27 Apr 2003, 16:20, JIM HOLSONBACK (1:123/140), wrote to MATT MC_CARTHY:

Hi JIM.

 JH> Hi, Matt.
 JH> You apparently posted this on 4/24, and this is one that I was 
 JH> missing.
 JH> I finally went back to Docsplace and reset msg pointers yesterday 
 JH> 4/26, and downloaded the last 375 or so msgs in this Echo, and it 
 JH> showed up this time.

I often wonder why that happens.  Ever since I've been in Fido, I often see
snippets of quotes in replies, and no original anywhere to be found.

 JH> -=> MATT MC_CARTHY wrote to MARK LEWIS <=-
.............

 MM>> With a lot of care, you CAN use a car charger, but you MUST use a
 MM>> current limiter.  11AH batteries should be charged at 1.1A for 10
 MM>> hours.

 JH> In my ignorance here, I have an old Sears Craftsman 6A automotive 
 JH> type charger, 6V/12V, and I've used it a number of times in 
 JH> testing/charging gel cels. From memory, when I connected to the 
 JH> commonly-used 12V 7AH batteries which would take a charge, it would 
 JH> typically start out showing about 3A on the meter, then taper down 
 JH> pretty quickly to 1A or so. AT those charge rates, battery would 
 JH> typically get warm, but not hot enough to split the case, or any other 
 JH> disasters.  I've heard of gel cels 'blowing up" when being charged too 
 JH> rapidly.

I've had plenty of lead-acid batteries blow up, but never a gel-cell,
although I have several that are dead and now at least an inch larger than
their original dimensions.    

 JH> FWIW, and IME, 'bad' gel cell batteries I've seen would generally 
 JH> not even start to accept a charge - the bad ones I've seen seem to 
 JH> have failed to an "open" condition.

That's generally the type I 'play' with.  They take a LOT higher charging
voltage to get started charging.  I've got a little 6V 4.2AH sitting in
front of me now that I took out of 'something' several years ago and put a
check mark on top, as it was a good one.  A month or so ago when I found a
use for it, it read -0- volts.  I put it on a 6V charger for a week and
couldn't even read a microamp of charge.  I switched it to a 12V charger,
and it began charging at about 23 microamps, and after two days went up to
40ma and began warming.  The 6V charger still would do nothing, so I kept
it on the 12V charger and put an auto stop light bulb in series with it,
which cut the charge to 30ma.  It's been charging that way for three weeks
now, and I can put my test light (a 6V 25ma bulb) on it and it will run
three days before dropping below 6V.  Not much of a load, but it's just a
test for fun anyway.  Maybe I'll just use it for a night light...   :-))  

 JH> So I think Mark is pretty much safe if he sets his charger to the 
 JH> 2A rate.

 MM>> Gel-cells have a thermal runaway problem when they get near their 
 MM>> full charge point, they will overheat and then short out, killing 
 MM>> the battery and causing a lot of swelling of the plastic case.

 JH> I haven't seen that problem with this Sears charger - maybe it 
 JH> has current-limiting circuit which keeps the thing from getting away? 

That's possible, but since the 6V position usually has a LOT more amps
output for the old 6V car battrties (at least twice what the 12V batteries
will take), I doubt it.  The problem is inherent to gel-cells, in that as
they heat, their internal resistance drops.  That's one of the main reasons
for the complexity of the chargers in UPS units.

 JH> The only swollen and even cracked battery cases I've seen were in 
 JH> batteries already apparently 'dead', which wouldn't begin to take a 
 JH> charge.

Once they are swollen, they _are_ history!

 ml>> what's the best way to load test them to see if they are still
 ml>> good?

 MM>> Put a 12V automotive bulb on them that draws about two Amps, like 
 MM>> a dome light bulb.  A tail light will run about three Amps, a stop
 MM>>  light about 5 Amps, and a headlamp about 8 Amps.

 JH> Hee. Our Director where I volunteer, after they appear to be 
 JH> recharged, just says to put them in a UPS and plug a 3/8" electric 
 JH> drill into the UPS.  If the drill spins OK, he says that is a good 
 JH> battery.  I don't always agree with that fellow, so I put them into a 
 JH> UPS, plug in a computer and monitor, and then unplug the power - - if 
 JH> the 'puter keeps running, I figure that is at least a "good
enough" 
 JH> battery  for the stuff we give away.

_Your_ test is more in line with what we do.  Not many customers want to
run a drill when the power goes off...   :-)

 ml>> what does the 11Ah (Amp hours) really signify?

 MM>> Exactly what it says, Amps x hours.  That 11 AH should put out 
 MM>> 1.1 Amp for 10 hours, or 11 Amps for one hour, or 5.5 Amps for two 
 MM>> hours, etc.

 JH> OK but I think the battery capacity isn't that linear - - 
 JH> probably tapering down during more rapid discahrge rates.

It does taper off a bit, but it also starts out almost an equal amount
higher.  If the batteries are properly made and marked, the AH rating
_should_ be taken at the half-discharge point.  In effect, it evens out.

 JH> So what could it be expected to put out for 15 minutes or so, 
 JH> which I think is about what most of these consumer/small office UPS 
 JH> units are designed for?

Hmmm...  My dealer has a APC UPS-650 box sitting on the floor next to my
chair, and it has that label on a sticker on the top of the box.  I glanced
at it several times Saturday night, but kept forgetting to get off my butt
and actually read it!  Will make a note to try reading that tonight when I
go over

 JH> The APC 420 Smart UPS Pro unit I have here just uses a 12V 7AH 
 JH> battery - - just 84 watt-hours of power. Figure in my system unit and 
 JH> this 17" monitor, and if those suck up 240W or so of power, I figure 
 JH> I'm down in the 15-minute range of reliable backup power, with this 
 JH> battery now a couple years old. Looks to me like this type of UPS unit 
 JH> is clearly designed just to give a user time to shut things down just 
 JH> after the power goes off, not to provide multi-hour protection and 
 JH> continued operation.

Exactly, just enough time for an orderly shutdown is what was intended.

 JH> Where to find a 2 Volt charger?  

I cheat!  I run mine from a Variac.   :-)

 JH> I dunno if Mark's charger has an ammeter on it, but I wouldn't 
 JH> hesitate to hook up the good old Sears 6A charger here to one of those 
 JH> 12V 25AH battery packs, and watch the ammeter carefully, esp at the 
 JH> start.  If all went well, I could go back and check individual cells 
 JH> later.

That would work well for those heavier batteries/cells, but it is still
difficult to get a good balanced charge on cells in series (or parallel for
that matter).

 ml>> when it all comes down to the end, i'm wanting to tie these all
 ml>> together in parallel to go for 12volt, ??Ah so as to boost my
 ml>> uptime on my UPS' when commercial power is off...

 MM>> Sounds good.  You have three 12V {at} 11AH, for a total of 12V {at} 
 MM>> 33AH, and two packs that together will give you 12V {at} 50AH.  
 MM>> With_everything_ connected, you would have 12V {at} 83AH.

 JH> And a rat's nest mess of 5 batteries and batt-packs spread all 
 JH> over the floor?

That's one reason I switched to car batteries, but he apparently wants to
use what he has on hand already taking up space.

 MM>> Your computer will need about 120 Watts to run with monitor, a 
 MM>> wee bit more to start up, but lets not worry about that, as it 
 MM>> will already be running in most cases.  Excluding lots of annoying 
 MM>> formulas, Watts is basically Volts times Amps.  To get 120 Watts 
 MM>> from a 12V system, you will need to be pulling 10 Amps.
 MM>>  If you do that for two hours, you have used 20 AmpHours.  If all 
 MM>> your cells are working and fully charged, you should have about 
 MM>> 83AH, and get about eight hours running time for the FULL 
 MM>> computer.  Triple that with the monitor turned off, or about 24 
 MM>> hours.

 JH> Sorry, this sounds pretty optimistic to me.  All cells fully 
 JH> charged, and all able to deliver AH as specified for new ones? 

I didn't notice that the age of the cells he has was mentioned.

 JH> And where do you get 120W for a full system with monitor?

From the meter on my power strip.  The monitor takes about 80 watts, and
the computer about 40.


     Good luck...  M.

--- Msged/386 TE 06 (pre)