This is one that seems so simple…how fast does the manufacturer say I can charge the cell? And it often is that simple, one number you can use to decide what to set your charger to.
But there can be situations where that one number is no longer good to use, or can even be dangerous. Let’s take a look at what the charge ratings mean and what situations are to be avoided.
Many cells can have two charge ratings, standard and rapid (or fast). The standard rate is pretty much just what it sounds like, something you can use every day without causing the cell to age faster than normal.
The rapid or fast charging rate though is harder on cells. It heats them up more (which cells hate) and it causes more of the materials inside to break down faster. This causes a greater loss of capacity and increases the internal resistance of the cell (which causes more voltage sag in use).
You don’t have to avoid fast charging completely, it’s just too convenient sometimes. But I recommend not doing it every day unless you want to replace your battery pack or cells more often.
If the datasheet for the cell or the manufacturer of the device that uses the cells only mentions one charge rate then there’s not much we can do. Hopefully it’s not a level high enough to really speed up aging of the cells. You can always use a slower charge rate though if you are unsure.
A good way to tell if your device, cells, or battery pack is being charged (too) quickly is to feel if it gets more than a bit warm. Heat is the enemy of all cells and if we want the longest cell life then we need to avoid creating that heat.
For battery packs, especially those wrapped in layers of abrasion-resistant insulation and shrink wrap, the temperature of the cells in the middle of the pack can be MUCH higher than the pack’s external temperature. Especially at higher charge (and discharge) rates. Hopefully you have a battery management system (BMS) device in place that you set up to monitor the temperature of the cells in the middle of the pack. Stay below 45°C for good cell life and never exceed 60°C to help keep the risks down.
For USB-charged devices like tablets, phones, and powerbanks we can often charge them at a slower rate by using a USB charging adapter that has a lower power rating. For example, you can use a 15W adapter (used for phones) to charge a tablet that might normally use a 45W adapter. It will take a lot longer to charge but this slow charging is a lot better for the battery.
For vaping devices, PEV’s like skateboards and e-bikes, and higher power flashlights we can use the ratings given by the manufacturer of the individual cells we’re using and decide how fast to charge them. The charge ratings are in the datasheet for the cells or, hopefully, are provided by the manufacturer of the battery pack you are using.
If you have no info about how fast you can charge the cells or pack then start slow! For ultra-high capacity 18650’s, over 3000mAh, start at 0.5A per cell (1A if two are in parallel, etc). For ultra-high capacity 21700’s, over 4200mAh, start at 1A per cell.
See if they get more than a bit warm. If they do then stop there, do not charge them any faster. Reduce the charge current a bit and try again. If they aren’t getting warm then you can probably increase the charge current. Be aware that many AC-powered chargers, not ones that use a USB charging adapter, run quite hot and can heat up the cells even if you are charging them slowly.
Use your best judgement here. Nothing I say can be applied blindly to every possible charging scenario. If something doesn’t seem right, if a cell or pack or the charger is acting weird or getting hot even at a slow charge rate, then stop!
You can use a fan aimed at the charger to help keep the cells cooler but do not use this as an excuse to use something like a 3A charge rate for a cell rated for 0.5A charging!
Another way to extend the life of your cells is to not charge them fully. Charging to only 4.1V or even 4.0V can significantly extend their life. This would be about 80% or so for devices like phones, tablets, powerbanks, etc., that just show you battery percentage. But you will get less run time from them since you stopped before they reached 4.20V (and then topped off). There is always a tradeoff!
A compromise for charging individual round cells can be to stop the charge as soon as they reach 4.20V. Removing them from the charger before they are fully topped off can help extend cell life a bit since they will drop down to 4.15V or so (depending on how fast they were being charged).
So when can charging be dangerous?
- The charger is set to a voltage that is too high. This damages the cells and can cause them to eventually short-circuit internally if it’s done often enough. Never charge a cell or battery pack above its voltage rating. This is typically 4.25V maximum (4.20V preferred) for standard Li-Ion cells and 3.60V for LiFePO4 cells. Check the datasheet for the cells to be sure though. If your charger stops at something like 4.25V or 4.28V and the cell doesn’t quickly settle down to below 4.20V then I recommend replacing the charger. There’s almost no extra capacity being added to the cell and you’re just aging them faster.
- Charging cold cells. Never charge any Li-Ion cells when they are below 0°C/32°F. This can cause lithium metal to be plated onto different parts of the cell internally and can eventually lead to an internal short-circuit which can cause the cell to vent, catch fire, or possibly burst.
- Charging much too fast. This heats up the cells and speeds up their aging. You’re adding lots of energy to the cells and if anything goes wrong, a cell has an internal defect or has reached the end of its life, then you increase the risk of a cell failure or fire.
- Charging without balancing. This can be a hotly debated topic for battery pack owners. Balancing a pack means making sure all of the cells come up to the same full charging voltage at the same time…or as much as possible. This prevents any cells from being overcharged and helps make sure we get the best performance from the pack. Most battery management system (BMS) circuit boards/devices are able to balance a pack automatically and in my opinion should be used. But if you have well matched cells (both capacity and internal resistance) in very good condition and you are not constantly stressing them then you might need to only balance occasionally or perhaps not for a long time. As all cells age though the small differences between them grow and eventually you will need to balance them. If you do not balance the cells in a pack, and you do not have a BMS that can prevent a cell from dropping below its 2.5V rating, then you risk a cell eventually getting so far out of balance with the others that it can drop to zero volts as the pack is used. If you continue to use the pack you can actually end up bringing the cell down to a negative voltage, damaging it badly. There is a real risk of this cell catching fire if this happens.
- Old cells. As cells age their internal resistance goes up. This can cause more heat to be created when the cell is being used and especially when the cell gets near its full charging voltage. As your cells or battery pack gets older be sure to occasionally check how warm it gets near the end of its charging. If it’s getting warmer than it used to then that cell might be nearing the end of its life or one or more cells in the battery pack might be doing the same. You might be able to get a bit more life out of a cell by lowering the charge current rate (so it doesn’t get as warm) but for a pack I would consider retiring it. The pack’s performance is limited by the worse performing cells and the safety risks go up as the cells age unless you have a very, very good BMS that is set up well for the pack. Even then it’s just not worth it IMO to continue overheating some cells for a bit more use of the pack.
Charging Li-Ion cells can be as simple as plugging in the adapter that came with the device or it can involve a complicated setup of a BMS you’re installing in a battery pack and the sourcing of the right power supply, connectors, and fuses to safely charge the pack. But no matter how you charge there are a few things we can do to extend the life of our cells and help prevent any risks growing.
