How to restore and charge Lead Acid batteries

Depending on usage, a lead acid battery can be expected to last for up to four years. However, only a third will actually last this long. Here are the secrets of lead acid battery restoration.

The lead acid battery contains plates of lead and lead oxide immersed in a solution of water and sulphuric acid termed the electrolyte. A good battery will contain 35% acid and 65% water. As the battery is discharged, this solution becomes weaker. A chemical reaction takes place to release electrons and deposit sulphur on the plates of the battery. When the battery is recharged, the process is reversed and the sulphur returns to the electrolyte restoring the acid.

A hydrometer may be used to test the condition of a battery. This measures the proportion of sulphuric acid in the battery.

For basic testing you can measure the voltage at the terminals, on and off load. For a 12V auto battery you could use a light bulb as a test load. Expect 12.7V for a fully charged, good battery off load. A discharged battery should float around 11.9V When the charged battery is loaded, the volts shouldn’t drop much below 12V.

How to charge a lead-acid battery.

We do this in 3 steps:

Step 1 – Bulk Charging

Apply a constant current set to a high value to rapidly recharge the battery to 80% of it’s capacity. When the battery volts hit 14.4V we go to the next step.

Step 2 – Absorption Charge

Hold the charging voltage at 14.4V The battery continues to charge with reducing current. When the current has reduced significantly, the battery will be 98% charged and we go to step 3.

Step 3 – Float Charge

We reduce the charging voltage to 13.4V The battery will then gradually reach 100% fully charged. The float charge will then hold the battery in a ready state for as long as you like. The float charge current is low enough not to damage the battery by overcharging.

The above technique is used in computer-controlled smart chargers. A very simple variant would be to have a voltage regulator which is switchable between 13.4V and 14.4V to select charge or float mode. Between the output of the regulator and the battery put a series resistor scaled to provide the bulk charge current.

The very cheapest battery chargers simply apply a constant voltage direct to the battery. The charging current starts off high and gradually reduces as the battery is charged.

Looking after a lead acid battery.

• Keep contacts greased so they are not corroded by battery gasses.
  
• Keep the battery charged and minimise the time the battery is left discharged.
  
• Keep the plates immersed in electrolyte.
  
• Don’t add tap water, use distilled or mineral-free water.
  
• Avoid deep discharging of the battery.
  

Reviving dead batteries.

Sulphation is the main cause of failure in lead acid batteries.

One of the popular ways to shift this sulphation and return it to the electrolyte is to use a pulse charging method.

The current pulses are of short duration and applied at a 2kHz rate.

When I have time I'll publish instructions on how to build a current pulser for this purpose ;-)

It'll use an inductor in a flyback configuration. The coil will be energized and connected across the battery every 0.5ms. The properties of inductors mean that the battery will be forced to pass this current no matter how many volts it takes!

To restore the battery to full health, another technique (see US patent 6,130,522) involves connecting a small load across the battery at a 2kHz rate. The effect is to impose a small step change in the charging current which supposedly cleans the plates of the battery.