On 5/19/2017 9:33 AM, Jim Wilkins wrote:
I've finally acquired enough equipment to measure the remaining
Amp-Hour capacity of my Lead-Acid and Lithium battery collection. The
first result that jumped out is that older batteries suffer from
rising internal resistance as they discharge, enough that the
automatic low voltage cutoff trips short of rated capacity, and then
the battery slowly recovers to well above the full discharge voltage
given in the specs.

http://www.power-sonic.com/images/po...hManual-Lo.pdf

The 5-year-old 12v 4.5Ah UPS battery I tested this AM delivered 2.45Ah
at 3A, which is the average current my laptop draws while browsing.
Table 2 shows in the 1 Hour Rate column that it should be good for
2.75Ah at 2.75A current.

Does anyone know a good reason why I can't measure the true remaining
capacity in two steps by first discharging to 10V at the fairly high
current of my typical loads, then continuing at the 20 hour rate AGM
batteries are specified for until the voltage drops to [the
appropriate endpoint] again?

The run time for a typical load tells me how useful the battery still
is, but it combines the effects of capacity and resistance. I'm
wondering if also knowing the Amp-Hour capacity at the 20 hour rate,
with less interference from the internal resistance, would indicate
how well my long-term maintenance procedures work.

-jsw


I gave up. Never got any predictive information.

Most of my testing was done with Lithium batteries in laptops.

I consider a laptop battery bad when it won't run the laptop
long enough. How vague is that? ;-)

Started with bad battery packs and tested cells.
At low current, I almost always got something like specified
capacity. The electrons are in there, but the laptop won't
let you have them.

I don't know what the sampling interval is, but the laptop
wants to shut down at some voltage so you don't lose data
and call the vendor. Subtract the peak voltage across the ESR
from the battery voltage. If it dips below the threshold,
the laptop wants to shut down.

Turn off the power features that warn of impending low voltage
shutdown.

The symptom is that the battery gauge decays slowly for a while
then drops instantly to a much lower number. The laptop senses
impending doom, but you've blocked that.
I've had laptops run two hours past the point when the battery
gauge hit zero. Problem is that when it dies, you lose whatever
you were working on.

I've never had any success trying to fix the ESR. That's probably
the same problem you have when your car fails to start. Never been
able to do anything about that either.

The higher the peak current, the fewer electrons all those
protection circuits will let you have.
Even with a new battery, capacity is a strong function of
load current.

Numbers from browsing the web won't help much when
Microsoft decides to do an update and runs all your cores
at 100%.