Need help to understand battery internal resistance pls - Page 1
Forum section: electronics / beginners

author: Chriss
author_rank: Frequent Contributor
timestamp: October 27, 2020, 12:46:50 pm
content: Hi!I'm "playing" with measuring of lead acid battery internal resistance.On som datasheet it is writing lets say for a 12v 7Ah UPS battery the internal resistance should be ~18m.On the net I found writing's like "high internal resistance is leading in bad battery..." etc...This is not really clear for me.If a battery have a low internal resistance that means it is more close to short circuit.If the internal resistance is let's say in Mrange it means for me far away from short circuit.Short circuit means for me a state where the battery internally discharge itself.High internal resistance means for me the battery will last longer cos it will not discharge itself so fast.So, I'm a bit confusing with this.Can somebody clarifie it a bit for me please?Thank you.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 27, 2020, 01:24:19 pm
content: Quote from: Chriss on October 27, 2020, 12:46:50 pmNo, because the internal resistance of a voltage source is considered to be inseries(not in parallel) with the battery.

author: ledtester
author_rank: Super Contributor
timestamp: October 27, 2020, 01:36:07 pm
content: QuoteA short circuit usually means a low resistance path from (+) to (-) created by the circuit attached to the battery.If there was a low resistance path from (+) to (-) within the battery itself wouldn't last very long - it would get hot and soon become a dead battery.Here's some articles on internal resistance and self-discharge:https://batteryuniversity.com/learn/article/rising_internal_resistancehttps://batteryuniversity.com/learn/article/elevating_self_discharge

author: Chriss
author_rank: Frequent Contributor
timestamp: October 27, 2020, 02:07:43 pm
content: Thank you so much!I understand it completly how you interpreted it.How high resistance would be in a bad battery?I mean, if I measure and calc the internal resistance, how can I know if it is ok or not?Thank you very much.I got it now.

author: TimFox
author_rank: Super Contributor
timestamp: October 27, 2020, 04:15:42 pm
content: You need to consider how much current the battery supplies to your application. As the internal (series) resistance increases, the voltage at the load deceases until it is too low for your purpose.

author: Chriss
author_rank: Frequent Contributor
timestamp: October 27, 2020, 04:47:56 pm
content: Thanks guys for the help.It is completely clear for me now how the internal-series resistor is acting in the battery.At the moment when I realized based on your writing does that internal resistance is in series actually then Iunderstand why is it bad if this resistance is high...

author: tunk
author_rank: Super Contributor
timestamp: October 27, 2020, 04:49:27 pm
content: It will also heat up the battery.

author: tooki
author_rank: Super Contributor
timestamp: October 27, 2020, 08:59:02 pm
content: Quote from: TimFox on October 27, 2020, 04:15:42 pmWell said!There’s even the case where a high internal resistance is actually desirable: built-in current limiting. The perfect example of this is putting a single LED on a 2032 button cell. The 3V nominal voltage would theoretically fry many an LED. But the internal resistance limits the current, meaning that in practice, you can put any LED* on it without a current-limiting resistor, making a perfect LED tester or mini-flashlight.*Even traditional green LEDs with a Vf of just 1.7V!

author: Chriss
author_rank: Frequent Contributor
timestamp: October 30, 2020, 08:24:03 pm
content: Ok guys here I am again with some confusing think.I ordered one new Tp12-7 type battery for my UPS and played a bit with...I can not find a datasheet for this battery, but similar batteries datasheets is writing in the datasheetan internal resistance of the battery around 18mOhm.I didn't get anything close when I was measured.Here is how I made the calc and what result I got, which is for me a bit high, but probably the error is laying by me.I used a voltmeter and a resistor RL= 330 Ohm/16w.I measured the battery voltage without the RL= 330 Ohm and it was Ub1=12.92VAfter that I attached the RLand measured the Ub2=12.88VThen I calculated the current I = Ub1/RL=0.0392AThen I calculated the Ud= Ub1- Ub2=0.04VThen I calculated the internal resistance of the battery Rs= Ud/ I =1.02OhmSo, I come to a result does my new battery on ambient temperature of ~22C has an internal resistance of 1.02 Ohm.That is for me somehow really high, or maybe I'm wrong? Or maybe I did something wrong in my calc?

author: TimFox
author_rank: Super Contributor
timestamp: October 30, 2020, 09:15:04 pm
content: Did you measure the battery voltagedirectlyat the battery terminals, not downstream at the load resistor?

author: Chriss
author_rank: Frequent Contributor
timestamp: October 30, 2020, 09:16:38 pm
content: Yes, on the battery terminals.

author: TimFox
author_rank: Super Contributor
timestamp: October 30, 2020, 09:39:23 pm
content: Technically, the current I = Ub2/R, but that makes no practical difference to your calculation.Your voltmeter resolution is not very good for this measurement, but if the resistance really were 0.018 ohm, the difference in voltage would be only 0.0007 V, and the meter couldn't measure it at all.Did you verify the resistor value? Was the battery fully charged when you did the measurement?

author: Chriss
author_rank: Frequent Contributor
timestamp: October 30, 2020, 09:46:27 pm
content: What you wrote about my DMM makes sense. The battery is fully charged. I asked the company who shipped the battery if they will full charge the battery or not before shipping. And they told every battery are full charged and tested before shipping.I also measured the load resistor and the result was 332 ohm what makes not a big difference.What you prefer what resistor should I maybe use maybe to make the measurement more accurate?

author: TimFox
author_rank: Super Contributor
timestamp: October 30, 2020, 10:41:07 pm
content: To measure the resistance, you should pull as much current as is safe, for as long as it takes for the meter to read the loaded voltage. Do you have access to a voltmeter with more than 3-1/2 digits? How much current is expected from the battery in normal use? Of course, the load resistor will need to be a high power part.

author: Chriss
author_rank: Frequent Contributor
timestamp: October 30, 2020, 11:08:36 pm
content: I will see if I can manage one dmm as you preferred.In normal use the battery will be loaded around 6-7A.

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 31, 2020, 01:14:30 am
content: According tothis articlethe internal resistance of a lead-acid battery is a function of the load current. For low currents the internal resistance is high and decreases exponentially until approx. 0.69C. Since C is 7Ah for your battery, according to the article, you should test it at around 5A. Internal resistance also changes with charge, time and temperature.

author: BravoV
author_rank: Super Contributor
timestamp: October 31, 2020, 05:31:25 am
content: Quote from: Chriss on October 27, 2020, 02:07:43 pmSee the internal resistance as the bottleneck size, the smaller one (high resistance) will slow down the flow, and it will affect the flow no matter how big the bottle's capacity.Of course the bottleneck size only matter when you pouring out (drain) or pumping in (charge) the bottle (battery).If bottle sits idling, not in pouring nor at filling state, then it does NOT matter what the bottleneck size is, even the neck as small as human hair size.Now to add more complexity as bsfeechannel pointed above, the bottleneck size diameter will change depends on how fast you pour/drain the bottle, and that is the nature of that lead-acid battery.Hope this analogy helps.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 31, 2020, 08:24:21 am
content: M&M analogy is great, but let's also put that back in electrical numbers avoiding ohms if that's causing more confusion than helping:Case 1: Assume a good imaginary battery reading 12.0V with no load. You connect a 10A load, and the voltage drops to, say, 11.9V. It's quite good.Case 2: Assume a worse battery reading 12.0V with no load. You connect a 10A load, and the voltage drops to, say, 10.0V. It's quite bad, isn't it?Further, we can calculate heating in the battery:Case 1: 10A * (12.0 - 11.9V ) = 1WCase 2: 10A * (12.0 - 10.0V) = 20WAnd finally, to put that in ohms:R = dV/dI = 0.1V / 10A = 10mOhms;R = dV/dI = 2.0V / 10A = 200mOhms.There is no "actual" series resistor inside the battery; this is an imaginarymodel. The simple resistor we draw includes all effects inside battery causing voltage drop and heating relative to current drawn.The simplest somewhat usable model of battery is a perfect voltage source + a series resistor. We have just modeled this imaginary battery using this simple model of 12V voltage source plus a series resistor of 10mOhms (or 200 mOhms).Real batteries behave in much more complex ways, and more complex models are needed, but for many beginner purposes, such simple model is good enough.Do note that the battery equivalent series resistance is highly dependent on temperature, state-of-charge and battery aging.

author: Chriss
author_rank: Frequent Contributor
timestamp: October 31, 2020, 08:50:03 am
content: I really appreciate your time what all of you spend to write such nice explanations.I understand every part you are explained, and it is fine for me now.Here is my new thinking relation which is not clear for me.For first I was just playing to learn, but now I'm trying to use in practical meaning what I learnedto lets say manage the problem "how to know if a lead acid battery is good or bad based on the internal resistance."Why I come to this idea is because I know and I own a car battery tester with huge load resistor on, something like this tester:Schumacher BT-100 100 Amp Battery Load Tester, but I also saw something like this:MOTOPOWER MP0514A 12V Digital Battery Tester etc.Some of them can also calculate the internal resistance and will show on a display or print out.The Schumacher BT-100 is a robust tester with a heavy-duty resistor and will really pull the s..t out of a battery.But the mentioned Motopower MP0514A is a toy compared to the BT-100 tester.On the Motopower tester the wires from the clamps to the tester are so thin does they can't even hold 10A or less.So, they can not pull a huge amount current even for 1sec. The whole unit would be burn down to hel.But the Motopower tester is still a tester and the manufacturer guarantee does the unit will meet the purpose to test batteries.Then I was thinking how the hack does this lightway testers are designed and how they can do the test.What comes to my mind was:1. they can somehow calculate the internal resistance. ( but how they do it? )2. they measure the actual voltage of the bat and if it is under 12.2v or so then they will say "bad", "recharge" or similar......But now, as I can see, it is not simple to measure and say what is the real internal resistance of the battery, how thissmall like a toy battery testers can do the measurement and how precise they are in real?on the net

author: Siwastaja
author_rank: Super Contributor
timestamp: October 31, 2020, 11:05:46 am
content: Well, it gets really complex and ugly, and as EEVblog forums have shown a few times, almost religious sometimes. People who have bought, or even just seen a Youtube video of a nice 1kHz AC ESR meter have hard time admitting the numbers it show might not be particularly useful, or the context needs to be understood.The truth is, the simple model of battery being an ideal voltage source + a series resistor does not catch the actual battery behavior very well.Hence, there is no single well-defined value for "battery internal resistance" that is good for all uses.If you want to test if the battery is able to supply some 200A cranking current to the engine starter motor at -30 degC,obviouslythe best way to measure is to pull 200A at -30degC and see if the voltage is enough to turn the starter properly.It's also equivalently obvious such test would be tedious to arrange.Using very short pulses to measure the voltage drop allows quick measurement and lower power dissipation. Going short enough, we can easily alternatively pull current from the battery, then push it back, while storing the energy in the capacitors of the tester. Say we do quick millisecond long discharge and charge cycles, measuring the voltage drop both ways. Done with sine waves, this approaches a so-called 1kHz AC ESR measurement. Which produces a nice value that can be compared against, but it doesn't capture the reality how much the voltage actually drops under a load that lasts for longer than 1ms; it captures the capacitor-like behavior of the battery.So there is no simple answer. Many ways of measuring battery ESR exist. My favorite is running three pulses with different discharge currents, say at 5A, 10A, and back to 5A, say 10 seconds each, then measure the relative voltage drop between the end of the 5A steps and the 10A step, and calculate R = dV/dI. This could be approximately called a 0.1Hz ESR, and for many purposes, it's "DC enough".And of course, sometimes you want to know how well the battery can supply 1ms pulses. A GSM or Wifi device might do power-hungry transmission of pulses at that length, and the designer doesn't want to spend in capacitors handling this 1kHz power need; they would need to be quite big! In this case, the 1kHz ESR value could be an usable parameter, and the fact that 1kHz ESR is less affected by temperature and state-of-charge than DC ESR plays in the advantage of the designer here.OTOH, if the designer doesn't know that, and tries to use 1kHz ESR value to determine undervoltage lockout value or maximum power dissipation for DC application, they can be shocked to find real DC ESR is easily 2 times higher under specific conditions (near the end of discharge, exactly where the voltage drop is very important to understand properly for triggering undervoltage lockout not too early and not too late).

author: SpecialK
author_rank: Regular Contributor
timestamp: October 31, 2020, 02:42:43 pm
content: Quote from: Siwastaja on October 31, 2020, 08:24:21 amThis is very helpful. Do note that you would need a 1.2 ohm resistor that can handle 120W (at least) to pull 10A for any significant amount of time. This is very different from the OP's 330R resistor. Also, the 200mR battery ESR becomes a significant voltage divider and would throttle that 10A to 8.6A. I suspect you could just compare the current produced between known good battery and the test subject battery.

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2020, 03:11:36 pm
content: The voltage directly across that 1.2 ohm resistor will give the current, but a separate measurement of the voltage at the battery terminals is needed for the change in voltage. A simple voltmeter suffices for the voltage at the resistor, but a higher-resolution meter should be used (simultaneously) for the battery. At high current, the resistance in the connecting wires can give errors that must be avoided.

author: Chriss
author_rank: Frequent Contributor
timestamp: November 01, 2020, 08:19:59 am
content: What about to use a scope vs a dmm?

author: TimFox
author_rank: Super Contributor
timestamp: November 01, 2020, 02:43:46 pm
content: You can get 4.5 digit resolution from a reasonable DMM, that allows a much better measurement of a small decrement in battery voltage than with a DSO (usually 8 or maybe 12 bits). 8 bits is only 2.5 digits (roughly).

author: Chriss
author_rank: Frequent Contributor
timestamp: November 02, 2020, 04:15:53 pm
content: Hmmm....Conclusion, there is no simple way to quick test an ordinary car or UPS battery for it's healthy.The best way is to check how old is the battery and if it is several years old, and giving some issue to any of the systemsjust order a new one and be sure does the battery is no more the reason for any possible issues in the system where it is installed.So, a car battery should be changed in every 3-5 years and in a UPS for PC every 1 - 2 years.As I can stich out from your writings, reading on the net and testing by me, this is the only secure option to be sure the battery is ok.

author: SpecialK
author_rank: Regular Contributor
timestamp: November 02, 2020, 09:41:25 pm
content: Quote from: Chriss on November 02, 2020, 04:15:53 pmI don't agree.It's very easy to tell generally if you car battery is getting weak. You test it every time you start your car. No need to replace it on a set schedule depending on your individual situation. If you live in San Franscisco, where it gets neither hot nor cold, that presumably would be very easy on a battery. Cars are very easy to start in those conditions too. That battery might be good for 10 years. If you live in Nome or Anchorage and vnter is coming, you might want to replace every year just in case. The consequences are probably dire if your car doesn't start.Now UPS batteries are something else. I'm not sure how well they self-test. I guess it would vary a lot by manufaturer and model. I do think we need a solution here. But yeah batteries are often cheap compared to the testers.

author: Chriss
author_rank: Frequent Contributor
timestamp: November 02, 2020, 10:20:15 pm
content: QuoteA weak battery could be a discharged battery but also a battery which cannot held the load.If we talk about a car battery then we have to take in count how many times the engine was cranked and how long was the charging time between the engine start end stop.Here in my areal we very often start and stop the engine, small distances, city driving etc. In this situation even a new battery would last for several weeks and it would show up one day during the engine cranking like a weak battery, but it is just an empty battery...QuoteFrom my experience, battery testers are not really good. They very often show does the battery is in a good condition but in real they are not. For example, yesterday I had a situation where a customer asked to test his car battery.As I mentioned earlier in mone of my post, I have a nice an robust battery tester for car batteryes.what I did:- took out the battery from the car and fully charged with a good quality car battery charger- after charging let it rest for 24h- after 24h I measured the voltage which was around 12.7V- I hucked up the battery tester, and the tool told also the battery is in perfect conditionBingo!The battery passed the test. I put the battery back to the car and told the customer does his battery is in really good condition.What was happened in week or 10 day when the weather was cool down to ~5C?The customer give me a call to help him cos his battery wont crank the car.We bought a new battery and everything is fine.Conclusion, even with a good battery tester we can not guarantee what is the real condition of the battery.I had also almost the same situation with UPS batteries testers.I bought several of them and all of them are failed.I can not be sure does the battery in the UPS are good or not.I have to full charge them, put a PC to them and test the whole UPS.That is really time consumable task.And then I started to learn about the concept of lead acid battery, how they are made etc. and ended up here with questions in hopeI will find a solution even if I have to make calculation etc.But I can see, even that is not an accurate result and task to do, cos the result can be very inaccurate...

author: mikerj
author_rank: Super Contributor
timestamp: November 03, 2020, 12:56:46 pm
content: Quote from: Chriss on November 02, 2020, 10:20:15 pmConclusion, the current delivering capability of a battery diminishes with reducing temperature, and more so with an old battery.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2020, 05:10:08 pm
content: Testers can be OK but that requires both good tester design AND correct setting of test parameters & interpreting the results correctly.How can a tester say "pass/fail" if you use the same tester for a small 10Ah motorcycle battery with 20A cranking current or a 200Ah truck battery with 1000A cranking requirement?You can measure how much the voltage drops over, say, fixed 10A test current during 10 seconds of test, but then depending on the battery size and the vehicle, your pass/fail threshold should vary, and take into account the temperature the battery will be used at. For example, if you actually run the vehicle 40 degC colder than the test temperature and expect 20 times higher cranking current than the test current, your expected voltage drop during the test should be tiny.

author: Chriss
author_rank: Frequent Contributor
timestamp: November 03, 2020, 05:15:29 pm
content: On most good quality testers you have to set up the tester before you start the test process. On one of my testers the tester won't even start the test if the battery is not fully charged. etc...But however, I think testing a battery is very complex think what is not really possible in DIY version.As I wrote the example before when the tester show the battery is in good condition then after 10 days on 5C outdoor temp the battery has to be changed...