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Topic: Primary and rechargeable cells in handheld application (Read 21 times) previous topic - next topic


Currently i am designing two handheld devices, one on the back burner and one active as a platform for various projects.
Consisting of a main PCB with MCU, voltage regulator, display, human interface, storage and RTC wich has a header for a "expansion board" that will hold the circuitry for a specific task.
The PCBs are not routed yet, only the schematic is done.
I thought for the longest time i had everything sorted out, the prototype has been build and right now i am working on the case.
But i forgot about something...

It was designed to work of two AA cells in series and drain them until the power fails, the startup voltage of the regulator is 0.9V and it will work down to 0.3V.
Now i thought about using rechargeable cells and that is the problem, they will not like that.

A simple undervoltage lockout will take care of the over discharge but will prevent squeezing every last drop out of the primary cells.
But below 1V the alkaline cell will have not that much charge left and a NiMh as well, a cut off at about 1.8V to 2V would mean the primary cell gets discharged reasonably low and a NiMh will not be damaged.

Would i be right with that assumption? In that case i would only have to put some sort of very low power device (supervisor, reset ic, comparator) at the enable input of the boost converter and i can call it done.

Additionaly i decidet to use a polyfuse and big TVS diode for a reverse battery protection since it could be possible to put batteries in the unit that have been discharged low enough that the gate-source voltage of a p-channel fet will not be high enough to disable it.
At least that is what the datasheet predicted and my test have shown. :)

If you ask yourself, why i do not want to use more cells or even a lithium cell...
Ease of use, swapable cells, readily available replacements, size/case restrictions and the "big brother" uses a lithium cell with integrated charger.
So i would like the little brother to be as low cost as possible :)


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