Hi
The iDSD micro requires a battery to be present and to hold at least some charge in order to turn on. If the battery fitted is totally dead, it will not turn on.
However, it is perhaps important to understand the definition of 'bad battery.' Generally a modern battery is considered 'expired' once its capacity has dropped to 70% of rated capacity. It means the battery is still functional, but no longer meets the minimum specification on capacity.
If operated in 'cyclic' mode, that is charged fully and discharged fully, around 300 - 400 complete charge cycles are possible before this point is reached. To reach this cycle life it is important to avoid frequently charging the battery when very low (< 15 Degrees) or very high temperatures (> 35 degrees Celsius) prevail, as this will shorten lifespan/number of cycles. As iFi products use modern batteries, as they lack the so-called 'memory effect,' so there is no penalty from partial charge cycles (read: we recommend to keep topping-up!).
The iDSD micro will operate in 'cyclic' mode if turned off 'hard' and recharge the battery to 100% charge state for maximum capacity in (portable) use. So if the battery is charged and discharged every day it will show appreciable loss of capacity after about 1 year of use, but will likely remain usable at reduced (and further declining) capacity for several more years.
An alternative mode is (incorrectly) referred to as 'float-charging' from the old lead acid batteries used in this mode. Here the iDSD is meant to be always powered and thus the battery is re-charged only to around 80% charge state. This charge was in independent research found to give the overall largest increase in 'shelf life.' This is used automatically if the iDSD micro remains turned on during charging. If enough external power is available (e.g. 1.5A from the iUSB3.0 or from a BC 1.2 Compatible charger or Hub) the battery will very rarely or never draw upon to deliver power. Under such conditions a 'floated' lifespan (with the same definition of 70% capacity) of 3-5 years over a temperature range of 10 to 40 Degrees Celsius is expected, longer with less extreme temperatures.
It should be noted that at 70% capacity the battery is still fully functional and has only lost 30% of its capacity (due to chemical reactions). So it is likely to last at continuously reducing capacity for more than time span that it took to get to 70% capacity given continued identical use.
So in practice, an iDSD micro that is used at normal room temperature and run of a permanent power source with enough current1 and is permanently switched on, should operate trouble and maintenance free for likely at least 7 - 10 years if not more. Replacement batteries are available, just ask your retailer/distributor to have the iDSD serviced.
One last caveat, try avoiding hard drops of the iDSD nano or micro, lithium based batteries tend to take excessive mechanical shock quite badly, in addition to the mentioned avoidance of excessive heat or cold during charging. For discharging fairly high temperatures of up to 60 degrees Celsius and fairly low temperatures down to as low as - 10 degrees Celsius are acceptable but will decrease lifespan as well, so better to keep Lithium Polymer batteries at a reasonable comfortable room temperature.
1In this context 'power source with enough current' means a power supply or charger that offers at least 1000mA under a recognised USB power limit signaling, e.g USB BC1.2 (the official USB standard). This means most modern high current chargers except some older Samsung, most Chinese domestic model chargers and anything Apple, as these all operate non USB-Standard methods of indicating a dedicated charger with high power attached, as opposed to a standard 500mA USB Port, which are not recognized under BC1.2.
