Battery Longjevity
 

How does one store their batteries? My battery manual says a 90% charge is optimum storage for a night, and 50% charge for long periods of time. Does this accurately describe the charge requirements to save the life of your battery? or does it sound phooey?

I am using an Sony XDCAM EX, but I suppose it is a lithium ion like most batteries.

Storage temperature and charge
 

Storing a Li-ion battery at the correct temperature and charge makes all the difference in maintaining its storage capacity. The following table shows the amount of permanent capacity loss that will occur after storage at a given charge level and temperature.

Permanent Capacity Loss versus Storage Conditions Storage Temperature
40% Charge 100% Charge
0 °C (32 °F) 2% loss after 1 year 6% loss after 1 year
25 °C (77 °F) 4% loss after 1 year 20% loss after 1 year
40 °C (104 °F) 15% loss after 1 year 35% loss after 1 year
60 °C (140 °F) 25% loss after 1 year 80% loss after 6 months
Source: BatteryUniversity.com[28]

It is significantly beneficial to avoid storing a lithium-ion battery at full charge. A Li-ion battery stored at 40% charge will last many times longer than one stored at 100% charge, particularly at higher temperatures.[28]

If a Li-ion battery is stored with too low a charge, there is a risk of allowing the charge to drop below the battery's low-voltage threshold, resulting in an unrecoverable dead battery. Once the charge has dropped to this level, recharging it can be dangerous. Some batteries therefore feature an internal safety circuit which will prevent charging in this state, and the battery will be for all practical purposes dead.[citation needed]

In circumstances where a second Li-ion battery is available for a given device, it is recommended that the unused battery be discharged to 40% and placed in the refrigerator to prolong its shelf life. While the battery can be used or charged immediately, some Li-ion batteries will provide more energy when brought to room temperature.

From Wikipedia

I think a visit or an e-mail to your battery manufacturer might be a more accurate source than a general info LI-ion page on Wikipedia.

For example, here is the IDX PDF on how to store batteries:

http://www.idxtek.com/pdf/tech_info/T-005.pdf