Re: (ET) re: battery testers

["David Roden (Akron, Ohio, USA)" <roden ald net>]

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