[gnome-battery-bench] Improve the approximation of power from current

[Owen Taylor <otaylor src gnome org>]

commit fa6052918a4dfa770062ac55ef1f2311c9d0dc6d
Author: Owen W. Taylor <otaylor fishsoup net>
Date:   Tue Jun 30 16:30:10 2015 -0400

    Improve the approximation of power from current
    
    P = Integral from Q1 to Q2 of V(Q)dQ
    
    Use:
    
     (V1 + V2)/2 * (Q2 - Q1)
    
    as the trapezoid rule approximation of this instead of:
    
     V2 * Q2 - V1 * Q1
    
    which doesn't have a clear meaning. If V is constant, both are
    the same, but for example, if we had a capacitor with V(Q) = Q/C,
    then the old expression would over-estimate by a factor of 2,
    while the new expression is asymptotically right for small changes.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=747370

 src/power-monitor.c |   14 ++++++++++++--
 1 files changed, 12 insertions(+), 2 deletions(-)
---
diff --git a/src/power-monitor.c b/src/power-monitor.c
index 8946b16..7a70f3c 100644
--- a/src/power-monitor.c
+++ b/src/power-monitor.c
@@ -455,14 +455,24 @@ gbb_power_statistics_compute (const GbbPowerState   *base,
         }
     } else if (current->charge_now >= 0 && time_elapsed > 0) {
         double charge_used = base->charge_now - current->charge_now;
-        double energy_used = base->charge_now * base->voltage_now - current->charge_now * 
current->voltage_now;
+
         if (charge_used > 0) {
             statistics->current = 3600 * (charge_used) / time_elapsed;
-            statistics->power = 3600 * (energy_used) / time_elapsed;
             if (base->charge_full >= 0)
                 statistics->battery_life = 3600 * base->charge_full / statistics->current;
             if (base->charge_full_design >= 0)
                 statistics->battery_life_design = 3600 * base->charge_full_design / statistics->current;
+
+            /* We can approximate the power used using the current and the voltage; the
+             * more the voltage is constant, the more accurate this will be. We could
+             * improve this by looking at intermediate statistics, but since reporting
+             * capacity in watts is clearly preferred by the relevant standards, too much
+             * complexity to improve this doesn't seem to make sense.
+             */
+            if (current->voltage_now >= 0) {
+                double average_voltage = (base->voltage_now + current->voltage_now) / 2;
+                statistics->power = 3600 * average_voltage * charge_used / time_elapsed;
+            }
         }
     } else if (current->capacity_now >= 0 && time_elapsed > 0) {
         double capacity_used = base->capacity_now - current->capacity_now;