Trickle-charging & Maintaining your 124 Spider Battery

Go directly to:   Other Battery Info         Alternative Lightweight Battery                                           Effects of the Security Alarm System & Solar charging

Keeping your car battery trickle-charged during the winter season is much different than charging a battery that is discharged and unable to start your vehicle.  Not only are the charging devices different, but so too are the battery connection procedures.  Battery chargers should only be hooked to your battery posts using clamps (red to the  + post and black to either the post or a good ground connection on your engine or car body) and should never be used for extended periods of time to “keep your battery charged”.  That is the job of a trickle charger or a battery tender.

Battery cells should be charged gently so that there will be no chance of immediate gassing and temperature rise over 100°F.   Regular battery chargers supply much more current, which if not regulated properly causes high temperature rise and excessive gassing resulting in heavy loss of water and sometime overflowing of electrcomputerom the battery cells, leading to permanent damage to the battery cells.  During charging of batteries, continuous monitoring is required, if gassing starts and the temperature reaches the said limit, reduce the rate of charging. If after reducing the charging rate, the temperature is still approaching to the limit, it indicates the completion of charging process because even the normal rate of charging may produce high temperature rise if the battery approaches to the fully charged condition.

There are several other manufacturers and brands of battery tenders, but just like everyone associates the word Kleenex® no matter who made the facial tissue they’re holding, Deltran’s Battery Tender® trickle chargers are the most commonly known brand. Battery tenders are gentle trickle chargers that have circuitry designed to prevent the battery from being overcharged, which could kill your battery over a period of 2 or 3 months.   Like charging your phone from a USB port on your computer rather than a dedicated 2 amp charger, battery tenders’ low current output may be unable to bring a dead battery back to full capacity; that is the job of a dedicated battery charger.  Since the current flow is much lower, a battery tender doesn’t have to be hooked up directly to your battery posts, although that still is the most preferred method.

Battery Tender® trickle chargers usually come with a SAE standard “disconnect“, so that different harnesses can be attached, depending on your specific needs.  The preferred option is to install the battery to port harness to your vehicle, and simply attach it to your 12 volt disconnect port when put into use.  The temp clamps are another alternative, but the clamps could undo themselves or prevent you from closing your hood, etc.  Gasses are emitted during charging and you don’t want to risk causing sparks near the battery!

Cigarette lighter plugs used to be a convenient access point for battery tenders, but they usually are no longer “hot”, meaning they are only ON with the Ignition in the Accessory or ON position, rather than all of the time.   The OBDII plug (at the bottom of your dash panel to the left of the steering column)  on the other hand is usually “hot”, so it makes for a convenient hookup point to the battery, but only for trickle charging – not for rapid charging of a dead battery!  In fact, trickle-charging via the OBDII connector is now the common method for auto technicians to keep 12 volts on your various modules and the Infotainment system while servicing your battery, so that you won’t be inconvenienced with losing all of your radio station settings and your emissions testing readiness.

So, as a third alternative for those using a Battery Tender® during the winter season, consider purchasing their optional polarity-correct OBDII connection cable, shown in the photo at the top of this article.   Want to be able to charge your phone while the vehicle is not in ACC or ON mode?   As long as you already bought the OBDII cable, and again, since the OBDII plug is “hot”, you can purchase their USB 2.1 amp charger that plugs into the SAE disconnect, and it will charge your USB device while the vehicle is OFF.  They also sell a female cigarette adapter that plugs into the SAE disconnect that again, will be on all of the time.


I am not affiliated with Battery Tender®, just an admirer of their product offerings and hopefully this article helps some of my blog readers make the correct decisions involving battery charging.

Other battery info + maintenance

fiat battery 37 point 35 lbs vs Miata battery 25 point 15 lbs.jpg

Fiat 124 battery                                            Mazda MX-5 battery

Apparently if you attempt to start the vehicle and the ambient temperature is below -22ºF  (-30ºC), the ECU module will prevent the starter from engaging with the engine, as indicated to the driver by a flashing yellow “Cold Start Disable Indicator Light”. However, between that limit and the +teens (ºF) are some very cold temperatures, and starting your vehicle on those days is when you’ll really count on the Cold Cranking Amps (CCA) that your battery is capable of.

The factory battery used in the 124 Spider is much heavier and is capable of more CCAs (520) than the batteries used in the Mazda MX-5 (350).   Perhaps one of the reasons that Fiat chose to select this heavy-duty battery is because of current-drawing motor(s) that may run for up to around 15 minutes after the engine has been stopped and could somewhat deplete a smaller capacity battery.  An electric pump insures proper cool-down of the turbo bearings by circulating coolant through it after the engine is stopped. In both the MX-5 and the 124 Spider, if the coolant temperature is above 223º F, the radiator fan will also run after the (hot) engine is stopped to aid in bringing that temperature down quicker.

The battery is designated a Q85*, “Start-Stop” battery, 520 CCA, 65 Ah [20 hr rate], 52 Ah [5 hr rate], and weighs 37 lbs (16.8 kg).   It is a serviceable “Wet Cell” (flooded with electrolyte, which is sulfuric acid and distilled water), Lead-Acid battery with six exposed, threaded, vented caps, as compared to the MX-5’s 25-pound, “Maintenance Free” type with six recessed, threaded, sealed caps.   This merely means that it is much easier to monitor the electrolyte levels in the individual cells  by unscrewing the caps and looking into the cells (with good lighting available), at least once per year:

  1. If the electrolyte level inside the battery cell comes down, it must be replenished with distilled water (not tap water which has calcium and many other minerals) up to the level marked on the cell itself. This is to compensate for the loss of water due to evaporation.  The lead plates within the battery should always be completely submerged in electrolyte – hence the term flooded.
  2. While topping-off cells with low electrolyte levels, insure that the level in the battery cells not exceed the fill-level line. Otherwise there may be a chance of  electrolyte overflowing during gassing of the battery, which besides being very corrosive, could cause softening of the sealing compound on the top cover and subsequent leakage/contamination of  electrolyte.

*  Q85 Specs


In the coming years when it comes time to replace your battery, unless you are on a mission reduce your vehicle’s weight**, look for a 90D23 or 90D23L battery and know that it will fit in the factory designated location.

  • L is standard, don’t get R
  • higher than 90 has even more capacity

Note:  I have heard reports that a 51R battery fits nicely as well, and they are spec’d at 500 CCA, which is very close to the Q85, and are said to be available at a much cheaper price.

Effects of the security alarm system

If you have the standard immobilizer rather than the security alarm system with the microwave sensor (it’s located under the armrest tray), you probably don’t have a hood latch switch, unless you have a European car (it’s called a bonnet switch there), so you are able to leave the hood open when trickle charging using battery clamps and/or simply leave the vehicle for 2 or 3 months without even worrying about charging the battery.  I notice I have a harness going to my hood latch switch, but nothing is plugged into it, unlike the diagram below.


testing parasitic draw is in the “Battery Inspection” section of the service manual

I’m told any number of things can interact with the microwave sensor, including a phone stored in (or setting on) that armrest tray, windows open, hood unlatched, a moth flying around in the car, and even an overhead ceiling fan can keep the security system from going into long-term sleep mode, resulting in a significant current draw which could result in a dead battery within a couple of weeks.

This would be the person that may need to seriously consider a trickle charger with the OBDII connector as discussed above!  (and perhaps consider disabling the microwave sensor as well)

Solar charging

If your vehicle will be stored in a location that lacks electricity, but is exposed to sunlight, you may want to consider solar charging your battery.  In addition to needing a solar panel, you’ll need a solar panel charge controller  connected either directly to the battery, or suing the OBDII connector discussed above, which will control the rate of charging much like the trickle charger does, and it will also keep the solar panel from discharging the battery during the non-daylight hours when it is too dark for charging.

Alternative Lightweight Battery

**  Good-Win Racing has a Weight Saving battery installation kit that accommodates the ultra light Battery Tender® LiFePO4 (Lithium Iron Phosphate) family of batteries.

battery-tender-batteryThe DELTRAN BTL24A360C, which along with all of the mounting brackets, weighs 3.35 pounds and replaces over 39 pounds of battery, tray, surround and mounting brackets.  At 360 CCA, I don’t know if it would serve your needs on the coldest of winter days or not.

The DELTRAN BTL35A480C weighing 1.15 pound more and capable of 480 CCA (vs. 360 CCA), is Good-win’s recommended weight-reducing battery for the 124, because of current-drawing motor(s) that may run for up to around 15 minutes after the engine has been stopped as well as the parasitic drain of other always-on electronic modules like keyless entry and the anti-theft alarm system.  Bulldog66 shared his short-lived experience with this battery in his 124 Spider, and came to the conclusion that the battery aligns more with racing rather than average irregular driving, and he ended up putting the factory battery back in.

If you go this route, you’ll need a pair of  post adapters  with M6 threads so that the harness will attach conventionally to the battery without modifications.

By switching your aluminum alloy wheel rims with premium magnesium alloy rims that are at least 10 pounds lighter each, along with this lighter battery, you could knock 75 pounds off of your 124 Spider in one day 😉 , but my advice for most people is to stick with the OEM battery.


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