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United Kingdom
Asked — Edited

Lipo Battery Auto Cut Off

Something I've been looking at since I undervoltaged one of my LiPo batteries is an auto cut off circuit when it get's below around 6.2v

I think this will do it but could do with some confirmation since this is not my area of expertise so am always concerned. Any comments and suggestions would be extremely appreciated.

If it is OK you can expect to see this replaced with a tutorial:)


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Zener Diode is rated for 6.2V
Mosfet is rated at 85A

I realise a small load may pass through R1 when shut off but I assume it will be so small it will not matter (unless left for a very long period)


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United Kingdom
#3  
Well mine will be cutting off the motors and everything but I didn't show that for simplicity (my schematic for power distribution is huge!). That's why I went for an 85A mosfet, it should be big enough to handle any load I will be running.

I was told Mosfets don't generate anywhere near as much heat as transistors and are extremely efficient, but will take your comments into account and throw on a heatsink at the very least, if it seems to be getting too hot in practical tests I'll also add a small fan too (although reluctant due to noise and power usage but we will see).

Thanks:) I'll order up the parts and give it a real world test. I'm also tempted to throw in an on/off switch on the ground between the mosfet and the battery for manual shut off, not that I plan to turn my robot off once it's done.
#4  
I like using this http://www.cainetworks.com/products/webcontrol/index.html to monitor low voltage in each cell and to charge them. LiPo batteries chould be bad if not charged the right way and the cai plc only job is to watch the batteries,and if there is a problem it can email/text me so i can check on them from my phone or even turn it all off.

I have 2 of these running my hottub and home alarm system for a year with no problem
United Kingdom
#5  
That's not really what I'm trying to achieve.
Monitoring is already in place, through the EZ-B. What I am trying to get is extra safety by cutting the power to everything if it drops below a certain level, in this case 6.2V (3.1v per cell, possibly 3.0 & 3.2 if slightly unbalanced).
#6  
Do you have cell balancing leads on battery? then you can monitor each cell if you have enough ports on ezb
United Kingdom
#7  
I already do, I've done a tutorial for the LiPo monitoring board and script. Each cell is monitored through the balance port on to the ADC but that's only good when ARC is running. What I want is a fail safe for if I forget to disconnect the battery.
United Kingdom
#8  
Perfboard/Stripboard layout for anyone who wants to try it out... bearing in mind this is untested.

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Netherlands
#9  
If you'd like, I could test the circuit this saturday at our local hackerspace :D
United Kingdom
#10  
That would be great if you could, I've not had chance to finish this one yet.:)
Netherlands
#11  
Hi Rich,

Sadly, your circuit did not work. In stead of 'snapping' off, the current just went down slightly with the voltage, I had a little talk with Ite, a very experienced EE and he recommended either a comparator or a specialized IC.

I'd also found this circuit, which I will be testing next:

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(more info)

I just need to adjust the voltage divider for it to work with 2S Lipo's in stead of 3S.

Edit: we don't currently have the TL431 in stock and a simple cutoff IC would be cheaper
United Kingdom
#12  
Thanks for testing it. I guess my theory is slightly off:)

I had assumed that if the voltage was above the rating of the diode it would allow it to pass therefore energising the transistor/mosfet base that allowed current to pass to ground, completing the circuit...

Hopefully we can find a solution, with LiPo batteries becoming more and more popular I'm sure it'll be something a lot of people will want - especially if they do what I did and leave their bot on.
Netherlands
#13  
This might be easier/cheaper for most people.
#14  
Its OK , it will get figured out soon enough. I will need a voltage cutoff on mine even when I'm still using lead acid so it doesn't get so far drained it takes a long time to charge back up.
United Kingdom
#15  
@Rich.

Where did you get with this topic, with your own projects to incorperate a power cut-off circuit/off shelve solution? Was it something you followed up on?
United Kingdom
#16  
No, this one never went anywhere. By the time I had time to get back on it the V4 was out which has it's built in low batter monitor which does the job just fine.
Great Britain
#17  
Hey guys, I am Alex and I am quite new to electronics. I am building a micro quadcopter on the base of some chinese no-name board that does not have lipo cut-off. I am flying 1s lipos and I have already ruined one. My aim is to create the smallest possible lipo auto cut-off. I searched web for quite a while and was not able to find a solution. Could you help me with this one please? THANKS!
Great Britain
#18  
Probably should add, I dont know exact power drain, but the battery is 600mAh 1s
Great Britain
#21  
Hi Steve, yes it is exactly what I need, I just to make it even smaller!
#22  
I've been looking for a similar circuit (hence me running across this thread)

The basic idea is sound... in fact it is similar to the under-voltage lockout circuit design shown in TI's datasheet for the LM2576.

But it needs some type of Schottky trigger. Otherwise, the battery bouce-back voltage (after the load is removed) will cause the voltage at the Gate to turn it back on. Let, it's my understanding that it will just ripple until it dies.

I haven't found the right solution yet.
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