How do you recharge a dead car battery?

Answer 1

Depends how dead it is.Start off by checking the cell levels and topping them up with distilled water,nothing else,if one cell is MUCH lower than the others it can indicate a dead cell that has an internal short caused by sulphate deposits between the battery plates,if this is the case the battery is knackered.After topping up,leave the cell caps off and connect the + and - of the battery charger to the battery,making sure you have connected them the right eat + to+ - to -,plug the charger in then switch on,the battery if flat/low will draw a high current at the begining of charging (The ammetre will swing to right if a electro/mech gauge from 0 to about 10 Amps,depending on the capacity of the charger and the state of the battery)if its a charger with charging lights,more lights show a high current charge,the battery will pull a high charge for about 5 hours and as the battery charges up the amps it draws will slowly decrease to about a couple of amps.On average it takes about 12hours to fully charge a flat battery,if the battery has been excessively discharged it can take upto 72 hours with a 10 amp charger (no kidding!)Fast charging batteries should be avoided as it can and does damage them.

The ONLY way to check that a battery is fully charged you will have to use what is called a hydrometer to check the electrolytes charge in each cell,insert the hydrometer in the first cell and draw up some of the electrolyte with the bulb ontop until the float starts to float (You are checking the fluids specific gravity here) most floats are colour marked red,orange,green,the higher the float sits in the liquid the higher the charge,look at the level of the fluid against the floating float,if fully charged the fluid level will be accross the green portion of the float,check each cell one at a time and rinse the hydrometer between cells with distilled water,just suck some up with the hydrometer and the squeeze it out.

Each cell should hold about the same charge when fully charge,any cells that show low levels of charge may be dead say if you get 5 cells in the green and one in the red after 12hrs of charging the battery is knackered,it will start a car,but it wont hold a charge as the dead cell will cause a slow short circuit in the battery,replace the battery.Sometime if you charge the battery for a couple of days you may be able to recover a battery with a dead sell as the sulphate deposit might be dissolved removing the short,but best not to risk it though,nothing worse than a battery you cant trust!DO NOT USE A HIGH CURRENT CHARGER (50AMP+) TO TRY AND "BOIL" A BATTERY TO LIFE,YOU WILL OVERHEAT IT AND BUCKLE THE PLATES!!)

When tha battery is fully charged,check the fluid level and top up if nesescary and replace the caps on the cells,leave the battery to degas for half an hour or so before using.Fully charged voltage after sitting for an hour after charging should be 12.8-13.8v,any lower replace battery.
simply with a battery charger. you can also try jumper cables to charge it up enough.