Calculating Battery life

Falcon

I have designed a sensor to monitor whether a window is open or closed. It uses a BS1, and a CIP-8E Encoder and TWS-434A Transmitter from WWW.RENTRON.COM The Master unit includes the compatible Receiver.

The loads for these are as follows:

Active Sleep
BS1 1mA 25µ
CIP-8E 0.8mA 0.2µA
TWS-434A 10mA N/A

The BS1 will check to see if the state of the window has changed from open to closed or vice versa. This takes about 0.5 seconds. If no change then it goes to sleep for 26 x 2.3 = ~60 seconds. It also keeps the CIP-8E Encoder in sleep mode for the same period. The BS1 is “awake” for about 1 second each minute if no change has been made.

If the window state has changed the BS1 will wake the CIP-8E encoder and power the TWS-434A transmitter for 1.5 seconds. This will consume ~11.8 mA for that 1.5 seconds. Then it turns off the power to the Transmitter and puts itself and the Encoder to sleep for another 60 seconds.

The windows are opened only about three times a week so the Encoder/Transmitter would only be active about 9 seconds per week. The BS1 will be active about 0.5 seconds per minute so about 6 minutes per day.

I want to power this circuit with as small a battery pack as possible, but still get a reasonable life from the battery.
The battery I have in mind is a SY073-ND from WWW.DIGIKEY.COM which indicates the capacity is 1.4Ah @ 10mA.

Is there a way to estimate battery life, or calculate the capacity necessary to power the circuit for 1 year on a single battery pack?

falcon

Mike Green

It's very simple. You calculate the total current required for the amount of time involved and average that over a typical period of time (like a week). Keep in mind that there will be some leakage when you're dealing with microAmps, so add a bit for slop. The baseline current for the BS1 is 25uA for 168 hours/week or 0.0042Ah/week. Worst case, the Encoder/Transmitter would be active 9 seconds / week at 10.8mA (0.000027Ah/week) and the BS1 will be active at 10mA for 42 minutes per week (0.007Ah/week). The total average drain is 0.011227Ah/week. With a 1.4Ah battery, you'll get roughly 125 weeks of operation. I'd figure on something over 2 years of operation if all these calculations are correct, certainly over a year. Your battery is a lithium primary battery which maintains its output voltage until near its end of life. This helps. You might also check the quiescent current of the regulator on the BS1 module. I don't know if the BS1's current ratings assume that the regulator is not used (or not).

Falcon

Mike,

I appreciate the information. Is there a more efficient way to regulate the 6 VDC from the battery pack to 5 VDC, and connect it to the Vdd pin instead of relying on the BS1's regulator via Vin? I've read about some chips that are more efficient but, if I recall, they require several external components.

Thanks,

Falcon

Mike Green

I think the open circuit (no load) voltage of your battery is 6.0V ... you need to check this with the manufacturer's datasheet! A single diode like a 1N4001 or equivalent will drop about 0.7V regardless of the current drawn leaving 5.3V which is ok to use. If the open circuit voltage is more than that, you may need two diodes in series. You could just connect one end of the diode (the anode) to the (+) battery terminal and the other end of the diode (the cathode ... with the band marking) to the Stamp's Vdd input.

I looked at the graphs in the datasheet and it looks like the peak voltage is 6.1V. With one diode drop, you should get no more than 5.4V at the Stamp's Vdd terminal which is acceptable. Over time, that will drop to where the Stamp will mostly see around 4.9V.

Mike Green

I looked again at the datasheets for the Energizer and Panasonic versions of this battery and it looks like the voltage drops quickly under load to 5.5V and, from these graphs, doesn't appear to rise above 6V. The microprocessor in the BS1 should be happy with Vdd up to 6.5V, but the EEPROM is only good up to 5.5V, so you'd be better off with the diode. I'd probably use a diode with a slightly lower voltage drop like the 1N914

Falcon

Thanks for the follow-up(s) Mike. I'll add the 1N914 to be safe.

Falcon