Question for lead acid battery experts - Page 1
Forum section: electronics / general

author: jrmymllr
author_rank: Regular Contributor
timestamp: March 31, 2025, 02:20:15 pm
content: I got this car battery analyzer made by Chinese company Autool, model BT360. I got it for free for evaluation so no comments about paying for itWhen I first saw the thick black and red leads between the instrument and clamps I assumed it was heavy single-conductor wire like most of these testers. But no, each cable contains two separate wires, and each wire goes to one half of the clamp in a Kelvin configuration. So I think I can assume one lead is for loading the battery, and the other is for voltage measurement. Fair enough.I put a 0.01 ohm resistor inline and put my scope across that so I can see what it's doing during test. It's applying a ~2.5A load for 4ms, then off for 4ms, then repeat for almost 500ms. The same thing occurs regardless of the cold cranking amps I set the analyzer to.How accurate is something like this? They are obviously taking the voltage measured and applying some algorithm, or using a lookup table, to determine battery state of health, SOC, and internal resistance. Even if correctly implemented, can this method result in an accurate state of health for a car battery? I tried it on my car and it claims it needs replacement which could be correct even though it is still starting OK.

author: tom66
author_rank: Super Contributor
timestamp: March 31, 2025, 03:14:34 pm
content: My guess is it is measuring battery impedance and then it knows either via an equation or look up table what impedance is needed for a given CCA (e.g. battery can still maintain >9V at 220A crank when cold = means a maximum of 15mohm impedance - just my guess).In terms of accuracy if you load the battery with 2.5A then a 10mohm impedance produces a 25mV drop, well within a 12-bit ADC range (1 count = 3.6mV or 1.4 mohm resolution). So they are probably using something like a 12-bit ADC and then averaging the samples to get a more accurate measurement of impedance.It would have difficulty if the car was still connected and loads active, e.g. alarm system, as those may periodically pull current from the battery. Maybe it looks at sample to sample variation to determine if the test is valid or not.In terms of state of health, the number one thing that kills lead acid batteries is plate sulphation, which directly correlates with an increase in battery impedance.

author: thm_w
author_rank: Super Contributor
timestamp: March 31, 2025, 07:37:32 pm
content: It will never give you an exact reading of the battery capacity, eg how many Ah it can actually supply. You'd need a load test with counter for that. But its usually good enough for average people, and good enough estimate for aging as mentioned above.You can see here some examples where internal resistance method within 10-20% error, with one 60% outlier:https://www.sciencedirect.com/science/article/abs/pii/S0378775321012052https://ut3-toulouseinp.hal.science/hal-04828450v1/documenthttps://www.reddit.com/r/AskMechanics/comments/16fa9ip/how_accurate_is_the_battery_tester_at_autozone/

author: edpalmer42
author_rank: Super Contributor
timestamp: March 31, 2025, 08:04:28 pm
content: I have a similar meter. I don't believe the actual numbers that it gives me, but I use them as an indicator of how the battery is changing over time. e.g. if the battery impedance starts at 9 milliohms and is still at 9 milliohms after 2 years then that's okay.I have found that you must give your battery a good charge before testing it. If you don't, it will always say that the battery is weak and/or needs to be replaced.