Index: sys/dev/acpica/acpi_cmbat.c
===================================================================
--- sys/dev/acpica/acpi_cmbat.c
+++ sys/dev/acpica/acpi_cmbat.c
@@ -36,6 +36,7 @@
 #include <sys/module.h>
 #include <sys/bus.h>
 #include <sys/ioccom.h>
+#include <sys/sysctl.h>
 
 #include <machine/bus.h>
 #include <sys/rman.h>
@@ -62,6 +63,7 @@
 #define	ACPI_BATTERY_BST_CHANGE	0x80
 #define	ACPI_BATTERY_BIF_CHANGE	0x81
 #define	ACPI_BATTERY_BIX_CHANGE	ACPI_BATTERY_BIF_CHANGE
+#define ACPI_BATTERY_BTP_WARNING_LEVEL 20
 
 struct acpi_cmbat_softc {
     device_t	    dev;
@@ -69,7 +71,9 @@
 
     struct acpi_bix bix;
     struct acpi_bst bst;
-    struct timespec bst_lastupdated;
+    struct timespec bst_lastupdated;    
+    bool            acpi_btp_exists;
+    int             btp_warning_level;
 };
 
 ACPI_SERIAL_DECL(cmbat, "ACPI cmbat");
@@ -87,6 +91,8 @@
 static void		acpi_cmbat_get_bix(void *arg);
 static int		acpi_cmbat_bst(device_t, struct acpi_bst *);
 static int		acpi_cmbat_bix(device_t, void *, size_t);
+static void		acpi_cmbat_btp(void *);
+static int 		acpi_cmbat_btp_sysctl(SYSCTL_HANDLER_ARGS);
 static void		acpi_cmbat_init_battery(void *arg);
 
 static device_method_t acpi_cmbat_methods[] = {
@@ -526,6 +532,90 @@
 }
 
 static void
+acpi_cmbat_btp(void *arg) {
+
+    struct acpi_cmbat_softc *sc;
+    device_t dev;
+    ACPI_HANDLE h;
+    ACPI_STATUS as;
+    uint64_t newtp;
+
+    ACPI_SERIAL_ASSERT(cmbat);
+
+    dev = arg;
+    sc = device_get_softc(dev);
+    h = acpi_get_handle(dev);
+
+    /*
+     * Try to set a default warning level.
+     * If successful, we assume that _BTP exists,
+     * and create the sysctl. 
+     */     
+
+    sc->btp_warning_level= ACPI_BATTERY_BTP_WARNING_LEVEL;
+    newtp = sc->bix.lfcap * sc->btp_warning_level / 100;
+    as = acpi_SetInteger(h, "_BTP", (uint32_t) newtp);
+
+    if(as == AE_NOT_FOUND)
+	sc->acpi_btp_exists = FALSE;		
+    else if (ACPI_FAILURE(as)) {
+	ACPI_VPRINT(dev, acpi_device_get_parent_softc(dev),
+	"error setting _BTP --%s\n", AcpiFormatException(as));
+	sc->acpi_btp_exists = FALSE;
+    }
+    else {
+	sc->acpi_btp_exists = TRUE;
+	struct sysctl_oid *cmbat_oid = device_get_sysctl_tree(dev);
+	SYSCTL_ADD_PROC(NULL, SYSCTL_CHILDREN(cmbat_oid), OID_AUTO,
+	"warning_level", CTLTYPE_INT | CTLFLAG_RW, 0, 0,
+	acpi_cmbat_btp_sysctl, "I" ,"battery warning level");
+    }
+}
+
+static int acpi_cmbat_btp_sysctl(SYSCTL_HANDLER_ARGS) {
+	
+    struct sysctl_oid *parent;
+    struct acpi_cmbat_softc *sc;
+    device_t dev;
+    ACPI_HANDLE h;
+    ACPI_STATUS as;
+
+    long battery_index;
+    uint64_t newtp;
+
+    ACPI_SERIAL_BEGIN(cmbat);
+
+    parent = SYSCTL_PARENT(oidp); 
+    battery_index = strtol(parent->oid_name, NULL, 0);
+    dev = devclass_get_device(acpi_cmbat_devclass, (int) battery_index);
+
+    h = acpi_get_handle(dev);		
+    sc = device_get_softc(dev);
+
+    if(req->newptr) {
+	/* write request */
+	SYSCTL_IN(req, &sc->btp_warning_level, sizeof(sc->btp_warning_level));
+	
+	/* correct bogus writes */
+	if(sc->btp_warning_level < 1 || sc->btp_warning_level > 99)
+		sc->btp_warning_level = ACPI_BATTERY_BTP_WARNING_LEVEL;
+
+	newtp = sc->bix.lfcap * sc->btp_warning_level / 100;
+	as = acpi_SetInteger(h, "_BTP", (uint32_t) newtp);
+
+	if(ACPI_FAILURE(as))
+		ACPI_VPRINT(dev, acpi_device_get_parent_softc(dev),
+		"error setting _BTP -- %s\n", AcpiFormatException(as));
+	}
+    else /* read request */
+	SYSCTL_OUT(req, &sc->btp_warning_level, sizeof(sc->btp_warning_level));
+
+    ACPI_SERIAL_END(cmbat);
+
+    return (0);
+}
+
+static void
 acpi_cmbat_init_battery(void *arg)
 {
     struct acpi_cmbat_softc *sc;
@@ -578,6 +668,10 @@
 
 	valid = acpi_battery_bst_valid(&sc->bst) &&
 	    acpi_battery_bix_valid(&sc->bix);
+	
+	if(valid)
+	    acpi_cmbat_btp(dev);
+	
 	ACPI_SERIAL_END(cmbat);
 
 	if (valid)