7
\$\begingroup\$

Currently I have two AA cells in series powering a 3.0v bus line for my application.Recently I am finding that one of these AA cells gets completely dead before the other. What could be the possible explanation for one battery getting depleted with respect to the other when both are in series?

Don't the batteries in series have to deplete equally?

\$\endgroup\$
  • \$\begingroup\$ They don't have to start equally charged - there will be natural variance in the capacity. \$\endgroup\$ – pjc50 Jan 30 '13 at 14:06
  • 1
    \$\begingroup\$ Have you tried different brands? Perhaps the internal resistance of the brand you're using varies significantly more than another brand. \$\endgroup\$ – JYelton Jan 30 '13 at 17:22
  • 1
    \$\begingroup\$ Voltage under load changes very quickly with charge condition near the end of the discharge curve. Normal variation in capacity (perhaps a few percent) could lead to large differences in voltage, but really they're both used up about the same percentage. If you're getting about the right ampere-hours out of the batteries, it's nothing to fuss about. \$\endgroup\$ – Spehro Pefhany Jun 20 '14 at 13:32
  • \$\begingroup\$ It is interesting that none of the primary cell data sheets I have seen, give any information on the spread of capacity between units and batches. I wonder if anyone here has done any testing and analysis? \$\endgroup\$ – Henry Crun Jun 25 '18 at 23:32
6
\$\begingroup\$

You have two identical cars, both with a full tank of gas. You drive them on the freeway side-by-side at identical speeds. No matter how closely the experiment is controlled, one car will run out of gas first. The other car may run out of gas soon after, or it may go for many miles before stopping.

There are many variables in battery manufacturing; no two batteries have exactly identical capacity. Like the cars, one will run out first even if they are discharged identically. If you are using cells from a reputable manufacturer, it is reasonable to expect that their capacities will be reasonably well matched. In this case, the remaining capacity in the cell that is not depleted should be small.

There are also variables associated with battery discharge that may exacerbate the issue, though these may not apply to your situation.

For example, some battery powered devices require different supply voltages internally. A cost-effective way to do this is to 'tap' the battery pack - Two series AA cells can supply both 3V and 1.5V, but in this configuration, one cell will supply the load current for both supplies, while the other cell will only supply the 3V current. In this case, the cell providing both supplies will always be depleted first.

In some devices, the physical location of the cells may cause one cell to be externally heated more than another (imagine one happens to be right under a heat-generating IC, while the other is near a keyboard). Two identical cells under the same discharge current but at different temperatures will have different discharge characteristics.

\$\endgroup\$
  • \$\begingroup\$ The cars example in your post is for batteries in parallel. I have seen huge difference (one cell 1.2v and other 0.9v) in battery capacity even when they are connected in series. \$\endgroup\$ – Chetan Bhargava Feb 14 '13 at 7:20
  • \$\begingroup\$ No; the car example is for series-connected cells. Each series-connected cell provides nearly identical total Wh (a measure of energy) during discharge but one runs out first. Two cars with the same amount of initial energy (volume of liquid fuel), operating in near-identical conditions of energy consumption, yet one runs out first. \$\endgroup\$ – HikeOnPast Feb 14 '13 at 20:24
4
\$\begingroup\$

Assumptions:

  • One of the two batteries in series (say the one closer to the + contact) is consistently running out before the other.
  • There is no center tap in the battery housing, so it is assured that both batteries provide exactly the same current throughout device operation.

One likely cause is temperature difference of the locations of the two batteries during operation.

For instance: See this excerpt from the datasheet of Panasonic LR6XWA AA alkaline cells:

Temperature Characteristics of AA cells

Note that the expected service life for a 50 Ohm load goes from around 110 hours, to around 150 hours, for temperature going from 20 to 45 degrees C.

The graph below, from the Energizer E91 AA alkaline cell datasheet, is even more explicit in service life differences:

Energizer E91 service life

Conclusion: It is fair to expect huge differences in service life from a given AA battery under different temperature conditions.


Therefore, it would be useful to examine the battery housing of the device for one part running warmer than the other during normal operation - that would explain the difference, as well as suggest a solution: Switch the batteries around halfway through their expected service lifetime.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.