Test car battery with oscilloscope during cranking - Page 1
Forum section: electronics / projects

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 16, 2024, 11:40:39 pm
content: My car battery (12V lead acid) is around 7 years old, and I'd like to check its health.I plan to measure the voltage at the battery terminals with an oscilloscope while starting the car. No plan to measure current because my biggest current clamp tops out at 150A.Does anyone have recommendations / rules of thumb for how low the voltage can go during cranking and still be ok?The car is a 2017 Toyota Yaris if that makes any difference.EDIT: The cold and warm crank voltage profiles athttps://www.monolithicpower.com/en/learning/resources/from-cold-crank-to-load-dump-a-primer-on-automotive-transientsseem instructive. I'm guessing they're worst-case low voltage since they come from industry standards for automotive electronics.

author: moffy
author_rank: Super Contributor
timestamp: November 17, 2024, 01:07:39 am
content: Just personally, anything below 7V-8V during cranking at the battery terminals would be suspect.

author: bdunham7
author_rank: Super Contributor
timestamp: November 17, 2024, 01:59:06 am
content: You may get a very short (a few ms) dip to fairly low voltages (7-8V) as the solenoid engages with the starter armature not moving, but as the engine cranks over you'll see "lumps" in the waveform that progressively get higher and shorter as the engine cranking speeds up. The trough immediately after the first lump should be over 9.6V, probably even somewhat higher if your engine is warmed up and the battery is freshly topped off as it would be if the engine had just been running. This is all assuming you measure across the battery terminals directly. I think a Yaris has a group 35 battery which is plenty big for that little motor, so I wouldn't expect marginal results. Please post what you find!

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 17, 2024, 10:39:20 am
content: I don't remember exact requirements at the moment but it is something like a voltage must not drop lower then 8-9 V with a healthy battery. It can be measured with a usual DMM if it is not too slow. It can be measured by your hearing (without any other meter) if you know a normal starter speed.You can have a battery resistance meter and measure battery internal impedance. There are universal meters like YR1035+ miliommeter. It can measure resistance of wires in a milliohm range, so it is quite helpfull and universal device.

author: Psi
author_rank: Super Contributor
timestamp: November 17, 2024, 10:45:58 am
content: Quote from: slugrustle on November 16, 2024, 11:40:39 pmIf you have some high current cable you can cut it into 6 short lengths, wire them all in parallel and put your 150A clamp on one of them.Then just multiple the current you measure by 6.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 17, 2024, 10:52:56 am
content: Starter current in a summer can be no more than 150-200 A with a good thin oil (for 1.5 L engine). It's winter that needs more current (possibly to 250 A for a usual 1.5 Litre sedan). I mean you usually don't nead 1000 A measurent range to measure this current.

author: johansen
author_rank: Super Contributor
timestamp: November 17, 2024, 07:23:00 pm
content: Ive measured on the order of only 100 amps for several 2 liter engines.As for the 9volts. Thr cranking amps on the battery are given for the terminal volts above 9 volts for 30 seconds.So a brand new car battery at 600 amps (30 amp hour capacity) (2400 amp short circuit) is far, far in excess of what is needed to start the car.But 7 years later it will be marginal.

author: johansen
author_rank: Super Contributor
timestamp: November 17, 2024, 07:28:32 pm
content: As for measuring current, you can measure the voltage drop across 1 foot of the typically 4 gauge wire that runs to the starter motor.

author: floobydust
author_rank: Super Contributor
timestamp: November 17, 2024, 07:47:31 pm
content: OP I think your test is no good.Cranking drain varies too widely. Mainly with temperature and if a cylinder fires or not, health of the starter motor, engine oil viscosity - you can't depend on it as a fair load for a test.If it's the original (flooded) Toyota made in Japan battery, these last much longer if you add water as needed and do a manual charge every few months. Up to 14.4-14.6V then float at 14V for a while. I find Toyota uses lower (ECU controlled) charging voltages I measure around 13.3V in two vehicles although it's up just after starting for maybe 15 minutes.After a period of time, the battery cranks slower and I've just done a top up charge and it's good for a couple months.People have used some of the car's battery cable as a shunt resistor but this (copper) has a big tempco for resistance. I think they use a lower (headlights) drain to calibrate it.Alternatively, you can go to an auto parts dealer and do a load test or ESR test.

author: johansen
author_rank: Super Contributor
timestamp: November 17, 2024, 09:43:20 pm
content: Quote from: floobydust on November 17, 2024, 07:47:31 pmEngineers really need to get out of the office chair sometimesYes, copper has a temp co. You need to heat up the copper 100 degrees Celsius to get a 39% increase in resistance.It warms up.. maybe 1C during cranking if the engine does not start on the first 3 revolutions of the crank.If you are off by 20% all it means is you will be off by 20% in your estimate of how much longer the battery will last.Ive run them down to the point they can start the car..but not if the headlights are left on for 3 minutes

author: CaptDon
author_rank: Super Contributor
timestamp: November 18, 2024, 02:13:45 am
content: Three thoughts, 1. I would drive it to an AutoZone, Advance Auto, Oreillys, whatever and let them throw the tester on it. It uses high drain current pulses to analyze the condition based on rated amp hour capacity. 2. If you know what good battery cells look like and you can see down through the filler caps I would look for very black plates, and nice silver gray plates (or course they alternate in their position one being the positive plate the other the negative) and nice plate dividers, not warped of deteriorating. 3. I'd probably buy a new battery anyway. C'mon, 7 years, why trust it? It'll probably take a shit on you when you need it most!!! I took the 7 year old AGM battery from my Prius and put it in my ham shack to power my 12vdc radio gear. It is always float charged and may survive in that role for another 10 years. Of course I had to bite the bullet on the 'core' charge without a trade-in.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 18, 2024, 02:18:53 am
content: Quote from: slugrustle on November 16, 2024, 11:40:39 pmI wouldn't even bother checking. At that age, the battery is well past its service date.To avoid being stranded, replace the battery ASAP.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 18, 2024, 02:48:59 am
content: Back in the day, before cars went all "high tech", many manufacturers used an 8 volt ignition coil, with a series resistor to reduce the coil current from the battery under normal operating conditions.Under starting conditions, the resistor would be bypassed, so that when the battery dropped to around 8v during starting, it would still deliver a full spark.Those cars that didn't use this system were noticeably harder to start during cold conditions.As to the oscilloscope, once the engine is running, a 'scope is useful to diagnose alternator faults, as any discontinuity in the ripple will show up such problems as an open diode,

author: wilfred
author_rank: Super Contributor
timestamp: November 18, 2024, 03:55:19 am
content: 7 years is pretty good for a battery. I'd be keeping an eye out for when batteries go on sale. But here in Australia we mostly don't have to worry about super cold weather. I'd say you've had a good run so don't push it.

author: Psi
author_rank: Super Contributor
timestamp: November 18, 2024, 06:48:25 am
content: Quote from: Vovk_Z on November 17, 2024, 10:52:56 amDepends what data you want to get, the constant draw is in the 100-250A range yes but the peak can get up to 700A or so when you first apply power and the starter motor is not spinning.If you want to get a complete waveform being able to measure up to 900A is a good idea, if you don't care about the peak then 300A range is fine.

author: armandine2
author_rank: Frequent Contributor
timestamp: November 18, 2024, 07:46:34 am
content: ...internal resistance seems the simplest measurement to do - I was hoping for a better reading on a couple of 7ah 12V batteries for use as a kWeld spot welder power supply - but at about 90mOhm each these two won't now supply the required 800A.

author: bdunham7
author_rank: Super Contributor
timestamp: November 18, 2024, 02:51:22 pm
content: Quote from: floobydust on November 17, 2024, 07:47:31 pmIt certainly isn't a test with well defined pass/fail criteria, but in my experience it as as useful as any of the "gizmo" tests that don't involve a carbon pile and an actual load. It does, however, give a good answer to the question "will my car start", especially if you have a known-good test of the same or similar car to compare to.

author: johansen
author_rank: Super Contributor
timestamp: November 18, 2024, 06:59:33 pm
content: Quote from: armandine2 on November 18, 2024, 07:46:34 amtypical short circuit current is 30 to 40 times the amp hour rating.your 91 milliohms is 18 times the amp hour rating, a little on the low side.if you want 800 amps you're going to need a new 35 amp hour battery.

author: floobydust
author_rank: Super Contributor
timestamp: November 18, 2024, 07:37:45 pm
content: Quote from: bdunham7 on November 18, 2024, 02:51:22 pmCowboy copper-resistor tech has already been tried- on wires and PCB traces as well. I see it as going down a rabbit hole in search of some psychological assurance. But you have no control over the pulsating load that cranking is, so I am pessimistic about it. Looking forward to a 4W measurement in dirt, corrosion and oxides. I see carbon-pile load tests are kind of a moving target with the voltage sliding down with time, and they assume the initial State of Charge is 100%. But I'm seeing modern cars keep them at 80% to achieve higher mileage numbers. During decel they will regen charge the battery, bump up alternator setpoint until you tip in. It's not working for a few car makers due to their battery model failing with age. A manual top-up charge is needed.For the labour involved here, it's cheaper to take it someplace for a test, or just buy a new battery. I knew one guy he kept a spare in his car. I'd used a small 17Ah gel-cell for boosting when I kept leaving my car stereo on.I've gotten over 10 years life with original Toyota Japan flooded batteries (Panasonic, Yuasa) but this is in Canada where it's much cooler and the "winter package" includes an oversized battery. Yaris is 1.5L and even a dinky Group 51 works for Honda, but I think OP's is bigger Group 47? which seems huge for the car, Toyota does not skimp there.Hot climates are an enemy of car battery life so Australia or southern US I hear they have different warranty/construction and are used to shorter overall life 1/2 of the north.Also the parasite drain greatly eats into battery life - Subaru are pushing 75-150mA and class action lawsuits over the 3-year life people are getting.I don't have much experience with newer ESR testers, they seem to use lookup tables to estimate CCA and are not great.johansen speaks of a ESR multiplier based on Ah rating but I have not seen where that's from.

author: johansen
author_rank: Super Contributor
timestamp: November 18, 2024, 09:05:35 pm
content: Quote from: floobydust on November 18, 2024, 07:37:45 pmread the datasheets from reputable LA manufacturers.also note that the 600 cranking amps of a typical car battery at 9 volts at the terminals corresponds to a 3.5 volt drop across the battery.. which corresponds to about 1/4th of the total short circuit current. then go back and observe that the battery is about 60 amp hours. as such, it follows the 30-40x AH rule.if you read this paper here:https://www.osti.gov/servlets/purl/1188216the short circuit current of these rather large thick plate 4 volt batteries, is about half the current that a thin plate starting battery can deliverhttps://cantecsystems.com/products/enersys-powersafe-2gn-23-detailthey listed 9000 amps short circuit for those 1100 amp hour 4 volt cells. when you multiply the 9000 amps by the .00023 ohm resistance, you get 2 volts. this is consistent with other talk in that report that the measured short circuit current was about half of theoretical, due to losses in the buss bars shorting out the battery.the higher 12volt cells have less of that problem.but its no supprise that 0.2" thick plates are going to have a lower short circuit current per amp hour than the .07" thick plates of a car battery, or shitty agm cells.

author: John B
author_rank: Frequent Contributor
timestamp: November 18, 2024, 09:09:17 pm
content: You can also use a multimeter with min/max sample logging.Generally I get a bit worried if the battery drops below 7V, but in the end you can just use your ears to hear how easily the motor starts.Battery terminal clamps, wiring and the starter relay are all sources of resistance which can degrade over time and have the same effect as an ageing battery.

author: armandine2
author_rank: Frequent Contributor
timestamp: November 18, 2024, 09:58:13 pm
content: Quote from: johansen on November 18, 2024, 06:59:33 pmIt's simpler than that --- divide the battery open circuit voltage by the internal resistance

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 23, 2024, 06:30:06 am
content: Quote from: bdunham7 on November 17, 2024, 01:59:06 amThanks for this. I ran the test on a cold start. The terminal voltage drops to 6.8V in the starting pulse and recovers to about 10V during the lumps; see attached scope shot. This is better than I expected. The national weather service said it was around 40°F just before I went out to run the test.I usually work from home and drive very little. I put this battery on a maintainer when I think to do so. It looks to be an original Toyota battery (bought the car used in 2019).Quote from: floobydust on November 17, 2024, 07:47:31 pmThe battery says "maintenance free" on it.

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 23, 2024, 07:13:14 am
content: Quote from: Andy Chee on November 18, 2024, 02:18:53 amFor now, it looks like she's a keeper.

author: moffy
author_rank: Super Contributor
timestamp: November 23, 2024, 07:26:41 am
content: Quote from: slugrustle on November 23, 2024, 07:13:14 amAt 40F that's pretty decent performance.

author: TaylorD93
author_rank: Regular Contributor
timestamp: November 24, 2024, 10:08:02 am
content: I have a 2016 Toyota Auris, and the workshop manual actually quotes a minimum permissable voltage during cranking. Certainly with Toyota UK you can pay a few £ to access the entire workshop manuals online for a few minutes.Another thing to be aware of when looking at the cranking voltage however is the "relative compression test". It is a test where you use an oscilloscope to measure the battery voltage during cranking (usually done with the injectors disabled), as the engine rises on the compression stroke, the starter motor is put under more load and therefore the battery voltage will sag at each compression stroke. so if you have a cylinder which is lower compression relative to the other cylinders, you would see a smaller drop in battery voltage on that compression stroke.If the 2nd channel of the scope is set to monitor a coil pack pulse (often can be held on the back of the coil pack, if coil-on-plug setup) you can then find which cylinder is actually low.My point being, engine mechanical health will impact your readings.

author: ugawoxoto61
author_rank: Newbie
timestamp: January 22, 2025, 09:10:38 am
content: Quote from: slugrustle on November 16, 2024, 11:40:39 pmTypically, the voltage should not drop below 9.6V during cranking, even under load. A healthy battery usually stays between 10V-11V during a cold start, depending on factors like temperature and the starter motor’s demand.

author: Psi
author_rank: Super Contributor
timestamp: January 22, 2025, 12:40:58 pm
content: Yeah, there's so much variation that will affect the current.- Cold vs warm state.- The ambient temp in your part of the world.- The size and compression of the engine- The thickness of the engine oil your using. (also temp related)- The number of accessories you have attached to the engine with belts.