Attempting to make an automated LiPo discharge device

[razor02097]

Hello all, being who I am I like to tinker... a lot! This is not article material yet as the following device is barely worthy of prototype status...

Currently I have a Electrifly charger that does not have the capability to discharge batteries to storage voltage (or 3.8v per cell). I thought about just using the discharge function and putting a cheap voltage monitor on the balance plug so it alarms when it is time to take it off but number one the charger gets hot when discharging and number two it is plugged in the wall so not portable... I don't want to be stuck inside the room the whole time!

I looked around on the internet to see what other people had for ideas on making a discharge rig to properly store batteries. I did find one DIY rig...but it involved a $30 cell logger device with an alarm output... Which makes it simple but I'm not content with that! Especially when I can use stuff I already have! I decided I want to see if I can make a cheap hobby king voltage monitor turn off my homemade discharge rig.

Here is the prototype... Yes it is made of cardboard... it is a prototype. Yes I have a fire extinguisher with me too




Here is the (badly drawn) circuit done on microsoft paint... because that is all I have...



Note the LED and it's resistor isn't on the schematic on purpose.

One thing about these cheap voltage monitors... as far as I can measure they output about 3.3 volts in very short bursts to activate the chirping speakers. I tapped in to one of the speakers so the other one still works. I had to use an NPN transistor to make the output run the relay.

This rig will discharge 3S batteries all day no problems! Now for the downsides... The relays are 12V and while I can start to discharge a fully charged 2S battery once the voltage drops below 8V the unlatching relay will not actuate causing a potential unsafe condition if ever left unmonitored (which should be never...). However the alarm will still sound continuously even so. I also don't know if the relays would be okay on 4S. I know a car can get up to 14.8V but a fully charged 4S can be upwards of 16V.


I am not an electronic engineer or anything. I tinker with electronics as a hobby so I'm hoping for some feedback to refine my design? Any input or constructive criticism I welcome! Thanks in advance

 

[ZoomNBoom]

I can only commend you on your Paint skills. Very impressive, I wouldnt be able to draw a scheme like that in paint .

 

[razor02097]

lol thanks I tried to fire up multisim to draw out the schematic but the version I have isn't Windows 7 compatible


I did some more testing on it and thinking to get this thing to work with 2S a simple 5V linear voltage rectifier and a 5V relay for the trigger...I'm not worried about efficiency with this circuit (obviously, it is a discharging circuit!). I may have to scrounge around...I know I have a voltage rectifier but not sure if I have a 5V DPST relay. The nice thing about protoboards... I can swap out components easily!

 

[PHugger]

I'm not sure I saw any power resistors. Is that one hiding below the relay over by the 'battery vortex' hole?
10 watts should be fine on a 12v setup like yours. 30ohm will only draw around 5 watts.
Personally, I'd worry more about the resistor catching something on fire more than the battery with such a gentle discharge.


Best regards,
PCH

 

[razor02097]

I'm not sure I saw any power resistors. Is that one hiding below the relay over by the 'battery vortex' hole?
10 watts should be fine on a 12v setup like yours. 30ohm will only draw around 5 watts.
Personally, I'd worry more about the resistor catching something on fire more than the battery with such a gentle discharge.


Best regards,
PCH

Ah sorry the resisters are hiding just under the fan.


Once I get this thing working like I want to, I will build a better concept with more photos

 

[razor02097]

I've messed around a little. 400mA is fine and all but since I have 10W resistors I'm going to up the draw and reposition them.



Here is an updated circuit. It now discharges at about .9A which is a little under 11 watts at 12V. The three 20 Ohm resistors I have in parallel are rated to dissipate 30 watts so I should be okay up to 5S. With GOOD active cooling....maybe 6S.

 

[razor02097]

So I'm trying to nail down a design that will accommodate a wide range of voltages and provide reliable operation for a long time.

After some testing I found my 12V automotive relay will still latch above 8 volts and stay latched all the way to the 7.6v ride. This means I can use a common automotive relay for 2s without a second power source. However once the nominal voltage is reached the unlatching relay will not actuate at 7.6v so I need to use a 5v relay instead.

I am going to try using a voltage regulator LM7805 to produce a 5V signal for the unlatch relay. This should provide the necessary signal with 2s to unlatch the system. I am also going to use a computer heatsink and fan for the active cooling element. I am toying with a simple 2 stage controlled by a mechanical thermal switch where when it trips it sends full battery voltage to the fan. I have messed with computer fans a lot... I have put more than 30v though one without it blowing up so I don't think it should hurt for a little while...that is what experimenting is all about right? I will also be adding some current limiting resistors so I don't blow up my relay if I did go to 5 or 6s.

Here is another circuit I drew up with the new elements and even a chart calculating amp draw and wattage calculations! The only bad part is I don't think my resistor bank would survive 6s...



Anyone bored yet?

 

[razor02097]

Well this past week I've been working on and off with this project. I finally started the actual prototype! :applause:

I want to have something that is all inclusive and completely portable...so I decided a 30 caliber army surplus ammo can should do the trick!


Next is the active cooling element...a old style HP computer CPU fan should work!


Plus look how nicely it will fit. Both are 3.5" wide so it will be perfect!


The first thing I had to do is to start cutting on the can so I can fit the heat sink in...



Then weld it all up!


Turns out it is a really tight fit so I just...adjusted....until it fit right.


Don't forget the resistor bank!


Wiring it up...


Finishing it up!




A few changes I made were I didn't bother with the 5V relay or anything I am actually using two automotive style relays. After playing with the circuit I actually blew up the battery checker I had I had plenty more but they worked differently... I had to adjust a few things to make it work. Now the control relay latches by operation of the battery monitoring device. This is also good because it solves the 2S problem of not unlatching the main relay! I also added another element of safety with a resettable breaker. I didn't use the temp switch either, I found the fan didn't spin at all when I put a resistor in series and a 2S battery was hooked up. Maybe not a bad thing necessarily the temp switch would just one more failure point.

This project was to create a simple, reliable discharge rig that will bring a fully charged battery to storage charge of approximately 3.8V per cell. All in total I have spent about $10 and used existing stuff I had that I would estimate to around $25. I'm pretty happy with it, I'll be sure to post any changes or additions I do. I hope someone else will take a crack at it and improve on it