Embedded Freaks..

October 1, 2007

Facts about Battery

Filed under: embedded-tips — Tags: , — kunilkuda @ 9:11 am

I’m designing battery powered embedded systems. Here’s some lessons that I got in my daily work.

Energizer E91

  • 1000mAH battery should last 10 hours under 100mA continous load, right ?

Whoops…Read carefully on the battery datasheet —if you are serious about your work, you should have serious battery supplier vendor which supports you with real datasheet and/or battery sample =).

1000mAH claim is based on how much continous current draw that your system take. Most of the batteries will die sooner, if you load them more than the datasheet requirement. I’m taking Energizer E91 battery as example.

Battery Capasity vs Current Consumption

Under 25mA continous load, the battery has 2750 mAH capasity. Under 100mA continous load, the battery has only around 2000mAH.

This is the characteristic of battery’s chemical reaction. Battery’s cathode has limited ability to absorb ion from the anode. Therefore, under heavy load, there’ll be a layer of ion covers the battery’s cathode. The heavier the load is, the thicker the layer will be. This layer will prevent other ion from anode to reach cathode. Hence, reduce the total capasity of the battery under continous load.

So, to predict how long your system will run, you will need to average your current draw, and match it with the battery’s datasheet.

  • Slew rate and battery

Like I said, the battery has limited capability to absorb ion from the anode (or battery has limited current that can be delivered in a time). To load battery with higher current, you need to increase the load gradually and slowly. If, you push high load suddenly, the cathode may be broken. This is the explanation why rechargeable battery that was shorted (and has not exploded or burned yet) cannot recover its full capasitance.

The same principle also applies to the slew rate. High speed systems usually have higher slew rate. Therefore, it’s able to draw current higher than the battery’s cathode can supply in a time. This may damage the battery. If the battery is rechargeable, this will reduce its charging life.

My suggestion is use capasitors to reduce the current spikes in your system. My experience with this is using the oscilloscope with the resistor in series with the battery. Then, adjust the capasitors (by soldering/desoldering them on board) until the spike slope is reduced. It’s not easy, but designing the best products have never been easy anyway.

  • Can we extract more power with boost regulator ?

This is the thing that I bet you’re thinking when designing battery powered system. If the battery is low, can we use boost regulator ?

Be careful with the boost regulator. Boost regulator has great efficiency (90%-98%) only if the difference between input-output voltage is low (refer to the boost regulator datasheet). If the difference between input and output is too high, the efficiency is greatly reduced (from 60%-80%..Refer to the datasheet). It means that if the battery is low, more power is needed to boost the voltage than doing the job.

The solution for this is to design the system with minimum operational voltage. The next step is to design the boost regulator to shutdown the system (stop regulating) if the minimum operational voltage reached. This is important, since most of batteries will have zero voltage right after their capasity is finish (its voltage will not gradually down again).

Battery VCC Characteristic

In the diagram, the boost regulator should stop working before 0.9V reached.

In battery application, after the system is off for some time, the battery voltage will rise again. This may trigger the system to active. However, since there’s no capasity left, the battery will drop dead soon. So, it’s like bouncing ball..the system is on, then off, then on, then off again..This may create trash data logged into the system.

To prevent this kind of problem, add delay in the boost regulator. So, the system will on, only if the battery is stabile for some time.

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