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As I am planning to go ahead with my electric tractor conversion, and will use 12V SLAs (for low cost, where more weight is good, and usage will be only occasional). So I want to make a fairly simple but effective charging system and BMS for a string of 4 to 24 SLA batteries or even a single higher capacity FLA battery. So here is my design idea at present:

This will have several functions. While connected to a battery, it will monitor the voltage constantly with a low current drain (100 uA maximum) which I realize will drain an 8 Ah battery in 80,000 hours (9 years) so that is not a problem. While the battery is fully charged, it will flash a green LED with 10 mA at 5 second intervals with a 1% duty cycle for an average of 100 uA. As the voltage drops due to self-discharge it will change to yellow (by alternating the current to light both the red and green), and then when it is really low it will flash the red LED.

There is also a 12 ohm 12W resistor which can be switched ON to test the battery under a nominal 1 ampere load. This may be done automatically for a very short time at a very low duty cycle (10 mSec per hour), which is long enough to take a voltage reading under load, but an average current drain of less than 3 uA. This will give a better indication of battery health, and could be reported by means of a different flash pattern such as alternating red/green.

For charging, I want to be able to use just about any DC source, such as a 12-15 VDC unregulated wall-wart or a laptop computer supply which are usually 15-20 VDC and 3 to 5 amps or so. They can be obtained for as little as $4 to $12 each: PS/ PS/ PS/

This raw voltage will then be controlled by means of a simple buck switching circuit which uses PWM and a small inductor to provide the desired current and/or voltage. The current is detected by means of a 0.01 ohm shunt which gives 100 mV for 10 amps and a 0-2 VDC from the high side shunt monitor INA197. I will actually probably use an INA213 which is bidirectional so it can measure both charging current and discharge.

I show a PIC12F675 which appears to be only just able to perform the functions desired, and leaves no extra IO for control functions. So I will probably redesign this using a PIC16F684 which has PWM functionality, and maybe even PIC16F1454 or 16F1825 which has USB and USART, so I could easily add Bluetooth communications using the serial port.

For right now, I just need a charger/monitor for the four 12V 12Ah batteries I got last year, and they need to be properly charged and monitored. I think I'll make as many as 10 PCBs (or possibly 3 PCBs with 4 identical circuits on each). I may think of some other options to add but I want to use the KISS principle because I often keep adding things and make it overly complex and never use the extra circuitry. I think the basic circuit can be built for about $10 in parts, so maybe $25 per battery if you include the PCB and the external charger.

If anyone is interested (or if you see any design problems or have suggestions) please let me know. I could order some extra PCBs and have them available for purchase and use for anyone who has SLAs or FLAs. ;)


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