Help to replace a coin cell battery with rechargeable battery and wireless charging coil

Thread Starter

JackAD

Joined Jun 23, 2021
4
Hi all

I am not an electronic engineer (hence this post). I have this circuit (designed by someone else) and I need to replace the battery holder (BT1, 3v) with a rechargeable battery and wireless charging coil (54 mm diameter max) to charge it with.

(btw, the J* components are just jumpers to allow traces to cross as this is a single layer board).

1624435584855.png
1624436204335.png
Is there anyone who would be kind enough to tell me how to do this? Recommend the components I need?

Thanks a lot!
 

Tonyr1084

Joined Sep 24, 2015
6,460
Welcome to AAC.

Few things here - you're asking about changing the type of battery to be used. The 555 circuitry you show is not relevant to the project at hand; so it doesn't much matter to how we answer. However, what IS important is what kind of battery you want to use. Another relevant issue is how much space you have. We could put a car battery into the circuit, but if it won't fit in your pocket then our answer would be useless.

If you choose a NiCad cell - you'd need three of them. NiCad is good because the way they are charged is simpler than if you used a LiPo (or other lithium based battery). Lithium batteries must have a charge controller to control the current feeding the battery. A good controller will also disconnect the battery from the circuit before its voltage drops to a harmful voltage. Lithium batteries can destroy themselves if their voltage is allowed to drop below a certain point. I THINK it's 2.8 volts but it's been a while since I messed with them, that number could be wrong.

A final, possibly critical issue is how much money you want to spend. As I mentioned lithium batteries, you can buy cheap controllers for their recharging. But the cheap ones probably don't have a disconnect (or LDO - Low Drop Out). Meaning the circuit disconnects when the battery reaches a low limit voltage. Very useful because, and I know this from experience, if you discharge a lithium battery then it probably won't recharge. I put one in an LED flashlight. Forgot to turn it off. Days later it was dead. AND it would not take a charge. In fact, I suspect it hurt my battery charger, possibly because the battery may have gone internally shorted. And yes, that's a real possibility too. And at low voltages those internal shorts can act as a heater, heating the battery to the flash point. No, that didn't happen to me, but it IS a possibility.

So we need more information from you. Battery type; allowable working space; how much you want to spend.

Again, welcome to AAC.
 

Tonyr1084

Joined Sep 24, 2015
6,460
Looking at your 555 circuit - how are you getting battery voltage to pins 4 & 8? Is this a circuit you found on the internet? If so - it's not correct. Looking at the TI Data Sheet it says the voltage supply is [edited] "Max 15V" but doesn't state a minimum voltage. "Minimum voltage is 1.5V" I'm questioning whether the chip will operate on 3V. Not saying it won't, just I haven't found any data stating a minimum voltage. [end edit] What I AM seeing is supply current at 1.5V needs to be 50µA. OK, that looks like it will operate at 1.5V and a coin cell CAN supply 50 micro amps (0.00005A). But now I question whether a coin cell can power 5 LED's in parallel circuits.

I'm easily confused - and I AM confused. Not withstanding, the question at hand is charging a battery wirelessly. Not the 555 circuit. So ignore my tangent.
 
Last edited:

Thread Starter

JackAD

Joined Jun 23, 2021
4
Welcome to AAC.

Few things here - you're asking about changing the type of battery to be used. The 555 circuitry you show is not relevant to the project at hand; so it doesn't much matter to how we answer. However, what IS important is what kind of battery you want to use. Another relevant issue is how much space you have. We could put a car battery into the circuit, but if it won't fit in your pocket then our answer would be useless.

If you choose a NiCad cell - you'd need three of them. NiCad is good because the way they are charged is simpler than if you used a LiPo (or other lithium based battery). Lithium batteries must have a charge controller to control the current feeding the battery. A good controller will also disconnect the battery from the circuit before its voltage drops to a harmful voltage. Lithium batteries can destroy themselves if their voltage is allowed to drop below a certain point. I THINK it's 2.8 volts but it's been a while since I messed with them, that number could be wrong.

A final, possibly critical issue is how much money you want to spend. As I mentioned lithium batteries, you can buy cheap controllers for their recharging. But the cheap ones probably don't have a disconnect (or LDO - Low Drop Out). Meaning the circuit disconnects when the battery reaches a low limit voltage. Very useful because, and I know this from experience, if you discharge a lithium battery then it probably won't recharge. I put one in an LED flashlight. Forgot to turn it off. Days later it was dead. AND it would not take a charge. In fact, I suspect it hurt my battery charger, possibly because the battery may have gone internally shorted. And yes, that's a real possibility too. And at low voltages those internal shorts can act as a heater, heating the battery to the flash point. No, that didn't happen to me, but it IS a possibility.

So we need more information from you. Battery type; allowable working space; how much you want to spend.

Again, welcome to AAC.
Hi and thank you so much for the detailed reply.

Space - this is the space reserved for the charging coil and battery(ies) together:
1624452561336.png
Something else I forgot to mention - the battery holder(s) and any other components need to be surface mounted devices.

Sounds like the best option might be to use three NiCad cells then? If not, and I use a LiPo battery, then I appreciate I'd need to spend enough to get a decent controller for it. In terms of cost, it doesn't matter hugely (within reason) as these are going to be produced in small numbers.

Hopefully that answers the questions?

Many thanks again
 

Attachments

Thread Starter

JackAD

Joined Jun 23, 2021
4
Looking at your 555 circuit - how are you getting battery voltage to pins 4 & 8? Is this a circuit you found on the internet? If so - it's not correct. Looking at the TI Data Sheet it says the voltage supply is [edited] "Max 15V" but doesn't state a minimum voltage. "Minimum voltage is 1.5V" I'm questioning whether the chip will operate on 3V. Not saying it won't, just I haven't found any data stating a minimum voltage. [end edit] What I AM seeing is supply current at 1.5V needs to be 50µA. OK, that looks like it will operate at 1.5V and a coin cell CAN supply 50 micro amps (0.00005A). But now I question whether a coin cell can power 5 LED's in parallel circuits.

I'm easily confused - and I AM confused. Not withstanding, the question at hand is charging a battery wirelessly. Not the 555 circuit. So ignore my tangent.
Not as easily confused as me! The circuit was designed by a trusted friend who I know is an excellent electronics engineer, so I'd be surprised if it's wrong, but of course everyone makes mistakes!

"how are you getting battery voltage to pins 4 & 8?" - no idea - I will pass this question on to him.

"But now I question whether a coin cell can power 5 LED's in parallel circuits." - I will also ask him this!

So fundamentally you don't think the circuit will work?

Thanks again!
 

Tonyr1084

Joined Sep 24, 2015
6,460
I'm WAY NOT the expert in electronics. The LED's may be drawing 20mA each, so 5 in parallel is 100mA. The data below is something I grabbed off the internet when searching 1632 coin cell current:
1624458051420.png
I'm not seeing how five parallel LED's drawing 100mA can be supported by this battery. It's PEAK current is only 6.8mA. FAR far short of the 100mA draw of JUST the LED's. Even if you were operating the LED's at 5mA, five in parallel is still 25mA. Again, exceeding the 6.8mA capability of the 1632. AT BEST the coin should be able to power five LED's at a current of 1.36mA each. That's not very bright.

As for why I ask about how the 555 is getting power to pins 4 & 8 - it might be a simple oversight on the schematic where lines cross at the below point:
1624458355934.png
It might just have been forgotten.
 

Thread Starter

JackAD

Joined Jun 23, 2021
4
I'm WAY NOT the expert in electronics. The LED's may be drawing 20mA each, so 5 in parallel is 100mA. The data below is something I grabbed off the internet when searching 1632 coin cell current:
View attachment 241988
I'm not seeing how five parallel LED's drawing 100mA can be supported by this battery. It's PEAK current is only 6.8mA. FAR far short of the 100mA draw of JUST the LED's. Even if you were operating the LED's at 5mA, five in parallel is still 25mA. Again, exceeding the 6.8mA capability of the 1632. AT BEST the coin should be able to power five LED's at a current of 1.36mA each. That's not very bright.

As for why I ask about how the 555 is getting power to pins 4 & 8 - it might be a simple oversight on the schematic where lines cross at the below point:
View attachment 241990
It might just have been forgotten.
Good work and well spotted!

Well, I said that my expert friend designed the circuit, but I didn't mention that I'd made a couple of modifications to it. One was to the schematic layout which I tried to do without changing the connectivity, but as you spotted I missed off a connection - thank you!

Regarding the battery - he originally included a CR2032 but that was too big so I swapped it for a CR1632 which I thought would be ok as they were at the same voltage - clearly another mistake! Looking at it now, maybe even a CR2032 is a little small - I could easily get rid of one or two of the LEDs.

But if I'm going to ditch the coin cell battery anyway....

If you would be kind enough to recommend a charging coil, battery (and controller if required) then feel free to base the recommendation on 3 or 4 LEDs instead of 5, if it makes things easier.

Many thanks again - really appreciate it.
 
Top