lithium thionyl chloride cell in a tek 2430 scope replacement

[m3vuv]

Hi all i have this scope with a dead battery for the cal values etc,its 3.6v normaly,could it be replaced with either a 18650 cell or a cr2032 cell,ie 3.7/4.2v  or 3v for the cr2032?

[tooki]

Whether a 2032 cell works depends on the circuit, but most likely yes. It just won't last as long as the 3.6V lithium cell.

I wouldn't use an 18650 Li-Ion, since the maximum voltage is too high, and you'd have to take it out frequently to recharge. (And you absolutely must use a cell with built-in protection IC that'll stop it from getting deep discharged.)

What's wrong with getting another 3.6V lithium cell? They're not particularly hard to get (by mail) and last forever with a very stable voltage until the very end, which is why they're used for these applications.

[David Hess]

The CR2032 will work but the lower voltage will likely trigger the CMOS battery low warning.

The voltage of the 18650 lithium-ion cell is higher, but will not damage anything.

Lithium thionyl chloride cells are not expensive compared to an 18650 and will perform better.  I bought one not long ago which would be suitable and would last at least a decade:

https://www.mouser.com/ProductDetail/667-TL5155P

[m3vuv]

in the end i used a 18650 charged to 4v in a holder,that way easy to remove to charge,its a 1.5ah so should last ages before it needs charging,now all self tests pass fine.,i may get the correct battery when funds permit.

[David Hess]

in the end i used a 18650 charged to 4v in a holder,that way easy to remove to charge,its a 1.5ah so should last ages before it needs charging,now all self tests pass fine.,i may get the correct battery when funds permit.

The self discharge rate of a lithium-ion cell is high compared to a lithium thionyl chloride or lithium manganese oxide cell, so operating life will be much lower than predicted by the capacity.

A simple charging circuit could be added with an operational amplifier and transistor.

[tooki]

A simple charging circuit could be added with an operational amplifier and transistor.

That is really bad advice. Lithium ion cells need more complex charging methods than that, and they hate being kept at full charge for extended periods of time.

Of course, I wouldn’t recommend a lithium ion as a backup cell anyway, it makes no sense for the application.

[David Hess]

A simple charging circuit could be added with an operational amplifier and transistor.

That is really bad advice. Lithium ion cells need more complex charging methods than that, and they hate being kept at full charge for extended periods of time.

Of course, I wouldn’t recommend a lithium ion as a backup cell anyway, it makes no sense for the application.

They need a current limited constant voltage charging circuit, and there is nothing which requires charging them to their maximum capacity so the charging voltage can be set lower, for like a 60% capacity for maximum life.

[scopeman]

You could use a standard lithium primary cell (not a rechargeable) in that instance. The only difference would be the operating life would be different if the cell capacity is not the same as the one that you are replacing. You might look at the Bromine series from Panasonic.

You should be able to find the same chemistry cell from DigiKey as I have purchased them before. The only issue is that they will not ship these cells by Air only by surface shipment.

Sam
W3OHM