Battery charging schematic - Page 1
Forum section: electronics / beginners

author: mike_mike
author_rank: Super Contributor
timestamp: May 01, 2023, 05:07:17 am
content: Hi ! I made the schematic from the link on a piece of bread board but it seems that the output voltage does not go under 7V and the maximum voltage is about 9V (from the potentiometer).I would like to know if this schematic is functional ?I need it to charge a PB acid battery from a 12V 10W solar panel, and using another circuit to light up some leds at night.https://www.electroschematics.com/dual-mode-battery-charger/

author: xavier60
author_rank: Super Contributor
timestamp: May 01, 2023, 07:45:30 am
content: With the divider values shown, it should adjust from 5V to 7.5V. Check for oscillations and TR1 should be fully on, very little C-E drop. If in doubt, bypass it.When a TL431 is regulating, there will be 2.47V on the REF pin.

author: iMo
author_rank: Super Contributor
timestamp: May 01, 2023, 08:01:48 am
content: I've been using that schematics for charging my 4S 16.8V li-ion battery for many years already (had been published long time back on an rc-model site, afaik). I do not use the transistor and my divider uses a larger value resistors. Using a schottky 3A diode and I do not use capacitor across the 431. The R4=1.2Ohm/5W in my case. You want a larger heatsink on the 317.PS: setting up the charger is easy - do not connect the battery and set the output voltage to the max voltage you want (ie 16.8V in my case).

author: Damianos
author_rank: Frequent Contributor
timestamp: May 01, 2023, 08:12:06 am
content: The presented circuit is not a dual mode charger, it is an "unnecessarily complicated" constant voltage with current limit. A dual mode Lead Acid battery charger switches from the "cycle charge" to "float charge" automatically. In the data-sheet of LM117/317 are shown some simpler circuits, to get ideas.

author: mike_mike
author_rank: Super Contributor
timestamp: May 01, 2023, 10:23:51 am
content: I made the circuit on a little piece of pcb, because my breadboard was not good.I was needed to eliminate TR1 and I made a direct connection between the C and E of TR1. Now the output voltage is from about 5V to 7.5V, as@xavier60said.The voltage on ref pin of TL431 is 2.48V.Both led's are on.I used my power supply to power the charger.Should I look for oscillations on K of TL431 and at the output of LM317 ?LE: I didn't test with battery connected at the output.

author: xavier60
author_rank: Super Contributor
timestamp: May 01, 2023, 10:51:24 am
content: Although all seems well, check all 3 pins of the LM317 for oscillations. Im not sure if it's ok to have no bypass capacitor on the output. Check the data PDF for bypassing requirements.

author: mike_mike
author_rank: Super Contributor
timestamp: May 01, 2023, 11:00:05 am
content: It looks like an output capacitor is needed (Onsemi datasheet).ST Microelectronics datasheet specifies the same.But in the charger schematics from datasheet, there is no output capacitor (ST datasheet).LE: I checked for oscillations, on all 3 pins of LM317 and it looks like the circuit is ok.

author: iMo
author_rank: Super Contributor
timestamp: May 01, 2023, 11:09:59 am
content: The battery connected is basically a large capacitor. I put a 220uF/25V at the output (see my above picture) for when the battery is disconnected..

author: mike_mike
author_rank: Super Contributor
timestamp: May 01, 2023, 11:16:51 am
content: My battery will stay connected to the charger during charging and when the night will come, then a relay will connect the battery to a few leds. So during the night, the charger will not have any load connected.Does this mean that I should use a 220uF/25V capacitor in parallel to the output of the charger ?

author: mike_mike
author_rank: Super Contributor
timestamp: May 01, 2023, 12:03:31 pm
content: Also, is this charger compatible with gel batteries ? I know that the gel battery is a type of lead acid battery, but will the charger be compatible with gel battery ?

author: iMo
author_rank: Super Contributor
timestamp: May 01, 2023, 01:41:15 pm
content: I would put the capacitor there. The charger first feeds a constant current into the battery and when it approaches the set voltage it reduces the current to almost nil, thus having the constant voltage at the battery. You have to ask in "Power/Renewable Energy/EV's" section on the types of batteries and the optimal charging method, imho.

author: Damianos
author_rank: Frequent Contributor
timestamp: May 01, 2023, 05:40:00 pm
content: Quote from: mike_mike on May 01, 2023, 12:03:31 pmThis information is in the data-sheet of the battery.I used a circuit with a 7805 and a red LED in series with its ground pin (total voltage about 6.8V), to charge continuously a battery (see attachment) as a basic UPS for a telephone. I didn't used any capacitors or other "fancy" components. Note that for the "float charge" there is not needed any current limit. The battery lasted a little more than 5 years.

author: vk3em
author_rank: Contributor
timestamp: May 04, 2023, 11:54:20 pm
content: If you want maximum capacity and life, I am not a big fan of these chargers - I do encourage you to consider the UC3906 (now known as as the BQ24450). You can consult the datasheet and app notes attached.