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

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

Warmer battery chargers faster due cell internal efficiency. This is
with constant power charger (until BMS naturally toggles it down at
near full). Less voltage climb higher are the amps. And ampere hours
are quite accurate with Lions (as most of you know already).

But drawback is the cumulated aging effect of Lithiums when heated.
Effect is there always but the steepness of the death is proportional
to the heat.

Later on you'll need even more heat to get same resistance.

If you know you need power it is more reasonable to have the heat
generated in controlled manner instead of just sucking the amps.

External heaters have issues (even heat distribution inside the cell).
Internal heating methods are more efficient.

-Jukka


2010/8/28 Cor van de Water <[email protected]>:
> 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 =3D=3D> 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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