I would hate to waste good new Lithium batteries by an over-discharge on the first round of use.
In some cases, the microcontroller can monitor and shut down the battery, but not always.
Make sure and check out the battery minder I mentioned in post #21. It makes a very loud sound if one of the cells drops below a set voltage. I doubt it would work well with LiFePO4 cells or other cells that are not in the correct voltage range.
For something that costs less than $2, it could save a person a lot of money from ruined batteries.
I have been digging. I finally located my 14500 cells that I purchased near the start of the thread. They happen to be 1200ma at 3.7V and after all this time are still over 4.0 volts. I have wanted to use these with my SXchips as they will tolerate variable voltage as long as I don't clock to fast and have the brownout shut down set low or off.
Also, I have been digging and digging for low voltage cutout circuits, but mostly were ones for 12 volt gel cels and such. They often suggested a comparitor or an op amp. That is way too complex.
But I finally found a set up that is a couple of transistors and one Mosfet on an r/c airplane site. That looks exactly like what I want. I will have to build a proto-type and test its shutdown, but the main thing is it is all easy to get components and with a 2N7000 it would only be for about 200ma of power. But with bigger MOSfets, one can use it for nearly any size Lithium battery pack.
Now I am trying to figure out if there is a low power MOSfet out there that is similar to the 2n7000, but with substantially less Rds than 1 ohm at 4.5V. By inserting it, I am wasting about 3-4ma.
For a low voltage cut out, I'd use a MN1381 ( or here ) driving a N-channel Fet in the ground connection to the load. Other power supervisor chips should work as well. Through-hole I'd suggest a IRLD014PBF-ND FET. If you're willing to use surface mount parts, 568-6540-1-ND works well for a "slow" switching application like this.
Thanks, that first chip looks fine at 35 cents each, but I'd have to buy a reel of 3000. So, it I need just one, it is $1000USD; if I need 3000, they are 35cents each.
The second one is 92 cents in single quantity, plus shipping. But they seem to want to sell 2500 or more. And so it goes.....
Since I suspect that part availability is going to cloud the issue, I am trying to DIY a differential amplifier that can be built with easy to get components. But I will add these items to my shopping list for when I visit a local parts house. They are quite used to telling me "Don't have it. Never heard of it."
I used to spend a lot of time locating the best part, and then finding out I couldn't get it in single quantities. My projects tend to be single production items.
Thanks, that first chip looks fine at 35 cents each, but I'd have to buy a reel of 3000. So, it I need just one, it is $1000USD
I'm pretty sure Digikey will sell you the MN13812SGUCT-ND in single piece quantities. At least that's what the price table says. So if you need one you'll pay 94c (plus shipping).
If shipping cost is a problem RS Components could be an option. It looks like they offer free shipping in Taiwan.
Search for "Voltage Supervisory" on their site - minimum quantities per part are usually 5 or 10.
.
I'd posted a circuit using the MN13812 as a battery cutoff for the Basic Stamp. I bring that up to follow up on Lawson's comments, because it took me three extra resistors to set the threshold and hysteresis, and a capacitor for stability.
The LP701 in the circuit is a lateral p-channel fet. There are more up-to-date parts that would be better in the same circuit--The MN13xxx series is listed as obsolete. My favorite mosfet for low power shutoff (up to a couple hundred mA) is the FDN304, with 70mΩ at 2.5V Vgs. Look up "trenchFET" or "powerTrench" for logic level mosfets in a large range of ratings & packages from Fairchild, Siliconix, Vishay and others.
For small lithium-poly cells (<1Ah) in my micropower data loggers, I'm using the LTC4071 as a combined charger and battery cutoff protection switch. It's an unusual parallel charger circuit meant primarily for energy harvesting or backup applications.
@Tracy Allen
Thank you, a schematic makes it all the more useful for everyone. I think there is another device, a TL431 - Programmable Shunt Regulator, that will do the job.
I'll have to do some local shopping as it just isn't worthwhile to pay shipping to Taiwan for anything these days unless it is a big order. And tonight, I will tuck into bed with a bit of review of BJT differential amplifiers in the context of a Low Voltage Cutoff. The part count for that is not much more than for your schematic. I am wondering if I can get a LP0701P or FDN304 Mosfet as well.
And @ Lawson, thank you as well.
I still need to adapt this to a single 3.7V LiFePO4 cell, or two at 7.4V with precise cutoff voltage. My 14500 is 3.7V and rated at 1.2AH, but I have seen some with odd fittings welded to the ends that go as high as 2.4AH. Maybe Tracy would find these useful.
There is lots more to come in Lithium rechargable batteries now that we have several generic sizes and shapes. It is tempting to throw out all my NiCads and NiMh as it take two or three to provide similar capacity, AND they are more temperamental in some ways.
If you're planning on experimenting with differential configuration circuits, I'd skip strait to an op amp or comparator with a <1uA idle current. For instance the MCP6141 also available in a DIP package if you can find them. (MCP6541 for the comparator version) Ti, Linear, etc. have similar parts if you local distributor doesn't stock Microchip parts. The biggest bit I haven't found is an inexpensive voltage reference with a similar ~1uA idle current under all circumstances. A 1.2v MCP1700 regulator gets close, but my testing of higher output voltage parts indicates it'll draw >10uA if the supply voltage drops below 1.2v. (not a big deal with battery power, but a pain running from super-caps and solar)
I agree that the MN1381 isn't particularly accurate. It's also reaching end of life. I suggested it because it still has one of the lowest idle currents I can find, and is a good place to start looking for newer chips.
Is there ANYTHING that Tracy Allen doesn't know about?
A big shoutout to one of the most knowledgable and helpful forumistas ever. I met him briefly at Expo in April and he's not only smart, but a very personable and likeable fellow as well. I hope our paths cross again soon.
I found the 1381 chip and Maxims 8212 at Solarbotics in single quantities. And most embarrassingly, the reason I looked there was because it occurred to me that I have one or two 1381s on hand for a BEAM project that never got done.
@ Lawson
My impression was that having a power supply that was dropping down to as low as 2.5VDC might play havoc with all but very special op amps. So I became interested in the discrete component differential amp solution. That MCP6141 looks perfect as I only need a comparitor and it will power at 1.4volts.
And now, having read an introduction that claims you are nobody in electronics if you don't comprehend differential amplifier design, I am wanting to dig into this for knowledge's sake.
Even if I build the differential amp version, it is going to consume much more power than either of these chips. So it is just an academic exercise and learning experience.
@Tracy.
There are some 'half-sized' 3V Lithium batteries, the 16555s which will fit two in one 18500 battery holder. These total 6V, not 7.2V at nominal operation. They might be more useful to you than the 14500s, which I hope to one day use to power a Propeller.
I thought maybe the original description on ebay said the batteries included battery holders and the battery holders were just sent late. After checking the original listing I don't see any mention of battery holders so these (and the shunts) where just gifts (and nice ones too). Kind of an amazing seller to go this far out of his way.
All I can figure is that some nogginhead used 'em like 1.5V AAs and complained when some magic smoke got released and the seller did a proactive, preemptive mailing to avoid more complaints from other Americans with room temperature IQs.
Room temperature IQs? Would that be Fahrenheit or Centigrade? Or does it matter to you?
It is NOT easy to use these like AA cells as they are .5mm longer. I had to force two of them into an AA battery holder. I am happy that I can though as it seems that no one is making something else for them.
Two of these in series are great for the BS2 as you have a voltage regulator to adjust the use. And Pololu sells tiny voltage switching regulators if the linear are too wasteful for you.
I have made a bit of progress with my low voltage cut off. I am abandoning the discrete transistor differential amplifier approach and looking at a LOW Voltage comparator chip - the LMV331 for use with ONE cell (not the LM331 which requires 5 volts power and is a voltage to freq pulse chip). This chip drains less power than anything I could build myself.
One can use the TL331 (single package) or 1/4th of the LM339 (quad package) if you are using two batteries in series as it would cut out before ever reaching 5volts. I still have to bench test something before posting. I've been hung up with another project using SLEEP in the SX28 chips and the Watchdog Timer. So please be patient.
Comments
Make sure and check out the battery minder I mentioned in post #21. It makes a very loud sound if one of the cells drops below a set voltage. I doubt it would work well with LiFePO4 cells or other cells that are not in the correct voltage range.
For something that costs less than $2, it could save a person a lot of money from ruined batteries.
Also, I have been digging and digging for low voltage cutout circuits, but mostly were ones for 12 volt gel cels and such. They often suggested a comparitor or an op amp. That is way too complex.
But I finally found a set up that is a couple of transistors and one Mosfet on an r/c airplane site. That looks exactly like what I want. I will have to build a proto-type and test its shutdown, but the main thing is it is all easy to get components and with a 2N7000 it would only be for about 200ma of power. But with bigger MOSfets, one can use it for nearly any size Lithium battery pack.
Now I am trying to figure out if there is a low power MOSfet out there that is similar to the 2n7000, but with substantially less Rds than 1 ohm at 4.5V. By inserting it, I am wasting about 3-4ma.
Lawson
The second one is 92 cents in single quantity, plus shipping. But they seem to want to sell 2500 or more. And so it goes.....
Since I suspect that part availability is going to cloud the issue, I am trying to DIY a differential amplifier that can be built with easy to get components. But I will add these items to my shopping list for when I visit a local parts house. They are quite used to telling me "Don't have it. Never heard of it."
I used to spend a lot of time locating the best part, and then finding out I couldn't get it in single quantities. My projects tend to be single production items.
If shipping cost is a problem RS Components could be an option. It looks like they offer free shipping in Taiwan.
Search for "Voltage Supervisory" on their site - minimum quantities per part are usually 5 or 10.
.
The LP701 in the circuit is a lateral p-channel fet. There are more up-to-date parts that would be better in the same circuit--The MN13xxx series is listed as obsolete. My favorite mosfet for low power shutoff (up to a couple hundred mA) is the FDN304, with 70mΩ at 2.5V Vgs. Look up "trenchFET" or "powerTrench" for logic level mosfets in a large range of ratings & packages from Fairchild, Siliconix, Vishay and others.
For small lithium-poly cells (<1Ah) in my micropower data loggers, I'm using the LTC4071 as a combined charger and battery cutoff protection switch. It's an unusual parallel charger circuit meant primarily for energy harvesting or backup applications.
Thank you, a schematic makes it all the more useful for everyone. I think there is another device, a TL431 - Programmable Shunt Regulator, that will do the job.
I'll have to do some local shopping as it just isn't worthwhile to pay shipping to Taiwan for anything these days unless it is a big order. And tonight, I will tuck into bed with a bit of review of BJT differential amplifiers in the context of a Low Voltage Cutoff. The part count for that is not much more than for your schematic. I am wondering if I can get a LP0701P or FDN304 Mosfet as well.
And @ Lawson, thank you as well.
I still need to adapt this to a single 3.7V LiFePO4 cell, or two at 7.4V with precise cutoff voltage. My 14500 is 3.7V and rated at 1.2AH, but I have seen some with odd fittings welded to the ends that go as high as 2.4AH. Maybe Tracy would find these useful.
There is lots more to come in Lithium rechargable batteries now that we have several generic sizes and shapes. It is tempting to throw out all my NiCads and NiMh as it take two or three to provide similar capacity, AND they are more temperamental in some ways.
I agree that the MN1381 isn't particularly accurate. It's also reaching end of life. I suggested it because it still has one of the lowest idle currents I can find, and is a good place to start looking for newer chips.
Lawson
A big shoutout to one of the most knowledgable and helpful forumistas ever. I met him briefly at Expo in April and he's not only smart, but a very personable and likeable fellow as well. I hope our paths cross again soon.
@ Lawson
My impression was that having a power supply that was dropping down to as low as 2.5VDC might play havoc with all but very special op amps. So I became interested in the discrete component differential amp solution. That MCP6141 looks perfect as I only need a comparitor and it will power at 1.4volts.
And now, having read an introduction that claims you are nobody in electronics if you don't comprehend differential amplifier design, I am wanting to dig into this for knowledge's sake.
Even if I build the differential amp version, it is going to consume much more power than either of these chips. So it is just an academic exercise and learning experience.
@Tracy.
There are some 'half-sized' 3V Lithium batteries, the 16555s which will fit two in one 18500 battery holder. These total 6V, not 7.2V at nominal operation. They might be more useful to you than the 14500s, which I hope to one day use to power a Propeller.
Now who wouldn't like to have those?
Amen, Brother.
I thought maybe the original description on ebay said the batteries included battery holders and the battery holders were just sent late. After checking the original listing I don't see any mention of battery holders so these (and the shunts) where just gifts (and nice ones too). Kind of an amazing seller to go this far out of his way.
Choirboy Duane
It is NOT easy to use these like AA cells as they are .5mm longer. I had to force two of them into an AA battery holder. I am happy that I can though as it seems that no one is making something else for them.
Two of these in series are great for the BS2 as you have a voltage regulator to adjust the use. And Pololu sells tiny voltage switching regulators if the linear are too wasteful for you.
I have made a bit of progress with my low voltage cut off. I am abandoning the discrete transistor differential amplifier approach and looking at a LOW Voltage comparator chip - the LMV331 for use with ONE cell (not the LM331 which requires 5 volts power and is a voltage to freq pulse chip). This chip drains less power than anything I could build myself.
One can use the TL331 (single package) or 1/4th of the LM339 (quad package) if you are using two batteries in series as it would cut out before ever reaching 5volts. I still have to bench test something before posting. I've been hung up with another project using SLEEP in the SX28 chips and the Watchdog Timer. So please be patient.