Re: [EVDL] Modified K&W BC-20 questions
The series connections in the primaries of several transformers is corrected
for connecting to a higher input AC voltage, or increasing the out voltage
on the total sum of the windings which will increase the secondary voltage.
Lets say you have two transformers that have a 120 volt primary with leads
mark L1 and L2 and a 120 volt secondary leads mark T1 and T2. This is
normally a 1:1 ratio transformer.
The normal way to connect to this transformer is to supply 120 vac to the
primaries to each transformer, which parallels the primary leads L1 to L1
and L2 to L2.
The secondary of each transformer will have a output of 120 volts.
Now is we series the primaries of the transformers by connecting one leg of
the 240 volt input to L1 and connect L2 of the first transformer to L1 of
the second transformer and the second leg of the 240 volt input to L2 of the
second transformer, you will still get 120 volts out each transformer
If we connect the secondary of each transformer in series like we did with
the primary, you can also get 240 vac out or can get 120/240 volt out if you
center tap the secondary series connections that go between the two
Another way to connect the 120 vac input power to two transformers that
primaries are series together, is to connect 120 volts to the first
transformer primary L1 and L2. Connect the first transformer L2 to the
second transformer L1 and no connection to the second transformer L2.
You can apply the 120 vac power to the primary of the first transformer and
using a volt meter, you will read 240 volts between the leads of the L1 of
the first transformer and the L2 lead with no connection in the primary of
the second transformer.
This is what is call a boast circuit or some transformers call a potential
transformers which have several taps in the primary.
So the series connections you have for the 108 v battery pack should
increase for a 120 v battery pack.
Also check to see if you have the correct resistor for the 120 v battery
pack as listed on page 7 of the manual.
Tightening all the wire connections and inspecting the wire for crack
insulation and etc, should be ok to fire it up or give it a smoke test.
Using Uve's EV calculations, a 120 v battery psck of T-145's should give you
a range of 66 miles at 10%D0D, or 33 miles at 50%. A pack of T-105's would
be at about 15 miles 50%D)D at a speed of 60 mph with a vehicle weight of
4260 to 4460 lbs.
----- Original Message -----
From: "Hunter Cook" <[email protected]
To: "Electric Vehicle Discussion List" <[email protected]
Sent: Tuesday, October 02, 2007 8:18 AM
Subject: [EVDL] Modified K&W BC-20 questions
> Hello again,
> Some of you may recall that I've got an old K&W BC-20 charger trying to
> charge a 132v (used to be 144v) pack, which should not (and in my
> observation, does not) work very well. Some very knowledgeable folks
> have said it's good only to 108v, or 120 with the LB-20 booster. So far
> this has all sounded very reasonable, as in my experience the charger
> will only bring the pack up to about 140-145v.
> This morning I really got in and looked at the way things were wired up
> for the first time. It appears that it is wired with a larger
> transformer in an LB-20-style boosting role.
> Specifically, there is a Signal Transformer model MPI-900-40 wired up
> exactly the way the BC-20 manual shows an LB-20, except that the MPI is
> using 2 20v outputs together in series rather than just the one 20v
> output of the LB-20. Here's the pdf of the manual, which has a good
> diagram of it: http://evdl.org/docs/bc-20.pdf
> I also found a loose connection from the plug to the MPI. Complete with
> burn marks. Awesome. This may be related to the truck flipping the
> breaker this morning when I plugged it in, which is what prompted me to
> take a more serious look at where the wires were going.
> So...this brings me to a few questions:
> 1. Should this transformer + K&W setup work for a pack of my size? It
> seems logical enough I suppose, assuming he also changed out the
> internal resistor (haven't had a chance to check, and not sure what the
> value should be as the target voltage is higher than the table goes in
> the manual) and assuming the other components in the BC-20 can take the
> extra voltage. Since the vehicle is pretty old, I guess they can.
> 2. How bad is it that I've got minor burns on my input terminal strip
> for the transformer? I tightened up the connection and it works again,
> though I didn't try for long.
> 3. Is this dangerous?
> I'm still in the market for a new charger, no doubt about that. But I
> hope I can still keep limping around on this one for a minute. I do need
> to move the truck about 6 miles this week (from my old house to my new
> one) which believe it or not will probably require a charge in the
> middle. I'm not really equipped to tow it.
> Thanks again for all the help everybody's been giving me.
> For subscription options, see
For subscription options, see