# How to fully discharge a battery to determine capacity

Hey I had another post asking this question but I dont think my question was answered correctly due to me not providing enough information so here is an update.

I am conducting an experiment trying to expiramentally determine the capacity of a lithium ion battery (380mah 3.7V). For this I am trying to discharge the battery under a constant current and time how long the battery would last (so of the current was 20mA and the battery lasted 2 hours I could determine that the capacity was 40mAh). Now on the data sheet it says that to test this I need to discharge the battery at 0.2C which is about 76mA. The reason I decided to use diodes was because I could see when the battery dies and stop the timer accordingly although many people told me to use resistors instead. My inital thought is to get 4x20mA diodes and connect them together in parallel and time how long they will work for. Would this approach work, if not could you guys suggest something. If really new to this field but interested about this topic. Thank you.

• Does this answer your question? Building a Circuit that takes constant mA Commented Feb 28, 2021 at 4:07
• Just FYI, constant current isn't a particularly good way to do this since voltage drops as the battery is depleted, so you don't actually end up measuring capacity, but instead you measure a similar figure. To do this properly you need a "coulomb counter" circuit.
– K H
Commented Feb 28, 2021 at 4:14
• No. Diodes will not do what you want. You should take the advice already offered or search this site for 'constant current source'. You can make an adequate one for this purpose with two transistors and a few resistors - or use Spehro's far better one using a TL431. (Cheap and usually available). Commented Feb 28, 2021 at 4:23
• Do not let Vbatv fall below 3V. Commented Feb 28, 2021 at 4:23
• You are asking a question that has already been answered in the ratings of the battery. If you want to estimate the health of the battery compared to the rating of the battery, it's a bit more complicated than measuring the discharge time. Read this for some insights batteryuniversity.com/learn/article/… And no, you cannot regulate current with LEDs. Commented Feb 28, 2021 at 4:46

You can buy 4x 20mA CC chips , if that's what you mean but diodes aren't unless you use current limiting resistors. e.g. to a 2V LED and then it's approximate from 3.8 to 3.0 as the current changes from the voltage drop across the R.

How you measure capacity depends on your test method. If it differs from the OEM manufacturer's datasheet, you better have a good reason for it.

1. If you want to validate their quality, use their method.
2. If you want to improve lifespan or compromise lifespan and use the capacity faster, understand the parameters which affect capacity and also lifecycles.
• raising CV from 4.1 to 4.2 to 4.28 improves capacity but the time spent during CV also degrades lifecycles and depends on the cutoff current of 10% CC to 5%CC to 1% CC

This means 20% of the mA in the mAh rating and ONLY "Constant Current" and not constant R.

It also means 1/20% or about a 5 hr test if the result matches the rating but only for the CV charge and cutoff V in the mAh rating.

Note that using CV to 4.28CV and 1% CC maximizes capacity but if OEM still specifies 500 cycle lifetime and that is acceptable to you and you intead to use discharge it in the same rate as this regular charge time, then you ought to expect a long life of 500 cycles/

But if you intend to use it up in much 30 minutes, don't expect 500 cycle lifespan. Alternatively, if you would wish to have a lifecycle of 5x or 2500 full charge cycles and learn that reducing the CV maximum and raising the under V cutoff to say 3.5V instead of 2.85 or 3V gives allows you to use only 50% of its rated capacity, but then you can get 5000 charge cycles or 5x the total lifetime Ah storage.

What you are really doing is conducting a Design Validation Test or DVT to YOUR specifications with tradeoffs for best short-term capacity or best capacity for longer lifespan.

As professional Test Engineers we might do both to verify the lifespan improvement and OEM specs.

See an example OEM datasheet below.

Next you need a circuit to program a CC, CV which I assume you have chosen from available charger.

Then choose a design spec for your test.

• using your specs for: CV, CC charge, %CC cutoff, CC discharge, UV cutoff & Time counter method.

• How to design this depends on your parts available, your specs and your skills (copy or DIY design.)

The reason I decided to use diodes was because...

Regular diodes do not regulate current. It is possible you are confusing regular diodes with what were once called current regulation diodes. They are not actually diodes but FETs, usually with an added resistor, in a single two terminal package. They could regulate small amounts of current, but not very accurately. I have not seen these lately, as there are other, far more accurate ways to regulate current.