The best way to charge lead acid batteries
For best battery life, i.e. greatest number of charge/discharge cycles and years service most battery manufacturers recommend that you should aim to recharge the lead acid battery when they have reached around 50% discharge. This level of discharge, of course, must be measured according to the rate at which the batter is discharged, which as w have already seen, varies the available total capacity of the battery.
In addition, some battery manufacturers specify that best life will be achieved only if the batteries are discharged sufficiently for a 4 hour bulk charge to take place before the batteries are fully recharged.
A reasonable rule of thumb is that you should aim to charge the batteries only when they are between 70% and 40% discharged. If you charge them then they are only lightly discharged i.e. less than 40% you will end up boiling them unnecessarily which wastes energy in the form of heat and gassed off hydrogen and in turn shortens the life of the batteries. In effect the batteries are being overcharged which can cause degradation and buckling of the plates. In the process some active material is forced off the plates and drops down to the bottom of the battery. If this occurs frequently the eventual result is a build up of a bridge between the plates which in turn can cause a possible short across the plates. This situation leads to the destruction of a cell which then reduces the capacity of the battery.
To confuse matters further, a battery will operate at its most efficient the deeper it is discharged, up to around 75%. The bulk phase of the charge cycle is the most efficient and is proportionately longer the deeper the discharge.
There are many battery chargers available. Today, manufacturers supply automatic chargers which are supposed to ensure optimum battery life. There are numerous charge profiles available and now, with the advent of the electronic switch mode chargers, it is possible to have a fully programmable charger on board your car capable of charging almost any type of battery.
Lead acid batteries must be charged carefully. If the charge is too violent and uncontrolled the batteries can overheat and cause thermal run-away which can result in a possible explosion.
Too gently charging will take too long to get the batteries fully charged with the result that the batteries will end up being used in an under charged state eventually leading to premature failure due to sulphation.
The latest electronic chargers mostly make us the IUI charge profile for standard flooded lead acid battery. This means that the current drawn by the batteries is allowed to flow at a constant (I) rate while the voltage is allowed to rise of its own accord, which it will do as the battery starts to be charged up. This first part of the charging cycle is known as the bulk charge phase. When a preset voltage has been reached, normally the voltage at which the batteries just start to gas, the charger will switch into the constant voltage (U) phase and the current drawn by the battery will gradually drop until it hits another preset level. This second part of the cycle is really the finish charge where the battery just topped up to the brim very carefully at a much gently diminishing rate. Finally the charger will switch again into the constant current mode (I) and the voltage is allowed to rise again, up to a new higher preset limit, in order to achieve a successful equalization charge.
Source: www.leoch.com
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