LIC Protection - request for help

[GH Crash]

I need help in designing a simple LI protection circuit.

Circuit basis : LIC, lithium ion capacitors, have a minimum voltage limit. Discharging them and/or storing them with their remaining charge below that minimum voltage will damage or destroy the capacitor. The circuit to be designed would be placed between the capacitor and the load. It would interrupt the current flow when the capacitor voltage dropped to its minimum allowable voltage. The circuit will be used in environments where size and weight are a major concern. The LIC(power source) would have a capacitance between 10F and 100F with a maximum voltage rating around 5 volts. The minimum charge (voltage)limit of the LIC is around 2.1v. The load would be a small coreless DC motor typically rated below 10 watts at a maximum voltage of 4volts.

Here are the circuit requirements

1. The circuit shall not appreciable change the operating characteristics of the load. That is, that the motor's input voltage and amperage and operating speed should be virtually the same, within +/- 2%, with or without the protection circuit in the system.
2. The circuit must be capable of handling currents up to 5 amps.
3. The circuit should operate satisfactorily at LIC voltages between one point five and six volts DC.
4. Quiescent current flow should be minimized as much as possible. Quiescent current flow of for the entire circuit should be less than 10uA.
5. The completed circuit, with all components should fit on as small a PCB as possible, preferably under 15mm by 15mm square.
6. The completed circuit must be light, under 0.75 grams max weight, under 0.5g ideally.
7. Items, or conditions, not specifically mentioned in the requirements are left up to the designer's discretion. Items such as component availability, cost, specification tolerances have no significant bearing upon the circuit design.

Addition information: Supercapacitor (EDLC) have been used for years as an energy source for electric free flight model aircraft that use small coreless DC motors connected to the propeller. The normal flight consists of charging the capacitor, switching on the motor, and launching the plane. The motor powers the plane until the capacitor is completely discharged. LIC, lithium-ion capacitors (sometimes called hybrid capacitors) are starting to replace the supercapacitor due to their lighter weigh and greater power density. Unfortunately completely discharging a LIC to a zero volt potential sooner or later will destroy the capacitor. The LIC protection circuit would prevent the motor from completely discharging the capacitor.

Questions:
What type of circuit would suggest as the best approach to meeting the goals and requirements given? How would go about protecting the LIC?

I posted this help request in another electronics forum. One of the answers given was to use the same circuit used to protect Lipo batteries. That was tried and the results were mixed, seemed to work for one individual, completely failed for another individual.

 

[ZipZapOuch]

That all sounds rather specific. Will you be sharing your fees with the engineer who does your work for you?

 

[rjenkinsgb]

The principles are no different to a lithium cell protection module, just a slightly different voltage range.
There are thousands of versions of those already.

(I'm not suggesting using modules designed for batteries - I'm saying that the design principles are the same).

There are also dedicated supercap management ICs - this one looks like a possibly suitable controller; it also includes charge control and output voltage boost:

https://www.farnell.com/datasheets/1860688.pdf

Or, just a low power microcontroller switching a couple of power FETs for charge & discharge cutoff.
eg.

https://ww1.microchip.com/downloads/aemDocuments/documents/MCU08/ProductDocuments/DataSheets/40001799F.pdf

 

[GH Crash]

That all sounds rather specific. Will you be sharing your fees with the engineer who does your work for you?

Well..... yes and no. I'm still negotiating with myself to determine my salary.

It is rather specific. Past experiences have shown that forum threads, and projects, tend to drift off course if not well defined at the start.

 

[ZipZapOuch]

It seems to me that the LiC manufacturers pretty well define what is needed in the datasheets of these widely used capacitors. Why is it so much trouble?

 

[GH Crash]

Thanks for the suggestions JRW.

First glance it looks like the Farnell LTC may be for higher power applications. Its Vin is 4.5v to 35v and for this project the voltages will be in the range of 2.5v to 5v. Using your second suggestion would really push my electronics knowledge and skill levels but I will longer study of the information in the link..

 

[GH Crash]

It seems to me that the LiC manufacturers pretty well define what is needed in the datasheets of these widely used capacitors. Why is it so much trouble?

ZipZapOuch, I'm not sure to what you are referring. My experience is that all the LIC manufacturers specify a minimum voltage for the capacitor but do not specify/give the method(s) by which over discharge is to be prevented. Can you give a specific example where the LIC manufacturer specified the means to prevent over discharge?

 

[ZipZapOuch]

ZipZapOuch, I'm not sure to what you are referring. My experience is that all the LIC manufacturers specify a minimum voltage for the capacitor but do not specify/give the method(s) by which over discharge is to be prevented. Can you give a specific example where the LIC manufacturer specified the means to prevent over discharge?

They certainly give a maximum current, maximum voltage and minimum discharge. Read the datasheet. Also, to clarify your goals, are you looking for someone to...
- read the datasheet for you
- confirm your interpretation of the datasheet
- interpret the datasheet for you,
- validate that your circuit's design meets the requirements of the datasheet, or
- completely design the circuit that meets the requirements in a yet-to-be-specified manufacturer and yet-to-be-specified part number.
- completely design and fabricate the circuit that meets the requirements in a yet-to-be-specified manufacturer and yet-to-be-specified part number.

 

[GH Crash]

ZipZapOuch,
I suspect that you are not reading my posts correctly, or carefully enough. If you wish, I will be glad to clarify every statement of each of my postings.

I can read data sheets. Probably as well as you. I need no help with the data sheets, in any form or fashion.

Let me repeat: What type of circuit would you suggest as the best approach to meeting the goals and requirements given?

I'm asking how different individuals might approach designing a circuit. I'm not asking for a circuit design. I'm not asking someone to validate my design. And, I'm not asking for design or fabrication services.

This is a project for personal use. No considerations have been given to manufacturing or sales. At most, there might be a possibility that the design, circuit or finial device would be shared, without costs, to interested individuals.

I don't want a pissing war over datasheets. Yes, they give max current and voltage, and minimum discharge. As yet I have not seen a LIC datasheet that specified exactly how over discharge is to be prevented. This forum thread is an attempt to determine how other individuals have prevented LIC over discharge. If you wish to continue this discussion on reading datasheets, may I suggest that we do it through a private conversation.

 

[ZipZapOuch]

umm, I'd design it to charge using a constant current source until voltage is close to the peak as listed on the datasheet. I'd stop charging when the voltage is close to (at) the max recommmended level.

For discharge, I'd do the opposite. Likely controlled by a microcontroller with ADC and on-board voltage references.

 

[GH Crash]

Oops. It appears that my last posting got lost. I will re-post it soon as get a chance. Right now, I'm late for a meeting.

 

[ZipZapOuch]

Oops. It appears that my last posting got lost. I will re-post it soon as get a chance. Right now, I'm late for a meeting.

Only two kinds of meetings on Sundays

 

[GH Crash]

ZipZapOuch’s suggestion sounds good but, as usual, I think I didn’t do a very good job of explaining things; of explaining what I needed help with.

I’m not trying to design an entire LIC charge/discharge circuit. I’m just trying to add a small and simple over discharge protection circuit to an already existing circuit. It would probably help if I explained the entire project.

I’m using capacitors to power small free flight aircraft. There is a small circuit on the airplane, called the CMC (capacitor motor controller) that regulates power, voltage, from the capacitor to the motor. I want to add capacitor over discharge protection to that circuit. And that is where I’m asking for help in determining what might be the best approach and what has been done in the past along those lines. Charging of the capacitor is done by a completely separate circuit that is plugged into the CMC only while the capacitor. Again, what I need help with is adding over discharge protection to the existing CMC circuit.

Presently the CMC circuit exists in two forms. One is a simple voltage regulating circuit and is used in planes weighing less than ten grams and motors less than 10 watts. The other circuit is a PWM circuit with motor run time and constant power output functions built into the ATTiny85’s code. (I’ve attached simple circuit schematic for both versions of the CMC.) I want to add the over discharge protection to both circuits. What is the best way to do that? Hopefully, I can use the same over discharge protection setup for both CMC circuits.

ZIpZapOuch, what is the other kind of Sunday meeting?