Willie2 via EV wrote:
Somewhat against my will, I just did some lead battery shopping. T105s
are about $150. The "minutes at 75 amps" rating seems most appropriate.
That would be just under 100 ah. From what you say, it appears those are
usable amphours. SAMs batteries, with presumably less capacity and
longevity, are about $90. So, a pack of six T105s would be about $900.
So the T105 is $150/(6v x 75a x 1.75hr) = $0.19/wh.
The Sam's Club is $80/(6v x 75a x 1.75hr) = $0.10/wh.
Negatives: corrosion, watering, lower performance as SOC decreases,
perhaps 1/3 the life of lithium, "all or nothing" major replacement.
Life would be about 800 cycles for the Trojans, and about half that for
the Sam's Club batteries (with a 75a load, discharged to 1.75v/cell,
barring any abuse from over-charging, over-discharging, or incorrect
watering).
I'm not sure what your "all or nothing" replacement means. You can
replace individual batteries if they fail early.
Compare to 12 TS-LPF100s: about $1500 plus about $300 worth of BMS
stuff.
$1800/(12 x 3.2v x 100ah) = $0.47/wh. About 2.5 times the cost/watthour
of the Trojans, or 5 times the price of the Sam's Club. They would have
to last 2.5-5 times longer to reach the same cost/mile.
I used 100ah; but I doubt you can get even 75ah out of them with a 75a
load before the voltage falls under 2.5v/cell. I don't know how these
particular Thunderskys would test; but the older 90ah Thunderskys I
tested had significantly higher internal resistance than 6v golf cart
batteries. They weren't good for 75a continuous / 500a peaks; but more
like 25a continuous / 100a peaks.
Negative: nightmare of wiring maintenance, "all or nothing" major
replacement.
If you have a good BMS you should be able to replace individual cells.
Whether the BMS wiring is a "nightmare" depends on the situation.
Compare to 5 20ah ebike batteries @ $285. Total: $1425. Experience so
far indicates as few as 2 20ah ebike batteries can be used at a cost of
$570.
$1425/(5 x 36v x 20ah) = $0.40/wh. That's barely any cheaper than the
Thunderskys.
The longevity of ebike batteries is a BIG unknown here.
Yes; you'd have to test to know for sure.
I believe it is likely that the above can be scaled to larger vehicles.
If the golf cart projects are successful, I will be exploring higher
voltages and capacities on larger vehicles.
One unknown is the internal resistance of these small cell packs. Some
may be good; some horrible. Ebikes don't draw much current, but a golf
cart does!
I will not accept the contention that lithium batteries should be
limited to 50%.
OK; so that's your hypothesis. Now do the testing, and see if it's correct.
As you say, these Ebike batteries aren't all that expensive. Buy one,
and rig up a life tester. It would:
- Discharge the battery with a load representative of what your actual
load will be, until it reaches your desired "dead" cutoff voltage.
- For lead-acids, 1.75v per cell under load is usually used.
- For lithiums, there are no standards. Try 2.5v/cell as the cutoff.
Or go lower if you think life won't be reduced by deeper discharges.
- Charge the battery to whatever fully-charged criteria you expect
to use in your application.
- For lead-acids, "full" is typically when the current falls to 4%
of its amphour rating at 2.5v/cell.
- For lithiums, again there are no standards. Maybe use the "free"
charger/BMS that comes with the Ping cells? Or for LiFePO4 cells,
try 3.7v until the current falls under 4% of its rated AC capacity.
- Let it cycle, until the capacity falls to some reasonable fraction
of its original capacity.
- For lead-acid, 80% of original capacity is usually used. You can
obviously use them longer; but without a BMS, the capacity usually
falls fast once some cell starts getting weak.
- For lithiums, you can use the same 80% limit, or keep testing to
see what happens if you use them longer.
--
A truly excellent politician will tell you everything you want to hear.
A truly excellent engineer will tell you the truth. -- D.C. Weber
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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