Help me design a LiPo discharger with storage voltage cutout! - Page 1
Forum section: electronics / projects

author: ilium007
author_rank: Frequent Contributor
timestamp: December 29, 2012, 12:57:06 am
content: Hi guys - this is my first post here, looks like this is a fantastic forum ! I am based in Brisbane, Australia and whilst I am not an electronic engineer I have been around electronics since I was a kid and still love learning about and building electronics projects. I have experience in Arduino based projects (electronics and programming).I am wanting to build a LiPo discharger that can handle (initially) 3S 11.1v batteries that I use in an RC plane.Background: LiPo batteries must be stored at a safe storage voltage- approx 3.85v per cell otherwise they puff up and destroy themselves. When you charge up 6 batteries t go flying and then the weather turns bad you are then faced with the task of discharging these batteries to about 3.85v per cell. I want to build a high current discharge unit that will discharge at a reasonable current, say, 10A a 20A (these cells can handle very large discharge currents) and then remove the dummy load at about 3.85v.I have scoured the net and have found a few projects that people have implemented but none are perfect - I need help to make the perfect LiPo discharger !This one is quite crude but I like the use of the LM317 as a current drain rather than a big wire wound power resistor:Then there is this design that uses an op-amp as a comparator (I have no idea how the op amp works and need to research this):The values for the components are as follows:--------------------------------------------------------------------------MP1 - P Power MOSFET 5+V 5+AU1 - Input neededU2 - LM317 adjustable regulatorR1, R3, R4 - 1kR2 - 590R5 - 220R6 - 1ohm, 20w - sized to provide a 4A drawD1 - LED of your choice--------------------------------------------------------------------------And then there is this older design:I would like to get a better understanding of OP-Amps and an LM471 project would be cool and instead of wire wound, high wattage resistors I thought I could make use of the LM317's as a constant current load to drain the LiPo's.Any help at this stage would be greatly appreciated.

author: ilium007
author_rank: Frequent Contributor
timestamp: December 29, 2012, 02:15:27 am
content: As far as the constant current sink goes I found this design in the TI datasheet for the LM150 regulator:http://www.ti.com/lit/ds/symlink/lm350-n.pdfA 6A constant current sink would be enough for my needs - I would still need to sort out a low voltage cutoff circuit though.What are peoples thoughts on this 6A design ?

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 29, 2012, 03:23:25 am
content: LVCO is pretty easy to handle, have a voltage reference provide say 2.5v and a resistive voltage divider provide a voltage of 2.4999 when the voltage input is 3.85v, then use an opamp as a simple comparator to drive a mosfet, have the resistive divider powered from the put of the mosfet so that when it turns off, there is no power through the divider so it stays off, that stops it from oscillation.

author: ilium007
author_rank: Frequent Contributor
timestamp: December 29, 2012, 10:37:49 am
content: Quote from: peter.mitchell on December 29, 2012, 03:23:25 amOh my dear - I am now so confused :SI did however manage to build a simple LM388K current sink that is dissipating 5A - if I could get this up to 15A I would be happy. If I could daisy chain 3 of these together I could draw 15A - I would need a massive heat sink though ! Is there any risk in driving these with no resistor between Vadj and Vout on the LM388K (or LM317 as in the first circuit diagram I posted ) ?

author: IanB
author_rank: Super Contributor
timestamp: December 29, 2012, 11:00:57 am
content: The art of good engineering is not to over complicate things.So for instance, if your only wish is to discharge the batteries for safe storage you do not need any kind of complicated constant current circuit. The difference in voltage between 4.2 V and 3.85 V is only 8%, so even with a constant resistance load the current will not vary by more than 8% during the discharge. You will not even notice.However, it gets better. There is a very simple and cheap constant current load available that would be ideal for this purpose--it is the plain old 12 V automotive bulb. You can just gang them up in parallel to get the current you need.All you need electronics for is the control part of your circuit. You need FET transistors to use as a current switch and a voltage reference and a comparator to switch the FET(s). The comparator will compare the battery voltage with the reference voltage and will switch off the current when the battery voltage reaches the target.The nice thing about using the bulbs as the load is that they will go out when the circuit stops discharging giving you a nice visual indication.

author: IanB
author_rank: Super Contributor
timestamp: December 29, 2012, 11:04:26 am
content: Quote from: peter.mitchell on December 29, 2012, 03:23:25 amIt would be much better to usea comparatoras a simple comparator. Op amps are designed for analog regulation, not switching.

author: ilium007
author_rank: Frequent Contributor
timestamp: December 29, 2012, 11:22:37 am
content: Quote from: IanB on December 29, 2012, 11:00:57 amThanks - a lot of other people are using bulds but I thought it would be cool to have a play around with a LM317 / LM338 - its purely academic.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 29, 2012, 02:47:10 pm
content: Quote from: IanB on December 29, 2012, 11:04:26 amBy all means, if you have some comparators lying around, or if you are going to buy the parts online you can use a comparator, but if not and you have an opamp, in the project there isn't much reason to use one or the other.And while using a bulb as your load can be done with no issues, they aren't as easy to cool as say, some to-220s attached to a heatsink with a fan, so if left unattended you could burn something, nor are they as small.While bulbs don't NEED cooling generally, the packages DO need cooling, as their operating temperature range is smaller and the heat output is more concentrated, so the problem is basically a hot bright light vs a warm noisy heatsink and a bit more bother.

author: ilium007
author_rank: Frequent Contributor
timestamp: December 29, 2012, 02:52:23 pm
content: Thanks fo rthe reply - I intend on going down the IC path for this little project. I have been thinking about it all day and have realised another small issue. The LiPo batteries need to be balanced at the completion of the discharge. LiPo's come with the main leads connected to the battery (capable of up to, say, 60A) and also a small harness that has a common and 3 leads - these are the balance leads and allow a charger to monitor / discharge each cell individually to keep them balanced. I am thinking that It might be better to build 3 parallel circuits that each discharge down to 3.85v via the balance leads - but these leads are small and I am not sure I could draw 5A through them.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 29, 2012, 05:20:06 pm
content: I do not see how this is an issue, when discharging, through the main connector at a reasonable rate, I doubt the cells would deviate that greatly, and more so, the cells are all balanced by the charger when you charge your pack before you go and fly anyway.

author: ilium007
author_rank: Frequent Contributor
timestamp: December 30, 2012, 01:15:38 am
content: Yes - you are probably correct. As long as the cells are balanced to start with.

author: ilium007
author_rank: Frequent Contributor
timestamp: December 30, 2012, 03:02:57 pm
content: I am going to do some more research into the LM10 opamp as it has a 0.2v ref voltage builtin.If I were to decide to measure each of the three cells in my batteries could I have three of these circuits feeding the gate of a single mosfet ?