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On 5/14/2022 11:18 AM, Jeff Liebermann wrote:
In the olden days they used a high-wattage/low-ohm resistor, like 0.5
ohms at 300 watts to cause the battery to put out about 25 amps (though
as the voltage fell the current fell slightly). You time how long the
battery takes to fall to 10.5 volts and then you have the reserve
capacity. The resistor gets very hot of course.
A more accurate way to measure RC is to keep the load at 25 amps. You
can design an active load using multiple MOSFETs. Of course those get
hot as well, and you need to heat sink them well and distribute the load
between multiple devices.
For all intents and purposes, a high-wattage resistor is sufficient for
testing the reserve capacity of a lead-acid battery. DC load and AC
conductance tests only take a few seconds but they don\'t give any
indication of reserve capacity. There is little, if any, correlation
between the internal resistance of a battery and reserve capacity.
On Sat, 14 May 2022 13:01:08 -0400, nospam <nospam@nospam.invalid
wrote:
In article <3rjv7htsgmnt4dp2jboaqkqhr2saoj7ue9@4ax.com>, Jeff
Liebermann <jeffl@cruzio.com> wrote:
Older automobile electrical system testers used a carbon pile load.
Basically, a big resistor.
they did, except that those are now obsolete.
The mechanic who works on my 2001 Subaru has one with a carbon pile
load. I know because I\'ve repaired it a few times. Several of the
auto shops that I frequent also have ancient chargers. Sorry to
report, but the old stuff doesn\'t just disappear.
In the olden days they used a high-wattage/low-ohm resistor, like 0.5
ohms at 300 watts to cause the battery to put out about 25 amps (though
as the voltage fell the current fell slightly). You time how long the
battery takes to fall to 10.5 volts and then you have the reserve
capacity. The resistor gets very hot of course.
A more accurate way to measure RC is to keep the load at 25 amps. You
can design an active load using multiple MOSFETs. Of course those get
hot as well, and you need to heat sink them well and distribute the load
between multiple devices.
For all intents and purposes, a high-wattage resistor is sufficient for
testing the reserve capacity of a lead-acid battery. DC load and AC
conductance tests only take a few seconds but they don\'t give any
indication of reserve capacity. There is little, if any, correlation
between the internal resistance of a battery and reserve capacity.