Hello! I currently have a requirement to make a device that takes two 18650 Li ion batteries to make a 3.3v output. The device requires only about 3mAh of current. However, the system must work when one of the battery is taken out for charging externally. I tried connecting each battery to a MCP1700-3302 regulator, and they seem to work at first.

However, I later found that when one of the battery is at a lower voltage that the other, the battery will stop outputting current, and somehow has the same voltage as the one that is outputting. I need to know the voltage of each battery so I can alert users to take it out and charge.

Can anyone give me some hints at best way to design this? Should I put a diode at the output of the regulators to stop it from affecting each other?

Thank you in advance!

 

The device requires only about 3mAh of current.

Do you mean 3mA?

What is supposed to happen when two batteries are installed? Which one should supply the current?

 

Yes, sorry I do mean 3mA.

If possible, they should supply the system at the same time, sharing the load. The system is suppsed to tell the user it is time to charge, and the user can take one out to charge, put back and take the other out for charging, so the device will not stop working while one of the battery is out.

 

Yes, sorry I do mean 3mA.

If possible, they should supply the system at the same time, sharing the load. The system is suppsed to tell the user it is time to charge, and the user can take one out to charge, put back and take the other out for charging, so the device will not stop working while one of the battery is out.

Just use two diodes, between the batteries and the regulator. Whichever battery is highest charged will supply the load, so that eventually both batteries will become fully discharged at the same time. At that point, which one do you charge?

 

The 2 Cells will work just fine when wired in parallel,
but they should always be kept in parallel,
and therefore, always at the same level of charge.

They should be installed as a brand-new pair, with equal hours of use.

Why not just install a USB-Charger-Board inside the device ?
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Just use two diodes, between the batteries and the regulator. Whichever battery is highest charged will supply the load, so that eventually both batteries will become fully discharged at the same time. At that point, which one do you charge?

Thank you very much! Haha, you are correct, it is a very valid question The device actually is installed between 2 rooms, so users may not have access to both batteries. When they see low battery level, they would change the one on their side.

 

Thank you very much! Haha, you are correct, it is a very valid question The device actually is installed between 2 rooms, so users may not have access to both batteries. When they see low battery level, they would change the one on their side.

Just hope that two people in the two rooms don't both decide simultaneously to remove their battery for charging!

 

The 2 Cells will work just fine when wired in parallel,
but they should always be kept in parallel,
and therefore, always at the same level of charge.

They should be installed as a brand-new pair, with equal hours of use.

Why not just install a USB-Charger-Board inside the device ?
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I wish there is a charger board and a battery inside the device. The device is supposed to stay in a wall for at least 10 years and no way of opening after installation. So they want to make sure the batteries can be replaced, and charged by the user.

 

Just hope that two people in the two rooms don't both decide simultaneously to remove their battery for charging!

That is the catch! However, the batteries should work for months, and I hope to solve the issue with user interface on the app.

 

You might get 3 years from a lithium ion battery, rather more with a primary alkaline cell

 

This is not a good plan.
If any one of the Cells can operate 2 or more Stations,
then it will not be possible to know which Cell needs Charging,
unless You add Charge-Monitoring-Circuitry,
which will put an additional Load on the Cell.
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You might get 3 years from a lithium ion battery, rather more with a primary alkaline cell

That is why the client wants no battery inside the device. My first proposal (maybe stupid) is to actually use the external batteries to charge the internal one, so when both batteries are out, the device can still work.

 

This is not a good plan.
If any one of the Cells can operate 2 or more Stations,
then it will not be possible to know which Cell needs Charging,
unless You add Charge-Monitoring-Circuitry,
which will put an additional Load on the Cell.
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Yes, each battery will be fed into different ADC for monitoring of the voltage. Understand there is leakage using this approach, but I cannot find a better way.

 

You are designing a Commercial-Device with inadequate knowledge and experience.
This won't end well.
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You are designing a Commercial-Device with inadequate knowledge and experience.
This won't end well.
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You are correct. That is why I am trying to get some advice here. Most likely the real thing will be designed by someone with experience, but I still have to suggest an approach first. Since the design relates to the cost of the device, my role is to at least know if we are talking about adding diodes, or some expensive power guages etc.

 

You haven't provided a list of performance requirements yet.
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You are designing a Commercial-Device with inadequate knowledge and experience.
This won't end well.
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You haven't provided a list of performance requirements yet.
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True. To be honest I don't know either. It is still very early stage. Let's say without looking at the detail, this requirement from the client is possible, right? I was going to build a small prototype for my interest first, but sounds like it could be much more complicated than I can handle.

 

"" Let's say without looking at the detail, this requirement from the client is possible, right? ""
All I can say is ............ You've got to be joking, right ???

So far, nobody even has a clue as to what this supposed "thing" is supposed to do.

The first question is, why is it "permanently-mounted" in a wall ?

2) If it needs to be permanently mounted in a wall, why is it not AC-Mains-Powered ?
( 24VAC Door-Bell-Transformer to power all units ).
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