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I have a 3Cell LiPo battery which I am going to use with an RC Car, and I want to design a circuit to protect the battery cells from going under ~3.3v each. So what do you think of this design?

enter image description here

As you can see in the schematic, I am using the MCP111-315 which detects voltage drop below 3.08V (active-low, open-drain).

  • Diodes are used to increase the detected voltage level, so it will detect the power at 3.08 + Vf (3.3 is my preferred value). However, the MCP111 only draws 1.75uA (max) and most datasheets only provides Vf for as low as 1mA. Is there a more reliable way to get this ~0.2 voltage drop at such low current? maybe a voltage divider?

  • I am using optocouplers since I don't have a common ground between Cell 1 & 2 and the MCU

  • Any cell that falls below 3.3V should pull the MCU pin down (MCU Internal pull-up resistor is turned on on that pin)

  • C1, C2 and C3 are bypass capacitors for the MCP111, whereas R1 & R2 are series resistors for the optocouplers LED.

Any suggestions on this design or do you have a simpler idea?

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  • \$\begingroup\$ Not a direct comment on your design but I thought this part was pretty cool from linear tech cds.linear.com/docs/Datasheet/4071fc.pdf Looks like it's a charger as well as a battery protector and later on they show you can stack them for multiple cells. Just a thought, if it helps you. \$\endgroup\$ – Some Hardware Guy Oct 1 '12 at 23:32
  • \$\begingroup\$ @Some Hardware Guy, I think LTC4071 works best for charging the battery, and also for low current applications, and a bit expensive (~$5 x 3Cells). \$\endgroup\$ – Fahad Alduraibi Oct 1 '12 at 23:56
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    \$\begingroup\$ Yeah linear's always expensive :) Your circuits not bad at all in fact I looked at diagram for how the MCP111 works and it looks a lot like how you'd do it with opamps. Found this too, Seiko makes a whole line of protection ICs for LI-Poly batteries. There's a three cell one, I saw one cell ones on Digikey for less than a dollar. \$\endgroup\$ – Some Hardware Guy Oct 2 '12 at 0:27
  • \$\begingroup\$ How about if i use a voltage divider with R1 = 71.5K and R2 = 1M? I think that will give me ~3.08V out of 3.3V and a maximum of 4uA current leak at 4.2V \$\endgroup\$ – Fahad Alduraibi Oct 2 '12 at 11:22
  • \$\begingroup\$ @SomeHardwareGuy Seiko's IC are really good if I want to also integrate the charger into my design, the down side is that it needs 2 FETs and extra component which will complicate my design. Also the over-discharge voltage values are lower than what is recommended for Lipo batteries!, however they have a voltage detector S-80833C similar to the MCP111 but it detects 3.3V, so I won't need a voltage divider or a diode to detect 3.3V \$\endgroup\$ – Fahad Alduraibi Oct 2 '12 at 14:43
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There's battery monitoring, and battery protection. They're two different things, and different design considerations apply to each. Sending a signal to a microcontroller is a good design for monitoring -- you can alarm and tell the user their battery is low.

But generating a digital signal isn't stopping discharge, so it doesn't provide any protection to the cells. The only way you can read your digital signal is by powering the microcontroller... which continues to drain and damage the battery.

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  • \$\begingroup\$ @FAD: This answer above is important. Also, it's VERY likely that your LiPo already has undervoltage cutoff built in. But if your purpose is to keep it from even entering the deep depletion region, you have to remember that the LiPo is likely to go into it anyway because of its own self-discharge. So in fact, if you keep your microcontroller's current draw small enough (comparable to LiPo's self-discharge or lower), then it might even be useless to add undervoltage monitoring / power-off. (You can sleep most microcontrollers to minimize current draw to microamps) \$\endgroup\$ – OrCa Nov 1 '12 at 3:19
  • \$\begingroup\$ What you guys say is correct for general applications, however, I am using this in a remote control car which I have upgraded to Lipo. The Lipo I have doesn't come with protection circuit and I think it is the case with most batteries used for RC toys. Cell phone batteries however have such circuit built into them. Also my concern is over-discharging the battery while operating the car, so my circuit will interrupt the microcontroller to cut the power going to the car motor by turning off the H-Bridge and alert me with a signal so I can unplug it totally. \$\endgroup\$ – Fahad Alduraibi Nov 1 '12 at 19:50
  • \$\begingroup\$ @FAD: Then you have a battery monitoring circuit, along with a software-controlled power interrupter. So design a good battery monitoring circuit. Does your MCU have any unused ADC pins? \$\endgroup\$ – Ben Voigt Nov 1 '12 at 20:30
  • \$\begingroup\$ @Ben Voigt, yes I am using the ATmega328 which has 8 channel 10bit ADC and all are unused, been using the digital pins only. I would love to know your suggestion but why do you want to use the MCU ADC instead of what I am suggesting above? Is it any better? How much power will it consume? \$\endgroup\$ – Fahad Alduraibi Nov 2 '12 at 0:04

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