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Old 26th February 2013, 05:47 PM   #1
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Default Battery Over-Discharge Protection Circuit

Greetings,

Anybody have any suggestions for a battery over-discharge protection circuit?

I have been using home built NiMH battery packs in some of my projects. (for instance my most recent uses 10 C cells). However, after doing some research, I've learned that these packs (also LiPO) packs, can run into trouble if they become over discharged.

I think I'd like to learn how to incorporate a circuit design that would not allow itself to operate if the terminal voltage of the battery dropped below a certain point (for instance 10V for the 10 cell NiMH pack.)

I've done some searching around and have come across a few designs, just curious if you folks had some insight in the area.

My most recent project did incorporate a six LED battery meter using an LM3914 chip which I calibrated such that the last LED is lit when the battery is at 10V, so I know not to operate the gizmo beyond that point, but I'd like to develop an actual under-voltage lockout.

Thanks for any suggestions!
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Old 26th February 2013, 06:29 PM   #2
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The ones I usually encounter seem overly complex (I'm sure the designer has his reasons) but I don't know why a battery lockout couldn't be built around a low-power comparator and depletion-mode MOSFET.
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Old 26th February 2013, 11:14 PM   #3
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I'm definitely more interested in simplicity and functionality over absolute accuracy. For instance, if it cuts out power anywhere from say 10.0 to 10.5V, I should think that would be fine.

Someone on a different forum also suggested using a zener operated MOSFET.

I'm not new to electronics, but I'm certainly no expert. This definitely looks like a good avenue of research. I'll be looking into this "low-power comparator" and "depletion mode MOSFET" you speak of.

Thanks
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Old 26th February 2013, 11:42 PM   #4
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A TL431 and a big regular P channel FET would be pretty easy to cob together too.
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Old 27th February 2013, 12:30 AM   #5
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So the TL431 would be set to output my desired cutoff voltage, say 10.0V to the MOSFET (allowing current for the rest of the circuitry). Then when the battery pack dipped too low below the regulator output voltage, it would stop conducting and shut off the MOSFET (and everything else with it).

Am I understanding your approach Andrew Eckhardt?
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Old 27th February 2013, 12:51 AM   #6
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Pretty much. The 431 has an internal 2.5V reference so tune a divider to hand that to the reference pin from whatever trip point voltage you want. Tie the source pin of the P channel enhancement FET to B+. May as well be a fairly large transistor to minimize loss. You're not looking to regulate or do anything very dynamic so bias isn't too critical. Maybe a 10K resistor from gate to source, TL431 cathode connected to the gate, anode to ground. FET drain to load. You might want to add a 15V zener clamp to protect the FET gate. If the supply voltage can go higher than that you'll need to be sure the TL431 doesn't try to drive big current into the clamp. 1k series resistance would be fine. See the TL431 datasheet. Figure 24 in the TI sheet shows a bipolar based shunt regulator, where if you swapped in a PFET and instead of grounding the drain used it to feed the load, you'd have the general idea.

If you are concerned about power consumption you can increase the bias resistor and use quite high resistances for the divider network, but I'm not sure what the limits would be without looking at it more closely.

Last edited by Andrew Eckhardt; 27th February 2013 at 12:54 AM.
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Old 27th February 2013, 01:19 AM   #7
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Something like this:


SUM110P06-07L-E3 Vishay Siliconix | SUM110P06-07L-E3CT-ND | DigiKey
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Old 27th February 2013, 03:26 AM   #8
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NiMH are not spoiled by deep discharge. Any Lithium type most definitely is.
If you have 10 NiMH in a stack, you do need to prevent reverse charging when some cells discharge before the others. A reverse biased Schottky diode across each cell will do this.
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Old 27th February 2013, 01:46 PM   #9
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Interesting.

I've never had any problems with NiMH packs myself, but after reading some horror stories on the internet (batteries exploding, etc) I thought that maybe I had just been lucky.

There seemed to be a lot of consensus that (at least with the NiMH packs) that what happened was exactly as you describe, davidsrsb, that the pack had become discharged to the point that one (or more) weaker cells had become over-discharged and then became "reverse-charged" and thus permanently damaged . Although I don't quite understand the mechanics behind this condition, it seems that after the "reverse-charge" event, some packs would last between one or a few charges before they became overheated, began venting, and in some cases exploded...

(Please correct me if I'm wrong, or at least feel free to clarify if you understand the circumstance better than myself.)

Davidsrsb, can you clarify exactly what the reverse bias schottkys do to prevent this from happening? (I've never used schottkys before, I do have an idea of their makeup, but am not familiar with their application.) In addition to that, can you give some insight as to how the battery pack will behave when it reaches the discharge point at which one or more cells begin to "reverse-charge"? (In other words, does it just somehow cut out at a safe voltage level once one cell reverses?)

I like the schottky approach because of it's simplicity and absolute lack of a PCB footprint, but I need to understand it a little better.

-Thanks
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Old 27th February 2013, 03:47 PM   #10
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So, still examining options...

Most suggestions return to the MOSFET design. Now as for what is going to open and close the MOSFET, I recognize three good options:

1. A zener diode with appropriate voltage (~10.0V in my case, probably the simplest application)
2. A comparator (Haven't quite worked out the specifics of this yet, but I don't think I'm far off.)
3. A regulator (TL431 for instance, still working out this one as well.)

Do any of these jump out at you guys as a better option than the others. (Andrew, you suggested the 431, any thoughts on why or why not it would pull this off better than the others, i.e. power consumption, accuracy.)

-Thanks again
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