18650 2-cell battery pack

[MikeK]

I've had a couple of these battery packs for >10 years without use, but they still test at 7.4V.  I'm wanting to use one presently, but was concerned about continuing to serial charge them.  So I cracked one open.  The cells are Sony.  They test at 3.7V each, but should I just remove the protection circuit and charge them separately?

[ledtester]

I've done that to revive a 2S3P pack. Just make sure the batteries have the same voltage when you put them back into the pack.

If it's a high-quality pack then it probably balances the cells during charging.

So, yeah, you can do it, but won't it be kind of a pain to go through all that work every time you charge the pack?

[MikeK]

Well, that's what I'm asking...  Should I get rid of the protection/balancing circuit and charge/use them separately?  Or, glue the case back together and use the cheap Qualcomm charger.

Can that circuit balance charge without having access to all four battery terminals?  It's only soldered to the output ends.

[sleemanj]

I would use as is, no need for balancing, the battery pack manufacturer didn't intend for it to be externally balanced if there are only 2 connections.

[MikeK]

The battery pack is two 18650 cells in series, though.

[sleemanj]

The battery pack is two 18650 cells in series, though.

Again, if there are only two external connections then the battery pack manufacturer didn't intend for it to be externally balanced.  Do you know something the battery pack manufacturer doesn't?

Either

  1. it balances internally; or
  2. it monitors each cell internally and forgoes balancing as largely pointless (the best choice); or
  3. the manufacturer decided it wasn't necessary to balance or protect the cells individually and just protects the series pair as a single unit (not a great choice, but for matched cells which they are confident in, certainly possible)

Look for a connection running to the far end of the pack between the cells under the plastic you didn't remove, that will tell you if it's option 3 (no connection) or one of option 1 or 2.

The cells are clearly either very well matched in good health, or being balanced, otherwise the voltage on them would be different.

[beanflying]

The battery pack is two 18650 cells in series, though.

A lot of Laptop and similar consumer batteries run without a Balance charger such as is common in the R/C world. Your circuit board looks like a simple 2S BMS for cell protection and won't have anything really to do with the charger other than likely telling to stop when the first cell triggers the BMS voltage cutout or as a simple backup safety device. The Charger might also independently have it's own voltage cutout and as there is Sony cells in use likely the charger is better than some of the current junk in use.

Providing the charger which is likely outside the pack in this case away from the battery peddles softly above say 8V (switches from constant current to constant voltage) then you will get to the end point safely with both cells sort of getting a near full charge before a voltage level halt to charging.

Where this falls over and why you typically see a single cell in a pack 'dead' is the weaker/lower capacity one discharges first then the better ones get to charged first on the next cycle and bit by bit the weak one gets behind. As SleemanJ said as yours seem close in voltage likely they are still comparing 'ok' with each other so don't panic.

Not an ideal charge system by any means but very common in consumer electronics to save a $.

[MikeK]

Okay, thanks guys.

[CaptDon]

Those old Sony 18650's seem to have been very well made!! I have 4 that are 15 years old harvested from an old laptop. I have them in series and use them to run a little 6 channel audio mixer I built from scratch. The mixer draws around 60ma. and can run nearly 24 hours on a charge. My charger circuit has a hard limit of 16 volts and above 15.6 volts it cuts back to just 10 to 20ma. float charge. There is no equalizing circuit and if the mixer sits unused for a month or two the total voltage fades back to about 14.6  I spaced the cells about an inch apart and glued them down with RTV to an aluminum chassis. I doubt there will be a thermal runaway even if a cell finally shorts due to age because of physical separation and heat sinking.
 

[MikeK]

Those old Sony 18650's seem to have been very well made!!

I was surprised to find out that both cells had 3.7V after *at least* ten years of sitting in a box.

[mikerj]

I've individually charged the cells on numerous laptop packs to balance them just by inserting a thin (spade) type connector between the cells.  Should be no need to disconnect the management board.  Just ensure you don't go past 4.2v (stopping at 4.1v is safer) as these packs will blow the safety fuse and set an internal safety flag within the BMS IC if an individual cell gets overcharged.

[MrAl]

I've done that to revive a 2S3P pack. Just make sure the batteries have the same voltage when you put them back into the pack.

If it's a high-quality pack then it probably balances the cells during charging.

So, yeah, you can do it, but won't it be kind of a pain to go through all that work every time you charge the pack?

Hi,

You have to be very careful when you go to redesign something like this that has some history of danger.  You dont know what you could be getting into if you dont have a full working background knowledge of the subject area.  Connecting Li-ion cells together is a lot different than connecting some resistors together.  The cells have some limitations that must be respected or else some really bad stuff can happen like fire and even explosion.

In the case of protected cells, there are different reasons why they have a protection circuit built in. One is to prevent overcharging.  you can limit your charging and design a charger that does it right.  There is another reason for the protection though and that is to prevent over discharging.  Over discharging a cell causes some chemical reactions within the cell that could render it dangerous if recharged again.  You can look up more about this on the web.  So if you remove the protection circuit you also have to add your own mechanism to prevent discharging too deep.
You also have to limit the discharge current because these cells are made to run at a current that is set by the manufacturer.  Some cells can take 10 amps others can only take 750ma to be safe.
The protection circuit might also monitor temperature.  That would prevent the cells from overheating.  IF the circuit is removed, you lose that protection.

So if you want to redesign you've got a little more work to do.