Hi!
Much has been written on the subject I am about to address. But nonetheless I would like to ask for advice of the more experienced engineers on this forum on a concrete design. My problem has to do with wiring primary, i.e., non-rechargeable batteries (button/coin) in parallel. I've read about the pitfalls and dangers due to unequal voltages and resulting current flowing from one battery into the other.
After trying it out, nothing bad happened. However, since my conclusions on this matter may in the end influence the design of a commercial product, I would like to be sure the risks are next to none.
About the design:
- The desired battery type selection and parallel wiring is unfortunately the most optimal one due to physical constraints, i.e., space available for the battery.
- Currents drawn by the circuit at 3.0 V are going to be between a few uA (around 5 to 10 when device is in sleep mode) and 2 - 3 mA max. When operating, the draw will be about 1 mA on average.
- The idea is that batteries will come in a custom casing (battery pack) so there is no chance the user could insert one full and one empty battery or reverse the polarity of one or both cells.
- The parallel connection of both batteries would only be made once the battery pack would be inserted into the device.
- Adding Shcottky diodes in series with each battery would cut away valuable operating hours and should be avoided.
With above in mind, is simple parallel connection of two coin/button batteries really such a bad idea? Currents will be relatively low. These batteries have relatively high internal resistances (15 Ohms and above). Voltage differences between new batteries are expected to be under 0.3 V (given past experience with these batteries it would more likely be in the range of 0.1 to 0.15 V). This assumption, combined with high IR would mean that equalizing currents are under 20 mA and power dissipated in the more empty battery about 6 mW.
One thing I am not sure about is how much of a problem it would be that the battery wasn't made for reverse current's. In other words, is forcing current into a non-rechargeable battery a problem only due to power dissipation or is it a chemical issue and will cause problems even if dissipated power is small. Will capacity effectively decrease because of that?
Thank you in advance for all your help.
Much has been written on the subject I am about to address. But nonetheless I would like to ask for advice of the more experienced engineers on this forum on a concrete design. My problem has to do with wiring primary, i.e., non-rechargeable batteries (button/coin) in parallel. I've read about the pitfalls and dangers due to unequal voltages and resulting current flowing from one battery into the other.
After trying it out, nothing bad happened. However, since my conclusions on this matter may in the end influence the design of a commercial product, I would like to be sure the risks are next to none.
About the design:
- The desired battery type selection and parallel wiring is unfortunately the most optimal one due to physical constraints, i.e., space available for the battery.
- Currents drawn by the circuit at 3.0 V are going to be between a few uA (around 5 to 10 when device is in sleep mode) and 2 - 3 mA max. When operating, the draw will be about 1 mA on average.
- The idea is that batteries will come in a custom casing (battery pack) so there is no chance the user could insert one full and one empty battery or reverse the polarity of one or both cells.
- The parallel connection of both batteries would only be made once the battery pack would be inserted into the device.
- Adding Shcottky diodes in series with each battery would cut away valuable operating hours and should be avoided.
With above in mind, is simple parallel connection of two coin/button batteries really such a bad idea? Currents will be relatively low. These batteries have relatively high internal resistances (15 Ohms and above). Voltage differences between new batteries are expected to be under 0.3 V (given past experience with these batteries it would more likely be in the range of 0.1 to 0.15 V). This assumption, combined with high IR would mean that equalizing currents are under 20 mA and power dissipated in the more empty battery about 6 mW.
One thing I am not sure about is how much of a problem it would be that the battery wasn't made for reverse current's. In other words, is forcing current into a non-rechargeable battery a problem only due to power dissipation or is it a chemical issue and will cause problems even if dissipated power is small. Will capacity effectively decrease because of that?
Thank you in advance for all your help.