# optimal mAH 18650 cell required for laptop

Good day

I will be replacing my laptop batteries with 18650's, however my question is as follows.

Stats :

Battery pack = 6 cell (18650 type Li-ion battery)

Time : +/- 3-5 Hours if memory serves.

My laptop power supply is 19V, 3.95A.

My laptop battery stats are 11.1V, 44mAh, 48.84Wh.

I have already done battery tests with my multimeter, seems like I have 3 banks of 2 since the voltages are as follows:

• 2 x 3.786V
• 2 x 3.808V
• 2 x 4.089V

This is a 2 part question:

1. With my battery stats, is there a method I can calcuate the maximum time my laptop can/could last from a full charge?

2. Can I use 18650 batteries that have e.g. 4000mAh?

1. Yes, but only if you know the capacity of the original cells or you can get a measurement the average current draw.

The basic formula is time = capacity / current. However towards the end of the discharge voltage drops and the laptop may draw more current to maintain a constant power drain. It will probably shut down before the battery is 100% discharged, so in practice you might get 80~90% of the rated capacity (assuming that the rating is accurate!).

1. No, because 18650 cells with a real capacity of 4000mAh do not exist.

18650 Battery Tests

18650 Battery Buying Guide (test on all from eBay below \$3)

Dangers of Ultrafire 18650 batteries

• Does the battery management board keep track of battery status and lock out a pack with dead cells? Because if it does replacing the cells is useless. – jms Feb 12 '16 at 1:43
• The battery protection circuit will 'lock out' the pack if voltages go too low, but it can be 'woken up' by recharging. – Bruce Abbott Feb 12 '16 at 2:42
• Why do you not want to call it a battery management system? It does identification, state monitoring, protection and cell balancing – jms Feb 12 '16 at 9:50
• Does it? Obviously not in this case because the cells are badly out of balance! – Bruce Abbott Feb 12 '16 at 17:44
• Each parallel group of cells has a bypass resistor (and a control transistor). During charging the BMS can thus bypass cells that have a higher state of charge, allowing the less charged cells to catch up. Since balancing is only done while charging, if the battery is left disconnected for long periods it can become unbalanced. Granted, many laptops have the balancing circuitry in the laptop itself. – jms Feb 12 '16 at 22:26

Rules on battery replacement when the battery has a charger:

1. Match battery chemistry
2. Match battery cells (these first two will also match the voltage)
3. You can go to a higher amp hour, your charging time will be longer

One problem that you might run into is matching the battery cells, a good manufacturer will match the charge discharge curve with the other cells. If you don't do this you could weaken one cell and have some major issues. This is done by either proper manufacturing or measuring the cells and 'hand picking' them to match each other.

If you have a 3000mah hour batter and it lasts for 3 hours, you have drawn approximately 1000ma each hour. So if you get a 4000mah hour battery you can last four hours but only in the ideal case because you can't draw down a battery beyond a certain voltage, you have to keep some energy in the battery: Battery University

You can always determine the cell exact type from the stated total capacity and voltage of the battery. The most typical cells in laptop batteries are of 2200, 2400 or 2600mAh capacity. 3000 and 3100mAh have started to be used recently.

4.4Ah means there are 2 cells of 2200mAh in parallel.

11.1V means There are 3 in series (11.1 / 3.7V nominal = 3).

So you have a total of 6 cells of 2200mAh each.

Now, you can replace with identical cells, or with higher capacity ones. Putting 2600mAh cells istead of the 2200 ones will be technically-valid and the only difference will be that the total capacity increases, therefore the battery will last longer (about 18% more time that it originally did). You will have 5.2Ah instead of 4.4, so yes, you can use higher capacity cells but 18650 4000mAh ones do not exists. The best ones on the market (original cells) are rated 3400mAh (3700s will follow soon in sufficient quantity). They are more expensive, though.