How to - Modify a Harbor Freight float charger for correct float voltage

[JC 1965 1,528]

Thanks Chuck  !!    I haven't used the maintainers yet, still new in box. Waiting to see what the fix is.  

[Save Old Iron 1,563]

Now the holiday is behind us, we can move forward with the conversation on this modification.

 

First off, a few cautions.

 

If you have any questions on your ability to perform this mod, do not have the proper tools, fear the mod may introduce a "fire hazard" in your home, or fear loss of the warranty on this ($5) product,

PLEASE DO NOT ATTEMPT THIS MODIFICATION.

 

Both  modifications involve cutting of wires to the positive battery lead and opening the charger plastic housing to exchange a component of the circuit board. These actions will require soldering skills and some finesse opening and closing the charger case.

 

Having said that, let's begin.

 

Why are we talking about modifying this product? Does it not work as advertised?

Yes and no.

 

Yes, the float charger will keep a lead acid battery in a full state of charge over extended periods of non use of the battery - BUT -

the manufacturer's use manual http://www.harborfreight.com/automatic-battery-float-charger-42292.html states the float charger voltage should be a maximum of 13.2 volts. On most of the float chargers I have purchased from HF, the output of the battery float charger is at 14 volts or above.

 

Output of voltage from the float charger at 14 volts or above will indeed keep the battery at full charge, but unfortunately can also promote unnecessary evaporation of the electrolyte in the battery and subsequent degrading of the battery, If the electrolyte levels drop and are not maintained while the battery is on a float charge, sulphation of the battery plates can occur and degrade the reserve capacity and cranking amps of the battery.

 

Replacement of the evaporated water in a capped battery is no problem. Maintenance free batteries can suffer permanent water loss in the cells and AGM style batteries can suffer permanent degradation

 

This float charger modification is aimed at setting the final voltage to the battery terminals to a 13.2 volt level. Why 13.2 volts?

It is at this voltage a full charge can be maintained on a battery without the problem of long term electrolysis of the water out of the electrolyte in the battery. Voltages above 13.8 will start the internal lead plates to "bubbling" which enables electrolysis of the water in the electrolyte, resulting in loss of the water in the cell.

 

So decide if this mod is for you and read on to the next post if you feel this will be of some benefit.

Edited December 29, 2015 by Save Old Iron

[Save Old Iron 1,563]

 

Scenario #1

 

The “Lucky Dog”

 

If you have your float charger already connected to a battery for more than 24 hours, measure the DC voltage across the battery terminals. If you measure 13.8 to 14.2 volts, consider yourself a “Lucky Dog”. The modification to bring the float voltage down to the 13.2 volt range can be easily accomplished without modification the the circuit board of the charger,

 

All that is needed is to properly install a standard  off the shelf 10 cent diode into the red lead of the float charger.

 

 

As I have mentioned many times before, a diode has an electrical property that allows a drop of 0.6 to 0.7 volts DC across itself anytime current flows thru the diode. So simply put, inserting a diode in the red charger lead will drop the voltage out of the float charger by 0.6 to 0.7 volts.

 

14.2 volts minus 0.6 volts = 13.6 volts -  a bit on the high side but acceptable

13,8 volts minus 0.6 volts = 13.2 volts – right on target

 

The mod consists of cutting the positive battery clamp from the coiled charger cable, stripping about a half inch of insulation off each end of the red charger wire

 

 

 

orient a 1 amp / 50 volt diode properly in the red wire

(SILVER/ WHITE BAND ON THE DIODE IS CONNECTED TO THE POSITIVE BATTERY CLAMP)

 

http://www.ebay.com/itm/25-x-1N4001-Diode-1A-50V-FREE-SHIPPING-/321024829020?hash=item4abe92265c:g:OaMAAMXQrNtRzT7C

 

 

I used 3 amp 600volt diodes as I had them on hand

http://www.ebay.com/itm/10-x-1N5406-Diode-Rectifier-3A-600V-FREE-SHIPPING-/321009349460?hash=item4abda5f354:g:SzMAAOxyhodRzTVt

 

solder the red wires to the diode leads (make sure you place the heat shrink tubing over the red wire before soldering the second wire to the diode).

 

 

 

and finish by heating the shrink tubing to seal off the electrical connections from the weather.

 

 

 

 

 

To check for the final battery voltage produced by the modification, the “surface charge” or the 14 volts that the battery was previously charged to, must be either left to drain off by itself over 24 hours or removed by consuming a small amount of power from the battery. An alternate method to the 24 hour wait is to connect the battery to say – the headlights of the tractor - and allow the battery voltage to drop down to under 13 volts.

 

Connect the modified charger to the battery and check battery voltage again in 8 – 12 hours, Expect to see the battery voltage stabilize in the 13.2 to 13.6 DC volts range.

 

 

 

Done and done.

 

Scenario #2 "The Fun Begins"

voltage above 14.2 or below 13.0

more to follow.

Edited December 29, 2015 by Save Old Iron

[mmmmmdonuts 274]

Awesome modification. Best part is it is super simple. Just something quick to add. If you wanted to get the voltage a little lower you could add a schottky diode in series as well which only has a voltage drop ~0.2- 0.4V depending on the schottky. Or if your voltage starts a little lower than everyone else's.

 

http://www.amazon.com/VISHAY-SEMICONDUCTOR-SB150-E3-SCHOTTKY-DO-204AL-2/dp/B011CQCOKO/ref=sr_1_1?ie=UTF8&qid=1451388860&sr=8-1&keywords=sb150+diode Or here:

 

http://www.digikey.com/product-search/en/discrete-semiconductor-products/diodes-rectifiers-single/1376383?k=sb150

 

 

[JC 1965 1,528]

Thanks for posting this info Chuck.       I have a question. When I checked my Harbor Freight chargers I just plugged them in to house current and checked the output . Will that give me an accurate reading, or does the HF maintainer have to be charging a battery when I check the output ?      

[Save Old Iron 1,563]

3 hours ago, JC 1965 said:

Thanks for posting this info Chuck.       I have a question. When I checked my Harbor Freight chargers I just plugged them in to house current and checked the output . Will that give me an accurate reading, or does the HF maintainer have to be charging a battery when I check the output ?      

Excellent question.

 

Float circuits without the diode mod will / should read their float voltage without being attached to a battery. If you measure the "open circuit" (no battery connected) voltage of a diode modified charger, the voltage will read the pre-modification output voltage. The diode voltage drop we expect from the "Lucky Dog" mod does not come about until a small current flows thru the charger to the battery.

 

 

[cod 116]

Great info Chuck!!!  Thanks for your continuing "electrical education program(s)".  Those of us who majored in electronics with a heavy emphasis in "skillful neglect" need all the help we can get...

[Tuneup 1,416]

Nice mod!.    Ah, the HP tech days. We had 6113s on the rack to aid in troubleshooting back when you had to locate the defective part and replace it instead of having the instrument tell you which board to change. I miss those days - the glory days of HP.

[Save Old Iron 1,563]

3 hours ago, Tuneup said:

Nice mod!.    Ah, the HP tech days. We had 6113s on the rack to aid in troubleshooting back when you had to locate the defective part and replace it instead of having the instrument tell you which board to change. I miss those days - the glory days of HP.

 

I still have great respect for the 1960 thru 1980 era of HP and Tektronix equipment. I grew up in that era using this equipment and still seek it out for daily use in my "near retirement" hobby workbench design sessions.

 

The old Harrison, HP and Power Design power supplies (on eBay for $30)  still have better specs than $1000+ supplies made today.

I am an old schooler much like yourself. Thanks for the kind words.

 

Edited December 30, 2015 by Save Old Iron

[Save Old Iron 1,563]

continuing on to the second scenario, but first the obligatory cautions

 

If you have any questions on your ability to perform this mod, do not have the proper tools, fear the mod may introduce a "fire hazard" in your home, or fear loss of the warranty on this ($5) product,

PLEASE DO NOT ATTEMPT THIS MODIFICATION.

 

 

The second modification concerns changing a resistor within the float charger circuitry.

 

 

This mod can be performed if the voltage output of the float charger exceeds 14 volts or is under 13.2 volts.

Why can there be such a variance in the output voltage?

HF has been producing this particular float charger for over 10+ years and has apparently gone thru several styles of circuit boards. Some of the original boards I had contained circuitry that required hand selection of a resistor value to set the output at 13.2 volts. Understand we are talking about a product that routinely sells for under $5. Any time assemblers had to hand select a resistor value to adjust the output to 13.2 V DC - well, that just becomes too costly. If the value needed to output exactly 13.2 VDC fell in between standard, readily available resistor values, several resistors had to be placed in parallel to come up with a "custom" value in order to achieve 13.2 VDC. Some of the early HF boards had several slots to insert these parallel resistors to achieve 13.2 VDC.

 

This concern for additional manual labor to custom adjust the float chargers every time a new batch of transistors were received became a cost concern. A new circuit board was apparently designed to allow the user of a variable resistor - a potentiometer - to be inserted onto the circuit board for final voltage adjustment.

A quick screwdriver tweek of the potentiometer would set the outp[ut voltage at 13.2 VDC. Done and done.

Well not quite. Potentiometers cost about 20 cents each and a resistor costs about 1 cent in quantity, sooooo....... the bean counters must have said something about the additional 19 cents in cost. Bean counters usually win and the product was assembled with a 1 cent resistor that was "close enough". This results in a possible explanation why some of the early model chargers put out a precise voltage and some not so precise.

 

The second scenario mod will change the 1 cent "close enough" resistor back to the 20 cent potentiometer design. This mod will allow the float charger output to vary between 12 to 15 volts. You as the user will have the final say in what the output voltage is - you should strongly consider setting the voltage around 13.2 VDC for proper float charging. This mod also had the advantage of being tweeked slightly if you need say 13.6 for gell cell float voltage.

 

Pros . Cons

 

Pro - only a 20 cent modification - ability to adjust final float voltage for wet lead acid, glass mat and gel cell variants of batteries.

Con - solderiing / de-soldering skills are needed - you need to order a potentiometer - the potentiometer may not be as reliable in a damp or wet environment as a resistor - rough handling (tossing the float charger or dropping it to the floor) may change the output voltage and require a quick resetting of the potentiometer.

From my internet searches, I find at least 3 versions of this circuit produced over the years, The mod I am presenting was performed on chargers in the 2008 era as seen by the stamping on the power supply cases. My circuit board looks like the one below.

 

 

 

 

 

 

I am currently placing an order for the most recent offering from HF as I believe this charger is going to be on clearance for the 2016 year .

Having addressed cautions and expectations, I will begin the mod in the next post

 

 

Edited December 30, 2015 by Save Old Iron

[pfrederi 17,114]

Does input  AC voltage affect the chargers out out DC Voltage?? Depending where you live you  household current can vary considerably 110v to maybe 125v..  Out here in the boondocks at the end of the extension cord you could be on the low side...closer to a substation may be higher.