18650 battery charging issue

I recently built a series of LED fairy lights for my grandaughter. They are controlled by an arduino MEGA clone- i installed two 18650 batteries in series to power the board and LEDs- after a couple of weeks the unit did not run, so I put the batteries in the DIYfull charger, but neither seems to be charging. They are reading .2 and .7 volts respectively. Any suggestions?

If the circuit you used to power the lights did not include battery protection, then it is likely that the batteries have become over-discharged. They might be recoverable. See here for some information, or search using the terms from that page.
Restoring/Recharging Over-discharged LiPo (Lithium Polymer) Batteries! : 6 Steps - Instructables

Some batteries advertised as ‘18650’ include charge/discharge protection, but most do not, so it is important to determine whether or not the batteries you are using require an external protection circuit. Batteries used for mobile phones, on the other hand, often include protection as part of the battery construction.

Thanks Jeff- very much appreciated.

Also - do you have a suggestion for a low voltage protection/alert module I could add to the two batteries running in serial?

Hi Robert, Jeff
If the cells are reading

I think I would change

to " it is pretty certain".
It is unclear how much current the set up requires. Are you sure the battery capacity is sufficient for a reasonable operating time.
Cheers Bob

If your lights will operate from 5V this is a solution that provides convenient mounting for the batteries, easy charging, and protection for both charge and discharge.

At the component level, this is a typical device:

Core doesn’t seem to have a 2 cell BMS listed, but they are available.
2s 8a Li-ion 7.4v 8.4v 18650 Bms For Li-ion Lipo Battery
However Core does stock 2-cell battery cases which you could use with a 2-cell BMS.

Hi Jeff, Robert
Be aware that cells with a nipple on the positive end may not fit holders with this type of metal bit on the ends. I can guarantee that they will not fit into that 3 cell holder with power jack fitted as I have one. Core advised on an earlier query that they only sell cells that fit into these holders. That is flat topped cells which are 65mm long. Most of the nipple types are 69 or 70mm in length.
Cells with built in protection are mostly in the 69mm category as they have to accommodate the protection circuitry in the top.
Cheers Bob

Thanks both- it seems that I’ll need to buy more 18650s- however if I continue with 2x 18650 batteries outputting 7.4v is it possible to get a simple voltage shutdown device to connect between the battery holder and the microprocessor?

Hi Robert

See Jeff’s link above regarding 2 cell BMS.
Cheers Bob

Hey Robert,

Regretfully we don’t currently carry a 2 cell BMS. Though Jeff’s link should be exactly what you are after.
The other options for it are using a lipo power booster/charger module. Though that device is more useful for a single 3.7V Lipo.


On a related note, I tried to revive a dead laptop battery this week…

It’s a 4s, with rectangular cells; I went to the trouble of removing the circuitry before juicing up the cells from dead flat to about 11V (nominal voltage 14.4V), before the epic hassle of re-soldering the board back on… Nada.

Then I was like stuff it, and just soldered a couple of wires onto the pads, and gave it a bit more charge, still nothing doing. Had another go, got the pack up to like 14.2V, put it back in the laptop again, and the tray icon showed it was charging for a few seconds, but then nothing, and it still showed 0%.

Not worth the hassle at all, considering a new battery can be had for as little as $45, but I gave it a go because it’s a pretty old model, and I figured a ‘new’ battery could well be far past its shelf life… Time to find out about that, I guess.