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### Topic: LiPo battery and safe operation (exhaustion) (Read 3572 times)previous topic - next topic

##### Feb 07, 2012, 02:54 pm
So, I have a 11.1V LiPo battery, a small circuit with a LED (LiPo Saver they call it) that tells me when I reached its limits (low voltage <3V per cell). Now, I'd like to run the complete project (arduino, motors etc.) via some sort of circuit that will only work if I have enough power in the battery.

What I was thinking (discard if bullshit): I take an optocoupler with triac (MOC3041 or something) that I'll try to run via a voltage divider that will deliver the minimum amount (1.3V) at the maximum (11.1V) voltage (or a better equation) e.g. if the voltage of the battery drops, the voltage divider cannot supply enough voltage the drive the LED on the optocoupler...
Or, I use this LiPo saver somewhere in between with a transistor npn in parallel with its LED... ...something (you have probably guessed my knowledge level at this point...)

Better suggestions are MORE than welcome. My main question comes down to: "What is the most save way of protecting my LiPo battery from undercharge?"
I prefer to shield the stuff as much as possible from each other.

#### PeterH

#1
##### Feb 07, 2012, 03:14 pm
There is an Arduino clone that has a Lipo battery charger built in. I  don't remember whether it also has the low voltage protection you're describing, but I think there is a good chance it has. I don't remember the name of the clone, but I'm sure Google would find it for you.

#### retrolefty

#2
##### Feb 07, 2012, 03:26 pm
Your lipo is a 3 cell battery. Maximum charge voltage is 4.2 X 3 = 12.6vdc, which is the cutoff voltage value for any charger you are using with the battery. Minimum voltage is 3 X 3 = 9vdc, but many recommend using a 3.5 vdc lower cutoff value for longer battery life, so 3.5 X 3 = 10.5vdc. The better (and a little more expensive) lipo battery packs have a small built in circuit to cut out the battery if either lower or upper battery voltage is sensed, however you must reference the batteries datasheet to see if it includes this optional protection circuit, and if not assume your pack has no such self protection.

As far as having your arduino being able to sense the battery back voltage and shutting off when lower voltage limit is reached that requires two functions:

1. Voltage sensing circuit. This is usually a simple two resistor voltage divider circuit which then wires to an arduino analog input pin. Your sketch code would include a function that periodically reads the analog input pin and when the reading is lower then the low cut-off value activates a:

2. Voltage cut-off circuit. This is normally either a small relay or a MOSFET power switch that wires to an arduino output pin. when the voltage reaches the lower cut=off value the arduino output pin activates and turns the 'power switch' off. This kind of power switch normally requires a manual push button switch to turn power back on, that is the arduino sketch and power circuit can only turn off the power not turn it back on without manual assistance.

Lefty