diff --git a/degradation_model/batfuns.py b/degradation_model/batfuns.py
index a352c3d709..e605a86747 100644
--- a/degradation_model/batfuns.py
+++ b/degradation_model/batfuns.py
@@ -578,8 +578,8 @@ def dydt(t, y):
         dnli += parameter_values.evaluate(3600/param.F)*C_p_loss*np.average(c_save_p)
         dy2 = dy
         dy2[0] = dnli
-        dy2[1] = dCn
-        dy2[2] = dCp
+        dy2[1] += dCn
+        dy2[2] += dCp
         # dy2[3] = dy[3]
         # dy2[4] = dy[4]
         # dy2[5] = dy[5]