Can I Put a Rechargeable Battery in my Hearing Aid?
Most personal electronic devices these days use rechargeable batteries, so it seems odd that hearing aids are behind this trend. To understand the hurdles in this area for hearing aids, it is helpful to first review some terms we see in the specification sheet for these batteries.
Nominal voltage is the theoretical or rated voltage, based on the electrochemistry of the battery, when the battery is discharging. For example, a nickel–metal hydride battery (NiMH) has a nominal voltage of 1.2 V, but when in real use the voltage can range from 1.0 V to 1.4 V. The nominal voltage depends on the electrochemistry of the battery, not the battery size. For batteries with the same chemistry, making the size smaller will decrease the capacity of the battery, not the voltage. This is why size 13, 312 and 10 zinc air batteries all have the same voltage.
Capacity is a measure of how much energy is stored in the battery and is reported as the current multiplied by the number of hours that current can be supplied under standard conditions. For hearing aid batteries, this is typically given in milli-ampere hour (mAh). A NiMH size 312 rechargeable battery with capacity of 23 mAh could last about 15 hours in the hearing aid if the hearing aid has a current drain of 1.5 mA: 23mAh/1.5mA = 15.3 h. Real world battery lifetime can vary considerably from this theoretical calculation due to varying operating conditions. For the standard-sized hearing aid batteries, the capacity of the battery is determined by the energy density of the battery chemistry used. The energy density describes how much energy is contained in a certain amount of mass (Wh/Kg) or a certain volume (Wh/l). For a rechargeable battery to be useful in a hearing aid, it should last at least one day without needing to be recharged.
NiMH rechargeable battery: Hearing aids are designed to operate in the 1 to 1.6 V range, and the nominal voltage for NiMH falls right into this range. So naturally this is the first rechargeable battery technology for hearing aids that appeared on the market. There are two main disadvantages of NiMH. One is that the energy density of this technology is low. So for a given battery size, it will not be able to power a hearing aid for anywhere near as long as a zinc air battery of the same size. The other disadvantage of the NiMH battery is that it may have a memory effect when charged and discharged. This reduces the capacity every time the battery is charged or discharged. Since the capacity is not all that high to begin with, this is a double-whammy. Furthermore, all rechargeable batteries are worn-out a little bit every time they are charged and discharged. This effect is called capacity fading and reduces the so called cycle life of the rechargeable battery. The battery may operate well in the first few months, but then the battery capacity will reduce gradually to an unacceptable level. Battery manufactures are of course aware of this problem, and address it by improving the recharge accuracy. That is, when the battery is charged to a certain level, the circuitry in the charger will detect it and then stop the charging. This means that every time the battery is charged, it will always be charged to the same level. This prolongs the life of the battery before it needs replacing.
So what’s the bottom line on using NiMH rechargeable batteries as a substitute for zinc air? Generally speaking, if a zinc air battery in the hearing aid lasts 10 days or more, then you can probably use the NiMH rechargeable battery. However, if the hearing aid has digital wireless capability, NiMH may cause unstable performance. This is because the capacity is “used up” much earlier than with a zinc air battery during use.
Silver-Zinc (AgZn) rechargeable battery: The AgZn is a newer rechargeable battery technology for hearing aids. This battery chemistry has at least two times the energy density of NiMH. So for the same volume AgZn should last at least two times as long as NiMH. Although this sounds promising, the capacity is still much lower than Zinc-Air. However, the greater capacity compared to NiMH makes it more likely that AgZn could power a hearing aid for at least a day on one charge. The main issue with AgZn is the nominal voltage; it is 1.8 V, which is too high for many hearing aids. Therefore, it can be only used in hearing aids which are specially designed to handle the high voltage. A voltage regulator must be added to the hearing aid circuitry, so the 1.8 V can be lowered to the level that the hearing aid can use. The use of a voltage regulator will in essence “eat” some of the battery capacity and thereby reduce the initial greater capacity this battery type is born with. Another issue for some hearing aid users will be the cost: considering that most people would probably like to have a spare or two charged and on hand, the initial cost can exceed several hundred dollars. Since the batteries will need to be replaced before the hearing aids, this makes the cost comparable to just using non-rechargeable zinc-air batteries.
The bottom line? You cannot swap your zinc air batteries out with AgZn batteries. The hearing aid must be specially designed to work with this battery technology. Users who don’t like the idea of hearing aid batteries ending up in a landfill may be interested in acquiring hearing aids that use AgZn batteries (which are recycled by the manufacturer), but they offer no advantages in terms of convenience or cost. In fact they may be less convenient. You must take them out of the hearing aids, put them in the charger, take them out of the charger and replace them in the hearing aids on a daily basis. This means handling the batteries much more often than would be needed with zinc-air batteries.
Lithium Ion (Li-Ion): There is one rechargeable battery technology that costs less, has reasonably high energy density and reduced memory effect. The Li-ion rechargeable battery is used pretty much everywhere but in hearing aids. Li-ion has a nominal voltage of 3.7 V, which would destroy hearing aid circuitry. Just as with the AgZn technology, the hearing aid would need to be specially designed with internal circuitry to regulate the voltage down to about 1.4 V. Li-ion batteries for hearing aid use must be specifically qualified for hearing aid use to provide the required level of safety.
A hearing aid has unique power supply requirements due to its small size, current consumption, and safety requirements. Currently, only NiMH rechargeable battery can advantageously be switched out with zinc air batteries, and then only if the current consumption is such that zinc air batteries last a long time and the hearing aid is not wireless. NiMH rechargeable batteries should last about 1 year if used appropriately. When making a decision, people may want to compare it to the yearly cost of zinc-air batteries. AgZn rechargeable batteries have some advantages over NiMH, but cannot be substituted for zinc air batteries as the hearing aid must be specially designed to use them.