Re: [EVDL] warm batteries ==> warm charger and cord ends

Re: [EVDL] warm batteries ==> warm charger and cord ends

My experience is that warm batteries charge at lower voltage
so if your charger is current limited then it will produce
less power (same current at lower voltage) which is one of
the main reasons that warm batteries charge more efficiently.

Now, depending on the design of your charger, it may use a
"lightdimmer" approach to get power from the grid to charge
its internal capacitors to produce the output voltage, which
is now lower and thus the time that the input is active must
be shorter. But since the power has only dropped slightly,
the average effective input current must be about the same
but it is running in shorter bursts, so the losses are higher
and the wires get hotter, which could even cause the current
consumed from the grid to be higher (the higher losses cause
the voltage drop to be higher and the higher current compensates
for the power loss of the voltage drop).
Total efficiency of the charger is lower due to its design which
is not optimized to deliver lower output voltages efficiently.

It may not be intuitive that wires get much hotter when having
the same average current, but think of it as when the pulses are
half the time, then the current in the pulse must be twice as
high. This however creates four times as much loss during the
pulse (double the current in the same resistance is double the
voltage, so the power of the loss is double voltage times double
the current, or four times the power)
but since the pulse is half the time, the total power loss is
half of those four times the power so twice the energy compared
to the situation with the original pulse duration.

To use a Power Correction and always get optimal efficiency,
either your pack voltage must be higher than the peak of the
AC grid voltage, or you must use a two-stage approach, where
typically the first stage brings the AC to approx 400V DC, so
it can work from any voltage up to approx 260V AC and the second
stage is a DC/DC converter (often including isolation) to bring
the raw 400V (with ripple from the PCF function) to a stabilized
DC voltage of your own liking.

Regards,

Cor van de Water
Director HW & Systems Architecture Group
Proxim Wireless Corporation http://www.proxim.com
Email: [email protected] Private: http://www.cvandewater.com
Skype: cor_van_de_water IM: [email protected]
Tel: +1 408 383 7626 VoIP: +31 20 3987567 FWD# 25925
Tel: +91 (040)23117400 x203 XoIP: +31877841130

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Chuck Hursch
Sent: Thursday, August 26, 2010 1:35 AM
To: Electric Vehicle Discussion List
Subject: [EVDL] warm batteries ==> warm charger and cord ends

Been in the midst of a mini heat wave here in the Bay Area. Was in the
mid-80s here at the apt for the hot point yesterday, 90s at work (San
Rafael), and got up to 104 (I heard) in Novato (further north and
further from the Bay or Pacific). I expected my pack to be warm, and
charger and cord ends / connectors to be warmer also.

Pack was sitting at about 94degF (the warmer batteries were probably
closer to 100) shortly before I plugged it in for the night. Usual 12.6
mile roundtrip to the evening job. Get the bluetooth connection set up
for monitoring the charge voltages from the apt. upstairs. Went back
down after half an hour or so to check on things before turning in for
the night.

Yep, charger (Zivan K2) warmer than usual. AC connections quite warm,
warmer than usual. KillaWatt meter reads low 13A range instead of mid
to upper 12s early in the charge cycle (putting about 9A DC into the 96V
floodie pack), power factor read from the KillaWatt in the high 0.60s
instead of the usual 0.70-0.75. Odd: wattage seemed a bit low: ~1050W
instead of the usual 1060-1100W; AC line voltage under load was its
usual 115-115.5V, give or take. Everything warmer up to and including
the plug in the outlet in the ceiling over the carport. I bet the
wiring all the way up to the circuit panel in the apt. was a bit warmer
too. Could be an unknown greater crest factor, or temperature sensitive
KillaWatt meter, but things ARE warmer.

This always seems to be the case when things are warm. More so than
just it being 10 or 20 deg warmer outside. Many years ago someone told
me that the pack is stiffer (lower internal resistance) when warm, and
I've been comfy with that for quite some time. I also heard many years
ago that a good way to burn up a charger was to put it across an Optima
(which are very stiff). I assumed in that case the charger was not well
current limited, so the current would go quite high as the Optimas
lapped it up. But my Zivan is more or less constant current in the bulk
charge mode, so it's DC current will stay within its usual range. I
would think a stiff pack in that case would be easier on a charger and
keep the cords cooler, since the packs voltage would stay lower for a
given DC current, thereby not making the charger work as hard to push
the current in. I know that as the pack voltage goes up, the AC side
current goes up as the DC current droops a bit (down towards 8.5A at end
of constant current bulk charge stage). But cooler charger and cords
are not what I feel on warm nights. What's the juice?

Oh yes, the warmer it is, the lower the charges' total kwh as reported
from the KillaWatt, which makes sense. Was 4.97kwh for the charge,
which ran midnight to 6AM (timer shutoff). Pack is cooler, the kwh will
be 5.10 to 5.25. Lower internal resistance of the warmer pack implies
less energy wasted as heat. Also have to factor in the Zivan's
temperature correction for the 2nd stage constant voltage, and how that
might be affecting current and total kwhs.

Muttering in my thoughts, hopefully not too dumb, Chuck

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