# Battery charging circuit

Can anyone assist me with correcting this circuit? I'm trying to build a 12 V lead acid battery charger that would have a green LED indicator when charging and a red LED indicator when fully charged.

I feel the collector should send enough voltage only to allow a flow of current through the emitter if the battery voltage is less than 13.5 V and once it get to 13.5 V (when the battery is full), that should be able to trigger the base which would let the red LED go on.

I thought the capacitor should act as a filter.

I go the charge indicator circuit from Google but missed the site. But I designed the charging circuit from my little like no knowledge of electronics.

And also how can I test if the battery is charging?

I'm an amateur in circuitry. I just love electronics

• (1) "I thought the capacitor should act as a filter." Did it not? What is not working or what do you think is wrong with your circuit? (2) Can you post a cropped screen grab with the grid turned off? Welcome to EE.SE. Nov 24, 2017 at 19:36
• I'm confused. Cos there was still some fluctuations after the capacitor causing instability in the rest of the circuit Nov 24, 2017 at 19:38
• 100uF is pretty light, you will still have quite a bit of ripple at that especially before D1. Nov 24, 2017 at 19:41
• What are you charging? Lead acid battery? Nov 24, 2017 at 19:49
• OK Trevor, what you suggest please Nov 24, 2017 at 19:50

The big problem with your circuit is that most of it does nothing to regulate battery charging.

Figure 1. Main current path from rectifier to battery shown in red.

There is no current limiting in your circuit. Every time the bridge rectifier output exceeds the battery voltage a large, unregulated current will flow into your battery. This is unlikely to give you a well controlled charge.

simulate this circuit – Schematic created using CircuitLab

Figure 2. The basic "charger" part of the circuit has no current limiting.

We'll tackle the problem in steps.

• Lead acid batteries should be charged from a voltage limited source. You have a 12 V battery with 6 x 2 V cells.

Anything above 2.15 volts per cell will charge a lead acid battery, this is the voltage of the basic chemistry. This also means than nothing below 2.15 volts per cell will do any charging (12.9V for a 12V battery) However, most of the time a higher voltage than this is used because it forces the charging reaction at a higher rate. Charging at the miminum voltage will take a long long time. As you increase the voltage to get faster charging, the voltage to avoid is the gassing voltage, which limits how high the voltage can go before undesirable chemical reactions take place. The typical charging voltage is between 2.15 volts per cell (12.9 volts for a 12V 6 cell battery) and 2.35 volts per cell (14.1 volts for a 12V 6 cell battery). These voltages are appropriate to apply to a fully charged battery without overcharging or damage. If the battery is not fully charged you can use much higher voltages without damage because the charging reaction takes precedence over any over-charge chemical reactions until the battery is fully charged. This is why a battery charger can operate at 14.4 to 15 volts during the bulk-charge phase of the charge cycle. Source: Powerstream Technology.

So you need to regulate the charging voltage to 14.1 V maximum. Your circuit has no regulator so you are in danger of overcharging. There are many circuits for this on the web.

simulate this circuit

Figure 3. The charge indicator circuit.

... a green LED indicator when charging and a red LED indicator when fully charged. ... I feel the collector should send enough voltage only to allow a flow of current through the emitter if the battery voltage is less than 13.5 V and once it get to 13.5 V (when the battery is full), that should be able to trigger the base which would let the red LED go on.

To turn on Q1 we need to pull the base lower than the emitter by about 0.7 V. The voltage drop across D1 might be enough to do that.

D5, the red LED, could only turn on if current flows through Q1, D4, D3 and D5. That would require at least 2 V for the green, 0.7 V for D3 and 1.8 V for the red. 2 + 0.7 + 1.8 = 4.5 V. D1, however, ensures that you could never have more than 0.7 V. The red LED will never turn on.

• Thanks so much. So you suggest I bring a current limiting resistor to that path before the diode.? What also have you got to say about the indicator part Nov 24, 2017 at 19:47
• Why don't you edit your original question, (1) explain what exactly you're trying to build, (2) what type of battery, (3) what charge voltage and current you're aiming for, (4) how you think your design should work, (5) what the LEDs are supposed to indicate and (6) where the design came from. I've numbered the points so you can't miss any. Nov 24, 2017 at 19:54
• (1) Answered. (2) Not answered. (3) Not answered. (4) Not answered. (5) You've explained what you want the LEDs to show but not how you think they are supposed to work. (6) Not answered. Score 1.5 out of 6. Could do better. Nov 24, 2017 at 23:44
• Thanks for the step by step guide transistor. I hope to learn a lot from you on my first use of stack exchange. You can check again please Nov 25, 2017 at 5:25