Spurred on by this recent post about battery life (
A way to measure batteries or chargers? - PentaxForums.com), I’ve made some measurements of the charge and discharge behavior of a Pentax D-LI90 lithium battery.
I have three of these (genuine Pentax) batteries. They are dated 201102, 201306, and 201601, and probably arrived with my K-5 (March 2011), K-3 (March 2014), and K-1 (July 2016) respectively. Somewhere back in the K-5 era, I bought an extra, which went with the K-5 when it was sold.
The data presented here are for the 201601 battery (the newest). I will test the other D-LI90s, as well as some other camera batteries (OEM and other) from my Canon G15 and Q7 when a new data logger shows up (in the next few days; see below).
I drilled out a piece of wood so that the battery would slide part way in, making a snug fit. I drilled three small holes through which I inserted wires that make contact with the battery terminals. Using clip leads, I can connect the battery to either the Pentax charger power supply or to a resistor to discharge the battery. A voltmeter connected across the battery + and - terminals lets me monitor the voltage of the battery. I can also connect other meters to measure the current. I have not measured the charging current, though, and only checked briefly the discharge current.
Here’s a shot of the test set-up in charging mode (next to the bathroom sink!):
Initially, when I took the battery out of the K-3, it was partially discharged - it had been used for a while, but I do not know for how many frames. I charged it back up to where the charger green light went out. That gave a voltage of about 8.43 V after about 2 and a half hours. This charge cycle is represented by the red dots in the first graph. After the discharge cycle discussed below, I charged it back up to “full” again. This time the voltage maxed out at about 8.45 V when the green light went out. This charge cycle is shown by the blue dots. Since the charge state at the beginning of the original cycle was unknown, I have lined up the two charge cycles by offsetting the first cycle by an eyeballed-for-best-fit offset of 143 minutes. The two curves overlay reasonably well. For an essentially completely discharged battery, at about 6 V, the voltage jumps up immediately to almost 7 V when the charger is applied, and then takes about 4 hours to get to full charge, around 8.4 V. All 3 batteries are rated at 1860 mAh. If they can be fully charged in 4 hours, the current supplied by the charger must be at least (1860/4) = 465 mA (inefficiencies in the whole process require rather higher current). As noted, I did not measure the charging current. I will do that the next time I perform similar tests.
Here’s the graph of the charging process:
To measure the discharge curve, I placed a 53 ohm resistor (comprised of various resistors I had lying about) across the battery terminals, and measured the voltage from time to time. Upon application of the load, the voltage dropped by a tenth of a volt, and then began an almost linear discharge for about 12 hours. Near the end, the voltage dropped fairly quickly. As many have noted - a battery near the end of a charge cycle can go quickly from usable to kaput. I don’t know at what voltage Pentax considers a battery to be no longer usable. When I do further testing, I will check what the camera thinks of the battery at various voltage values.
Here’s the discharge plot:
For my 53 ohm resistor, I can readily calculate what the current flow should be, using Ohm’s law: I = V / R , where I = current, V = voltage, and R = resistance. Using my V and R values, the initial current should have been about 156 mA, whereas I measured 138 mA. I’m not sure why the difference, but I trust the voltage reading from my Radio Shack meter more than I trust the current reading from my Harbor Freight freebie meter!
If I take the calculated current and the time intervals between meter readings, I can total up how many milliampere-hours came out of the battery. If I do this for the discharge cycle, I get 1856 mAh - astoundingly close to the rated 1860 mAh! I don’t really believe the similarity. Again, there are losses in the system, as well as some uncertainly attached to the readings themselves. But, the agreement does encourage me to believe that I have measured something close to reality.
My discharge rate was around 0.08 C (where C is the mAh capacity, assuming a one hour discharge; i.e. pulling a current of 1.86 amps from our little D-LI90). I do not know how much current a K-3 or K-1 draws during various functions, but it is probably generally more than this, perhaps approaching 0.5 C during some operations. I am trying to kludge up something to stick into the battery compartment to let me measure the current used by the camera.
As you can see from my first picture, my measurement setup is a bit of a kludge - multiple clip leads. And, I have to read the voltmeter periodically (every 10 to 20 minutes, typically), which is a drag.
I have ordered, and should receive very soon, a data logger which can store time-tagged voltages. With this, I can set up a test and then go away until it is finished (although I need to figure out a way to stop a discharge test before the voltage drops too low). Once I’ve got the logger, I will perform similar tests on all my D-LI90 batteries, repeating some to check for consistency, and to see whether age has affected them (although none have been all that heavily used - I would guess less than a few hundred cycles on any of them), as well as the other camera batteries that I have.
Stay tuned.