[BC] Lead acid batteries.

[RichardBJohnson at comcast.net]

Lead acid batteries:

At Analogic, I helped design the world's first portable CAT scanner. CAT scanners require about 10 kW of X-Ray power, far greater than one can obtain from a hospital power outlet. However, they do not run the X-Ray generator continuously.

Therefore, by storing energy in lead-acid batteries, it was possible to design such a scanner that plugs into the typical 1500-watt outlet for operation.

I designed the charging algorithm and wrote most of the energy management software. Those interested in the details may read about it here http://www.route495software.com/Devil.html .

The algorithm compensated for and worked around the following problems with lead-acid batteries:

Lead acid batteries, when fully discharged, have a high internal resistance so when you apply a charger, and read the terminal voltage they seem to be fully charged! This means that you cannot determine the discharge state by reading the terminal voltage. You need to read the voltage under load, which, too often, is too late.

When fully discharged, batteries may take no current from a charger, therefore preventing any attempt to charge them! You can "fix" them by connecting a one-ampere diode in series with a 100-watt incandescent lamp and the battery and plugging it into a 120-volt outlet until the lamp starts to light. This forces a current through the battery, starting the required chemical reaction(s). Note that one must completely disconnect the battery for this and using 120-volt AC power can be dangerous.

When batteries are connected in series, there is most often at least one cell that will discharge before the others. This means that the first cell to discharge ends up being reverse-charged by the current from the other cells. This is NotGood(TM). You prevent this by not discharging such a battery more than 66% of its rated ampere-hour capacity.

One can prolong the life of lead acid batteries by noting and observing the following rules:

The fully charged voltage of a "12-volt" battery at 25degC is 2.2 volts per cell or 13.2 volts. Most chargers open-terminal voltage exceeds 14.7 volts, the absolute maximum allowed to "quick-charge" the batteries. If possible, the charger (alternator) should be adjusted to 14 volts. This will also make incandescent lamps last longer.

No not discharge the battery more than 66% of its rating and do not allow a discharged battery to stand overnight in below freezing temperatures. A fully-charged battery will handle -20degC temperatures. A discharged one will freeze at -10degC or even higher.

Low maintenance batteries contain antimony and a recombinant structure to combine evolved hydrogen and oxygen back into water. Over-charging such batteries may dilute the sulfuric acid in some cells and increase its concentration in others as water transfers between cells using the recombinant mechanism. This will seriously reduce the life of such batteries.

Lastly, properly used lead acid batteries will last at least ten years. The telephone company used lead acid batteries for many decades and even repaired them as necessary, cleaning cell jars and replacing parts. It is a reliable technology.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Joseph DiPietro" <rfengineers at gmail.com>

My car runs on ten 180-Amp-hour 12-Volt deep cycle batteries connected 
in series.  Keeping the charge balanced is difficult to say the least. 
I keep close records on each battery and I can attest to the fact that 
the ones that are discharged the deepest fail first.