That drawing shows the true, real, “internal resistance” the resistance that will warm up the battery with current flow. The resistance that limits short circuit current. The resistance that causes a small voltage drop when powering something.
The thing that can be thought of as the “effective resistance” that limits charging current is an imaginary thing. It is actually counter emf rather than a real resistance. You could calculate it and draw a curve showing voltage across the cell and current vs time. From: dmmoff...@gmail.com Sent: Wednesday, June 7, 2023 1:20 PM To: ch...@go-mtc.com ; 'AnimalFarm Microwave Users Group' Subject: RE: [AFMUG] battery overcharging These are supposedly measurements from a wheelchair battery: Resistance dropping as open circuit voltage increasing. Maybe this guy can’t read his meter right. From: ch...@go-mtc.com <ch...@go-mtc.com> Sent: Wednesday, June 07, 2023 3:13 PM To: dmmoff...@gmail.com; 'AnimalFarm Microwave Users Group' <af@af.afmug.com> Subject: Re: [AFMUG] battery overcharging I guess that should be effective resistance is rising. Got it backwards. From: dmmoff...@gmail.com Sent: Wednesday, June 7, 2023 1:07 PM To: 'AnimalFarm Microwave Users Group' Cc: ch...@go-mtc.com Subject: RE: [AFMUG] battery overcharging I get what you’re saying. Sounds like the worst outcome I can get is warm batteries. Where I’m confused is I’m reading that resistance decreases as state of charge rises (and also decreases from heat). If resistance is dropping, and we’re at a constant voltage, why is the current also dropping? I’ve certainly seen a car battery charger do that, so it’s not that I’m disbelieving it, but what’s the mechanism causing it? -Adam From: AF <af-boun...@af.afmug.com> On Behalf Of Chuck McCown via AF Sent: Wednesday, June 07, 2023 2:03 PM To: AnimalFarm Microwave Users Group <af@af.afmug.com> Cc: ch...@go-mtc.com Subject: Re: [AFMUG] battery overcharging Another metaphor would be inflating a tire to 32 lbs with the compressor pressure set to 32 lbs. Once the tire is full the air will stop flowing. From: Forrest Christian (List Account) Sent: Wednesday, June 7, 2023 11:58 AM To: AnimalFarm Microwave Users Group Subject: Re: [AFMUG] battery overcharging Ignoring the overvoltage for a minute.. A normal battery, when charged at a normal voltage, will take less and less current until full. It doesn't matter how much current is available, it will only take what it needs. Generally you want to limit the current based on the size of the array, but that's for the start of charge, but the end of it. That is, an empty battery array will take all the current you can give it, and too much can cause damage to the battery and maybe wiring. As far as the overvoltage goes, that's far more dangerous as overvoltages tend to cause "boiling" of the battery which is just another way to say that the battery is producing lots of hydrogen and oxygen which can cause explosive atmospheres around the battery, or in an extreme case can cause sulphuric acid steam to escape the battery. Combine this with no overcurrent or time limit and at the bare minimum you should expect swollen and destroyed batteries. On Wed, Jun 7, 2023, 1:05 PM <dmmoff...@gmail.com> wrote: So I was looking at a rectifier config and trying to imagine the worst possible thing I could do. With some creativity I could start an equalizing charge that will run for 48 hours at 58.5 Volts. Normally it would stop when charge current hits a configurable fraction of the C10 Ah rating of the battery, but by intentionally misconfiguring the size of the battery and that fraction of Ah I could ensure that the equalizing charge runs until the maximum time limit…..which I can set as high as 48 hours. I could also disable the battery current limit, disable the over temp shutoff, and disable the temperature compensation. If I’ve disabled the current limiting feature in the rectifier, what determines the current that will go into the battery when I’m on a constant voltage charge? The charger can do up to 100A, but would it put 100A into the battery for the full 48 hours, or would some other factor limit it? The spec sheet for the batteries lists an internal resistance of 3 milliohms. V=IR tells me they could take thousands of amps at 58.5V. Is there any other limit here besides resistance. If nothing else stops this machine from dumping it’s full 100A into the batteries for 48 hours I’m pretty sure I could start a fire or explosion with this rectifier if I wanted to. I’m not trying to commit an act of sabotage by the way, I’m just wondering if a creative idiot could do something terrible. -- AF mailing list AF@af.afmug.com http://af.afmug.com/mailman/listinfo/af_af.afmug.com -------------------------------------------------------------------------------- -- AF mailing list AF@af.afmug.com http://af.afmug.com/mailman/listinfo/af_af.afmug.com
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