50% vs 50%?

[cowtown]

We usually say that a 50% DOD gets the most mileage out of a pack, but  
does it matter *where* that 50% is taken? Would you get more from a  
pack cycled from 30% to 80% SOC than you would with the same pack  
cycled 50% to 100% SOC? The charge cycle would be more efficient and  
faster.


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[Jon Glauser-2]

I have always assumed when we say Depth Of Discharge, it means from 0%
or maximum charge (given by a certain specific gravity of acid in a
lead-acid battery). Pulling more juice from the battery drops its
specific gravity to a point where chemically it shortens the life.
Remember a battery is a chemical energy storage device. Mess up the
chemistry equations and the energy output is different.

-Jon Glauser
http://jonglauser.blogspot.com
http://www.evalbum.com/555


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[zyewdall]

The problem with doing that with a lead acid battery is sulphation.  It
seems that this really isn't a problem with EV's, but if you never fully
charged them, it could be. Any time it isn't at 100% SOC, it's sulphating...
very slowly... if you recharge it the same day, it's not even noticeable.
But... if you Never charge it back up to 100%, I think you'd start seeing
sulphation.  This is a big problem with off-grid PV systems in the winter,
you start cycling between 40 and 70% or something like that, instead of 70
and 100%.... and it starts sulphating.

I think that with Lithium's, it might be better though... I gather that some
types of them -- not sure if the LFP ones, are not happy being float charged
at the 100% full voltage, and keeping them to no more than 90% might be
good.... there are probably people on here who actually know the answer  :)

On 7/23/08, cowtown@... <cowtown@...> wrote:

>
> We usually say that a 50% DOD gets the most mileage out of a pack, but
> does it matter *where* that 50% is taken? Would you get more from a
> pack cycled from 30% to 80% SOC than you would with the same pack
> cycled 50% to 100% SOC? The charge cycle would be more efficient and
> faster.
>
>
> _______________________________________________
> For general EVDL support, see http://evdl.org/help/
> For subscription options, see http://lists.sjsu.edu/mailman/listinfo/ev
>
>

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[cowtown]

> The problem with doing that with a lead acid battery is sulphation.  It
> seems that this really isn't a problem with EV's, but if you never fully
> charged them, it could be. Any time it isn't at 100% SOC, it's sulphating...
> very slowly... if you recharge it the same day, it's not even noticeable.
> But... if you Never charge it back up to 100%, I think you'd start seeing
> sulphation.  This is a big problem with off-grid PV systems in the winter,
> you start cycling between 40 and 70% or something like that, instead of 70
> and 100%.... and it starts sulphating.

Thanks, that makes sense. Does the same hold true with AGM and gel-cells?


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[Gordon Niessen]

That depends on your battery chemistry.  NiMH do better when you don't
got to 0% or 100%.  The Prius trys to keep the battery in the 60%-80%
range and expect the life of the battery to increase more then 100x.  
Lead Acid like more of a charge, but even on them the last few percent
put the battery under more of a stress.

cowtown@... wrote:

> We usually say that a 50% DOD gets the most mileage out of a pack, but  
> does it matter *where* that 50% is taken? Would you get more from a  
> pack cycled from 30% to 80% SOC than you would with the same pack  
> cycled 50% to 100% SOC? The charge cycle would be more efficient and  
> faster.
>
>
> _______________________________________________
> For general EVDL support, see http://evdl.org/help/
> For subscription options, see http://lists.sjsu.edu/mailman/listinfo/ev
>
>  

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[Lee Hart]

cowtown@... wrote:
> We usually say that a 50% DOD gets the most mileage out of a pack, but  
> does it matter *where* that 50% is taken? Would you get more from a  
> pack cycled from 30% to 80% SOC than you would with the same pack  
> cycled 50% to 100% SOC? The charge cycle would be more efficient and  
> faster.

For lead-acid, aim to keep them between 50-100% SOC, and be sure the
most hours are accumulated with them as close to 100% as possible. Mild
brief overcharging is necessary for best life.

For lithium, stay between 20-80% for best life. It is vitally important
to *never* let a cell go dead or overcharge it.

For nimh, aim for 40-80% SOC for best life. Mild brief overcharging is
OK. Running dead is OK.

For nicads, aim for 20-100% for best life. Mild continuous overcharging
is OK.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net


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[EVDL Administrator]

On 23 Jul 2008 at 23:08, cowtown@... wrote:

> Does the same hold true with AGM and gel-cells?

Yes.  The issue is the fundamental principle of a lead battery.  It doesn't
matter whether the electrolyte is free to slosh around, is absorbed in
spongy stuff, or is filled with silica gel glop.  

It's not clear to me, though, that there's no way to manage this problem.

Suppose, for example, you charged your EV to 80-90% SOC all week, and then
to 100% on the weekend (perhaps equalizing too).  You should gain cycle life
from not overcharging, but might lose some capacity to gradual sulfation
from chronic undercharging.  Is the net a gain or loss in total amp-hour
throughput?  

David Roden - Akron, Ohio, USA
EVDL Administrator

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