More than 5.5V on VMODEM?

Hardware
1

We have a design where we would really like to use 4 Energizer AA Ultimate Lithium L91 cells in series. When fully charged, the open-circuit voltage of each cell is a max of 1.8V. That’s 7.2V max. Realistically, most of the life of these batteries, their per-cell voltage is closer to 1.5V, or 6V in series.

The Notecard’s datasheet says that the VMODEM input max is 5.5V. Is there any safety margin in there? What would happen if we hooked these 4 AAs in series up to VMODEM?

2

Hi @daterdots,

We generally recommend avoiding the use of L91 batteries in series for cellular applications (or any applications where high current draw bursts may occur). The middle ones will reverse voltage and fail when the voltage gets too low. There is also no safety margin (5.5V is it). We recommend using a cheap LDO regulator.

Thanks,
Rob

3

Hey @RobLauer - thanks for the heads up that the 5.5V is an absolute max. However, I am confused what you mean about “middle ones will reverse voltage and fail.” Are you talking about cell reversal? I have heard about this in theory, but I have never experienced it in practice as long as all the batteries are replaced at the same time. Have you seen problems with Notecards and AA batteries in series?

4

Also, to be honest, I have never heard of cell reversal being an issue for primary non-rechargeable cells. I have built/seen/used so many products that are comprised of 2 or 3 AA or AAA batteries in series that it’s hard for me to imagine that this is a common issue. However, the ~2A peak demand from the modem is relatively high relative to most products, so I am curious if you have seen failures in practice. Something we could do is just force the whole product into a low-power mode when the battery voltage drops below a certain threshold or if/when the cell voltages start to diverge from one another.

1 Like
5

Hi @daterdots,

I haven’t seen this happen in practice, personally. This is more of a warning of what could theoretically/likely(?) happen when voltage gets too low! Your plan sounds like a reasonable one to me.

Rob