Battery tester. (discharge only)

Hello,

I am trying to make a battery tester. (yes I thought lets try a simple project for a change)

When I got to the part to actually discarge an 18650 lithium ion (an old 850mah according to my liitokala) I noticed there is a big difference in the battery voltage with load and without load. It is about 0.4/0.5 volts.

So I thought lets measure the voltage when the load is off. It turned out the battery needs time to settle. I paused discharge for like 1 second every 5 seconds to get an unloaded reading. This is still not right if I wait like 10 secongs the voltage still goes back up but it's a lot closer to the real battery voltage than the reading under load.

How would my liitokala do the tests? Even more important: How do BMS boards test?
A BMS cannot interrupt the load to test the real voltage and would only know the voltage under load. Maybe they compensate a little and have lower cutoff voltages. But if they do than batteries might get damaged if they have a load that draws only a little current.

If I would test my batteries using the unloaded voltage I would fool my self into thinking I have higher capacity batteries than I actualy have because he bms just cuts it off based on the voltage under load. Right?

What about the batteries. Is that lower limit with or without load?

I found this project. Reading the code he also uses the unloaded voltage. So trying to use the voltage without load is not such a crazy idea.

I am very confused now what is the proper way to read the battery capacity.

I am using a 5 ohm 10 watt resistor which is switched via an irl44z and a nano. The load is arround 600 ma I measure the voltage via a modded ina3221.
I do not have any capacity test results yet since I got stuck at which values I should use. I tested with about 3.87 volts which than quickly drops to 3.3 underload and the reading withoutload (after 1 second pause wil be 3.66).

Any advise how to proceed?

Thx

Hans

I think you can do it either way but I'd guess a measurement under load will give a more accurate indication of charge state.

If you look at a battery datasheet they should give you a discharge rate such as 1000mAhr at 100mA. Or sometimes they give you multiple graphs at different discharge rates. If you look at those graphs showing the voltage discharge over time, they don't remove the load to make the voltage measurement.

Sometimes it's hard to find the "end of discharge" voltage. I'm not sure if there's an industry standard but I think it's usually 60 or 70%. ...The mAhr rating doesn't assume you're going all the way to zero but a 3.7V battery might still be considered good at 2.25V (more than 60%).

macron0:
It turned out the battery needs time to settle.

For measurements on a lead-acid battery one should wait 8 to 12 hours for the voltage to settle.

I suspect that battery management systems make assumptions based on the test conditions so that their decisions are equivalent to measuring the settled voltage.

That suggests to me that the on-load cut-off voltage will be somewhat lower than the voltage the battery will then settle at - but this is just a guess. My electric shaver may represent an example of this. It will stay running while the battery warning light is on but if I switch it off it won't restart until it is recharged.

By the way the exact same problem exists in reverse when a battery is being charged.

...R

DVDdoug:
If you look at those graphs showing the voltage discharge over time, they don't remove the load to make the voltage measurement.

I think that is a very convincing argument. So measure under load it is.

Thank you.

Robin2:
By the way the exact same problem exists in reverse when a battery is being charged.

Yes I noticed that too. But i thought lets focus on one problem at the time and (for me) the problem is less of a problem. If I just make sure my charger voltage is never higher that 4.2 volts I can not go wrong (right?). But it does take a lot longer to complete a charge.

Working with the question about remaining power in large, 1000 Ah, 1 ton, lead-acid traction battories the company invited a professor from a battory manufactoring company. The answer was that it's impossible to judge about the remaining capacity from measuring the voltage, with or without load. The capacity heavily depends on how current had been used. Currents like 200 - 300 Amps were frequently used during start of the drive- or hydraulic- motor but less than 100 Amps, C10, later. Our strategy was to integrate the running time of the motors and subtracting this. The system "recalibrated" itself when an unloaded state for a certain time occured.

In order to protect the battory, especially LiPo battories as I understand, is to measure during load and "cut off" the consumption at a voltage level according to the data sheet from the manufactorer. Display the voltage and let the prediction of remaining time be done by the operator.

macron0:
If I just make sure my charger voltage is never higher that 4.2 volts I can not go wrong (right?). But it does take a lot longer to complete a charge.

Charging a LiPo safely is not as simple as with a lead-acid battery or a NiMh battery. I strongly advise that you use a purpose-built charge controller - for example the Microchip MCP73831/2

...R

Robin2:
Charging a LiPo safely is not as simple as with a lead-acid battery or a NiMh battery. I strongly advise that you use a purpose-built charge controller - for example the Microchip MCP73831/2

…R

@Robin2
I fully agree. LiPos are sencitive I have been reading. I once burned down too many NiMaH cells using a quick charger. Slow charging worked better…
Each battory type has its own special characteristics.

Have a look here, I've built this and it works perfectly, It can also be used with PC software to control it and puts it in a graph. It's quite simple to no load resistors required.

The guy as done a great job all credit to the author of it

http://www.vwlowen.co.uk/arduino/battery-tester/battery-tester.htm

Steveiboy:
Have a look here, I've built this and it works perfectly, It can also be used with PC software to control it and puts it in a graph. It's quite simple to no load resistors required.

The guy as done a great job all credit to the author of it

Arduino - Rechargeable Battery Capacity Tester

Impressive work but using C 1 or C 0.5 looks wrong to me. I my mind the standard for measuring capacity is using C 10.
Alternativly using a load current that matches the current consumption of the actaul project could tell how long time the project will run.
And yes, LiPo cells, and LiIon deteriate when being cycled a large number of times so remeasuring the capacity is a good idea.

My commercial tester is showing weird results lately and retesting allready tested batteries give very different results now.

That's why I thought it would be cool to try and make a tester myself. A very good test project for a beginner.

But when reading the reactions here and reading more online it looks like most tests are sort of relative.

Some 18650 lithiom ions turn out to handle 2000mah when testing at 300ma load but collapse to 1000 mah when tested onder 1 amp load. It is probably sometimes true the other way around too.

2000 mah is what my liitokala reports. But what are it's empty and full limits?
You are not supposed to discharge to 0 volt. So 2000mah is what the device measured going out. But (honest) manuafactorers might sell it as a 2200 mah. 2000 is the useable value. Does that mean that even if I make the perfect battery tester it should never return the value thats written on the battery?

Now I got an other issue. I was again trying to record the mah using only the values onder load. But again mah at what voltage?

Should I "index" to 3.7 volt mah? So when reading 2.93 volt and 553ma I records +- 399.91 ma at 3.7?
Or is every body just ignoring this and I should just add all the mah readings? I think it is a big difference. Ie a 5 volt usb tester might float between 4.75 and 5.20 volts. But 2.9/4.2 is a big difference. Would it average out in the end?

In the end my main reason to test batteries is to figureout which batteries are more or less the same so I can put matching batteries together to make one big battery. If test them all with the same tester than I should be okay I guess.

I should have bought 2 different testers just to see how much they agree.

Thank you everyone for replying to my question :slight_smile:

macron0:
Some 18650 lithiom ions turn out to handle 2000mah when testing at 300ma load but collapse to 1000 mah when tested onder 1 amp load. It is probably sometimes true the other way around too.

2000 mah is what my liitokala reports. But what are it's empty and full limits?
You are not supposed to discharge to 0 volt. So 2000mah is what the device measured going out. But (honest) manuafactorers might sell it as a 2200 mah. 2000 is the useable value. Does that mean that even if I make the perfect battery tester it should never return the value thats written on the battery?

Now I got an other issue. I was again trying to record the mah using only the values onder load. But again mah at what voltage?

It has since long time been a standard to use C10. That means that a battory made as 2000 mAh is discharged by 200 / 10 = 200 mA and the time down to a defined cut off voltage is reached. Who/What sets that limit?

The difference occuring between 200 mA and 1 Amp is normal for lead acid battories. To tell how much capacity is left is neccesary to know how capacity has been decreased, by what currents. The chief of machine in submarines have very good equipment keeping track of that.

Down to what voltage should the measuring go on? Ask the manufactorer! When the voltage starts to drop there's not much energy left.

I have been told that LiPo, and similiar battories lives the longest kept between 20% and 80%. I ask how those powerbanks work. Does 0% displaed mean that internally the cells are at 20% and when 100% is displayed the cells are at 80%.... Who knows.

macron0:
Some 18650 lithiom ions turn out to handle 2000mah when testing at 300ma load but collapse to 1000 mah when tested onder 1 amp load. It is probably sometimes true the other way around too.

Discharging a battery has a lot in common with discharging your personal energy.

  • You can probably walk at a steady pace for a number miles without any difficulty.
  • However if you jog you draw down your energy faster and can't go as far.
  • And if you sprint as hard as you can you may only manage 100 metres.

If you discharge a battery at a fast rate the energy will be supplied by the chemicals nearest the discharge point. Their energy will be replaced by energy moving from the chemicals deeper in the battery and that rate of replacement will depend on the battery chemistry and on how much of the chemical material is close to the discharge point. You will probably find that if you allow the battery to rest for a while after a heavy current is drawn it will, to some extent, recover and be able to provide more current.

I would not expect any battery to be able to deliver more total energy at a high discharge rate than it can deliver at a lower discharge rate.

...R