Re: Combo Optima and Floods make 84v pack.

[Bill Dube <billdube@killacycle.com>]

At 12:05 PM 7/31/99 -0700, you wrote:
> Has anybody run together Optima's and floods, I was thinking of adding
>7 Optima's across my 14 6 volt floods. 

	This has been discussed on the list at some length in the past. No one has
actually tried it yet, however. 

	There are two separate issues. Of course they are charging and discharging. 

	When you draw from this "composite" pack, the batteries with the lowest
impedance (the Optimas) will supply the majority of the current. When you
lift you foot, there is no real difference of potential to motivate the
flooded pack to recharge the AGM pack. Thus, the AGM pack will get hammered
and will be very deeply discharged before any significant discharge of the
flooded pack occurs. The Optimas will be ruined in short order. 

	There are two approaches to this problem. You can connect your controller
to the Optima pack and then connect a smaller (custom) controller from the
flooded pack to the Optima pack. The Optima pack will have to be lower
voltage than the flooded pack so that you can flow a steady modest current
from the flooded pack into the Optima pack. 

	It has also been proposed that you can simply run the flooded pack
open-circuit voltage at the float voltage of the Optima pack. With your
set-up that would mean that the flooded pack would have one additional 6
volt battery (or the optima pack would have 6 12 volt units and 1 6 volt
unit.) A the beginning of you drive, you would connect the two packs
together. The higher voltage flooded pack would charge the Optima pack if
the Optima pack SOC became significantly lower than the flooded pack. This
is MUCH simpler than the two-controller set-up, but would not be quite as
controllable.

	No matter what discharge approach you take, charging is an additional
complication. The finish charge requirements on flooded batteries are
starkly different than the requirement for AGM batteries. The packs must be
separated for at least the finish charge. In both of the above scenarios,
the AGM pack is at lower voltage than the flooded pack. Thus, you can start
the charging process with the packs in parallel if you like. Once the
cut-off voltage for the AGM pack has been reached, you can disconnect it
from the flooded pack for it's finish charge.

	If you selected the two-controller set-up, you can use the small "tie"
controller to completely control the charge of the AGMs while the flooded
pack is directly connected to the charger. Basically, the tie controller
will do a similar job to the one that it does during discharge. Aside from
a more modest current limit, and a sequence for the finish charge, there
will be no difference in it's duties during charge or discharge.

	I think that composite battery pack are probably what will end up in
production EVs if the market ever becomes truly competitive. You can
squeeze a lot higher energy density out of a battery design if you only
must supply a modest power density. The opposite is true as well. Cost per
watt and cost per watt-hr are reduced in both cases. Thus, a composite
battery pack comprised of a design optimized for power density and a second
design optimized for high energy density will reduce the cost and at the
same time increase the range and performance of the vehicle. The
electronics gets somewhat more complicated, however. Once it all sorts out,
I believe that composite battery systems will be the likely winner.


	Now, aren't you sorry you asked?

Bill Dube
billdube@killacycle.com
Quickest Electric Motorcycle on Earth
(303) 355-4275
(303) 377-5664 fax