Lead Acid Charging Circuit in Car

[D_Men]

I have designed a circuit for in car use that operates at 12VDC. However I need to have the circuit have a backup power supply separate from that of the car battery. I have chosen a small 12V Lead Acid Battery. The problem I have run into is trying to charge the battery. I have been looking at charging circuits using the LM317 and so on but because of the voltage drop these won't work. The around 13V that I can get out of the cigarette lighter of the car when running should be enough to charge my battery, but I need a circuit that will stop charging the battery when it is fully charged, so I don't overcharge the battery. So a circuit that can take input of about 13V and charge the 12V battery is what I have spent hours looking for with no avail. Can anybody recommend anything or have any different advice on how to charge the battery.
Thanks In Advanced,
Dave

 

[kchriste]

It may be OK to float charge the battery indefinitely at 13V. Can you post the spec sheet for the battery you'll be using?

 

[desmondlai]

i also doing this kind of charger

View attachment battrey..pdf
this is the battery datasheet im using ! ny charger output is 13.29v do it possible to charge the battery in this state?

 

[Hero999]

View attachment 40178
this is the battery datasheet im using ! ny charger output is 13.29v do it possible to charge the battery in this state?

Don't ask the same question more than once.
https://www.electro-tech-online.com/threads/12v-lead-acid-backup-circuit.104858/

 

[tcmtech]

Don't ask the same question more than once.

Looks like two separate applications to me. One is for a line powered charging system and the other is for an automotive powered charging system.

Your vehicle works with a 12 volt lead acid battery system already and the charging system is designed to work with that type of battery so just charge your other battery directly from the vehicles electrical system with a single diode in line from the charging source so that it wont back feed to the vehicle if the engine is off.
Also if your vehicles charging system is working properly you should see around 13.8 - 14.4 volts as a normal voltage when at driving speeds.

 

[crutschow]

Your vehicle works with a 12 volt lead acid battery system already and the charging system is designed to work with that type of battery so just charge your other battery directly from the vehicles electrical system with a single diode in line from the charging source so that it wont back feed to the vehicle if the engine is off.

A standard diode will reduce the charging voltage by about 0.6V which means the battery will likely not be fully charged from the generator. A schottky diode will reduce the drop to about 0.4V but the charging may still be marginal. Even better is an ideal diode circuit such as this.

 

[tcmtech]

Over the years I have had a number of portable devices that used the small 12 volt LA batteries and where rechargeable from the cigarette lighter. All of them just used a single diode to prevent reverse conduction when the engine was off.

There is the ideal theoretically perfect charging method and then there is how things will actually work in reality. Given the less than perfectly stable and uniform charging systems in vehicles they still do just fine with rather wide working voltages.

 

[Hero999]

Looks like two separate applications to me. One is for a line powered charging system and the other is for an automotive powered charging system.

Do you think?

The schematics look identical to me.

 

[tcmtech]

I looked at the other thread first and saw the AC input on the schematic and assumed he was referring to using a line powered system possibly for charging whatever it is at home.
Then I saw the title of this thread referring to a car and assumed that he was also needing a charging method that worked in his vehicle as well.

This is what happens when people dont elaborate on what they want or are trying to do!
We all start making assumptions and taking guesses.


As far as my thinking goes, I stopped that a long time ago. I got tired of being the outsider in society and decided to try and fit in by being like most everyone else.

 

[MikeMl]

The battery voltage in a modern car sits at 12.65V+-0.1V while the car is parked with nothing turned on; it is at 14.2 to 14.7V while the engine is running fast. A 12V SLA battery should get 13.7V+-0.1V while being "floated"; and 14.7V+-0.1V while being "charged". If the SLA is simply paralleled across the car's battery, then it will be discharged while the car is parked, and will likely become overcharged if left connected if the car is driven for more than two hours per day.

Using a relay to connect the SLA to the car's battery only when the car is running will solve the back-discharge problem, but it still leaves the potential overcharge problem if the car is driven a lot. A circuit consisting of a voltage reference, a comparator, with hysteresis, which pulls-in the relay at SLA<=13.6V, and turns off the relay at >=14.4V, would prevent the overcharge.

Another simple way is to use a Schottky rectifier diode between the car battery positive and the SLA positive. That prevents the back discharge, and drops the charge voltage by about 0.35V (from 14.7V to 14.35V) which the SLA will tolerate for a couple of hours per day.