Connect with us

Reducing battery charger output

Discussion in 'Power Electronics' started by HomerDodd, Feb 23, 2020.

Scroll to continue with content
  1. HomerDodd


    Sep 6, 2017
    Good afternoon. I have a standard car battery charger, which when set on the lowest setting outputs 12Volts and 2amps, which is great for slow charging larger car batteries. However, I also have a much smaller motorcycle battery which will literally get warm or hot if charged with 2 amps or more. My question can I somehow reduce the 2 amp output to 1 amp or even .5 amp , which will then save me the money of buying a motorcycle charger ( I will if I have to). My first thought is a simple resistor pack connected inline between the chargers positive output cable and the motorcycle battery positive post. Just want to make sure I do this correctly, if possible. Is there another way (caps and/or resistors). Do you have any size recommendations ? Thanks !!
  2. Bluejets


    Oct 5, 2014
    Charge current should be able to be reduced using some means of voltage control.
    i.e. the smaller the voltage difference between the charger and the battery, the smaller the charge rate.

    Problem could come from just what type of charger you have if considering any active electronic component as some of these simple battery chargers are really just pulsed dc with some degree of ac component.

    If thinking along the lines of a resistor, the cheapest most effective would be to experiment with a range of 12v auto bulbs until you get the desired effect.
    You could , for example, start with say a brake/tail incandescent lamp which I think is 21w on the brake section.
    If too much drop, perhaps a headlamp which normally comes as 55w/75w hi and lo beam.
    Connect the bulb in series between the charger and the battery.

    Failing that some nichrome wire mounted on a air cooling frame with one end fixed to the charger output and the battery under charge via a slider crocodile clip.
    Obviously a multimeter set on hi current range to initially monitor the rate.

    Finally there are some pretty cheap plug pack trickle chargers on Ebay etc. for motorcycle batteries.
    hevans1944 likes this.
  3. duke37


    Jan 9, 2011
    My first thought is why do your batteries need charging? It would be better to make sure the vehicle system is working properly. I last used a charger about thirty years ago when the brushes in my car dynamo had worn out. The charger is now used to drive a coil winder.

    12V lead acid batteries should be charged to 14.0V The current will be lower in a small battery than in a large battery. Charging with a constant current will overcharge the battery thus lowering its life,

    I have been given a couple of alarm boxes which contained a 12V battery. They were fitted with LM317T regulators with the voltage set to 14V. The batteries were supposed to be changed every ten years, These have now been used to provide 24V for a railway slave clock system.
  4. HomerDodd


    Sep 6, 2017

    The battery drained slowly over the past few months of the motorcycle not being run on the winter. It is actually quote common. I am buying a Battery Tender to keep it charged during the off season. I am going to use my car charger, with a 12v lamp in series between the positive charge cable and positive battery post. That will reduce the charging voltage to a safe level.
    hevans1944 likes this.
  5. dave9


    Mar 5, 2017
    I'd measure resultant current when that is put in place with the battery fully charged already. If it's more than a couple dozen mA it is too high. Even that, maybe too high for long term storage. Even an AGM battery can have outgassing if the electrolyte is being liberated.

    As Bluejets mentioned there are different types of "standard battery charger" now and some have their own regulation. You might want to experiment with different resistances or use a linear regulator set to a target float voltage (using an LDO regulator if the voltage margin isn't enough otherwise which it may not be if the charger is regulated, or even a diode in series if the voltage margin is smaller still).

    Then again there are trickle or float chargers purpose made for this, and surely some DIY circuits floating around the 'net that need nothing more than an unregulated little 12VDC wall wart as a power source. Being unregulated their low load voltage would float up above the needed battery float voltage, including enough voltage margin for an LM317 regulator in series at the low float current rate you'd need for that size battery.
    HomerDodd likes this.
  6. duke37


    Jan 9, 2011
    No, the resistor will reduce the current but will not restrict the voltage.
    If you charge through a resistor, keep a record of the battery voltage and stop charging when 14,4V is reached,
    If the battery has been over discharged it may be necessary to use a high voltage to change the electrolyte from high resistance water to acid. The battery will have very little capacity.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Yes, a properly sized lamp will do the job, but the voltage drop across the lamp will decrease as the battery voltage approaches the charger voltage and the lamp resistance will therefore also decrease. You need to make sure that the charger voltage remains low enough to prevent overcharging if the battery remains connected for months at a time.

    During the winter in Dayton, OH, I would remove our motorcycle batteries and bring them into our heated basement for storage and as-needed charging. It was an inconvenience to re-install the batteries, but if weather permitting wife and I decided to "suit up" and go for a ride, that is what we did. We were always properly dressed for motorcycle riding and almost always wore our helmets. Do it right and you can tolerate even the coldest of days for a few hours, even if your motorcycle battery doesn't!

    I used to have a Variac connected to the primary of a really large low-voltage transformer that in turn was connected to a large, convection air-cooled, silicon bridge rectifier. This combo could be used to start my vintage 1968 Mercury Cougar with its small V8 engine, delivering a hundred amperes or more to the starter motor... I didn't have a shunt to measure the starter current at that time in the 1980s when I acquired those parts.

    Later on, after ruining a 50-0-50 ampere D'Arsonval meter movement equipped with an internal current shunt, I removed the meter and the Variac and substituted a length of nichrome wire in series with the positive output and used the transformer/rectifier combination as a battery charger for 12 V DC car batteries. The nichrome served as a crude current-limiter, but I never did try to use it to measure the charging current. Nichrome, like tungsten light bulbs, has a positive temperature coefficient of resistance, so as it gets hot the resistance increases. Good thing, too, or electric heaters and light bulbs would quickly "run away" when connected to a constant-voltage power source such as the usual house wiring.

    Anyhoo, this lash-up was not a good thing to be left unattended. I always monitored the battery terminal voltage by periodically disconnecting the charging wire and waiting for the voltage at the battery terminals to stabilize. If it was anywhere near 12 V after waiting five or ten minutes, I would try to start my car with the newly recharged battery. If the car started, I checked the battery terminal voltage to make sure the alternator had taken over the charging function. If the alternator appeared to be working, I rolled my "charger" back into the garage to be used another day. I figured the car alternator circuit was "smart" enough to finish charging the battery... assuming the battery hadn't been ruined by the deep discharge cycle it had just experienced.

    It seems to me that anyone can sometimes forget to turn something off that isn't turned off by the ignition key... so, the next morning they get up and discover their car won't start. That's why I kept the kluge charger around, even though I have a AAA road-side service subscription. Most of the time we had two vehicles and I could use the battery in one of them to jump-start the other if the "dead" battery still had even a little charge left in it. Now that we are down to one vehicle, I guess I will resurrect the kluge charger, which I disassembled because I wanted to equip it with larger wheels purchased years ago for that purpose but never installed.

    The suggestions given above regarding charging circuits appear to be good ones, but since I don't ever see the need to "trickle charge" a car battery, like Grandfather did to make sure his Dodge would start on a cold West Virginia or eastern Tennessee morning, I doubt I will make any "improvements" to my beast of a battery charger. Best way to take care of your motorcycle battery is to keep it warm and recharge only as necessary. Use a hydrometer to make sure the cells are okay and remember to adjust the hydrometer readings for electrolyte temperature.
  8. HomerDodd


    Sep 6, 2017

    Thanks. I flushed the battery with hot distilled water. I could not see any obvious plate sulfation. Then refilled with Epsom salt and distilled water, which is homemade electrolyte. It's base voltage was only about 9 volts, which is very low discharged. I then put the charger and lightbulb inline and measured the amps. It appears to charging at about .60 amps, which is lower than I wanted (I wanted about .9 or even 1 amp). I may get a lower wattage bulb which may raise the amps slightly, and charging voltage, which is about .750v .However, I will just give it some time and once it reaches about 14 volts, I'll give it a try.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day