Miata Mailing List: January 1997, Message #250
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   From: (none)

Subject: Re; Battery and Idle... related?

   Date: (none)



Miata-battery related erratic idle is caused by its gel battery

characteristic of refusing to come up to full operating voltage until it

has actually accepted a change.  For this gel battery it takes charging

time, and not just more charging current as do the standard wet cell car

batteries for the operating voltage to be achieved.  



This characteristic has needlessly caused many Miata owners to buy a new

battery when the tow truck jump didn't get the car going.  If these gel

batteries are down it makes almost no difference in actual stored charge

if the charging current is 2 amps or 50 amps.  All the 50 amps will do

is over heat the battery, but it still will not charge until it's ready

to do so.  I've found the quickest way (around 14 minutes) to get a

charge into these completely dead batteries is to watch the voltage and

raise a continuously adjustable charging current as the voltage rises,

but this type charging equipment is usually not available.   So, the

next best thing is to only charge a real dead Miata at about 2 or 5 amps

for 30 minutes.  This is usually enough to get, and keep, the Miata

going (put an overnight charge on it when you get home).  Tow trucks

don't know to do it this way -- they will sell you a new battery you

don't need.  

  

If the car sits unused, the fix for the Miata's gel battery is to keep

it charged up with a low level charge (continuous ½ amp is near ideal in

cool weather).  If this occurs with a new battery, the first fix is to

not buy an new battery.  If you have purchased a new battery (that you

surely didn't need) then the fix is to put the new one on a 2 or 3 amp

CONTINUOUS charge for about 10 hours.   If you park your car where you

can't keep a charger on it, then go back to where you bought your new

battery and get your old one back -- it's still good.  Keep that old

battery on charge and swap it out with the new one when the new one goes

dead in the car from sitting.



Miata-battery related erratic idle seems to be caused by the alternator

(at a distance from the battery, i.e., at lot of electrical resistance

in the connecting wires and ground-loop currents) running for all it's

worth trying to overcome the gel-battery's reluctance to raise it's

voltage.  This seems to set up a feed back path that the alternator's

regulator has a hard time keeping up with and the engine's computer just

can't quite figure out what to do when this occurs.  I've spent a little

time looking into this situation but I haven't necessarily found all the

answers yet.    



I no longer have this problem in my 91 that sits through the winter -- 3

years ago I replaced the Miata gel battery with a reqular type sealed

(excpet for vent tubes) car battery ($30 at K-Mart) and vented it.  I

have no problems at all now -- no acid smell, no acid mess, no erratic

idle, etc, nothing at all in the way of a problem.   



>>KENWALK@frmail.rosemount.com, Fri, 3 Jan 1997 11:52:48 wrote: 

     I'm on my third battery now and I have a question.  When I've had a

     battery go completely dead (due to my own negligence or extended

     travel) the idle behaves erratically after I replace or charge the

     battery.  Specifically, when I put the car in neutral to

decelerate,

     the idle falls to about 50 or just short of a stall.  Then it jumps

     back to 800 where it should be.  After a week or so on the new

     battery, the idle seems to stabilize and things are normal again.



     I just replaced my battery last weekend, and the same thing

happened.

     The idle hunts for a second or two when returning to idle from

     3000-5000rpm.  Its starting to balance out, but I'm curious; what

     causes this?

     kjw &

     ICE-CUBE<<

___________________________________________________________



-- 

Jeff Anderson                 @   @     @     @   @@@@@   @      

Arlington, VA                @ @ @ @    @    @ @    @    @ @

91 Silver, 92 Blue          @   @   @   @   @ @ @   @   @ @ @ 

MCA, Capital Chapter       @         @  @  @     @  @  @     @

M Club #97

 1) Curer of the Miata's bad head rest sound.

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