I have designed a micropump driver powered by 2x AA (Li) batteries, the pump is piezoelectric and requires 27V.
The pump requires a switching waveform, at the battery side this draws (including inefficiencies and driver consumption) 400mA and 900mA at the battery voltage which is nominal 3V (depending on the pressure being created by the pump), the batteries won't really see the switching because I am using a H-bridge and so there are two signals that are orthogonal to eachother and other than the small (ns) deadtime between them the boost converter will only see a DC drain (and even the deadtime will get lost in the feedback loop response time of the SMPS).
Several problems have arisen:
After a failed life-cycle test I measured the voltage across the batteries, one was sitting at 0.2V the other 1.47V, they are used in series, so why would one battery have such drastically different voltage? Does this suggest damage? or simply one is much more drained that the other?
I'm using boost solution that can boost from ~1.5V up to the required 27V. I have noticed that the someway into a test the battery voltage drops off during a pump cycle, very quickly dropping below 1V and after the cycle ends quickly recovering to 2.8-2.9V, does this suggest I am drawing to much current out of the battery? or that the battery is damaged or simply running dry?
I assume my design is damaging or stressing the batteries somehow, are there any standard practices to be used in this scenario?
Based on a constant discharge test with reference to the battery manufacturer I estimate that the batteries that drop off in voltage drastically after a period of time being used are less than 1/3 discharged. I.e with a load drawing a defined constant current and measuring teh voltage across the terminals and comparign this with a the discharge curve (http://data.energizer.com/pdfs/l91.pdf)
Unfortunately I can't upload schematics due to confidentiality agreements with the customer, however I'll provide whatever additional information I can.