Electronics > Beginners
Weird simple car battery charger
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Eldi4:
Hello,
Excuse me, it is probably a very very beginner question
I've been looking for a simple car battery/lead acid battery charger using 12V Linear Transformer to charge 1Ah to 100Ah (or literally any capacity) car battery/ lead acid battery, and then i found this one

I think there are something wrong with this circuit, my analyze is the relay will be cut-off when the voltage is around 11V, and when the voltage is above 11V the relay is on, and will never going off again except the voltage is going down to 11V, so i think it is more likely a Overdischarge protection circuit rather than a charging circuit,

But if i flipped the the op-amp non-inverting and inverting input, and change the +12V (which is a supply voltage) on the inverting input (after flipped the input) side of op-amp to battery voltage instead, the relay will be off when the voltage exceed 14.2V and turn on again when the voltage is going down to 12.2V.

Are my analyze is right?, this charger will be working when i flipped the op-amp input.

If you have any ideas please tell me, or you have a suggestion on a simple car battery charger.
ArthurDent:
Check the data sheet for the LM311 which isn't an opamp but is a comparator.
Ian.M:
A LM311 is a  differential comparator not an OPAMP.

There's something screwy about the circuit round it.  Normally you'd have positive feedback to provide hysteresis, but it looks like that arrangement has negative feedback via R1 (270K) and that's going to make its switching transitions mushy and may even make it oscillate due to C2.

With in+ > in- the LM311 collector output will be high (see datasheet p13 section 9.2.2.1 Input Voltage Range) and the relay will be on.  It looks like an undervoltage lockout circuit so it only connects the 12Vin to the battery output if the 12Vin is high enough to charge the battery.  Consider a caravan 'house' battery being charged from the tow vehicle.  When the engine is stopped the relay would disconnect the 'house' battery from the vehicle battery so loads on the 'house' battery (e.g. fridge) don't drain the vehicle battery overnight leaving you stranded in the morning. 

It gets away without an over-voltage cutout for the end of charging by having a transformer that at nominal mains voltage only just puts out enough voltage to reach fully charged.  However odds are its float voltage is too high for long battery life unless its not maintenance free and you keep it well topped up.  When you use its 12Vin, it relies on the vehicle'a alternator regulator to limit the charging voltage.

Unless you need the capability to charge from a 12V vehicle supply, this charger has very little to recommend it, and it is likely to drastically shorten the life of SLA and maintenance free batteries.
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