AP_Scripting: added get_cell_voltage() API and arming check example

this allows for cell imbalance to be checked as an arming check using
a lua script

libraries/AP_Scripting/docs/docs.lua

@@ -2718,6 +2718,12 @@ function battery:healthy(instance) end
 ---@return integer
 function battery:num_instances() end
 
+-- get individual cell voltage
+---@param instance integer
+---@param cell integer
+---@return number|nil
+function battery:get_cell_voltage(instance, cell) end
+
 
 -- desc
 ---@class arming

libraries/AP_Scripting/examples/cell_balance_check.lua

@@ -0,0 +1,48 @@
+--[[
+ script implementing pre-arm check that batteries are well balanced
+--]]
+
+local MAX_CELL_DEVIATION = 0.2
+
+local auth_id = arming:get_aux_auth_id()
+
+local NCELLS_MAX = 16
+
+function check_cell_balance(bnum)
+   local min_volt = -1
+   local max_volt = -1
+   for c=0,NCELLS_MAX do
+      local voltage = battery:get_cell_voltage(bnum, c)
+      if not voltage then
+         break
+      end
+      if min_volt == -1 or min_volt > voltage then
+         min_volt = voltage
+      end
+      if max_volt == -1 or max_volt < voltage then
+         max_volt = voltage
+      end
+   end
+   local vdiff = max_volt - min_volt
+   if vdiff > MAX_CELL_DEVIATION then
+      arming:set_aux_auth_failed(auth_id, string.format("Batt[%u] imbalance %.1fV", bnum+1, vdiff))
+      return false
+   end
+   return true
+end
+
+function update()
+   local num_batts = battery:num_instances()
+   local ok = true
+   for i=0,num_batts do
+      if not check_cell_balance(i) then
+         ok = false
+      end
+   end
+   if ok then
+      arming:set_aux_auth_passed(auth_id)
+   end
+   return update, 500
+end
+
+return update()

libraries/AP_Scripting/generator/description/bindings.desc

@@ -90,6 +90,7 @@ singleton AP_BattMonitor method overpower_detected boolean uint8_t 0 ud->num_ins
 singleton AP_BattMonitor method get_temperature boolean float'Null uint8_t 0 ud->num_instances()
 singleton AP_BattMonitor method get_cycle_count boolean uint8_t 0 ud->num_instances() uint16_t'Null
 singleton AP_BattMonitor method reset_remaining boolean uint8_t 0 ud->num_instances() float 0 100
+singleton AP_BattMonitor method get_cell_voltage boolean uint8_t'skip_check uint8_t'skip_check float'Null
 
 include AP_GPS/AP_GPS.h