How to measure the capacity of a 18650 battery (or an estimation)

Hello all,

First of all I want to thank the people that will take time to answer to my reply.

I'm asked to measure the total mAh of a battery pack for a review. The battery pack contains six 18650 batteries from a Chinese brand. I know nothing about the batteries except there 18650 batteries.

The question I have is the following one: how can i make an good estimation of the capacity of a 18650 battery?

I need information about: what tools do I need besides a multi-meter? a tutorial would be very nice.

I'm sorry if I ask something that is already asked more than a 100 times but i couldn't find this question in other recent threads.

Thanks you again for reading my post and taking the time to help me out.

What means do you have for discharging the pack (for instance, if the pack was for a bike light, do you have the light the pack powers)?

If you do have some kind of load to discharge the pack with, does that load have a cutoff when the pack voltage drops, and/or is there a low voltage cutoff built into the battery pack.
Do you know what current the load takes, and whether that current draw is constant, or whether it varies with the voltage of the battery pack - do you have any means to put your multimeter into a circuit with the battery and load to measure the current being drawn.

A Hobby charger, they start at around $20. You will also need a 12 to 18 volt power supply (A laptop one may be usable) and some way to connect the battery.

uk_caver said:

What means do you have for discharging the pack (for instance, if the pack was for a bike light, do you have the light the pack powers)?

If you do have some kind of load to discharge the pack with, does that load have a cutoff when the pack voltage drops, and/or is there a low voltage cutoff built into the battery pack.
Do you know what current the load takes, and whether that current draw is constant, or whether it varies with the voltage of the battery pack - do you have any means to put your multimeter into a circuit with the battery and load to measure the current being drawn.

Click to expand...

The purpose of the battery pack is charging phones and other wireless devices by USB. I know the maximum voltage for USB 2.0 is 5V. I must admit that I don't know what current an samsung galaxy s4, nexus 5 or iphone 5 takes. I suspect that even when I put the phone off the current draw won't be constant. I could use a light bulb perhaps for more constant power draw? I can take the battery pack apart so i should be able to put my multimeter in a circuit.

It's the following model i'm supposed to review:

eBay link removed.

I'm sorry if its forbidden to post ebay links, I realize i'm a new user and by no means do i want to use this link to spam. This link gives the following item discription.

Brand new and high quality.
Perfect appearance and performance.
Easy to take and charge conveniently.
Two USB output socket.
High capacity: 50000mAh.
Input: DC 5V-1A.
Output 1: DC 5V- 1A.
Output 2: DC 5V- 2.1A.
Charging time: about 7 - 8 hours.
High security: Protection for overcharge, overdischarge, overload, and short circuit.
Comes with connected cable for charging the power bank.
Mobile Business, Entertainment, Charging anytime and anywhere.
Compatible with Nokia, iPhone, iPad, MP3, MP4, and other mobile phones and ect.
Long service life,more than 800 times the charging and discharging.
Color:White.
Size: 15cm x 7.8cm x 2cm

The same type of model is all over ebay (it's the most popular model and comes in alot of different sizes and prizes) with crazy mAh, this 50.000 version contains six 18650 batteries, it can charge a iphone 4s (1400mAh battery like 5 to 6 times. So it's probably a battery pack of 10.000mAh. I hope you guys can help me a bit further with this information. I also will consider the second post=).

HKJ said:

A Hobby charger, they start at around $20. You will also need a 12 to 18 volt power supply (A laptop one may be usable) and some way to connect the battery.

Click to expand...

For my PC I have an corsair AX760 power supply. It has a 12 volt line so i think I'm good there, is there any hobby charger you or someone else with experience in this area would recommend?

If I have:

1. power supply.
2. hobby charger for 18650 batteries.

How do I measure the mAh of my batteries? Do i drain them with a low voltage source and then charge them up to full with the hobby charger and register the charged ammount of mAh?

Thanks for the replies=)!

schaapje said:

How do I measure the mAh of my batteries? Do i drain them with a low voltage source and then charge them up to full with the hobby charger and register the charged ammount of mAh?

Click to expand...

First you let the hobby charger charge the battery, then you use it to drain the battery while measuring the mAh (Read the link from my first entry, it explains what a hobby charger can do).
Some hobby chargers has a cycle setting, where it will both charge and drain the battery without any intervention.

Thank you HKJ, I did some more research and found the following charger:

turnigy accucel 6

Few questions:

1. If I fully charge my battery and then discharge it to 3.0v, does that give me a picture of the full capacity of the battery? In other words if i buy a 3100mAh (no fake one) battery and I discharge it from full till 3.0v will i get 3100mAh or more something like 2500mAh because it is not fully discharged but only to 3.0V?

2. Is there any other charger you would recommend below 50$?

schaapje said:

1. If I fully charge my battery and then discharge it to 3.0v, does that give me a picture of the full capacity of the battery? In other words if i buy a 3100mAh (no fake one) battery and I discharge it from full till 3.0v will i get 3100mAh or more something like 2500mAh because it is not fully discharged but only to 3.0V?

Click to expand...

You will not get the full capacity, here is an example (You can find many of these on my website):



One trick is to select another chemistry when discharging, where you can select a lower voltage.

schaapje said:

2. Is there any other charger you would recommend below 50$?

Click to expand...

I sometimes uses iCharger 208B, but that is not below $50.

For me, the actual mah range for the USE of the cell is useful....and if its a 3400 mah cell, but all the added mah is from allowing a lower discharge voltage than the light it will be used in can use, its not more USABLE MAH.

If the LIGHT cuts off, or can't support the drive level of importance, below a certain voltage, then testing down to THAT voltage is useful and relevant.

Many lights cut off at ~ 2.5 - 3.0 v or so. Many can't support turbo, etc, when below even 3.6 or so.

If charting the cell's ability to supply the light, you want to choose a range that matches the light(s) performance requirements, to see relevance as to if one cell might be better than another for it.

I bought a board on Amazon for about £2. Here is a picture of it in operation.