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By No Crust Racing · Posted
Can't remember the last time I saw a good, healthy battery dropping into the 9s during cranking, Too many variables to say anything about it with any certainty though. A float voltage of 12-12.2 is tired AF and given how AGMs handle high C rate and DOD, will just die faster and faster with each cold start. -
I find most vehicles dip into the mid to high 9 volt range, even with a good battery. Hitting the low 9s, and especially hitting into the 8s is pretty indicative of a dieing battery. The other part to look at is the battery voltage after the battery has been fully charged. They should be sitting at 12.8V, as lead acids are dieing, they head down low and lower. Often getting as low as 12.0 - 12.2. You'll start seeing this when the engine is cranking over slowly. Interestingly, depending on the vehicle, when batteries get really bad, you can see some REALLY funky things going on. From very low voltages, to higher than 16V too. Something I totally didn't expect! This part of the super low and super high voltages I've been seeing on motorbikes, I need to do some further studies on cars. I suspect cars will act a little bit different though for a couple of reasons. -
By soviet_merlin · Posted
Thanks! Supercheap has a sale on this weekend so might be a good opportunity to get a new one. I do have a decent multimeter so I'll give it a try either way. Hah, you got me! I do not. All I know is that the battery holds charge and sits at an okay level before I start the car. Doing the full charge and rejuvenation cycle on a trickle-charger seems to not really make a difference to the cold start behaviour. I'll try and measure it during cranking and see what it does. It's likely terrible; it certainly feels terrible. I'll be happy if it is really just the battery and I don't have to dick around with the alternator or starter motor. -
By No Crust Racing · Posted
how do you know voltage is fine? quick easy test is connect DMM up and put it on max/min mode, remove fuel pump fuse, crank car for 3-5s, go check min voltage You'd be surprised how many "healthy" batteries are showing under 10v during this test in cold weather. -
Not properly. You need to be able to dissipate 100 amps or so to doing it meaningfully. You can do it indirectly by watching to see how far the voltage falls during cranking. Unplug the coils or something else to prevent it from starting so you can get a good couple or three chugs. It also helps if you have the multimeter set up reading before you start, and that it has max/min functions. So you can catch the real minimum without having to watch the screen, which often doesn't update fast enough to show the real max/min in dynamic situations. Or use a digital oscilloscope, which can be obtained for <<$100 from Aliexpress (although I'd argue for paying up to ~$200 for a nicer one). A >4 yr old battery will very likely be well down the path to the knackery. Many only last 5-6 years these days. The cold weather lately will definitely make it worse.
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