Litium battery question

[Eric Page]

Note that the ETX680 and ETX900 share the same case size: "E". Either will fit in the same battery tray. If you have an electrically dependent engine and a single battery, you might consider using the 15.6Ah ETX900 instead of the 12.4Ah ETX680. Doing so will achieve longer alternator-out endurance, or the same endurance with less load shedding required. Weight difference is 0.8 lb.

Even without an electrically dependent engine, there are advantages to using the ETX900. It can supply higher starting current and tolerate higher recharge current. Those things, combined with its higher initial capacity mean it will likely last longer in the same environment than an ETX680.

Whether the longer lifespan will favorably amortize the $70 higher price is an open question...

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Edited to add:

If you have a 912iS (and presumably a 915iS), neither of these batteries meet the minimum battery ampere-hour capacity listed in the Rotax installation manual, but the ETX900 is nearly big enough.

ATTENTION
The use of rechargeable batteries with lithium–ion technology should only be
used in combination with a suitable battery management system. If such an battery
is used, the responsibility is up to the aircraft manufacturer. The approval of
the relevant aeronautical authority may be necessary. For installation of lithium ion
batteries refer e.g. to FAA AC No: 20-184

ATTENTION
Observe the specifications of the battery.
The size of the battery needs to be adequate for essential flight equipment and has to
meet the airworthiness requirements of its place of operation.
When sizing the battery ensure that during each operating state and also during the transition
between two operation states a sufficient supply of the display is guaranteed (e. g.
during engine start). Furthermore, it must be ensured that at least 30 minutes after failure
of the primary power supply the display is supplied with sufficient energy, if it is necessary
for a safe operation of the aircraft.

Nominal voltage: 12 V
Internal resistance: maximal 10mΩ at -18 °C (-0.4 °F)
Cold Cranking Ampere (cca): At least 350 A at -18 °C (-0.4 °F) (SAE J537)
Capacity: At least 16 Ah