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Re: [SPAM] Re: (ET) Charger repair.

Let me run outside with my P3 meter.
Ok, the tractor's been charging for awhile, all 3 of my LEDs are on (meaning 14.6 volts per pair of batteries) so it's fully charged. 

Line voltage: 118v
Line amperage: 2.13a
Watts in: 176
Power factor: .69
Watts in (PF corrected, sometimes called VA) 260 VA
So at full charge, the transformer does taper off to about 176 watts. At 45 volts (assuming a 90% charger efficiency) the pack is probably seeing about 3.52 amps; my guess is around 3 amps when you take the rectifiers into effect as well.
This kind of makes sense actually; if you continued to charge a 36 volt pack at 45 volts at 25 amps you would boil the daylights out of the electrolyte. One of the features of the transformer in the Elec-Trak (and why it was expensive at the time) is that it does the neat little trick of dropping current as voltage goes up. This is why it's not totally critical to measure the charge time; a bit of overcharge won't hurt the pack.
Oddly enough, this would indicate that the transformer could probably safely charge AGM batteries as well, since the charge current tapers off as the batteries hit full.
I'll check again tomorrow when I flatten the batteries, but my guess is it doesn't pull over 1000 watts when batteries are dead, and the power factor probably goes up as the voltage on the pack goes down. 

Chris


Markus Lorch wrote:

Ralph wrote:



I think the ET charger might work ok, but you may have to change the
capacitor or add a 
resistor in >series with it to tune it to 50Hz.   A 1000va step down
transformer would 
only give you about 8.4 amps at 120 volts.  I think you will need more
then that for 
bulk charging.



Ralph,

oops, I think I made a miscalculation. I thought our charger puts out 25A
for the
36V system which would be 900W, if I take an efficiency of 0.9 then I get
1000W.
What I didn't think of was that the charger actually does its bulk charging
around
39-40 V and probably topps out at 43-44V. Now from what I read somewhere
else the transformer based chargers don't supply full current anymore at the top-end voltage. Is this correct? Does it have a "natural" tapering built in that 
reduces
the charge current at higher voltages, when the batteries reach full (and
thus limits the total power in?). It if does put out 25A at 44V then thats 1100W which 
would
be too much for the step down transformer I had in mind. What happens if
such a small tranformer is slightly overloaded? Could I simply add a cooling fan? 

Or am I falling into the VA vs. Watt trap? I guess then a 1000VA 
transformer
may not be sufficient to supply a 900W load. My knowledge about AC inductance is 
a bit rusted, but I remember something about reverse current flow/phase
shift that doesn't add to the real power but still heats the wire. So if they 
count
this current into the VA rating on the transformer then it probably is 
quite
a bit to small. I found a larger one (15A 120V output) but its twice as
heavy
and also almost twice as expensive after shipping. Maybe I should go back 
to
using my small 12V "modular" charger array. 
Markus


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