There are yet other issues to consider relative to specific gravity. #1: I believe temperature has been mentioned already, but merits revisiting:
Specific gravity varies somewhat with the temperature of the electrolyte. Some of the more sophisticated measurement equipment takes temperature into account. Lower cost models don't. However, temperature can be included in your calculations by use of a simple thermometer to measure approximate battery temperature at the time the SG readings are taken, then factored into the readings. Cold batteries will yield higher SG readings than warm ones. #2: Another concern is stratification. Acid is more dense than water and thus sinks to the bottom of the battery. For more accurate readings, electrolyte must be "stirred" to thoroughly re-mix the solution *before* samples are taken for SG measurement. Given the construction of batteries, this is difficult (but not impossible) to do. Failing that, and using just an initial sample of electrolyte from the top of a cell, SG readings from the top may be lower than actual values. Jamie Surrette recommends adding distilled water to bring the electrolyte level up to the manufacturer's recommended level, then charging the battery to a point of light-to-moderate outgassing (with all precautions, of course) for a couple of hours. After the battery has rested for as long as possible (24 hours suggested), *then* take a SG reading. Hydrogen and oxygen get trapped in the electrolyte and will cause errors in any SG readings taken immediately after such a charging period. Since this is an off-grid home, one option might be to charge with the generator as late into the evening as possible, then take SG readings the next morning before any significant loads are applied. May be a Friday night/Saturday AM event if the owner works during the week. Dan --- On Mon, 10/24/11, Allan Sindelar <al...@positiveenergysolar.com> wrote: From: Allan Sindelar <al...@positiveenergysolar.com> Subject: Re: [RE-wrenches] intermittent battery problem; ...Battery Sulfation To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org> Date: Monday, October 24, 2011, 5:35 AM Ron,You have made a connection that I would not have made, and while I may be wrong here, it has got me thinking. I have never equated low water level with SG, nor have I ever read in battery maintenance guides that I should maintain a particular water level before measuring SG. I'm not convinced that electrolyte level has any connection with measured SG, although it would seem logical on the surface. I have always thought that if the correct concentration of acid was installed at the factory and not lost thereafter (such as due to a spill or chronic overfilling), the SC is only a function of SOC. I'm interested in others' opinions here. AllanPositive Energy
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