Catalog Ordering & Shipping Electrical Tech Work Shop Tips About M.A.D.


          With the charge rate selector increased to HIGH, the charger is delivering a 40 amp current flow to the battery and voltage at the battery rises to 14.2volts.  This is the rate that a powerful alternator would recharge a powerful model battery, at peak charge rate.  But in the car, the voltage regulator would limit system voltage to about 14.2volts, the amps would taper off as the battery became charged. 

With our charger, voltage level soon came up and surpassed 15volts at this charger setting, because the battery did not want this much current.  Expect that with this large charger set to HIGH rate, the battery will soon begin to produce a lot of gas from the vents.  And the battery temperature may rise to warmer than acceptable level.  This charger does not limit voltage to the 14.2volts as a regulator for an alternator would, and so voltage will soon climb above acceptable level.

It’s actually difficult to fully charge a battery at excessively high charge rates.  (It doesn’t give opportunity for chemical reaction in the battery to fully pentrate the plates.  (We shut it down just after this photo was taken.) 

          We have again connected the small 6amp charger, and let it charge the battery for a few hours.  Now as the battery begins to recover, it is accepting less current (about 3 1/4amps).  Also as the battery is becoming charged, voltage has come up to 13.8volts.

          In a previous photo at “Part 1” showing this small charger, the battery was at a less charged condition, and the battery was accepting over 4 amps with voltage at 13.1volts.

            Comparison of the above photo to the previous photo shows that; as the battery becomes charged, the AMPS are lower, and the VOLTS are higher.

          And after a couple more hours with the small charger connected, the battery is only accepting 2amps from the 6amp charger.  And battery voltage is now up to 14.1volts.  The battery is approaching a fully charged condition.

            Compare this photo to the previous photo, to see again that as the battery becomes charged; the AMPS are lower, and the VOLTS are higher. 

          Results with the large charger are about the same as with the small charger, now that the battery is approaching a fully charged condition.  In the photo above, the “Charging Rate” switch is set to “LOW 12volts.”  The amp gauge on the charger indicates a very small current flow to the battery, and voltage is at almost 14.1volts.

          These readings are about what we would find when testing an alternator-voltage regulator-battery system, with battery near fully charged and with normal alternator-voltage regulator function. 

          And here we have set the switch to “12volt MEDIUM” charge rate, and this photo was taken after about one minute at this setting.  But as the battery already charged, it does not want this much current (about 10 amps shown), battery voltage is nearly 17volts, which is excessive.

If this were a test of an alternator-regulator-battery system, an overcharge problem caused by lack of voltage regulator control would be indicated.  The probable cause would be a defective voltage regulator.

          The 12V HIGH (BOOST) switch setting would increase the voltage to a very abusive level—we did not bother to show it. 


          When the battery is fully charged, it does not accept much current if the voltage is limited at 14.0 to 14.5volt level.  Therefore, expect that when current at the battery charger gauge is down to a couple of amps, and battery voltage is above 14volts, the battery has reached about full charged condition.

(The “textbook” level for voltage regulator setting with most alternator systems is 14.2 volts.  And so expect very low current flow to the battery, when driving with a fully charged battery.)

  • When recharging a discharged battery, the amps will be high and the voltage will be a little low.
  • As the battery regains charge, the amps go down and the voltage comes up.
  • When the battery approaches a fully charged condition, the amps will be very low if battery voltage is limited to the 14.0 to 14.5 volt range.


Expect that re-charging at high rate is abusive to batteries.  High rate charging will cause excessive gassing from vents, and batteries can be overheated too.  (Generally battery temperature should not exceed 125degrees F when re-charging batteries.)

          Although it does take longer to re-charge a low battery with a reduced rate, slow rate re-charging is generally the least abusive to batteries. 

          If the battery has removable covers, it should always be tested with a hydrometer before recharging is attempted.  At least use the small, inexpensive hydrometer, which can be found at most Auto Parts stores.  The one shown in the photos above and below has four small “float balls” of different colors. 

  • All four of the balls floating at the top of the liquid means that the battery is fully charged.
  • Three balls floating and with one ball that sinks indicates a three-quarter charge condition.
  • And with two balls floating, and two sinking to the bottom of the liquid, the battery is only at about half charged condition.

When working with batteries that have removable covers, there is a lot of worth to testing a “questionable” battery with these simple hydrometers.

          There are larger, more expensive battery hydrometers that actually measure specific gravity of the liquid in the battery.  And the better of hydrometers will also have a built in thermometer, as temperature affects the reading.  But for most people, the simple inexpensive model with four “float balls” is fine.  If all the cells show a fully charged condition, then there is no need to charge the battery; and in some cases it is dangerous to do so.

And the hydrometer should verify that all cells are about the same. 

A defective battery is indicated when the hydrometer test shows one or more cells “dead” and other cells test at full charge.  (The hydrometer test is not valid if water has just been added, and also not valid if the battery has been charging at high rate, just prior to the test.)  When the hydrometer indicates a fully charged condition at all cells, then the battery should put out sufficient power to crank the engine—otherwise the battery is defective. 

          When a battery does not have removable covers, then it is best to observe voltage and amperage while charging, as we have in the photos with this feature. 


          We have left the removable covers in place while recharging the battery.  The covers are vented, and they serve as functional spark arrestors.  When the covers have been removed, any spark near the top of the battery may directly enter the battery where there is a high concentration of explosive gas.  And it’s easy to make a spark by touching a metal tool between the POSITIVE battery post and metal part of the car.

We do remove the caps to check liquid level before recharging a battery.  Also use a HYDROMETER to check the state of charge condition.


          If a battery does not have sufficient power to crank an engine, and also does not accept significant charging current while keeping the voltage within acceptable levels, then the battery is probably defective and it should be replaced. 


Also note that when a charger is first connected to a completely discharged battery, at first the charger is likely to show only a very low charge rate.  If the amp gauge on the battery shows a low reading when the charger is first connected, check it again after about a fifteen minute period of charging.  By then the charge rate will be nearly at peak amount of current.

And, some of the best battery chargers are polarity protected.  (ours is not)  Polarity protected chargers will require some voltage at the battery before charging will begin.  When this type of charger is connected to a completely discharged battery, a special procedure will be required.  With the charger connected to the discharged battery, and the charger switched ON, connect booster cables from another battery to the discharged battery.  The booster cables must be connected for at least a few seconds, voltage from the charged battery will “close contacts” in the charger so that the charger can go to work recharging the discharged battery. 


          It’s best not to push our luck around batteries.  Battery explosions are very violent and very dangerous.  Fast rate charging will generate excessive amounts of explosive gas.  Have patience and keep the charge rate low when recharging batteries.  It’s a lot safer; it’s less abusive to the battery; it causes less corrosion; and the slow rate charging more thoroughly charges the battery. 


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