I will try to answer here as it covers most if not all aspects and questions asked.
1) How much current does your application draw?
The "load' is in fact a joule thief with minimal current drawn, I would say 5mA at most under certain conditions and changes to its output.
2) How much current can you allow the protection circuit to consume?
It would be best to be as low as possible, certainly not in the range of 10's of mA ideally under 5mA
3) What is the duty cycle of the load - mostly on, mostly off or unpredictable?
Its a joule thief which most of the time likes to switch at around 100Khz but it varies with voltage etc so i'd say unpredictable LOL
4) Is it cost sensitive?
VERY!!!!! I am trying to find something as simple and cheap as a passibes around an under 10c ic/mosfet
5) How many cells do you need to monitor/protect? Is it a single cell or a number of cells in series which you want to monitor indiviually, disconnecting the load if any cell drops below 1V?
This is about a single cell application.. i could make it 2S if absolutely needed but i'd have to at least reduce them from AA to AAA or even 2/3AAA as this has to be as small and compact as possible too.
6) Is it space/volume/weight sensitive?
sensitive no.. but the smaller the better
7) Is the cell manually switched into the circuit or can it be switched on by some electronic signal?
The cell is on a charger IC but the load is always present.
8 ) Does the application use a microcontroller that you can use for the monitoring and protection?
No.. even if it did, it would have to operate with a singe NiMH cell and i am not aware of any and even if i did, i am not able to program them (yes, i have no idea really!)
9) What voltages are available in the circuit other than the single cell's 1V minimum?
5V from a microUSB when charging, not always present
10) Do you have any electronics design experience or are you looking for a ready made solution?
I can follow guidelines, read datasheet and think i can design simple stuff (hence why i deal with simple chargers to begin with LOL)
11) What size/capacity cells are you using?
Like mentioned, max size 1XAA capacity could be any available, i'd say in the range of 800-900mAh anything else would be lower than that and smaller size esp if the only way to offer some sort of protection to the cells is to go 2S
12) What temperature range will it experience?
The charger offers over-temp protection for the cells while charging but i would leave that one open as it could be used outdoors with -10 to +40 max range (there is no demand to work at this range as soon as it will not get damaged!)
13) Is this a one off, or is to be put into production?
I wouldnt dare to say production but i like to make 10pcbs for the prototypes and test the market with them if they work without changes needed. Then i might make 20-30-50pcs...
A typical solution is to use a transistor, usually a mosfet which can have very low on resistance, requires almost no power to maintain in the on state and has almost no leakage current in the off state. If you only have 1V available though you might not be able to find a suitable Mosfet with a sufficiently low gate threshold voltage in whch a high gain bipolar might be suitable.
I have tried with a P channel (AO3401) with Gate to negative Source to positive and Drain as the positive terminal of the "pack" ... i found it searching around for some clue but it didnt work... i tried with a power supply, not battery to adjust voltages and the strange thing was that once voltage dropped under 0.5V i got negative readings on the DMM!!!!! I wouldnt say it works but if i have to search for a suitable mosfet, what do i look for ?
Another possibility is to use a relay but a) you might not be able to find one that operates at 1V and b) will use a relatively large amount of power. You could avoid the latter by using a latching relay but the control circuit may be a bit more complex and you will need to have sufficient energy available to switch it. That could be stored in a capacitor so you are not reliant on the battery being able to provide it when almost exhausted. Latching relays probably wouldn't be suitable if the application may be subject to mechanical shock, vibration or strong magnetic fields which could flip the relay state.
A relay of any kind would be out of the question for cost and size alone!
If 1V minimum is too restrictive you could use a joule thief or charge pump to boost the voltage - several manufacturers make suitable low power, low input voltage devices intended for energy harvesting. You might even find one with a suitable voltage reference and comparator function.
I haven't though of using the JT output to power a protection circuit to be honest.... that would be interesting!!
I regulate the voltage to 5.1V at some point, that would be more than enough as soon as total current to operate the protection circuit is VERY low.. now that i think of it, i might have a few voltage monitoring ICs of very low cost and size (SOT23) that could possibly be used.. HMMMMMMMMMMM
A more radical idea is to use a fuse to supply the circuit and short circuit the battery+fuse to blow the fuse, assuming there is enough energy to blow the fuse. A circuit breaker would make it reusable but perhaps a bit bulky.
I dont know what you have in mind here but... HMMMM use the JT output... yes, that MIGHT work
THANK YOU and thanks to all who took the time to post here, i really appreciate it!