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For one of our upcoming projects, we have a battery pack consisting of 12 Energizer L91(Lithium /Iron disulphide AA Battery).

We divide this battery pack into three sets of 2 ,5 and 5. In order to have 3 voltage bus rails for the embedded system as the following.

  • Bus 1 - 3.0V
  • Bus 2 - 7.5V
  • Bus 3 - 7.5 V

Now as energy constraints are high for our embedded device, it is very essential to monitor these 3 voltage bus lines. Doing a research on the same, I came across TI BQ27500-V130. Going through the data sheets, I was not sure whether I can use this for the monitoring the bus lines in our case.

It would be grateful if you could give me an advice on whether I can use TI BQ27500-V130 for our application. Here are the constraints we are looking forward for the battery monitor we need to use.

  1. Consumes very low power
  2. Has UART/spi interface
  3. Estimate the remaining voltage on the pack.

I am looking for suggestions on what kind of battery monitors I can use for this purpose.

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  • \$\begingroup\$ You're guaranteed to deplete one of the three sub-packs before the others. Why not run a single 18V battery rail and down-convert from that? \$\endgroup\$ – HikeOnPast Jan 7 '13 at 22:09
  • \$\begingroup\$ yeah that is right.3V bus line is the RTC and two 7.5V bus lines are used with Xor. One 7.5V bus runs out and the next starts running. \$\endgroup\$ – Elsa Adams Jan 7 '13 at 22:13
  • \$\begingroup\$ Is your load constant current or constant power? \$\endgroup\$ – HikeOnPast Jan 8 '13 at 6:17
  • \$\begingroup\$ The load is varying. \$\endgroup\$ – Elsa Adams Jan 8 '13 at 13:17
  • \$\begingroup\$ I agree @HikeOnPast, there is very likely a design gotcha even in the question. It is very very hard to find a good reason for these three separate battery rails. Elsa, you may share your actual goals with us, and you may get a way better solutions. \$\endgroup\$ – Gee Bee Mar 16 '16 at 17:08
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The purpose of the TI fuel gauge chip is to determine "state of charge" (how full) of a one-cell lithium-ion or lithium-polymer rechargeable battery. What you're using is multiple cells, multiple packs, non-rechargeable, a different chemistry. Lithium-ion is not the same as lithium-iron (it's not a typo).

Roughly the way a fuel gauge works is to watch the battery drain to empty (reset to zero percent full), then watch by monitoring both voltage and current how much energy goes in during recharging, estimating a correcting factor for inefficiencies, differences in how the battery will behave under different temperatures, age, and so on. Then keep track of energy flowing out of the battery during usage to estimate the percent capacity remaining. The behavior model of a battery can get quite complex. Measuring voltage alone will tell you little useful information.

Meanwhile you're using disposable lithium AA cells. A fuel gauge would have no way to know their capacity because it's never watched them get recharged. Neither does it know if someone put in brand new cells or a mix of things found in the back of the kitchen drawer. There isn't really a solution to those problems without involving the user to manually specify the cell capacity upon insertion.

Which is not to say you couldn't program a microcontroller to watch voltage, current, temperature etc. and report accumulated energy flow, then subtract it from what you estimate the energy of a new battery to be. Just that an off-the-shelf solution is probably going to be oriented towards a somewhat different problem. You might want to consider changing your battery pack to one comprised of multiple Li-ion cells that would work with off-the-shelf battery management chips. The spec for Energizer L91 says 4 W-hr per cell, so 12 of them is about a 48 Watt-hour capacity, comparable to an iPad 4 or small laptop. The battery management circuit would then be comparable to what's in a small laptop as well, possibly too expensive and complex for your project.

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