Rechargeable or consumable?
I’m assuming that we’re talking about disposable batteries here and not rechargeable batteries. But in general, the way people have been able to estimate the remaining percentage of juice left in batteries overall (not just hearing aid batteries) is by measuring the voltage on the battery. For example, for AA Alkaline, if their voltage show 1.5 volts or more when measure, then they’re considered full. Then as they gradually degrade down to 1.4, 1.3, 1.2 volts, etc, then a remaining percentage estimate can be drawn based on the remaining voltage.
For disposable hearing aid batteries, they do make battery testers where you just put in the hearing aid battery and make contact, and there’s an LCD display of bars representing the voltage on the battery. The more bars you have (up toward the “full” end), the fresher the battery. Usually they don’t really have quantifiable markers in terms of percentage for these testers, because it wouldn’t be quantifiably accurate. It’s designed to just give you a rough idea by looking at the LCD bars that appear.
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consumable batteries. My 2014 HA’s use 312 batteries. Don’t know what my upcoming 2023 HA’s will use.
Being a retired engineer, I measure AAA, AA & occasionally C batteries all the time.
But old watch and HA batteries hold a surface charge. So a voltage measurement, tells little about the battery condition.
Zinc air batteries have a pretty constant voltage discharge curve.
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My resound omnia and Nucleus 8 last pretty much all day, So i really charge them once a day really, for the nucleus 8, I have 6 battery (4 inherited from the Nucleus 7), one is really old, doesn’t last long like 1-2 hours. the rest last a ridiculous amount of time as i wear them all day. I can’t tell you how long it last because it spans multiday battery life rechargeable wise and it is hard to keep track of it because i take it off if i go into the shower, there is going to be a gap there and multi day battery too makes me don’t want to track it…
I have 2 compact battery and 4 standard battery… I am finding all of them lasting a day except for that old battery…
edit: clarification on battery
Do they use disposable batteries? If rechargeable, that has nothing to do with disposable battery life.
Yeah, and that’s why I have several of these Zinc Air battery tester collected from each time I bought a pair of HAs at Costco, yet I never bother using them except to see if the battery is simply dead or alive, but never to gauge how much is left in in because the voltage discharge curve is so flat that it’s basically either a go/no-go and if it’s in between at all, you know that it’s going to be dead real soon.
Nevertheless, the question was asked about how to gauge the remaining charge of a hearing aid disposable battery, and the only way I know of is via a voltage measurement approach, no matter how crude it is.
I am not using disposable battery at all and OP doesn’t say anything about specificity about disposable battery life.
Direct quote from the OP. The context is definitely disposable batteries.
I beg to differ. it just says what he is using
Even if it’s about the rechargeable and not the disposable type, the measuring technique to gauge the remaining juice in rechargeable batteries remains the same, using voltage measurement. You can only measure voltage on a battery or current discharged by the battery. But usually for the current discharged through the battery, unless you can get fancy enough and keep track of the cumulative amount each time to get a whole picture from a fully charged state, you can’t really tell the battery percentage status if you’re only looking at a snapshot in time and not the entire discharged history that would need to be monitored and stored somewhere.
But the mfg can get more fancy in tracking the voltage measurements and maybe they can also monitor current discharge to get a more accurate picture of battery life. That’s probably why Oticon wants you to reset the batteries when you replace them with new ones, so it can keep track of the statistics to help determine freshness over time to factor that into the battery capacity measurement.
For laptops or smart phones, they also have the facilities to keep track of battery usage statistics in order to help them determine battery health (which is its max capacity when fully charged compared to when the battery was brand new). It’s possible that they also monitor current usage statistics in addition to voltage snapshot measurements in order to be able to give users a more accurate picture of the battery’s remaining juice, on top of the battery’s health.
You may notice that sometimes for Li-ion batteries based laptops, when you want to know what the battery health is, they may ask you to run the battery down to depletion and recharge it back fully before it can accurately give you an estimate of the battery health, because that’s the only way it can measure the amount of juice that can be put back into it, hence its health. Other smart phones may not require you to do this because they probably keep better statistics of your usage cycles through voltage and current measurements to be able to make a good guess of the battery health without needing to ask users to deplete and recharge fully the battery like some laptops do.
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Another general way to measure the health and charge capacity of a battery is to fully charge it from a discharged state and while doing so, measure the mAh required to fully recharge the battery (Volusiano mentions this w.r.t. built-in laptop circuitry). To do so, it’s handy to be able to insert a USB digital multimeter in the charging circuit pathway and set it to record mAh passing through. A variety of such devices are sold on Amazon.
Each Li-ion battery is rated in its capacity when discharging at an average voltage, usually 3.7 v, as specified on its label. Recharging a Li-ion battery is usually done at 5 v or higher (the USB multimeter will display the charging voltage in the circuit). So, to correct the 5 v mAh input into a battery to the 3.7 discharge capacity, one would have to multiply the input mAh by 5/3.7.
The other thing is that charging is not 100% efficient. Wired charging is about 95% efficient in charging a Li-ion battery, wireless charging only 60% to 70% efficient (the other 40% to 30% of input energy is wasted as heat, same for the loss in wired charging). So, you ain’t gonna get good figures for recharging your HA’s by wireless induction even if it’s possible to stick a digital USB multimeter in the circuit path.
You can also get battery health apps, at least for Android, that essentially do the same via phone circuitry. I’ve used one in the past, AccuBattery, for my Galaxy Note 8. AccuBattery recommends that one charge through at least a 20% state of charge difference to get enough significant figures in the measurement to accurately estimate remaining recharge capacity compared to the stated battery capacity.
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Aye Jim, I appreciate what you are saying, but all DC Multimeters weren’t created equal, I have several including a Fluke 87v, and a Fluke 374 True-RMS, I use both simultaneously when testing Li-on batteries, for discharging or indeed charging, most other cheaper multimeters are way out! And I could say, without fear or favour, I only trust Fluke, they are expensive kit, but they are calibrated, but as always you get what you pay for… Cheers Kev
Are you saying if it works it is a Fluke?? LOL
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Yes, Kev, I have several Fluke meters, too, including a computer interface to go with my Fluke 289 True RMS Multimeter, etc. I’ve found a ~$60 (going-out-of-business) Radio Shack True RMS Multimeter performs just about as well, and I use that most of the time rather than risking the 289.
But I’m not speaking of the traditional digital multimeter. I’m talking about a USB version: https://www.amazon.com/gp/product/B07X3HST7V/
I read the Amazon reviews before I bought the device and, IIRC, most reviews by knowledgeable folks said the USB device was very accurate, comparing the results obtained with traditional multimeters. I don’t remember what tests I did (probably in a previous post somewhere on this forum) but I didn’t find any problem with its accuracy. Most of the time, I just use it to charge up a device that doesn’t offer any metering itself to somewhere between 40% to 70% SOC just to maintain and store the Li-ion battery in the device at that relative charge and not have the battery go off by storing at 100% SOC. So, for most uses, I don’t need great accuracy. It’s hard to beat the price. $24 on Amazon right now.
Edit_Update: Well, here’s one previous post where I described in detail how to use it and how I found it pretty accurate in measuring the energy required to recharge a 25,000 mAh battery pack:
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Thank you Jim… It’s refreshing to know, you are using the correct equipment in your endeavour to get a more accurate picture of your batteries 
My interest isn’t particularly in rechargeable aids though, it’s more on LifePO4 in general, and capturing solar power, I have approximately 5kw of solar panels, and about 20kw hours of LifePO4 batteries, 2 x Victron 5kw inverters plus, another 4kw one, which is non-victron. I have many Li-on batteries for all my Dewalt power tools, I alternate these batteries fairly frequently… In truth, I am an amateur, with all the professional gear, I like to have the best I can afford, but I have an extremely keen interest, I read up via the net, sometimes for hours, if not days, it keeps me occupied… Cheers Kev 
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In other news, I had to replace pretty much all my rechargeable battery in my 5 years, they definitely have some reliability issues like expanding cell or just really short battery life after. 4 years of constant charging. I can’t wait to see more advanced battery that hit the market such as solid state battery. CI mfg really need solve the issues or find another way to power a totally implantable cochlear implant system.
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Perhaps they need a little larger capacity battery and better battery management The battery is rated on full charge-discharge cycles and can be damaged if kept at a low charge level. That is why a new battery usually has about half charge.
The Toyota Prius moved to Li-Ion batteries several years ago. I believe they only charge to 80% if the battery capacity and discharge to 20%. This greatly extends battery lifespan.
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I understand. There doesn’t seem to be enough time left in life to indulge in all the things we had to put aside to make a living!
Just an observation, though. No meter, not even a Fluke, is calibrated forever. Anyone doing mission-critical work, e.g., manufacturing medical equipment, needs to have their measuring instruments periodically recalibrated. For instruments used in environmental health and safety, the required recalibration interval was once a year for radiation detectors. We had spares, would rotate the instruments, and ship the idle ones when their number turned up off to a recalibration service that would furnish us with a certification that we’d keep on file, plus a new sticker on the instrument.
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The charts linked to in post #96 or 97 tell me what I wanted to know.
Measure the 312 cell voltage with a 1500 Ohm load for 1 mA current.
The no-load voltage changes little months after the cell has given up the ghost. At 9 months, one still reads 1.35 Volts.
On the subject of DMM volt meters. I have, lets see, 6 meters (the1980 Fluke 8020 screen is dead) the best is a Keithley 6½ one.
On the subject of calibration, I was part of a team that got an industrial test lab, NVLAP certified. Total pain sending out meters to be recalibrated. Only reasons for recalibrating modern meters are safety or legal.
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