If you have a 900-series translator, a spare battery may come in handy if you are outside the USA for an extended period. Note that this battery charges only when inside the unit, but is sized for a full day of use.
Note: this battery does not have a cradle charger. It charges inside the unit. Don't buy this if you intend to use one battery while the other charges, because that functionality isn't there. It is great as a spare and if you charge both batteries prior to a trip.
This rechargeable 3.7 V, 1400 mAh Lithium-Polymer Battery is just the thing to keep your P900 series device up and running. Use it as a supplementary power source and you will always be ready to go.
You may want a spare battery, if you are going to be outside the USA for an extended period or if we are shipping this to you with those international shipping charges. Otherwise, buying one just to have a spare is probably not the best use of your money.
There is no cradle charger for this battery. It charges inside the device. Therefore, you can't charge one battery while using the other battery.
You could charge one battery in the unit before a trip and insert the other one fully charged. But that really isn't necessary. The battery is sized for a full day of use. It should last you about three years (similar to a laptop or cell phone battery).
Mobile devices and battery drain
Mobile devices are always on. Nobody wants to reboot a smart phone just to make a phone call. Thus, a mobile device constantly drains its battery even if the screen is turned off or it goes into standby mode. After a few days, that drain can be significant. For most reliability, don't go more than a couple of days without charging for an hour or two.
How to have healthy batteries
When using any USB charging source, follow the charging limits of the li-polymer battery device you are using. Note that, typically, the manufacturer does not provide the correct information. So, we'll do that here for you.
Most mobile devices do not have robust charging circuitry, to save weight and space. Thus, the only way to prevent overcharging (and thus destroying) the battery is to avoid overcharging. Typically, this means to stop charging after 8 to 12 hours maximum.
The actual permissible time under charge is a function of the battery capacity in mAH and the charging rate of the adapter/charger. But it's not simple division to know that time. The output number on a charger is its initial rate. That rate slows dramatically as the battery charges--think of back pressure when you are trying to blow up a ballon and how it gets harder as the balloon fills with air. Same thing in a battery.
Li-poly batteries accept partial charges and do not develop memory. Charge them frequently for a couple of hours and they'll last a very long time.