Club Homepage Forums The Cessna 150-152 Fly-In Fly-In Questions Rechargeable hearing aid in a tent

I just got some new hearing aids that are rechargeable and need a power source when away from normal mains power for more than a day or two. Costco, where I got them, sells a battery for $100 that connects to the charging case to allow portable recharging. But since the input is a standard USB port, I knew that at least some of the inexpensive and readily available power bank "fuel" batteries, which I have anyway for charging cell phones and an increasing number of other items, would work.

These power banks contain a single Li-ion cell with nominal voltage of about 3.7 volts while the output has to be close to the USB standard of 5 volts. So they have an internal voltage converter. The converters are typically 80-90% efficient when delivering current, but still consume some power when there's no load at all. So the power banks are designed to detect how much current is being drawn by the powered device, and turn off when the current drops to some low value which indicates that the connected device has become essentially fully charged. Otherwise they'd slowly discharge from the converter current drain after the connected device is fully charged or even when nothing is connected.

The problem with my hearing aids -- and probably most others -- is that the current consumption during charging is so low that most power banks turn off within a minute or so of turning on, or don't turn on at all when connected to the hearing aid charger. (Worse yet -- if the hearing aids are set to turn on automatically when removed from the charger, they turn on when the power fails and drain while you sleep what remaining power they contain.) The good news is that not all power banks do that.

I found two which work with my hearing aids, and both can do double duty charging other USB powered devices. One is the Tenergy 5103x where x is a number that specifies color. Unfortunately, this has been discontinued but some might still be found. It continues working down to an exceptionally low current, and recharged my hearing aids for more than 20 nights before needing recharging. I think it's unlikely that a "substitute" proffered for this would have the same low current capability, so I'd stick to the original device if you can find one.

The other one has more capacity, works well, and is currently available, the Voltaic V25. It detects when something is physically plugged into the USB port and stays on as long as it is, even if nothing is connected to the other end of the cable. So you do have to remember to turn it off in the morning. This can be done either by unplugging the charging cable from the power bank or by a double press of its button. I tested this only for one night, but it has even more energy than the Tenergy bank.

All the better to hear you with, my dear.

Roy

I wonder if a small "pull up" resistor would help the automatic switching turn on and stay on?

"Pull up" resistor refers to a resistor connected from a signal or power line to a higher voltage positive supply. Without disassembling the unit you have access only to the power line and ground. But a "pull down" resistor from power line to ground (i.e., across the output) would consume extra current and if sufficient it would keep the power bank on. Of course, the power it consumes would be wasted but this wouldn't be a big deal if you could recharge the power bank regularly. I tested several power banks and the amount of current required to keep them on varied quite a bit. In general, higher capacity units required more current to stay on.

When looking for power banks that would work, I came across a number of DIY circuits that would load the output with a low duty cycle pulse. This wouldn't waste as much power but adds considerably more complexity.

The hearing aids charge in about 3 hours, so if you have a cell phone or other higher current device that takes longer, you could get an adapter that allows you to plug them both into the same power bank port and let the cell phone keep the power bank awake. Just remember to set the hearing aids so they don't automatically turn on when the power finally goes away.

I have a couple of other USB-charged devices that draw very little charging current, too -- an MP3 player and keychain flashlight -- although neither as little as the hearing aids.

Roy