the potential for errors can give poor results.
If the battery is known to be fully charged because it has been on shorepower for a couple of days the SG readings taken then will represent 100% charged. If these are all lower than the readings when the batteries were new that indicates they have lost
some of their Ah capacity. Every time the battery is discharged and charged again these SG values will reduce slightly.
The only thing I find a hydrometer is really good for is to tell you when cells are out of balance and when to stop equalizing. Your resting
voltage will tell you what the hydrometer does in terms of overall SOC.. Never seen it not agree, but the key is resting voltage.
I have seen many boat owners destroy their batteries thinking they need to check SG all the time. The elcheapo hydrometer they are using often resides in a drawer full of rusty tools & contaminates and each time it is inserted they are adding metals & other contamination to the electrolyte. The battery tops are also quite often filthy and loaded with contaminates.
It is not uncommon for me to see 1-2 year old batteries with cells gassing at float levels which is most likely due to metals contamination... When I ask to see the hydrometer, well, out comes this filthy tool dug from a drawer of rusty tools.......
If you own a hydrometer I have two words for storage
, TUPPERWARE CONTAINER. Please give your hydrometer its OWN clean storage
container. It also helps to flush it after use with distilled water
I then ask the procedure the owners use and nine times out of ten they are doing it incorrectly. This also leads to erroneous readings and defeats the entire purpose of exposing the cells to potential contamination..
I no longer use a hydrometer as I find a sight refractomter far easier, less invasive, less electrolyte to use, and less clothing
holes. It is also easier to read. After breaking my third fragile and very expensive Freas hydrometer I switched to a refractometer.
As Bill said an SG test tells you nothing about the current
capacity of the bank. The only way to determine this is with an actual discharge capacity test. Waiting on a resting voltage reading will tell you the "SOC" but won't tell you individual cell balance..
I have seen plenty of batteries that can put up perfect SG readings after a proper charging
yet they have almost no "capacity" left in them. I have two sitting in my shop right now where all cells are evenly balanced and after being charged they both show near perfect temp corrected SG yet the physical capacity is just not there anymore.
I use SG readings most often to tell me when the batteries are in need of an equalization
and when to stop equalizing.