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Re: (ET) New Member


On Wed, 2 Jul 2008, David Roden wrote:


On 2 Jul 2008 at 15:31, Michael S Briggs wrote:


... much lower internal resistance (higher net efficiency, no voltage
sag under load, no significant drop in performance as the battery
capacity drops, ability to rapidly recharge the pack, etc.).


AGM lead batteries fit that description pretty closely.  They have very low
internal resistance and can produce some ferocious currents.  Look here :

http://www.evdl.org/pages/plasmaboy.html

But it doesn't matter.  To characterize the internal resistance of even 
golf
car batteries as "high" is simply incorrect.  They can deliver thousands of
amps.  That's far more than an ET can ever use, especially if you're just
mowing the lawn.
High internal resistance doesn't mean a battery is *incapable* of producing large currents - it means that with high currents there is more of a voltage drop across the internal resistance, thus the battery gets hotter, and more energy is lost over the internal resistance (i.e. the efficiency is worse). For example, it's well known that while T-105s are rated at something like 200 AH at a 6 hour discharge rate - but at the rate current is pulled out of them in ETs (and EVs), the actual discharge capacity falls to more like 100 AH. Why? High internal resistance effectively translates into a high Peukert number, as more energy is lost over the internal resistance at high discharge rates (and high charge rates), and thus not available for use by the external circuit being powered. So sure, lead-acid batteries can still dump massive currents - enough to weld a metal bar to the electrodes - but that doesn't mean that the internal resistance is low, and thus the batteries have a high efficiency. That said - yes, AGM lead acid batteries are much better than flooded cells in terms of lower internal resistance (and thus higher efficiency). 


for basic mowing, the additional weight of lead-acid batteries should
be completely unnecessary.


Agreed.  I take it you never plow or move things with your tractor.  In 
that
case lighter is better.


Yup.


with the low internal resistance
of advanced cells, the result is that you can get a "roundtrip"
(charge-discharge) efficiency over 90%, even with high loads


The round trip efficiency of a lead battery is about 90%.  Most of the
losses are from heat and electrolysis in the absorption phase.  From 20% to
That is only true with a low discharge rate. Look at the voltage meter on your tractor - the voltage drops significantly as current increases. Why? Because more of the internal resistance - at higher currents, more voltage drops over the internal resistance, thus more power is being dumped over it.
I'm sure you are familiar with Peukert's number, and how the discharge capacity (in AH) drops significantly with increased discharge rate (current) with lead-acid batteries, right? That has a *big* impact on efficiency. Say you charge a T-105 to full capacity, which lets say is 200 Ah - and treat the voltage as 12 V for simplicity. If you discharge it slowly, you can get close to that full 200 Ah out of it (so the energy extracted is 12V*200Ah=2.4 kWh). If the charging process was 90% efficient (just to pick a number - you'll see where I'm going with this), then you would have used 2.4kWh/0.9=2.67 kWh to charge it. But, let's say you use it in your ET in a high power application, such that you are drawing maybe 15-20 Amps out of it, and the discharge capacity drops to 100 Ah. How much energy have you gotten out of your battery then? 12V*100Ah=1.2 kWh. So, while it would have taken the same 2.67 kWh to charge it, you now only got 1.2 kWh out, so an efficiency of 45%. That's the issue with lead acid batteries - the significant internal resistance (high Peukert's number) ultimately translates into a low efficiency when used in high current applications. You are correct though that AGM batteries are better than flooded batteries in that regard. 


NiCd is somewhat lower, 80-85%.
The difference here is that NiCds have much lower internal resistance (well, not all types, but some too - illustrated by the much lower voltage sag at high currents talked about by those on this list who have NiCds in their ETs) - so the efficiency doesn't drop nearly as much with high current as it does with flooded lead acids. 


The greatest share of the inefficiencies exist in the charger and will be
essentially the same for other chemistries.  These can be minimized, but 
the
cost to do so is usually more than most users are willing to pay.


That is only true for low current applications.


Again, that applies to *current* Li-ion batteries. I'm talking about
advanced Li-ion batteries with solid electrolytes and lithium iron
phosphate cathodes.


It's easy to make claims about something that isn't available yet!  I've
been following EVs since the late 1960s, and the "miracle battery" has
always been about 2 years away.

As the song says, there are liars, damn liars, and battery salesmen.   ;-)
And I am neither, and don't get my information from any of them - my information comes from closely following and evaluating the physics research going on in Li-ion battery development. Again, I'm not intending to tell people that *they* should hold off on replacing old batteries if they need to, or that there definitely *is* a "miracle battery" coming - I just figured people might want to know my view on it, since I am very familiar with the research in the field, and feel it is fairly likely to have an impact on electric vehicles within the next few years. 


I'm not telling people they should abandon lead acid batteries - I'm just
telling people what *I* am choosing to do, based on my knowledge of the
state of Li-ion development.


Go for it!  Be sure to report back.  We'll see you in 10 years.  ;-)

It sounds like you're an electrical engineer interested in lithium for its
"newness" and able to deal with the challenges of integrating it into an
existing device.  It IS fun to be on the leading edge of technology, but 
not
too helpful for the majority of users who just want to keep the grass cut
and the driveway clear.
Nope, I'm a physicist (former engineer) whose research revolves around energy production and storage (my primary focus is nuclear fusion and fission - but for the purpose of transportation, being able to store that electricity is critical. Thus the reason I also closely follow the development of new battery technologies). But yes, you are right in that my interest and motivation is likely different than other peoples'. I wasn't intending my post to be advice telling other people not to get other batteries, but figured that other people might be interested in what is going on in battery development. 

Mike