Outdoor LED / Solar panel / Battery Charger Design

Thread Starter

Engineer9001

Joined Dec 13, 2019
22
I'm working on an outdoor LED lighting project. It just basically works as a motion detection light (turns on when movement is picked up through a PIR sensor). The amount of LEDs I want to use brings the power consumption to 7.2W (12V, 600mA), but the LEDs will only stay on for 20 seconds per trigger from a motion detected (~40mA consumed?, can also limit it through the connected STM32 uC if battery levels are too low) It also incorporates a solar panel and battery charging devices (LTC4056).

I wanted to use multiple 18650 batteries (3-4) in parallel and charge them with the LTC4056, however, I read it's not safe to charge lithium batteries this way. It is best to charge them individually. The problem is I want to be able to use the capacity of multiple batteries to power the LEDs. It's possible that the LEDs will drain a single battery faster than the solar panel can fully charge it.

I came up with using multiple batteries, but multiplexed so that if the first battery drops below a reference voltage (3.7V), the circuitry will select a different battery to use. Also since each battery is connected to its individual LTC4056 it would be safely charged?

Outdoor Motion Detect LED Block Diagram 001.png

Is this design feasible? Or maybe there's a better solution/design to my problem?
 

KeithWalker

Joined Jul 10, 2017
1,216
I am not aware of any reason why you should not charge 18650 cells in parallel. It is a very common practice. I recommend that you stay with your original concept and keep it simple. The LTC4056 does not supply very much charge current for multiple cells but as long as your solar cells can supply the required power and you get enough sunshine throughout the day it will probably work OK. It depends on how often the LEDs turn on.
Regards,
Keith
 

Thread Starter

Engineer9001

Joined Dec 13, 2019
22
I am not aware of any reason why you should not charge 18650 cells in parallel. It is a very common practice. I recommend that you stay with your original concept and keep it simple. The LTC4056 does not supply very much charge current for multiple cells but as long as your solar cells can supply the required power and you get enough sunshine throughout the day it will probably work OK. It depends on how often the LEDs turn on.
Regards,
Keith
Are there any safety concerns if they are connected in parallel and one cell has more/less voltage than another cell? Or what happens when one cell starts to fail?

The LEDs would only turn on during night time. I'm thinking if it starts to trigger too often, I could program the uC to limit the LED ON-time duration to 2 seconds, or even disable it if the battery levels are too low and just send notifications over the ESP8266.
 

KeithWalker

Joined Jul 10, 2017
1,216
Are there any safety concerns if they are connected in parallel and one cell has more/less voltage than another cell? Or what happens when one cell starts to fail?
Before you connect the cells in parallel, charge each one up to a pre-determined level. Then when you connect them, the highest voltage one will not dump current into the lowest one. 18650 cells very rarely fail. They just loose capacity as they get older.
Keith
 

Thread Starter

Engineer9001

Joined Dec 13, 2019
22
Before you connect the cells in parallel, charge each one up to a pre-determined level. Then when you connect them, the highest voltage one will not dump current into the lowest one. 18650 cells very rarely fail. They just loose capacity as they get older.
Keith
Thanks! I'll try that.
 
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