Is there a relatively foolproof way to automatically sense alkaline or lithium primary cells?

Question

I have an application where alkaline or lithium batteries could be used, and I do NOT want to have the user answer which type they are, and new/used. That seems risky to me, and the consequences of a wrong choice could include death. (serious)

To be clear, I'm talking only about retail Lithium Iron Disulphide (1.8V) cells vs retail alkaline cells like Energizer and Duracell and so on. Li-Ion cells (unfortunately now available in sizes that will fit in common battery holders) are sensed by the overvoltage protect circuit, and Nicad/Nimh cells, are prohibited by undervolt protection. Nicad/Nimh cells would perform very poorly here and could get someone killed.

This query is ONLY about consumer Lithium Iron Disulfide cells (1.8V OC) vs consumer alkaline cells (1.6V OC).

There is no physical way to prevent inserting the wrong cells.

Loking at the discharge curves suggests this may be possible, but I'm not aware that it has been done without user input.

I am hoping there is some method, maybe involving impedance measurement, or some other technique I'm not aware of yet.

I really wish they had included some mechanism to tell the chemistries apart!

Answer 1

The answer to this question is "No." As far as I am aware, there is no foolproof way to distinguish which type of cells are inserted in general.

You didn't ask, but I will point out anyway, that what you are trying to do sounds unreasonable. You want to exclude the use of all battery types except alkaline (automatically) and then, based on the assumption that the battery type is alkaline, provide a battery life estimate. You also noted that loss of life could conceivably result from an incorrect battery life estimate.

There are so many different types of AAA form factor batteries, and combinations of batteries that what you are trying to do seems to be folly.

Just as one example, what if the user inserts two NiMH batteries and one Energizer Ultra Lithium? It seems that the OCV might be compatible with alkaline, but your battery life estimation will probably not be accurate through the course of the discharge.

A similar problem could occur if the user inserts two discharged alkaline batteries and one Energizer Ultra Lithium. The OCV may be in range, but the voltage will collapse much more rapidly than expected during discharge.

I have designed several battery operated products, but none of them were safety critical. We simply warned the user in the user guide that they must use similar batteries and replace them all at the same time.

Incidentally, I also think it is a bit of a cop-out on your part to not allow NiMH batteries. They have similar total energy to alkaline batteries, and as far as I know there is no application where NiMH performs much worse than Alkaline with the possible exception of extreme low power devices such as clocks and TV remote controls.

End of discharge voltages between NiMH and alkaline are largely compatible (depending on the load it can be 0.8 to 1 V per cell or so).

Answer 2

I have an application where alkaline or lithium batteries could be used, and I do NOT want to have the user answer which type they are, and new/used. That seems risky to me, and the consequences of a wrong choice could include death. (serious)

Seriously, if the consequences of battery failure may include death then you should not be using dry cells, which are inherently unreliable for several reasons:-

1. The contact system, which relies on spring pressure and makes contact at only a single point. Oxidation, contamination or physical shock can make the contact unreliable.

2. They leak, corroding the contact springs and eventually other parts of the device. Users will replace leaky cells without servicing the device.

3. The only way to measure capacity is to discharge the cell, which makes it useless. Therefore there is no practical way tell if it has acceptable capacity, and you have to rely on accuracy of the manufacturing process.

4. As you note, the device manufacturer has no control over which cells the user will insert. They might be Alkalines, but cheap poor quality cells with low reliability - and the device cannot tell until they fail.

5. The user may insert a mixture of fresh and partially discharged cells and/or different types, and your device cannot tell unless it measures the voltage of each cell individually (and perhaps not even then if any are lithium cells).

In chat you say

I didn't create the rules, I just have to follow them.

No, you don't. "I was just following the rules" is not a valid excuse when you know the product you designed is unsafe. Make it safe, or don't make it at all.

Answer 3

A typical alkaline cell is 1.55V or so when full and is considered empty at about 1.1V, both with no load.

Primary lithium cell voltages at full vary from about 3.0 to 3.6V but any of them would be considered empty at anything below 2.5V

Given that there is no overlap on the useful range of voltages, you can detect the type of cell based on voltage alone.

But to properly do this, it will take a lot more information than provided.