Care and Feeding of AGMs (Was: Honda City)

["Bill Dube'" <bdube@BOULDER.NIST.GOV>]

>Jeff Owen writes:
>>
>>
>> finishes. After phoning Hawker, I now switch the charger off shortly after
>> the CV stage when the voltage is about 151V - 153V. Hawker suggested letting
>> it occasionally running the full cycle (every 2 weeks)as the occasional high
>> voltage won't hurt. They also suggested not using regulators or pairing up
>> all the batteries.
>
>Any idea why Hawker recommended *not* using regulators or pairing the
>batteries?

        I doubt that Hawker would make this recomendation. It goes
completely against the grain. I suspect that your local distributor, not
Hawker, made these incorrect recomendations. Your pack will go south in no
time if you don't run regs. Hawker publishes a very complete EV applications
booklet. As I recall, it says to buddy up parallel strings at least every
other battery (24 volts.) I'm not sure if it says explicitly to run regs.

         Also, I can't see how you could possibly expect to stay within the
specified charging algorithm without regs of some sort. The voltage on
"early bird" batteries that fill up first will go wild. I believe that the
max voltage for charging is 15 volts in the spec.

        I know that Optima will only warranty batteries in an EV for a month
if no regs are used. Otherwise, they warranty the pack for a year.

        Buddying up your pack makes sense if you think about it carefully.

        Let's say that you have three weak batteries in your pack somewhere.
If you have two separate parallel strings, it is likely that you will have
at least one weak battery in each string. This means that when you get near
the bottom of the pack, these batteries will poop out and limit your range.
Worse yet, you could continue to drive and  then reverse a cell or two in
these weak units.

        However, if you had buddied up the pack, it is highly unlikely that
two weak batteries are buddies. This means that the strong companion battery
will bolster up it's weak buddy. Your range will be greater and you will be
much less likely to reverse a cell. Buddying up you pack only makes sense.

        The best way to buddy up the pack is to charge all your batteries
100%. You can do this by using a bench power supply to carefully top each
one up individuallly. After they are all charged, wire them all in series
and put a resistive load across the group. (I like to use a kiln or electric
baseboard heater.) Constantly check the voltage on each battery in this
string. Pick a "discharged" voltage of some sort. I like to use 10.8 volts.
When a battery in your string reaches this voltage, remove it from the
string and put it against the wall. Continue to discharge the remaining
string. Pick out batteries and line them up as they discharge.

        When you are all done, the batteries will be sorted from weakest to
strongest. None of the batteries should be remarkably weaker (>20%) than the
rest. If they are, you should consider turning them in for replacements.
(Recharge the batteries, but be sure to keep them in order.)

        Buddy up the batteries from opposite ends of the line-up. The
weakest with the strongest. The next weakest with the next strongest, and so
on. This is the best way to build a pack that will give the greatest range
and likely the best cycle life.


             _   /|
             \'o.O'              Bill Dube'
           =(___)=           bdube@boulder.nist.gov
              U

Care and Feeding of AGMs (Was: Honda City)