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Small (Parasitic) drains on the battery will require Battery Maintenance when our Hot Rods are parked for extended time periods   and  
When recharging a low battery SLOW RATE is the most thorough and gentle method.

High Powered alternators on modern Hot Rods can be abusive to partially discharged batteries.  /  And the partially discharged battery can cause alternator overheating too.

The AutoMeter battery extender in these photos has been much used and also much appreciated during the last three or four years, at M.A.D. Enterprises.  We had much need for it around here, and purchased this one from P.A.W. (Performance Automotive Warehouse).  Knowing that AutoMeter products are consistently first class, we actually saved the box, intending to offer a product report to friends.

We have a few vehicles, not all are driven frequently, and as with any car equipped with modern equipment, small “parasitic drains” kill the batteries when a vehicle is parked for extended time periods.  Even at a professional facility, concerned exclusively with automotive electrical systems, we found the battery situation expensive, inconvenient, and irritating in general. 


In the many years of testing and servicing cars, we have found that cars equipped with modern electronics will have small drains when parked.  Not that there is any electrical system defect, and the drains are of such a small amount that they would never be noticed if the car was driven significant distance at least a couple of times per week.

Even a small “entry level” radio with a station memory and clock will cause a small drain.  Add an alarm system, and more drain will be added.  Add a computer for electronic engine management (modern fuel injection) and the amount of the drain will grow larger yet.

BCI used to publish in their battery service manual that “a drain of more than 0.1 amps is a problem, and should be fixed.”  Battery Council International is the organization that gives us industry standards for battery sizes and levels of electrical power, as well as practical guidelines for testing and servicing batteries.  BCI gives us “group sizes,” and electrical specs such as Cold Cranking Amps to measure starting power, and Reserve Capacity rating to judge how far we might drive if the alternator fails.  The recommendations and standards of rating given to us by the BCI are in general very practical and useful.

However, in the “real world” the amount of drain that is “normal” and can be tolerated is a gray area–It’s not practical to draw a steadfast line for acceptable amount of small drain on the battery when a car is parked and shutdown.  An amount that is normal for one car might be excessive for another.  And the amount of drain that will be a problem largely depends upon how frequently the car is driven.  In example, a drain measured at between 0.02 and 0.03 amps might be normal and not a problem for a well equipped, late model, daily driven car.  But the same 0.02 to 0.03 amp drain will soon take its toll on a battery in a Hot Rod, if the car is only used for short drives on an occasional basis.

Therefore, we must use a little judgment and logic where drains on batteries are concerned.  We could disconnect the battery every time the car is parked, and reconnect the battery every time we go for a drive.  And if the battery is hidden, we could even install a “main battery switch” to make it simple to disconnect the battery.  But, expect that we will have to reset the radio stations, and the clock too (if we want the time set), and we cannot use the on board alarm system.  With modern factory electronic engine management systems, constantly disconnecting the battery may be worse yet.  Various systems will behave differently, but expect that any diagnostic codes will be erased.  And the “history of driving memory” which helps the engine perform well will also be erased.  (And it can take up to 20 warm-up/cool-down cycles for the “history” to become embedded in the computer.)

And also, we should consider that a battery lasts the longest and performs the best when stored in a fully charged condition.  Occasional charging at least a small amount helps to keep the lead surfaces within the battery clean, which helps a battery to keep its full level of power rating a lot longer. 

All factors considered, a special maintenance battery charger can be a very good investment.  Ordinary, small “trickle” chargers can be dangerous when left unattended, and they can also destroy a battery by overcharging when they are forgotten.  Ideally, a maintenance type charger will have special electronics to prevent overcharging when in use for long time periods.  And the latter is the special and sensitive part of designing and building the maintenance charger.  The good maintenance charger will also be fully protected from overloads to prevent fire.  Most small battery chargers have overload protection, although the automatic-reset circuit breakers can get dangerously hot after cycling for a while.  A fusible device would be a better choice with the maintenance charger, because it may be left unattended for long periods of time.

Our “Battery Extender” from AutoMeter has been extensively used, outputs have been measured, and as we expected from an AutoMeter product, it’s exactly what we like.  Re-Charge rate, taper-off, Voltage and Amperage outputs at maintenance mode, all seem perfect.

We have taken a few photos, to show some of the performance aspects.  Finding a battery charger that will put out a large amount of current when a battery is discharged is easy.  And so we did not bother with a photo showing the output in “Charge” mode.  However, we have measured several times, and it is as advertised; a 1.25 Amp maximum rate battery charger when in “Charge” mode.

  • The built-in electronics switch from Charge mode to Maintenance mode when the battery reaches exactly 14.2 volts.  
  • When it is in the Maintenance mode, battery voltage has never exceeded a 13.21 measurement on our test meter.
  • And in Maintenance mode, current output will taper off to as low as 0.01 amps.
  • Also in Maintenance mode, if small electronic devices on the Hot Rod use more power than the charger delivers, it will switch back to Charge mode when battery voltage drops to exactly 12.60 volts on our meter.  Then at 14.2 volts it will again switch back to Maintenance mode.

Performance, all functions, factory preset-by-electronics charging rates, and reliability, has all seemed perfect with the AutoMeter Battery Extender.

About the only test we have not done is to leave the same charger on the same battery for more than 3 weeks non-stop.  (And that is only because we have just one of these neat little chargers, and several cars with batteries to maintain.  So all the cars are wired for the quick-disconnect on the charger output cord, and periodically the charger gets moved from car to car. 

Here is the charger in the “Maintenance mode,” notice the green indicator light is glowing.  We included the tape measure in this photo, to show the approximate size of the charger.  Also notice that the charger does have a handy carry-handle, and convenient flush-mount brackets to make it a permanent garage fixture, where the Hot Rod is stored. 

This close-up photo shows the connector in the cord from the charger to the battery.  The insulated, round pin “bullet connector terminals” are the most reliable connector used for automotive.  The charger is thoughtfully packaged with two types of cord ends for the battery.  One of the cable assemblies has the traditional spring loaded clip-on ends (as typically found on small battery chargers).  And the other cable assembly has ring terminals, which may be permanently installed on the car.  The round pin bullet connector was a very good choice for a quick disconnect in a cable from a small battery charger, as they are very reliable.

TIP  Since the quick-disconnect (bullet connectors) at the charger output cord does not have a “retaining latch,” it can also serve as an automatic disconnect feature.  (1)  Mount the charger securely to the garage wall, and also strain relief mount part of the output cord to the wall with insulated clamps.  (2)  Install and wire the battery end of the cord permanently on the car.  Leave the connector where it will be convenient to plug in, but safe and out of sight when not in use.  And STRAIN RELIEF the charging wires securely to the car with insulated clamps.


And then, should the car accidentally be driven away while still plugged into the charger, the quick disconnect should merely disconnect with no damage to the car or to the charger.  (I did not space out! I was distracted!  Honest!)  At least all is well, thanks to AutoMeter for using this type of connection in the charger cord. 

Here we are measuring the battery charge rate in amps, in Maintenance mode, as the battery approaches a fully charged condition.  Eventually the charger will drop to a 0.01 amp rate, indicated on this meter, when the battery reaches full charge.  The charge rate is tapered off by electronics in the charger, to prevent over charging the battery.

And here we are measuring voltage at the battery, also in Maintenance mode.  This is a good level for charging a battery during extended periods of time.

Why we used this small battery as a model for our example test.

The small battery is a much smaller “electrical storage reservoir” than a big car battery.  Therefore, the little battery allowed the charger to trim off the charge rate in less time than a big car battery, so that we could better show in photographs the small charge rate when in Maintenance mode.  Output specs of the AutoMeter we were testing were well within the battery charging specs, which are printed on the battery.  As we mentioned previously, it’s easy to find a high rate battery charger that will over-charge and destroy batteries when it is forgotten while plugged in.  Finding a safe, effective, easy on the battery type of charger is not so easy to find.


We like the AutoMeter unit very much, and we recommend it.  We paid the full purchase price, plus tax, plus shipping, and got no discount when we bought it from P.A.W. Yet, it was a very good investment–and we wish that everything we purchase would work out so well.

No doubt there are many applications that could use this charger.  Motorcycles during the off-season winter months, riding lawn mowers, and boats too.  RV’s that nearly always have small drains, and that nearly always get parked for long periods of time between vacation trips.  Snowmobiles during the off-season summer months, and agricultural machines that are used only on a seasonal basis, are other examples of machines with automotive type batteries that may be stored for extended time periods–and that need battery charging maintenance.

However, before connecting this model charger to a battery for months-at-time, non-stop charging, contact AutoMeter.  We have not tested the charger for such extended periods of time, and we are not sure that this charger model is intended for such use. 

The Battery Extender charger and other great AutoMeter products may be seen at www.autometer.com  



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