DIYers...We have all been through the situation when our high end chargers are busy charging those lithium polymer batteries but you still need to charge that 12v lead acid battery and the only charger you got is a blind one …. Yes a blind one as it never know when its killing the battery by overcharging it…. Same goes while discharging the battery as you have no idea what charge state it's at.
Well I have got a solution for that situation as we are going to built a battery capacity level indicator by using a LM3914 IC and we are also going to add an important feature to our existing charger to cut off the charging current when the battery is fully charged.
If you are also facing the same problem then this intractable is for you.
Step 1: Designing the Schematic
The basic idea is to design a battery level indicator but after going through the data sheet I found that we can easily control the overcharging problem by adding a relay that switches off the supply to the charger when the battery hit the max charge level.
Even though its not exactly what controlled charging is but its better to cut off the charging current before we jump above the max voltage limit stated on the battery pack thats 14.4v.
Since we are going to use 10 LEDs to indicate the battery capacity level so each LED represents approximately 10% charge.
Moreover the current through LEDs is limited using the resistor across pin 7 of LM3914 IC so we don’t need to use individual resistors for each of them.
Besides that the variable resistors R3 and R4 are used to set the upper and lower voltage levels for the battery pack that you are going to use. For the lead acid battery its usually 10.8v fully discharged and 14.4v fully charged. More on that later.
The rest you see are a bunch of complimentary components as advised in the data sheet.
I have also added the Gerber file for the PCB in this step so be sure to check it out.
Step 2: Designing the PCB (Printed Circuit Board)
Well I love it when I make it neat and thats the one thing that I always prefer. So instead of messing up everything on a perfboard I decided to built this circuit over a PCB, so I designed one. Well this step is not mandatory but these extra efforts are going to payoff later and I must say you should give it a try.
As the layout is finalised I went to PCBWAY, checked out all the options that I want and uploaded the gerber files. The best part about their services is that they review your design within an hour and let you know if there is any issue with it.
We received the PCBs within a week and the quality pretty much talks on its own so guys have a look at their website as they made this project possible by sponsoring it.
Step 3: Tools and Material
After receiving the PCBs, we have decided to gather the tools and components that are used in this project.
LIST OF TOOLS USED:
- Soldering Iron
- Soldering Wire
- Multi Meter
Components that are used in this project are listed in the BOM (Bill Of Material).
Step 4: Assembling the PCB
Later we plugged the soldering iron, grab all the components and start soldering them. I will provide the links to schematic, gerber files and the list of the components in the description below. Now all the components are dropped in place as indicated on the PCB and thats the advantage of putting up time designing the PCB earlier.
Now once everything is soldered in place we just inserted the brain of this project thats the LM3914 IC .Make sure to place the IC with the notching correct direction as indicated. I always prefer to use an IC holeder that comes in handy if you burn the IC you could easy replace one.
Step 5: Final Results
The circuit is then connected to the battery and calibrated according to the upper and lower voltage limits mentioned on the battery.
This could be done using the two variable resistors as we have discussed earlier. Once we connected the load across the battery we can monitor the charge level and safely disconnect the load once the battery seems to run out of charge.
Later the supply of the battery charger is connected across the relay. As the battery reaches its maximum charge the supply of the charger is cutdown and thus adding a charge control feature to our so called blind charger.
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