Good price for a Milwaukee M18B5 Battery, works out cheaper if you select the 3 Units option.
Seller also has some good deals on other Milwaukee products.
Edit: Please read 1jwoz comments below before purchasing, seller may not be authorized Milwaukee dealer.
Milwaukee M18B5 18V 5.0Ah Li-Ion RED LITHIUM Battery
The M18 5.0Ah REDLITHIUM-ION BATTERY features superior pack construction, electronics, and performance to deliver more work per charge and more work over the life of the pack than any battery on the market. M18 5.0Ah REDLITHIUM-ION BATTERY features an advanced design, protecting the pack against harsh jobsite environments. This protection routes water away from the electronics and out of the battery pack, extends run-time and life by minimizing heat, and prevents pack failures from vibration or drops.
This unique to Milwaukee design also allows the battery to operate in extreme cold weather conditions (below -180C). Managed by Milwaukee’s exclusive REDLINK Intelligence, the battery features overload protection to prevent you from damaging your cordless power tools in heavy-duty situations, while the discharge protection prevents cell damage.
Features:
Powers more than 100 Milwaukee M18 cordless power tools
REDLINK intelligence provides optimised performance and over load protection
Best in class construction - offers long lasting performance and durability
Fuel gauge on board displays remaining run time for less down time on the job
All weather performance, delivers fade free power in extreme jobsite conditions
Specifications:
Voltage: 18V
Capacity: 5.0Ah
Charge time:
M12-18FC: 60 mins
M12-18C: 105 mins
Weight: 0.72kg
Technically it is genuine Milwaukee, however this seller splits up kits and sells the items separate and then gives out his own "1 year warranty". So what seems like cheap will bite you in the butt when you need to warranty the item.