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Application:

Our device wakes up 5 times a day. The wake-up period is nearly 30 s and during this time it takes a measurement from a sensor and sends data with a Zigbee, and then goes back to sleep.

Active-mode current is nearly 80 mA, and sleep-mode current is 1.6 mA.

We are using two new LiSOCl2 non-rechargeable batteries in parallel (the continuous current of one cell is 50 mA, so we use two cells in parallel). The cell name is LS14500 shaft battery.

What we observed is:

1- In some devices for a particular time (eg. on some days during we the voltage of this parallel battery pack was down to 3.0 V compared to a normal working day when it is 3.6 V, and during this time sometimes the microcontroller board restarted (Teensy 3.6). After some time the voltage switched to normal-condition 3.6 V again and continued to work normally.

2- We are also measuring tbe voltage for our reference of this cell pack using a BQ35100 IC. Here we measure voltage between wake-up and powering on all other modules (so only the Teensy microcontroller board is on) to avoid fluctuation in voltage. Here the same thing is observed: for some hours or days the voltage droops down from 3.6 V to 3.2 V or 3.3 V, and after some time switches back to a normal 3.55 V again.

I don't understand why on some days the voltage is stable, and on other days it drops down and after that goes back to normal again.

NOTE: I recently received one such battery pack from the site and for checking purposes I opened its plastic coating and checked both parallel batteries separately. One was showing an open-circuit voltage of 3.6 V and a running voltage of 3.0 V. The other battery showed 2.4 V in open circuit and the same when running (here I think the board is not powered on properly in this condition).

So we can say that despite being in parallel one cell is discharged more than the other (both are new cells when first taken into use).

Can some one help to understand this situation?

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1 Answer 1

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Ohhh boy.... Please immediately stop using these primary lithium metal batteries in parallel without additional protection. You must use a diode on every cell to prevent the current from flowing into that cell - because these are non-rechargeable chemistries that have a chance of catching fire when mishandled this way. You do not want to deal with a lithium metal fire in any capacity.

When you receive the batteries, you should de-passivate them - put a 20 ohm load across the battery for upto a minute (monitor voltage). In storage these batteries become passivated and need a wake-up before being put in service.

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