My 18650 Batteries get hot

Question

I have a few 18650 batteries laying around, so I decided to use them to make a power bank with more capacity. I proceeded to buy a small power bank which comes with one 18650 battery (almost all of them do).

Before wiring everything up, I used the module that comes with the power bank to charge the batteries separately but after a while the module and the battery gets hot. This doesn't happen when I use the module to charge the battery that comes with the power bank.

Time to recycle, or are there other options?

Answer 1

What you are attempting is dangerous practice. There are many different types of 18650 cells with different chemistries for differnet intended purposes. A battery must be built with cells of matched characteristics; internal resistance, charge retention, charge and discharge capabilities etc. Mixing different makes of cells within a battery is almost certain to create serious problems and unless you can be sure they are all of the same basic type and chemistry - should NOT even be considered - it could be dangerous!

You stated, "I used the module that comes with the power bank to charge the batteries separately".

First let's get the terminology correct. So you have a single 3.6v 18650 in your hand - that is not a battery, it's a CELL. When two or more cells are connected together they become a BATTERY.

Cell or battery overheating is an indication of improper charginng or discharging. You used the "module" (intended for charging a battery to about 19v) to charge individual cells of 3.6v! Is it any surprise that everything overheated? Would anyone use a 24v truck battery charger to charge a phone battery?

If the cells got too hot to touch (more than 50C), then you have already scrapped them, chuck them for recycling.

Putting together a battery of cells without matching (balancing) them may result in some cells rising in temperature; these wil be cells with the poorest characteristics. And they will not get any better, only get worse. It becomes a visious circle and the usable time on the battery will continue to deteriate.

it's very temping to reuse cells, I do it myself on less critical applications using 3 or 4 cells. But for rebuilding a laptop battery, unless you have a good understanding of 18650 cells and good ability to match cell charcteristics, don't attempt it, the results will be a big disappointment and could be dangerous. Better would be to buy a new laptop battery of the same voltage and with the same number of cells and swap the cells into your old battery case. At least you will then be using balanced cells. However, you DO need to buy from a reputable source. Remember - NEVER buy memory or batteries on ebay direct from china!

Battery manufacturers produce cells to specific intended characteristics. But the cells produced are NOT all the same. So the characteristics are tested into three categories: The best are set aside for customer with high-end needs at high prices. The mid range quality go to battery producers of laptops, power tools etc. The poorest quality go for sale as individual cells on the open market.

If you want the best, go to a good SPECIALIST supplier (NOT a retailer), who can provide a set of matched cells for making up a battery. But find one who can provide the cells ready built-up with welded tags - self soldering the cells should not be attempted.

Answer 2

Not all 18650's are the same! What chemistry is the battery and what is the charger intended for? If they are different (e.g. LiPo, LifePO4 ... ) the charging voltage will be wrong and you might cause a fire. (You DON'T want a lithium battery catching fire.)

Please be careful! Make sure the charger you use is correct for the battery type first.

Answer 3

Not knowing anything about your specific batteries it is hard to give a good recommendation for what you should do. But for sure you should not let the batteries be charged at temperatures over around 50C. If they are getting that hot, then you should not use them. In general, the fact that they get noticeably hotter than the original battery is a bad sign.

Answer 4

First thing to consider is the charging current. Check if the power bank is designed for rapid charging. In this case, there is a chance that your power bank's 18650 supports high charging currents, while the "other" 18650 doesn't. Always limit the charging current to the maximum recommended value.

Second, don't charge at full charging current when the voltage is below the recommended threshold: you should check the cell's datasheet for this value. If your cell is deeply discharged (well below the minimum voltage threshold), always consider a preconditioning, low current charge phase.