In my previous posting “Some Basics on Battery Ratings
and Their Validation” I discussed the importance of making certain you are
getting the most out of your battery as a key task for optimizing the battery
run-time of a mobile battery powered device. You do not want to just rely on
what is specified for the battery but you really need to validate it. Indeed,
when I did, I found a battery’s capacity to be 12% lower than its rated value.
That is a lot of unexpected loss of run-time to try to make up for! On further
testing and investigation I indeed confirmed it was the battery and not
something I did with inadequate charging or discharging.
Once you get a handle on the battery’s stated ratings,
based on recommended charging and discharging conditions, you should then
validate the capacity you are able to get under the loading conditions your
device subjects the battery to. Most modern mobile battery powered devices draw
high peak pulsed, low average current from the battery. Batteries subject to
pulsed loading deliver less capacity in comparison to being subjected to the
comparable loading that is only DC. The
amount of impact depends on the battery’s design and its ability to handle high
peak pulsed loading. Furthermore two different batteries with the same ratings
can deliver substantially different results in the end-use application. The
bottom line is you need to validate the battery under end-use conditions to
assess how much impact it has on the battery’s performance.
Creating end-use operating conditions for devices of
course depends on the type of device. In some cases it may be fairly simple but
in many cases it can be rather complex. A smart mobile phone, for example, requires
a set up that can emulate the wireless network it normally operates in and then
place it in a representative active operating state under which to run down the
battery. The battery’s run down voltage and current in turn needs to be logged
until the battery reaches its proper discharge termination point, in order to
assess the amount of capacity it delivers under end-use conditions. An example
of such a set up is shown in Figure 1.
As you are trying to assess battery capacity under
end-use conditions you will likely want to run trials several times and for
different batteries, you will want to control conditions as closely as possible
so that you can confidently compare results knowing they were done under
comparable test conditions. You also need to be careful about (not) relying on
the mobile device’s internal battery management system for end-of-life
discharge termination as it is a possible source of error. A technique I
resorted to was to record a representative portion of the end-use pulsed current
drain drawn by the mobile phone which I then “played back” continuously through
our N6781A SMU, acting as an electronic load, to discharge the battery. The
N6781A had the required fidelity, accuracy, and “playback” hooks to faithfully
reproduce loading of the actual device.
Further details on this record and playback approach are documented in a
technical overview “Simplify Validating a Battery’s Capacity and Energy for End-Use Loading Conditions”. The results of my validating the battery’s
capacity under end-use conditions are shown in Figure 2.
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.