Basics of Cell Electro-Chemistry

Electrochemistry
Discharge

Fully discharged: two identical lead sulfate plates

In the discharged state both the positive and negative plates become lead(II) sulfate (PbSO
4), and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water. The discharge process is driven by the conduction of electrons from the negative plate back into the cell at the positive plate in the external circuit.

Negative plate reaction:

Pb(s) + HSO−4(aq) → PbSO4(s) + H+(aq) + 2e−

Positive plate reaction:

PbO2(s) + HSO−4(aq) + 3H+(aq) + 2e− → PbSO4(s) + 2H2O(l)

The total reaction can be written as

Pb(s) + PbO2(s) + 2H 2SO 4(aq) → 2PbSO 4(s) + 2H 2O(l)
Charging

Overcharging with high charging voltages generates oxygen and hydrogen gas by electrolysis of water, which is lost to the cell. Periodic maintenance of lead-acid batteries requires inspection of the electrolyte level and replacement of any water that has been lost.

Due to the freezing-point depression of the electrolyte, as the battery discharges and the concentration of sulfuric acid decreases, the electrolyte is more likely to freeze during winter weather when discharged.
Ion motion

During discharge, H+
produced at the negative plates moves into the electrolyte solution and then is consumed into the positive plates, while HSO−
4 is consumed at both plates. The reverse occurs during charge. This motion can be by electrically driven proton flow or Grotthuss mechanism, or by diffusion through the medium, or by flow of a liquid electrolyte medium. Since the density is greater when the sulfuric acid concentration is higher, the liquid will tend to circulate by convection. Therefore a liquid-medium cell tends to rapidly discharge and rapidly charge more efficiently than an otherwise similar gel cell.
Measuring the charge lev
http://batteryworld.co.ke/battery-acid-water/

  • Electrolyte Temperature

    Electrolyte : Here is a copy of info from: http://www.trojanbattery.com/tech-support/faq/

    > 9. Can a flooded battery freeze?
    The only way that a battery can freeze is if it is left in a state of partial or complete discharge. As the state of charge in a battery decreases, the electrolyte becomes more like water and the freezing temperature increases. The freezing temperature of the electrolyte in a fully charged battery is -92º F (-69º C).
    >
    >At a 40% state of charge, electrolyte will freeze if the temperature reaches approximately 16º F (-9º C).

  • Hydrometers Intro

    I have ended up buying two hydrometers. I first called Plymouth Battery Centre who informed me they had a glass model at £19 which I thought was a bit expensive so when I went to town I thought I'd look around before I took the mile or more walk to Prince Rock.

    The Toolshed could get a glass model for £5 and the car shop next door had the same model for £2.99, which I bought. When I went to the Plymouth Battery Centre to buy some red and black covered multi-core cable I decided to buy the hydrometer they had anyway, which cost £14.50 eventually.

    The better model from Plymouth battery Centre is the one on the left, although you cannot see the coloured scale. It is supplied by Gordon Equipments under the name DURITE model 0-070-00 ; range 1.15 to 1.40; accuracy 0.01. All parts are individually replaceable.

    The difference in readings is notable.

    Concerns regarding the cheapo

    1. The float diameter is much smaller than the container diameter so the readings are at an angle.
    2. On the side of the container the float rests, the surface tension of the acid draws the liquid further up the side of both, giving a distorted view.
    3. The scale is on the small side so not so easy to read.
    4. And of course there's the idea that the calibration is off.

  • Specific Gravity

    A lead acid battery will have a ration of sulfuric acid and water as the electrolyte. The process of charging forces any sulfate from the battery plates back to the electrolyte which increases it's specific gravity. The specific gravity of pure water is set at 1. By dissolving substances in water makes if heavier per volume. The common amount of acid brings electrolyte specific gravity (SG) to 1.128 approx.

    Measuring the SG with a hydrometer can show if the acid is in the plates and they are sulphated.

    The two times this would occur is a) in the normal process of discharging and b) in the longer term build up of sulphated plates that do not respond to a normal charge and the battery is termed 'sulphated'

    The SG will also vary on the level of electrolyte as the water evaporates. As water evaporates the SG increase along with ration of acid to water. Adding water will lower the SG and show the battery is not as fully charged as it may seemed when the electrolyte was lower than the designed level.

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