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lead acid battery charger problem

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viperpaki007

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A 12 volt lead acid battery should be operated between 10.1 to 13.8Volts in charging and discharging cycles. I have a battery charger having output voltage of 19 volts to charge a 12V lead acid battery. When i connect the 12 volt battery to the battery charger the voltage measured at the terminals of the battery starts to increase with the passage of time. while the battery charger is charging the battery the voltage read at the terminals of the battery is 1 or 2 volts higher than battery open circuit voltage at that time.....i need to monitor the battery voltage so that i can terminate the charging at 13.8 volts...How i can terminate the charging process when the battery voltage while charging is not exactly measured.
 

You could have a comparator drive a FET on the charging line.
 

Depending on the currents required.

You could just set a LM317 regulator to 14.4V output and run a series resistor and diode from the output of this regulator to the battery.

The resistor can help to limit the current into the battery at the start of charge. This approach will likely require a rather large heatsink on the LM317.

oddbudman
 
When the charger is charging the battery the open circuit voltage measured at the terminals of the battery is not exact.....It is not static voltage and i can not guess the state of charge by measuring this voltage.
you suggested LM317 with series resistor.. this configuration do provide constant current source but does not help in measuring the open circuit voltage of the battery to determine the state of charge.

Added after 44 seconds:

thanks
 

Why do you want to terminate charge at 13.8V?
 

Its is because accroding to an article on Lead acid battery charging algorithm lead acid battery charger should not be charged above 13.8 Volts
 

Are you aware the battery won't be fully charged when cell voltage hits 13.8V during charge?

Its best to monitor the charge current and keep the charge voltage fixed. Once the charge current reduces to around C/100 or C/200 its safe to assume the battery is mostly charged. SLA Batteries take a long time to reach 100% charge, and they will happily sit with a float charge voltage applied permanently.


oddbudman
 
thanks oddbudman ....i have another question what is the max safe charge level for charging lead acid battery qucikly....most of articles suggest that it will be 30% of C ...will it be ok for fast charging
 

The current does depend on the battery.

The batteries I use allow 0.3C inital current. (I use 4.5aH SLA units).

Fast charging SLA batteries is not really all that fast.

You will find if you do put 0.3C into the battery you will hit the float voltage (or even the cycle voltage if you are cycle charging) quite fast, then the current will taper off. The battery really does dictate the charge somewhat.

oddbudman
 
simple dude, just make a voltage divider using resistors or use adjustable voltage regulator....
LM117 maybe...
 

viperpaki007 said:
what is the difference between cycle and float voltage
Click to expand...

The volatages vary slightly with ambient temperature, but at 25° C and a 12V battery they are:
Cycle 14.4 to 15V, Float 13.5 to 13.8 Most manufactures recommend limiting the initial charge current to 0.020 x C in amps, and when the charge current of cyclic charger drops to 0.01 x C in amps the charger should be removed or stepped back to the float voltage level.
 
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