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Re: (ET) re: battery testers
- Subject: Re: (ET) re: battery testers
- From: "David Roden (Akron, Ohio, USA)" <roden ald net>
- Date: Tue, 24 Aug 1999 01:21:59 -0500
- In-reply-to: <002201beedce$e68256a0$1f9d1e26@snaugler>
- Sender: owner-elec-trak cosmos5 phy tufts edu
Wow, lots of questions. I'll do my best with these. Corrections and
emendations are welcome.
Wombat's recommendation to note the specific gravity is a good one.
Every person who has an ET should probably own a hydrometer.
> 1.
> Wouldn't you need to actively adjust resistance to maintain 75 amps ...
> as the voltage drops ...
I guess for optimum accuracy, yes, you should. I've never worried much
about it, because I was always more interested in relative values from
battery to battery to determine the weak guy.
Actually, if that's all you want to do, there is an easier way. Load the
entire pack to 150 amps or so, then measure the voltage across each
battery. The one which is lowest is the weakest.
> 2. Would you test the 36 V pack, and then test individual batteries if a
> problem was found?
Usually your panel voltmeter will warn you if there's a weak battery in
the pack. You'll see it drop farther under load than it ought.
When you know there's a problem, it's probably more useful to test
individual batteries, but I think the method above is easiest.
> Is 75 A the correct testing current for our 6 V
> batteriess?
If all you are doing is comparing batteries, any high current typical of
operation is fine. I consider 75 amps a good value to use for golf car
batteries because that is the current at which their reserve capacity is
rated.
> How about for the 12 V batteries used in E8s?
Twelve volt batteries have thinner plates and are not designed to produce
the high currents that golf car batteries are. That's why they usually
don't last as long. You can test them at 75 amps, but 25 is the standard
for 12v reserve capacity.
You could argue persuasively that either the peak or average current the
batteries see in your ET would be a good test value. I have not measured
that current (sure wish my instrument panel ammeter were calibrated in
something besides red, yellow and green), but one person on this list
posted a while back that he'd measured around 150 amps while mowing,
peaking at 300 while climbing steep hills. That seems a bit high to me
(given the amount of time I can mow per charge), but I'd easily believe
75-100.
BTW, I hear some folks have upgraded their E-8s to 6v batteries. If it's
not too much trouble to implement, I'd recommend that change. Twelve
volt flooded deep cycle batteries are ~not~ suited to prolonged current
draws over around 50 amps.
> Is this published somewhere on the web?
For a partial listing of typical reserve capacity and other battery
specifications, see
http://www.trojan-battery.com/products.htm
or
http://www.usbattery.com/menu.htm
> Is the 10.5 V for a 12 V battery and 5.25
> V for a 6 V battery the correct test voltage for all liquid electrolyte
> lead acid cells?
That is the voltage (1.75 volts per cell) at which a lead battery,
measured under load, is considered discharged by BCI (Battery Council
International) standards. Flooded and VRR are equivalent for this. A
few manufacturers fudge their numbers by using 1.66 or 1.5 volts as a
"discharged" marker.
What "under load" means varies with the test. For the reserve capacity
test I mentioned earlier, it's usually 25 or 75 amps. For a 20-hour-rate
amp-hour capacity test, it might be 5 or 11 amps.
> 4. Has anyone built their own tester where they didn't
> need to stand next to the test rig constantly making adjustments and
> taking readings?
Let's distinguish here between load testers and cycle testers (battery
dischargers). Typically a load tester applies a known load to the
battery and measures its voltage. This works because as a battery loses
capacity, its internal resistance increases and thus the voltage at a
given current falls. (Note that internal resistance also increases as
the battery discharges.)
A cycle tester automatically discharges a battery at a constant current
until its voltage reaches some threshold (usually 1.75 volts per cell)
and measures the time (and thus the amp-hours) required to get there. A
sophisticated system will also recharge the battery and repeat the cycle
if desired, for measuring cycle life.
You can buy fancy computerized cycle testers, but they're pricey enough
to be beyond most individuals' budget (in the thousands of dollars). I
have heard of homemade cycle testers, but I've never seen one.
I can envision a way to make a quick and dirty load tester using a water
bucket and a voltmeter, but I've not done that either. I test my
electric car's batteries by loading down the pack with a heating element
from an old heat pump (switched with a scrounged contactor), then
measuring the individual batteries as described above.
> And related questions on chargers? 1. Has anyone tried
> one of these new battery desulfators yet?
I haven't tried one, but have heard mixed reviews. The skeptics say that
almost all battery chargers sort of pulse anyway, and the desulfators are
snake oil. On the other hand, some people are very enthusiastic. Home
Power Magazine seems to like them.
Others have reported good results from adding EDTA to the cells to
chelate the lead sulfate. There are always classified ads for this stuff
in the back of Home Power.
> 2. Has anyone tried some new technology testers yet? My
> research so far suggests that a decent modern charger new is priced from
> $300 - $400.
You mean "new technology charger?" I have recently gotten a
microprocessor controlled charger for my electric car, and I love it. It
is smart enough to squeeze significantly more charge into the pack
without overcharging it.
Smart chargers are much more important for VRR (valve regulated
recombinant, or "sealed") batteries than for the flooded batteries most
people use in their ETs. They will make any battery last longer, which
is crucial when the batteries cost $175 or $200 each as VRRs do. But the
increase in life takes much longer to pay back the extra cost (more like
$500 - $600 for the 36v smart chargers I've seen) when the batteries cost
$50 each.
Hope this helps. As mentioned above, those with ideas or experiences
contrary to mine, please post them!
David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1979 General Engines ElectroPed 24vdc
1974 Honda Civic 96vdc
1970 GE Elec-trak 36 vdc