If you really want to know at what point a battery is dead, for a particular device, use the voltmeter to see at what voltage the battery is when a battery dies in that device. So when the smoke alarm starts beeping, take the battery out, put it on the voltmeter battery tester, and note down the voltage at which that device thinks a battery is dead.
You may notice that different devices have different dead points. For example, I have a paintball hopper that uses two nine volt alkaline batteries and they test at 6.5 volts and still work (though it is not as powerful as it was when the batteries were full and has a higher probability of jamming if I overload it). If a smoke detector dies at, say, 8.1 volts, you still have life left in those smoke detector batteries, and batteries are expensive. I think a lot of partially charged alkaline batteries are thrown out each year because of this lack of knowledge that I didn't realize until I got a voltmeter.
Capacity is important. The larger the capacity, the longer it should power your device. Different technologies and brands have different mAh capacities.
Alkalines seem to have between a 1000 and 1400 milliampere (mAh) hour life. Lithiums seem to be worth twice the price at about 3000 mAh.
9 volt alkaline cart (on my voltmeter)
Good = ~9 volts-7.8 volts
low power = 7.7 volts - 6 volts
Replace = below 6 volts
9.6 Volt NiHM Powerex
Full = 10.42 volts (tested on a fully charged 230 mAh Powerex
What battery should I buy?
Info
Alkaline batteries discharge their voltage linearly. Rechargeable batteries initially drop to their rated voltage, then hold that rated voltage for a long time, then drop suddenly when they are dying. I propose that how dead something is is relative to the lowest voltage a battery can be and the device still operate. The back of the voltmeter says
A general 1.5 volt alkaline AA chart according to my voltmeter.
New = 1.52-1.57 volts*(as I've tested)
Good = 1.5 v - 1.28 v
low power = 1.27-1.15 volts
replace = below 1.15 volts (76% of rated capacity)
Starting AA voltages for new alkaline batteries by brand and age. Newer batteries tend to have more remaining voltage.
Brand Expiration date Voltage Birth date Age Shelf life left Qty Shelf Life
CVS N/L 1.52 © 2009... 4-5 yrs? N/L 1 ?
Kirkland Sig March 2016 1.52-1.54 (+one fluctuating battery) 2 years 5 ?
Energizer March 2016 1.54-1.55 ? ? 2 years 3 ?
Rayovac Dec 2016 1.55 Dec 2009 4 years 3 years 1 7 yrs
Rayovac Dec 2018 1.55-1.57* Dec 2011 2 years 5 years 5 7 yrs
Different devices require a minimum charge. A good way of knowing what that is is to take a battery tester to it when the device stops working to see what the minimum volt measurement for that device is.
For example:
Three Alkaline batteries at 0.99 volts each could not turn on a digital clock.
An old digital camera would not turn on at 1.4 volts. A rechargable battery at 1.2 v did not turn it on. So that is a super short battery life on that device.So I took those batteries out and used them in other devices.
A review on Amazon said that the devices a reviewer used would say an alkaline battery was dead at 1.4 volts (like the battery hungry camera I had). I don't know if 1.2 v rechargeable batteries would power a device that required 1.4 volts. Most cameras I've had since 2006 have been able to use rechargeable batteries though.
Though you can argue a battery is dead when a device can't use it anymore, though it may still power a different device, like a flashlight.
A 1.2 volt AA rechargeable battery freshly charged may start off around 1.3-4 volts, initially drop to 1.2 volts and stay there for a long while, about 85%-15% of its charge, and will be on the quick path to dying at 1.0 volts or so.
Here are some rough estimates, but maybe use these as a start as you get to know your battery. Look on the side of your rechargable battery to see what it's rated voltage is. Here is a guide for a standard 1.2 rechargeable AA.
A general rechargeable 1.2 volt NiMH AA chart
85%-100% =1.4 volts> and >1.25 volts
15%-85% = 1.10> and >1.25 volts (a bit under 1.2 v is still fine)
0%-15% = < 1.10 volts (about when the steep drop off may start)
While most devices that take AA batteries work with both 1.2 v rechargeable AA and 1.5 v alkaline batteries, I know of at least one camera circa 1996, a Japanese Minolta Dimage 7, that only showed low battery and refused to fully turn on with batteries at 1.3 v, but worked fine when the batteries were at 1.5 volts. That's when you get a new camera, because you can only use alkaline batteries that get to the 1.5 v and only from the 100%-90% of their supposed rated life. 1.35v / 1.5v = .9 Which means only 10% of the battery life was drained before the camera couldn't use the battery anymore. Kind of a waste.
This is a real concern for waste because other devices can use the remaining 90% of the charge, but I know I'd normally assume the batteries were fully dead and throw them out. Maybe that's why our moms kept half dead batteries in the refrigerator growing up; they would work in other devices.
Rechargable battery brand
Powerex was recommended to me in 2012 at Batteries Plus.
9v = 230 mAh
AA = 2700 mAh
Why? The mAh on these is among the highest of any brand, meaning more battery life per charge. I tried Duracel and Energizer with a rapid charger, but neither held their charge after maybe 15 charges on a 15 minute charger. A slow trickle charger seemed to bring them back to life.
There are some with low discharges, which hold more voltage over storage time, but they may come at the expense of a little capacity.
Best charger?
Apparently the rapid chargers can overcharge and burn out batteries. They are pretty convenient though.
In 2012, I went to Batteries Plus and asked for the most versatile charger at the best value. They sold me the 'Nurech-8' which will charge 4 x AAA, AA, C, D and/or two 9V batteries at a time. The user has to manually set the length of time of the charge, option A for small capacities, and option B for larger capacities. The battery life isn't really smart monitored, but maybe that's just one thing less to go wrong. I do feel a little bad about overcharging on option A if I am recharging a battery drained to 80% since it may overcharge it if I don't remember/set an alarm to manually take it out. Maybe it's not really an issue.
Option A will charge the batteries for 4.5 hours. Option B will charge for up to 9.5 hours. I don't know if there is a safety timer for 9 V.
9V NiMH 180 mAh takes 16 hours to charge. I use the 230 mAh Powerex 9 volt batteries, and leave them in there for over a day if they are dead dead. I can check on the voltage, but I'm not sure if taking them out resets the safety timer (if there is one). So while I'm not going to say it's the easiest one to manage, it works well.
I have been eyeballing this battery charger on Amazon that claims it can even recharge alkaline batteries up to 80% of their original charge.
