# Question on Current Sensing Resistor in Lead Acid Battery Charger

I was reading several threads on Lead Acid Battery chargers and encountered a circuit diagram given by Russell McMahon here https://electronics.stackexchange.com/a/152641/93631

First of all, I would like to know if this circuit prevents overcharging for 12V lead acid battery or not.

I also want to know the principle of the charger and how it prevents overcharging lead acid battery. I am specifically curious to know the function of Current Limiting Resistor. From the diagram it seems to be a constant current charger (since I am able to set the current limit). Is it true? Or is it the max allowed current limit (and not constant current limit)?

## 1 Answer

I would like to know if this circuit prevents overcharging for 12V lead acid battery or not.

Yes it does if you set the output voltage properly !

Overcharging for a SLA battery is done by charging it to a voltage which is too high. What you must do is set the maximum voltage to the right value so that the charger will not overcharge the battery. This is done with P1. Set the output voltage (with no battery connected !) to around 13.6 to 13.8 V using P1 and you're set. No overcharging possible as the charger will stop charging as soon as the battery reaches set voltage.

The current limiting is needed for when the battery is quite empty. Let's say the battery is 11 V, now we set the charger to 13.6 V. When we connect the battery to the charger the charger will supply as much current as it can to get the voltage up to 13.6 V. But it should not do this, it should just provide a certain current (which is set by the current limiting circuit) and charge the battery with that current until the battery voltage has increased enough and the current will become smaller.

The LM317 also has build-in current limiting already so for a charger using an LM317 you only need a current limiting circuit if the 2.2 A maximum output current of the LM317 is too high or you simply want to lower that maximum current.

So Russell charging circuit has current limiting and voltage limiting. The current limiting is used to charge the battery, the voltage limiting is there to prevent over charging.

I use an even simpler circuit (relying on the LM317's current limiting) to charge my car's battery when needed.

The LM317 will get HOT so it needs to be mounted on a cooling plate or metal box. It does prevent itself from getting too hot though but still cooling is needed.

• Hi FakeMoustache Your answer was really informative. I atleast now understand how it prevents overcharging. So what I understood from your answer is, there is no mechanism in this circuit to turn off charging before the battery is charged to 13.6v. How can I achieve that? I mean, I want to charge with 13.6v but want to stop charging when the battery is around 12.7v charged. Is it called an 'automatic battery charger' which can do this? And I am really eager to see your 'simpler circuit'. Could you please share a diagram? Jun 28, 2016 at 10:17
• If you stop charging a lead acid battery at 12.7V you aren't completely charging it. Why don't you want to charge it fully? Edit the question to explain that and you might get the answer you're looking for.
– user16324
Jun 28, 2016 at 10:29
• Hi Brian, 12.7 volt is just an example voltage which I referred. I don't have any specific requirement to keep it undercharged. I just wanted to know, what if I want to charge the battery to volt x by supply volt y where y > x. Jun 28, 2016 at 11:18
• @sribasu Maybe you think that the charging needs to stop abruptly, like a switch being opened for example. Well, that is not how this circuit works and also not needed. If the charger is set to 13.6 V and after some time, the battery is also 13.6 V, the current will become zero. For current to flow you need a voltage difference. If that difference is zero, the current will also be zero meaning: no charging. It is really that simple. Compare it with a capacitor in parallel with a battery, when the capacitor is full, no current flows anymore. Jun 28, 2016 at 11:38
• And like with a capacitor, the charging will be fast when the capacitor or battery is empty (therefore we need current limiting) but as the capacitor or battery charges, the current will gradually decrease until it will be zero. Jun 28, 2016 at 11:41