“When charging process, the battery's internal resistance (ric) is low and will
increase according with increasing the level of SoC. While at the discharging
process, the battery's internal resistance (rids) is low and will increase in
proportional to the decreasing of the SoC level.”
--From the abstract of a physics paper
So resistance increases when charging and also increases when discharging?
But it means the guy measuring the wheelchair battery is correct if he’s
measuring resistance while it’s discharging.
It does not seem intuitive to me that resistance goes up both when charging and
discharging…..but it makes actual battery behavior match up with Ohm’s Law.
I.E.: voltage dropping while discharging, and current dropping while
constant-voltage charging both make sense that way.
-Adam
From: [email protected] <[email protected]>
Sent: Wednesday, June 07, 2023 3:13 PM
To: [email protected]; 'AnimalFarm Microwave Users Group' <[email protected]>
Subject: Re: [AFMUG] battery overcharging
I guess that should be effective resistance is rising. Got it backwards.
From: [email protected] <mailto:[email protected]>
Sent: Wednesday, June 7, 2023 1:07 PM
To: 'AnimalFarm Microwave Users Group'
Cc: [email protected] <mailto:[email protected]>
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 <[email protected] <mailto:[email protected]> > On Behalf
Of Chuck McCown via AF
Sent: Wednesday, June 07, 2023 2:03 PM
To: AnimalFarm Microwave Users Group <[email protected] <mailto:[email protected]> >
Cc: [email protected] <mailto:[email protected]>
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 <[email protected] <mailto:[email protected]>
> 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.
--
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