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The starting idea was to build a battery charger for Pb battery used in car and moto;
the free time was only one day (sunday) so I've used what I've found at home.

Just two words about battery charger;
a Pb battery can survive up to  5 years but if used in warm place it stops at 37 months or less.
Any cell presents, when full charged, 2,11 V @20-25 C (it's higher if temperature is higher, lower if temp is lower);
So a 12V battery made by 6 cell will give you  12,66V.
When full discharged a cell give 1,8 V (lower value means problems in some cells) so a full discharged battery (6 cells) will give you at least  10,8V (unloaded);
Under load you can read also 9,3-9,9V @ 38 C
To Re-charge usually we use 4 steps:

1) BULK : battery absorbs constant current; voltage goes up until 14V; high current is used (usually no more than  20% of the max A/H of the battery, for example in a 50A/H battery the re-charge current shoul be limited at 10A).

2) ABSORPTION : voltage stay constant and the absorbed current goes slowly down until 1% of A/H; at this point you can read  2,43 - 2,48V on every cells (x 6 = 14,58 - 14,88V). Most battery charged stops here.

3) EQUALIZING : the re-charging voltage is set at + 5% (2,6-2,73 V per cell / 15,6-16,3V); this step is usefull to equalize voltage in all cells until they are all equal.

4) FLOAT :  Voltage is set at 2,18V per cell; in a  50A/H battery a good value is  13,2V/500ma; it's just a maintenance charge.


If battery temperature goes up until  43 C you must stop the charge.
Before re-charging, check the electrolyte level (it shoul be OK); at the end re-check it and add water until 1 cm under the top border of battery.

Let's see now my battery re-charger; I've used a slow (1/10 A/H) charge wich give a long life to battery.



After connecting the battery to the re-charger, press the Start switch; a comparator check the battery voltage and if it's less than 12V a timer starts and battery is recharged with a 1/10 current (2,5A in my project);
at the end, the timer rele' is switched off and battery is connected to a lower current circuit (500mA);
if voltage goes up over 14,5V the charge stops and restart when lower than 14,5V; 
If you connect a not full discharged battery it will use the low current recharge;
The circuit is ok, but I've not used the comparataor because the switching was not fine: in case you need, use a Schmitt trigger or, better, a comparator such LM311; by the way, there is a "Test" switch wich disconnect temporarily the battery so you can check the battery voltage;

I've set the chareg voltage at 14,7V but  if you need a shorter time you can use 15 or 16V.
The timer use an external knob to vary time from 5 to 8 hours; there is also a doubling swicth so you have 5 to 16 hours.
If you vary voltage-time- limitator resistor you can use this re-harger with all Pb battery.

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73 de iz7ath, Talino Tribuzio