Alkaline and lithium 9V batteries are both 9 V, and a rechargeable NiMH 9V is 8.4 V on Energizer’s sheet. They differ in weight, shelf life and the temperatures they work in.
Your 9V
Going into
What you have
The three types side by side
From Energizer’s datasheets
Alkaline (522)
Lithium (L522)
Rechargeable NiMH (NH22-175)
Nominal volts
9.0 V
9.0 V
8.4 V
Weight
45 g
33.9 g
42 g
Shelf life
5 years at 21°C
10 years
Not given
Works from
-18 to 55°C
-40 to 60°C
0 to 50°C
Capacity
Chart by load
Chart by load
175 mAh rated
Codes
6LR61, 6LF22, 1604A
1604LC
HR22, 7.2H5
Energizer’s and Duracell’s alkaline and lithium sheets give capacity only as charts, which change with the load. Duracell’s MN1604 is the alkaline 9V under Duracell’s own number, IEC 6LR61.
Smoke alarms
First Alert’s SMI100 manual lists four acceptable 9V batteries: Duracell MN1604 and MX1604, and Energizer (Eveready) 522 and 1222. It says to use the exact batteries it specifies and not to use rechargeable ones. Energizer’s 522 sheet and Duracell’s MN1604 sheet each include a smoke alarm test among their discharge tests. For any other alarm, its own manual names the batteries it takes.
Other things that take a 9V
Genie’s store says its Intellicode keypads, models GWKP, GWKIC and GWK, each use one 9-volt battery. It names no type.
Common questions
Are rechargeable 9V batteries really 9 volts?
Not on Energizer's sheet: its NH22-175 rechargeable NiMH 9V is 8.4 V nominal, against 9.0 V for its 522 alkaline and L522 lithium.
Can I put a lithium 9V in a smoke alarm?
Only if the alarm's maker lists one. First Alert's SMI100 manual lists two Duracell and two Energizer batteries, none of them Energizer's L522 lithium, and says to use the exact batteries it specifies.
Is 6LR61 the same as a 9V battery?
Yes, by Energizer and Duracell: both print 6LR61 as the IEC name of their 9V alkaline, the Energizer 522 and the Duracell MN1604.
Volts, weights, shelf life and codes come from Energizer’s 522, L522 and NH22-175 sheets and Duracell’s MN1604 sheet; the smoke alarm from First Alert’s SMI100 manual and the keypads from Genie’s own store. Checked against published data from Energizer, Duracell, First Alert and Genie.
First Alert, SMI100 smoke alarm user's manual (PDF)
The Genie Company, 9-Volt Battery product page
Battery sizes come from battery makers' datasheets and device makers' own manuals and help pages. Check your device's manual or the old battery before you buy. If anyone may have swallowed a battery, call the National Battery Ingestion Hotline at 1-800-498-8666 or your poison center at 1-800-222-1222 straight away.