How do rechargeable 1.5 V batteries work?

Question

Until the last years, my knowledge was that the rechargeable analogue of the 1.5 V batteries have only 1.2 V. This greatly decreased their usability (many devices did not work well with 1.2 V).

The cause of this lower voltage was chemical and it was considered a hard limit until some years ago.

However, today there are a lot of 1.5 V rechargeable batteries (lithium or nickel-based). I experienced it as a new development, in the last few years. They seem to do what they state.

Sometimes they are going so far that they even have an USB connector to charge it, like this:

These batteries can be charged by USB (much more than 1.5 V), but they still give 1.5 V. Other batteries, without an USB connector, can be charged with 1.5 V. In all aspects they look like 1.5 V batteries, except that they look chargeable.

How do they work? Maybe they have some integrated DC-to-DC transformer circuitry?

Answer 1

The batteries you show use Li-ion cells internally and include a step-down and a USB 5V to Li-ion charge controller. They’re essentially power banks that output 1.5V with a step-down instead of 5V or 12V with a step-up.

(Why must the battery be Li-ion? For this specific cell form factor [AA], a comparable NiMH battery is only 600-1000mAh (720 ~ 1200mWh.) So NiMH chemistry could not achieve 3500mWh at 1.5V (~2300mAh) in that form factor: it's just not dense enough. Not so with Li-ion, as we see.)

Such externally-charged '1.5V' Li-ion batteries have been around for a while, since 2014, initially made by the Chinese company Kentli. The Kentli battery and its competitors use a single 3.7V Li-ion cell, with the electronics housed typically in a 'hat' at the (+) end. Kentli's battery uses a recessed 'ring' that connects directly to the battery for charging as opposed to USB, and thus requires a proprietary charger.

Nonetheless, the most common '1.5V' rechargeable seems to be the micro-USB ones, like you've shown. Your example appears to be from Poover, an unfortunate company name transliteration if there ever was one. (Maybe it's "The official rechargeable of the Hundred Acre Wood." Yeah, that's it.)

On the other hand, here's a video teardown of a '1.5V' rechargeable Li-ion based battery that doesn't use USB or a hokey proprietary connector at all. It's more clever in my opinion (charges with 5V applied via a passive 5V-powered charger.) See https://www.youtube.com/watch?v=L62tCyOP06w and https://budgetlightforum.com/node/69034

Answer 2

How do they work? I think, maybe they have some integrated DC-to-DC transformer circuitry?

Correct. There's been multiple types over the last decade that tries to work like a ubiquitous battery. These are just the latest, thanks to Miniaturization of electronics. Yesterday's 4 inch pcb is today's dime size single chip IC.

While not the one you are asking about, here are two similar ones. From https://lygte-info.dk/info/BatteryDisassembly18650usb%20UK.html is a usb chargeable Li-Ion cell. Note it is 3.7V (nominal output). It has a pair of charging ICs plus protection ICs, but has not output regulation.

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While an older version required a 3rd non-standard contact as a ring around the positive contact for charging the Li-ion cell separately, but has a built in switching regulator for 1.5V output. From https://ripitapart.com/2014/12/06/teardown-of-kentli-ph5-1-5-v-li-ion-aa-battery/amp/

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If you combine these two circuits, you get the one you are asking about, a usb chargeable li-ion with a 1.5V output voltage switching regulator. The circuitry handles battery protection and low voltage lockout so you get pretty much a full life at 1.5V instead of the slowly dropping voltage of a primary or non-regulated bare secondary cells. No different than any given usb power bank, except the specifics of the voltage being used.

Answer 3

There are a number of USB-charged AA Lithium-ion batteries from China. They use a switched inductor to regulate the charging with an IC. Since the USB plug and charging circuit are inside the battery and take up space, then the cell itself is smaller than an ordinary AA battery. Therefore the battery capacity of these rechargeable Li-ion batteries is less that an equivalently sized NiMH battery cell.

Answer 4

Different battery chemistries have different nominal voltages and different discharge profiles.

Unfortunately nickel-cadmium and nickel metal hydride Cells have a nominal voltage of 1.2V which is somewhat lower than the 1.5V of traditional zinc carbon and alkaline cells. Most equipment would tolerate this, but some would not.

There were attempts over the years to bring different chemistries to market that more closely approximated the 1.5V of traditional non-rechargable cells. The two I remember were "rechargeable alkaline" and "nickel zinc" but neither seems to have been terribly successful in the market. The former suffered from short cycle life while the latter suffered from concerns that it's higher voltage (1.6V nominal, as high as 1.8V fully charged) may damage equipment.

The items you post in your picture take a different approach, rather than expose the raw cell directly, they generally use a lithium ion cell (nominal voltage 3.7v) and then include a buck converter to convert the cell voltage down to 1.5v.

The advantage is that the output voltage is no longer dependent on battery chemistry, so you get a stable 1.5V.

Obvious concerns are that the buck converter will produce electrical noise, which designers of battery powered equipment will not be expecting. Self discharge caused by the buck converter is another concern.

My other concern is that these devices seem to be sold almost exclusively through marketplace sites. The sad fact is that a lot of stuff sold through such sites cuts corners, either on safety or on actually meeting the specifications printed on it.