Battery & Power question

Thread Starter

Sigma229

Joined Apr 17, 2019
48
Hey Guys & Gals,

I have a couple questions about power for a device.
I have a small handheld computer/Programmer and it has USB-C port and has a 3.7vdc lithium wired battery.
The unit has the battery in the space where it could hold 4aaa batteries thou not wired for them & no contacts for them.

My question is if I did not want to use usb-c to charge device, could I use 3aaa batteries if I setup the slots to use them,
Or 3aaa and a dummy battery so all spots would be used? I am good at soldering and rework I just never tried switching a 3.7vdc lithium for 3aaa which is 4.5vdc.
I assume since usb-c is 5v and current battery is 3.7vdc lithium that 3aaa should work. Am I correct?


Thanks
Fred
 

MrChips

Joined Oct 2, 2009
30,807
Hi Fred,

Do you have the circuit diagram of the unit?
This would show how the battery supplies power to the rest of the unit.
 

Thread Starter

Sigma229

Joined Apr 17, 2019
48
The item was made in China and I can find no info on it anywhere.
Even the seller (not manufacturer) does not have any info on it.
The device has no info listed on the device or box for it.

Only info I saw was on the 3.7v battery which is why I know the voltage.

The Device was purchased so I can reprogram some rfid implants and some fobs.

Thanks
Fred
 

Thread Starter

Sigma229

Joined Apr 17, 2019
48
Hey,

I have had really bad issues with lithium battery packs from China in some other devices.
Some have expanded so bad and got so hot I no longer trust the pack batteries.
I had to take old ones to a special depot where I live to be desposed of.
I had expected the device to be powered by the 4aaa batteries.
But it seems I got what they call the upgraded model which has the pack instead of aaa.

So I want to use none lithium batteries for safety.
The USB-C wont be used again and will run of batteries only.

I know if I used 3 nicad and a blank that would bring device to 3.6v
But was hoping to use normal alkaline batteries but at 4.5v that would be .8v higher than the 3.7 of the lithium.

Thanks
Fred
 

wayneh

Joined Sep 9, 2010
17,498
I know if I used 3 nicad and a blank that would bring device to 3.6v
But was hoping to use normal alkaline batteries but at 4.5v that would be .8v higher than the 3.7 of the lithium.
The 3.6V of the nicad and 3.7V of the lithium are both the end-of-service voltages when they should get recharged. The nicad will charge to about 4.5V and the lithium to 4.7V. I'm pretty sure the full-charge voltage of the 3 alkalines in series is 4.5V and depending on the load you'll toss them out when they fall below 1V or so each.
 

Audioguru again

Joined Oct 21, 2019
6,692
The 3.6V of the nicad and 3.7V of the lithium are both the end-of-service voltages when they should get recharged. The nicad will charge to about 4.5V and the lithium to 4.7V. I'm pretty sure the full-charge voltage of the 3 alkalines in series is 4.5V and depending on the load you'll toss them out when they fall below 1V or so each.
No, Wayne.
A Lithium battery has a "storage" voltage and its selling voltage (on the shelf in a store for a few years?) at a half-discharged voltage of 3.7V. It is charged to 4.20V, your 4.7V will cause an explosion and fire.
Although most protection circuits disconnect a cell's load when its voltage has dropped to 2.5V I think that is too low, the disconnect should occur at 3.0V. My Lithium battery powered projects give a low voltage warning at 3.2V then they disconnect the load at 3.0V.
 

Audioguru again

Joined Oct 21, 2019
6,692
A lithium battery is stored at only 3.7V (half-charged) but is 4.2V when fully charged.
The lithium battery voltage drops only a little until it is dead (maybe down to 3.2V) but three alkaline cells voltage start at about 4.8V then fairly quickly drop down to 2.7V or 3.0V. A series diode will kill the alkaline voltage.
 

Thread Starter

Sigma229

Joined Apr 17, 2019
48
Hey guys,

Thanks for all the info and help.
The device will only be used a few times each month if even that.
For most of the time it will be stored away.

I will take apart item and see what circuit looks like and if there is a voltage regulator.

I will post back within a couple days.

Thanks
Fred
 
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