A jump start to a Miata battery can often be done just like any other battery without any problem, but it depends on the state of the battery at the time. This regular jump condition would typically be if your headlights glow at least fairly good until the starter is run.
The problem cases I've seen are when the lights have been left on all day and the battery has been completely run down and left under load while it was run down. The Miata battery in this condition will not in short order raise in voltage when a charge (i.e. jump) is given to it. The battery will draw lots of current but the voltage remains very low (around 5 to 7 volts if I recall correctly). A tow truck when he sees a regular battery do this will simply increase the charge current to raise the voltage enough to start the car. However, from what I've seen of the Miata battery in this condition, under reasonable high levels of current it will just plain refuse to increase voltage. Instead of accepting a high level charge, it'll just convert the energy into lots of internal battery heat, and not stored electrical power. If the over-strong charge current is removed the battery's voltage will quickly (in seconds) return to its low voltage state.
So, what needs to done in this Miata battery condition is apply a charge current of around 5 to 10 amps (maybe 15 at most if it's real cold out) for 5 to 10 minutes. Then try to start the Miata from its own battery power alone.
IF IT STARTS, drive the car home and put your 2-amp charger on for 16 to 20 hours (or at least overnight). The Miata's alternator system will then likely be able to do the rest as you drive after that.
IF IT FAILS TO START, put the tow truck charge back on at 5 to 10 amps (very max) for 10 to 15 minutes, then drop back to around 5 amps for another 10 to 15 minutes, and try again (tow trucks jump can be left on now when you try). By now it's almost sure to start. The problem with this method is that the tow truck will not want to stay around that long. So, he'll crank up the charge current, toast your battery, tow you back to the garage, spend hours trying to find a new battery from his suppliers with a high mark up before getting one from a Miata dealer at little markup for him.
WITHOUT A TOW TRUCK (best) and jumper cables under your own control, turn everything off in your Miata, then jump your Miata to the donor car while the donor engine is off. Start the donor engine and idle. Let the donor engine run at idle for 2 or 3 minutes and then increase its speed to just a little above idle for at least 8 to 7 minutes more. Then shut off the running donor car and try to start your Miata (battery cables still connected is best). Try to start no more than 3 short attempts. If you still don't start, turn off your Miata (everything) then start the donor car, and let it run at fast idle for around another 15 to 30 minutes. Now you'll almost for sure start, and then put your 2-amp charger on when you get home. This slow charge is important to do. The Miata alternator system alone will often not bring the battery back to full charge. This is not because the Miata's charging system is weak, just the opposite-it's strong enough to cause the heating of the battery without allowing charge stored in it.
But what if it still doesn't start-take the battery out, take it home, put 2 amps charge on it overnight, or longer up to 20 hours, then reinstall the battery, and start the Miata.
If it still doesn't start, buy a new battery, but this is very unlikely to be needed unless you've toasted the Miata battery by somehow getting too much high charge current to it. One indicator would be if you get no initial charge current when the 2-amp charger is put on, and there still is none after its been on for an hour or so. In this case, and if you're local to the D.C. area, and have the time, I may be able to fully restore the battery for you in two or three days. Many others no doubt have had other Miata battery jumping methods work quite successfully. It's hard to give just one method that will work for all the various conditions of the Miata battery-its charging requirements and operation are more variable than a regular liquid/acid type car battery. The methods I give here are mainly based on lengthy experiments I've conducted on the Miata battery.
One thing about what I wrote is that it's a hard call when jumping two cars together if the starter of one car should ever be actuated while connected to the other. This runs some risk of the starter motor's electrical spikes popping the computer in the receiving car, especially if its key switch is left "on". On the other hand, if the donor car is running when the two are connected together and (due to a mishap) the jumper cables are shorted together (or through the body of the car) the donor's alternator can easily be popped. So, there may be some controversy (including with me) about the procedure I indicate. The way the Miata's battery is recessed under the fender, it's a little easier than in most other cars to electrically short the jumper cables when connecting. So, that's why I opted to write the car-to-car jumping procedure the way I did -- for the Miata I think it's the lesser possibility of two evils. The best way depends on how careful you can be when connecting the jumper cables, but being careful enough is sometimes hard to do, especially when it's dark, cold, and you have snow blowing in your face.
| Back to the Electrical Page | 06 Feb 1998 |