Project Design may leave Lion battery fully charged for a long time: bad idea?

Thread Starter

Jim_PDX

Joined Nov 13, 2019
9
I’ve been working on this little gift that I want to give to each of my sisters. It’s basically a strip of WS2812 LED “fairy” lights controlled via an Arduino Pro Mini. It’s got a couple buttons which allow the user to change the color patterns and such.

My question today regards powering this project.

The LED light strip can be easily disconnected from the black box which contains the Arduino Pro Mini, a small li-ion battery and a TP4056 charger. It will have a standard 1A 5V micro USB adapter to charge it. It also has a P-channel MOSFET and diode for load sharing to allow the battery to be safely charged while also powering the Arduino and lights (similar to what’s described here).

I’m envisioning it could be used in either of these two situations:

1.) With a fully-charged battery, this device could be placed somewhere far away from a power outlet and run solely on the battery until it’s depleted (5-6 hours). You would have to recharge the battery in the black box before using again (obviously).

2.) If the LED lights are to be used somewhere near an outlet, this device could be permanently plugged into the outlet and powered from that. In this situation, the TP4056 would keep the battery fully charged for long periods of time.

Here’s my question: is it safe to use this device in situation #2 for long periods of time, say multiple weeks or months? The li-ion battery would remain connected to the TP4056 and fully charged for quite a long time. Is this safe?

I once had a repurposed old iPhone that I used as a security webcam monitoring my house. After several months of being constantly on and connected to its charger, the internal iPhone battery expanded so much that it burst out of its casing.

I suspect I could build some circuitry with another MOSFET to periodically disconnect the TP4056 from the power source and run off the battery for short periods so it stays conditioned. Would this be useful and/or necessary?
 

Audioguru again

Joined Oct 21, 2019
3,201
I could not find a datasheet for the TP4056 charger IC, just many Chinese modules that have it plus a protection IC.
www.batteryuniversity.com and model airplane websites say that a Lithium rechargeable battery is ruined if it stays fully charged.
They say to discharge each cell to a "storage charge" of 3.7V to 3.8V that is about half a full charge. I do that each winter so that my batteries last for a few years.

A Lithium rechargeable battery is also ruined if it is discharged to less than about 3V per cell. Many protection circuits disconnect the load at 2.5V which is too low.

Periodically running a fully charged battery wastes some of its limited charge-discharge cycles. Many people say to leave it sitting at a storage charge then charge it before using it.

An expanded Lithium battery is dangerous and could cause a nasty fire. Maybe your charger charged yours at a current that is too high or wrongly "trickle" charged it that are bad.
 

BobTPH

Joined Jun 5, 2013
3,318
As an aside, I have built projects using WS2812B, and I found that the quiescent current is quite high, 20 or 30 mA for about 30 LEDS, so maybe 1 mA each. If you want it to use minimal current when off, you need a MOSFET to disable power when not in use.

Bob
 

Thread Starter

Jim_PDX

Joined Nov 13, 2019
9
The TP4056 module I’m using is like this. It will keep the battery from over-charging and will also avoid over discharging (<3V). I've been testing it for a while and it seems to work well. Again, my concern is having a li-ion battery that’s constantly charged and connected to the TP4056 for long times without being used.

I’m starting to think that I cannot safely have a device that allows BOTH situation 1 and 2 (as described in my first post). It would have to be one or the other. If I design it just for situation 1, I would always disable power going to the Arduino and lights WHILE charging the battery (keeping the P-channel MOSFET but removing the diode that would have allowed the load sharing). If I design it just for situation 2, I would completely get rid of the battery and the TP4056.

I think my sisters will really like this gift, but I’d rather not have it burn down their homes. ;-)
 

Audioguru again

Joined Oct 21, 2019
3,201
Will your "little" battery (with no part number) survive the charging current of 1A?
Does the charger properly detect a full charge (not the voltage) then shut off?
 

Alec_t

Joined Sep 17, 2013
12,080
To overcome the problem of the battery being left in a fully-charged state for lengthy periods, IMO the simplest solution would be to have a circuit which shuts off the charger when the battery reaches about 50%-60% of full charge. Is your TP4056 module adjustable to allow that?
 

Thread Starter

Jim_PDX

Joined Nov 13, 2019
9
Audioguru: Battery is a 3.7v rechargeable protected li-ion in a 14500 package. Rated capacity is 1300 mAh, so charging at 1A is okay. TP4056 data sheet states “The TP4056 automatically terminates the charge cycle when the charge current drops to 1/10th the programmed value after the final float voltage is reached.” (programmed value being 1A).

Alec: Unfortunately the TP4056 doesn’t provide a method for charging any less than a full charge. I saw one example online of modifying it to allow that, but it’s too much additional circuitry than I want to deal with.
 

Audioguru again

Joined Oct 21, 2019
3,201
A 14500 lithium 4.2V battery is the same size as a common AA lower voltage 1.5V battery cell. They all come with a protection circuit to reduce the amount of fire when it it used in the wrong product.
 
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