LiPo over discharge protection

[shumifan50]

As promised the circuit schematic, artwork(not very arty), source files and some notes.
The code was a tight squeeze once I added the display( CW-LCD-02 serial display TTL), so I could not use the floating point library to calculate values more accurately, so the voltages are of by up to 0.3V, but for its purpose this will not matter to me.
I had endless grief with the ADC, where initially the values were all over the place with a 10K/5K6 resistor network. The current solution is the 0.1uF caps, but I am not sure that it is that good a solution. The problem is with impedance(I think) as the PIC requires a maximum 10K impedance. I don't want to lower the resistors too much as I don't want to draw excessive current.
I am currently using a relay to switch the battery, but would ideally like to do it solid state, but lack the knowledge of FETs etc.

Any pointers will help. Note that at a minimum it must be able to handle 40 Amps.


I just realised I uploaded the wrong source files. It is the version I was using to test. Instead of turning the relay on at startup and turning it off when there is a cell with a low voltage, it just turns on the 'relay' (LED during testing) when the low-voltage happens. The low voltages are set at 4.13V (for testing) so would have to be set at 3.8V (safe).

 

[(*steve*)]

I tried to open this with DesignSpark 6.0 but it won't open. Can you post an image?

 

[shumifan50]

@Steve:
The image would not be good as it becomes a mess with the silk screen and bottom layer printed together. You can download ExpressPCB for free and that links the schematic and PCB layout so you can check the net connections. If you prefer not to do this, I will post an image as best I can.

 

[(*steve*)]

Sorry, it looked like a designspark project. I prefer not having to download and install too much software. If I have to, it would be nice to know what I need (I don't think you mentioned it earler)

I might download that later...

 

[shumifan50]

Attached 2 jpg files -hope they are OK

 

[(*steve*)]

OK, that looks fine except for a couple of things:

1. The relay is not a viable option, it will consume too much power. However you're asking about a mosfet elsewhere, go with that.
2. Use the lowest clock speed you can to reduce power consumption.
3. Beware of how you use the MCLR pin. Check very carefully if your still permits in circuit programming (if that's what you'll be using)
4. The quiescent current drawn by the LM78L05 may be too high. It might be worth looking for a lower quiescent current option.
5. you may need an output capacitor on your voltage regulator

 

[HellasTechn]

One thing you may also want to use is Protected Lipo batteries. They are supposed to cut off when reaching critical level of discharge and also protect against over current draw (short circuit).

 

[shumifan50]

@HellasTechn:
Protected LiPos will be dangerous while flying the Quadcopter - rather lose a battery than crash a chopper. My radio/reeciver/sensors have telemetry and monitors cell voltage levels, raising an audible alarm when the cell/battery voltage drops below selected minimums(Taranis radio withXR8 receiver and voltage sensors). This exercise is for protecting the batteries while bench testing where I have forgotten to disconnect the batteries on some of the devices after a session. I have to use batteries during bench testing as my desk power supply cannot deliver the required amps to all the devices - quad, FPV screens/receivers.

 

[shumifan50]

@HellasTechn:
Flying a quad with a protected battery could be dangerous; I rather lose a battery than a quadcopter(several hundred pound vs £20). I use a Taranis radio with XR8 receiver and a cell/battery sensor which gives me telemetry on the radio with audible warnings when voltage settings are violated. This solution is for bench testing as I don't have a desk power supply that can provide enough power for testing a ll the devices(quad, FPV etc).

@Steve
Thanks for taking the time to look at this - much appreciated. I will take all on board and will post back the adjusted files.

An off-the-shelf solution would have been nice, but this has been interesting and a great learning experience. I wish I had done electronics rather than civil engineering, but in 1965 electronics was not all that exciting yet.

 

[HellasTechn]

Ok then just use protected cells for workbench testing. anyways it is always best not to forget... You are welcomed.

same here back at 60's but 1970-1980 was a golden period for electronics. You could work an everything !!! now with the microprocessors and the SMT's there are very very little you can do as far as repairing or playing with circuits.

 

[shumifan50]

microprocessors and the SMT's there are very very little you can do as far as repairing or playing with circuits.

I did this circuit with through hole only as I was going to post it here. Normally I make SMT boards with 0804 components and have even used TQFN micros on double sided boards etched at home - but it is finickity. and requires careful control of the etching process. Just wish I knew more about electronics I spend an awful amount of time finding example circuits or asking for help.

The forgetting is just age catching up with me