# How fast does a 65 watt charger actually charge the battery cells?

My laptop (A thinkpad t440s) has a 6-cell battery: three pairs of 18650 cells in series, for a total of about 11 V. My charger is 65 watts at 20 volts, which works out to 3.25 amps. My question is, do most laptops step this down linearly, so that it charges each cell with about (3.25 amps)/(2 cells in parallel) ~= 1.6 amps (which seems fairly reasonable for a high capacity 18650: "0.5C"), or is it more likely using a switching regulator to put the full power into the batteries, which would work out closer to 3 amps per cell?

I ask because I'm planning to replace the cells and want to ensure that the cells I buy are able to safely handle the charging current.

• It will most likely use a switching regulator. As for the exact charge current this may vary. Mar 12, 2022 at 3:10
• Your question isn't about most laptops but about a Thinkpad T440s. Lenovo has been using about 19 V to 20 V as a standard brick-voltage supply for quite some time. Especially with the Thinkpads. For two decades I know of, so far, and still going. It's likely that they have a very good charging system that is well-mated to the battery suppliers they use, but also general-purpose enough to allow them some range in choosing suppliers, too. Charging has several stages, but if I had to guess (I do) they likely use (in current-source staging) an amp, or less, per series section. Not likely more.
– jonk
Mar 12, 2022 at 3:10
• Likely it charges at 65w tops. But it depends on the charger and the pack. If you connect it to a compatible 120w power supply, your cells need to handle that too, if the charger and pack both allow charging at 120w. Just buy a new battery pack if you don't know the specs of the original pack or original cells. Mar 12, 2022 at 9:01

Let's make some assumptions:

1. the 6-cell battery has around 72 Wh which breaks down to single cells that have roughly 3.4 Ah each)
2. the charger is a switch-mode charger, so we must regard that the charger can push more amps to the battery as it get's on the input
3. he cells are all empty, they could be down to let's say 3.0 V that is 9 V for the series of 3 groups
4. they are just not yet considered deep-discharged so the charger does start in fast-charge mode (if they are lower than 3 V, according the the datasheet of samsung cells, IT equipment chargers should employ pre-charge with I translate to reduced current charge up to 3 V)
5. the computer is off - maximum power available to the battery
6. power supply is 65 W @ 20 V, with 90% efficiency that could give 58.5 W for charging, that means at 9 V no more current than 6.5 A is available in the first few minutes of charging
7. the efficiency may decrease a bit at lower load, but we ignore that
8. the cells are charged in a string 3S2P with balancing

Discussion

If we assume that the two parallel cells are in good shape they consume each half of the 6.5 A so 3.25 A in the beginning of the charging process.

If 3.6 V per cell are reached (pack at 10.8 V) the 58.5 W translate to 5.4 A max. so the current per cell is already below 2.7 A.

Justme's comment about larger power supplies is useful. The charging can and will be faster. But it is not said that it will charge with 120W, because the charging system will limit the current to what the original cells can handle (and the charging system itself also has a current limit!).

If it would use 120W for charging that would mean 12 A of current under the same assumptions as above, so 6 A per single cell.

There are also 170W power bricks available with the same jack so you can connect them to any mating Thinkpad - without harming the battery. (My P52 supply has 170 W (20 V and 8.5 A) and it can safely charge a Carbon X whose original supply has only 65 W (20 V and 3.25 A).

Conclusion

• The new cells should have the same rating than the original cells
• we see with the 65 W brick the cells are more or less charged with 1C
• we just can and do not know the max. charging rate limit of the original cells, but we can assume that with a larger power supply they may get charged with up to 2C
• If the original cell type is unknown, take new cells from a trustworthy supplier and manufacturer that allow continous charging at 2C rating. For 3.4 Ah cells this means 6.8 A max per cell, which means >12A for the string which is not possible from a 120W supply. Thus you are on the safe side.
• Good, but I would be totally confident that the charge rate would be safe, no matter the name plate rating of the PSU. Its a fair assumption it will be less than 65 W, but in my experience laptops never charge in an hour, more like 2 or 3 hours, so it's probably well under 65 W. It would be safe connecting the laptop to a 100 A capable 19 V power supply, etc. Jul 9 at 4:37
• Is that a question or a comment? But yes, in principle it is the laptops internal electronics (or partly the electronics in the battery) that limit the charging current if the PSU can supply more power. On the other hand, Lenovo PSU communicate their capability by an internal resistor value and there is no such value defined for a 100A PSU. Jul 11 at 17:49

Assume charging at 90% efficiency using a switching regulator. That is, the cells receive up to about 60W of power in total, with the laptop switched off and the batteries empty. That should be conservative enough. But leave yourself a factor of safety of 1.1, that is whatever currents you figure out must be multiplied by 1.1 before you use them to specify the cells.

• Good answer, but limiting to the assumption that it will only ever be connected to 65W supply is not entirely sufficient. Many computers come with " standard charger" but can be used with higher wattage supplies as well for faster charging. So the cells must handle whatever the original cells handle, so that the cells don't get damage if you e.g. connect a compatible higher wattage charger where available (at work, at friend's place, etc). Mar 12, 2022 at 9:05

That's easy enough to answer without looking closely: a linear regulator would turn almost half the energy into heat, and with the laptop switched off, no fan is running. To charge the batteries with so little heat production would require drawing only a rather small fraction of the available current, not matching the achievable charge times.

So you better plan for cells that are comfortable with the maximum available power, and it would be saner to not rely on the power being limited by the particular brick in use: better plan with the charging times achievable with a brick that is not maxed out.