Re: Sealed Lead Acid Battery Tester Strategy?
- From: "john jardine" <john@xxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 4 Jul 2005 02:01:55 +0100
"Mike" <mjb@xxxxxxxxxxxxxxx> wrote in message
news:da8b9k$v2m$1@xxxxxxxxxxxxxxxxxx
> I've designed and built a prototype PIC based Sealed Lead Acid Battery
> Tester, but I'm having confusing and conflicting results and would like
some
> input on whether the strategy I'm using is sensible ...
>
> It's based around a constant-current load, controllable by the PIC chip.
> The current can be set over a range of C/40 C/20 C/10 C/5 C/2 and C (where
> C is the nominal Ah rating of the battery). The unit can monitor the
> voltage of the battery. All the voltage measuring/current measuring and
> current setting is appropriately calibrated, so any problems are down
> to approach or duff assumptions on my part :(
>
> The basic idea is a "brute force" test of the capacity of the battery:
Load
> it up at C/n and time how long the battery takes to reach a discharged
state.
> In theory, it will take "n" hours for a 100% battery (aside from the
obvious
> that discharging faster will reduce apparent capacity etc.)
>
> From my research so far a fully charged SLA cell should read 2.16v at
rest,
> and 1.75v when discharged. So for a 12v battery, the useful capacity is
> found between terminal voltages of 10.5v and 12.96v. Right?
>
> The capacity of the battery can be estimated using this voltage range,
which
> I'm assuming to be a linear function (it nearly is...). So the first
feature
> on the tester is a battery capacity estimator, based on offload voltage.
>
> Then loading the battery at (some constant current) begins the discharge
> process, and the voltage starts to fall. I time how long it takes for the
> voltage to reach 10.5v, and then stop the timer, and compute the capacity
> actually achieved. It should be that simple, however ...
>
> I have a dilemma: When things say "don't discharge the battery below
10.5v"
> do they mean on-load or off-load? There is a difference!
>
> I noticed that loading the battery, especially at higher currents, causes
> the terminal voltage to fall, due to the internal resistance of the
battery.
>
> So I added a feature to try and measure this resistance at the start of
the
> discharge process. I did this by taking the off load voltage, the on-load
> voltage at C/n, and working out the voltage being dropped across that
> resistance. This gives me a figure of <100 mOhm on new batteries, and
> anything up to 4-5 Ohms on older "suspect" batteries. It also gives me a
> "delta voltage" to adjust the 10.5v limit figure downward by.
>
> My tester currently records two capacities. One: Discharge to 10.5v on
load.
> Two: Keep discharging until 10.5v MINUS the voltage-delta calculated
above.
> Obviously capacity 2 is always >= capacity 1. This seems to give
reasonable
> figures, of capacities that are <= nominal.
>
> The snag is that when this test terminates, the battery really *really*
> should be flat. As I've compensated for the voltage-drop across the
internal
> resistance, I should get 10.5v when the load is taken off, and the battery
> is empty, right?
>
> Wrong.
>
> The voltage bobs back up to an embarrassing "You have 60% left" level.
That
> shouldn't happen :) I flattened it to 10.5v on-load. I flattened it
further
> to 10.5v minus the voltage across the internal resistance ... how can it
> still have power in there?
>
> Trying to load the battery up further (repeating the discharge) just
causes
> it to immediately terminate as the voltage is too low.
>
> Is there anything particularly wrong with this strategy?
>
> Should I even be compensating for the voltage drop across the internal
> resistance? I know it's meant to be low order mOhm, but even that can
cause
> a significant drop at e.g. 30A test current on large batteries. So I think
> yes, I should be.
>
> And does the internal resistance significantly change, meaning I should be
> re-testing it at intervals? I thought the internal resistance was a
function
> of how sulphated the plates were, and would only vary if the battery was
> deep discharged and left, or charged and rejeuvenated.
>
> Why does there appear to be useful power left in the battery (from the
> terminal voltage) and yet there .. isn't!?
>
> If the discharge is genuinely flattening the battery, yet the terminal
> voltage comes back up so high, how can "battery fuel gauge meters" ever
> reliably indicate the state of charge of the battery? I'm beginning to
> think that they can't!
>
> Maybe I should take the load off every so often, wait for the voltage to
> bob back up and settle, and if it's > 10.5v, go on a bit longer?
>
> Here's an example of the kind of readings I'm getting. Each capacity is
> stated twice, once for "discharge to 10.5v" and once for "discharge
> below 10.5v by dV"
>
> Battery: Camden Europa Plus, 20 months old, UPS battery. 7.2Ah nominal.
> Internal Res: 94-126 mOhm (tested at 7.2 Amps)
> Measured Capacity @ C/1 rate: 2.44Ah/3.16Ah
> Measured Capacity @ C/10 rate: 5.11Ah/5.18Ah
> Est. Capacity after C/1 test - 67% (UPS software claims 63%)
> Est. Capacity after C/10 test - 50% (UPS software says 58%)
>
> Spookily, I seem to have drained 33% out of the battery (2.44Ah) and 67%
> is estimated to be left, which means I haven't flattened the battery yet.
>
> Battery: Yuasa NP1512, 5+ years old, 15Ah nominal
> Internal Res: 76 mOhm (tested at 15 Amps)
> Measured Capacity @ C/1 rate: 9.9Ah/10.5Ah
> Measured Capacity @ C/10 rate: 15.15Ah/15.0Ah
> Est. Capacity after C/10 test - 18%
>
> Battery: Sonnenschein Dryfit A300, 5+ years old, 3Ah nominal
> Internal Res: 270 mOhm (tested at 3 Amps)
> Measured Capacity @ C/1 rate: 1.05Ah/1.08Ah
> Measured Capacity @ C/10 rate: 2.7Ah/2.7Ah
> Est. Capacity after C/1 test: 50%
> Est. Capacity after C/10 test: 60%
>
> What have I screwed up? :)
>
> Mike.
> --
> --------------------------------------+-----------------------------------
-
> Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/
The terminal voltage between fully charged and flat is from about 13V to
12V.
www.yuasa-battery.co.uk/ind_vrlamanual.html
Has a really excellent technical write up on their SLA type batteries.
Use it as a good reference point for designing any high quality SLA
charger/discharger.
regards
john
.
- References:
- Sealed Lead Acid Battery Tester Strategy?
- From: Mike
- Sealed Lead Acid Battery Tester Strategy?
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