Changing Car Batteries

Changing a car battery is simple, right?  Not so fast.  How to change the car battery in your Miata.
by Ron Carmichael

I think we have all installed a new car battery during our lifetimes. It was a simple operation which called for unhooking the battery terminals, removing the old battery, installing a new one and reconnecting the terminals to the new battery.  Today, the once-simple task of changing a car battery can have serious consequences if not done properly.  With the advent of computerized systems in today’s automobiles, our car’s electrical system is not the only concern when changing a battery.   Late-model vehicles rely on computers that operate everything from the stereo to the transmission, and fuel mixture ratios.  If power is lost to the computers, the settings are lost and automatically reset to their factory default values.  If you’re lucky, you’ll only lose your radio presets and will need to enter a security code to reprogram them.  If you’re unlucky, the transmission, fuel mixture and other computerized settings which have been “learned” during the car’s operation will have to be “relearned.”  If settings are lost, the vehicle could feel different when driven, computerized controls will have to “relearn” previous settings and you may have to enter a security code to access the stereo. Even when the battery is unable to crank/start the engine, there is usually enough trickle current to keep these computerized systems energized.  That is why even when you jump start the battery, you may not have to reset the computerized systems.  However, when the battery is completely disconnected, loss of computerized settings is a real possibility.  Most of the memories and their associated functions which will be lost when you remove the starter battery can be easily reprogrammed or will be re-learned after 50 to 100 miles of driving. The exception, and the real reason you want to use one of these devices is the radio anti-theft device.

Using a simple memory saving device when replacing your car battery will help you avoid these problems.   Memory savers are used to save the stored settings in your car’s engine control computer, diagnostic system, seat and steering wheel adjustment mechanisms, radio station presets, and possibly the anti-theft circuitry in your car radio.

There are numerous videos on YOU TUBE that will show you step by step instructions for changing a battery with a memory saver.  I strongly suggest you look at a few of these videos before attempting to change your battery.  I think I’ve documented things correctly but can’t guarantee anything, so please make sure to check my instructions with your car’s owner’s manual and the YOU TUBE videos.  If you see anything that you disagree with, follow your own guidance.

The battery on my 2015 MX-5 is the original and is about 6 years old.  I purchased a Group 51R Interstate battery from the local COSTCO.  Make sure you check the specifications for the appropriate battery group/size for you particular vehicle.  There are a variety of memory savers available, but generally they look like the one in Figure 1.  Most have an OBD2 connection, and a 12 V cigarette plug.  The OBD2 port on your vehicle is the diagnostic port on your car that allows for connection and monitoring its operations, settings etc.  It is also the port that is used during your emissions control inspection.  The memory saver allows for a trickle current to continue to be flowing to your car’s computer systems while you change the battery.

I purchased my memory saver from Amazon for about $20-$25.  It comes with an OBD2 connection, a cigarette lighter plug and a cigarette plug adapter for connecting jumper cable connectors to a second 12 V power

source.  I used a separate jump starter device which has a 12 volt cigarette lighter plug as the second 12 V power source.  You can also connect the cigarette lighter end of the memory saver into the cigarette or 12V plug of a second vehicle.  MAKE SURE YOU HAVE POWER TO THE SECOND VEHICLE’S PLUG SO CURRENT IS FLOWING.  You may need to turn the second vehicle’s key to “ACC” setting for power to flow to the plug.

Here are the step-by-step directions I used to change the battery in my 2015 AE MX-5

1. Remove MX-5’s plastic battery cover.  There are snap connectors on the right and left sides of the battery cover.
2. Lift off the hose attached to the left side of the battery cover and place to the side out of the way.
3. Lift off the battery cover by depressing the snaps on each end.
4. Remove the battery hold down bracket using a 10 mm socket/ratchet or wrench.
5. Plug the memory saver (OBD2 end) into the OBD2 connector which is located under the left side of the dash. (See Figure 2)
6. Plug the cigarette lighter end of the memory saver into a 12 V outlet on your jump starter or a 12 V cigarette plug on a second vehicle. (Make sure there is power to the second vehicle’s 12 V plug). (See Figure 3).
7. A red light on the memory saver will indicate that current is flowing through the memory saver. MAKE SURE POWER IS FLOWING THROUGH THE MEMORY SAVER BEFORE MOVING TO THE NEXT STEP!
8. Loosen the NEGATIVE battery cable FIRST using a 10 mm socket/ratchet. Remove NEGATIVE cable from old battery.
9. Loosen the POSITIVE battery cable using the 10mm socket/ratchet. Remove the Positive terminal from the old battery.
10. DO NOT LET THE BATTERY CABLES TOUCH OR LET THE POSITIVE (RED) CABLE TOUCH ANYTHING METALLIC! I put a sock over the POSITIVE (RED) terminal cable.
11. Remove the old battery from the car. Be careful it is HEAVY!  Keep it LEVEL and watch out for acid!
12. Check cable terminals for corrosion.
13. I installed corrosion pads on the terminal of the new battery before installation.
14. Install new battery in the vehicle.
15. Connect POSITIVE cable on terminal FIRST. Use 10 mm socket/ratchet.
16. Connect NEGATIVE cable on new battery using 10 mm socket/ratchet.
17. Check both POSITIVE and NEGATIVE terminals to insure they are tight and secure.
18. Unplug the memory saver from cigarette lighter plug on the battery starter or the second vehicle’s 12 V outlet.
19. Unplug the OBD2 connection from the OBD2 connector under the dash.
20. Replace the battery hold down bracket. Make sure the “J” hooks are connected to the bottom of the battery tray.
21. Replace cover onto new battery.
22. Replace hose on left side of the battery cover.
23. Snap the battery cover back into place.
24. Start vehicle to make sure all radio settings and presets are working.
25. If they are, then you’ve successfully replaced your battery.