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Battery charger questions

Discussion in 'Power Electronics' started by Tha fios agaibh, Sep 9, 2017.

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  1. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Scenario 1:
    Someone asked me to repair a golf cart style battery charger. I found there was nothing wrong with the charger itself. The charger was not turning on because it didn't see its minimum cut-on voltage because the batteries were too dead.

    Q1:Why do they design a charger with a cut-on voltage that is so high that it won't recognize dead batteries?

    Scenario 2:
    Large batteries used in forklifts and similar equipment use scr type chargers that are able to detect voltage and battery capacity and automatically adjust the charge program to suit the battery that's connected to it.
    Q2: How does it detect the connected battery capacity (in ah), just by the 2 wire charging leads? I may have a 36v battery with a capacity of 510ah, and another 36v battery with a capacity of 750ah. How does it know the difference?
    Q3: If I have a 48v battery that is ran down to 36v how does charger know its a 48v battery and not a charged 36v battery?

    This 2nd type charger is an Ultracharge model multi-volt charger made by Amtek. Batteries don't have the optional memory chip (bid module) that the charger talks to. Just the pos and neg charging cables.
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    Q1 What's the point in trying to charge a battery that is beyond recovery.

    Others...usually the charger injects a burst of charge of differing levels to ascertain details from the battery.
     
    HellasTechn likes this.
  3. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    They are not beyond recovery, just over discharged. I charge them with another type charger and once they climb a few volts higher the original charger works fine and they hold a charge as normal.

    Can you elaborate on the latter? How?
     
  4. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    I'm guessing it applies a high frequency to the battery and then measures the voltage and current response. Kind of like an Esr meter?
     
  5. dorke

    dorke

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    Jun 20, 2015
    A1: This is probably due to the fact that you had on-hand a constant-voltage charger.

    These devices are the cheapest and simplest chargers.
    The charge current is determined by the difference between the charger voltage and the battery voltage divided by the internal battery resistance(+external series resistance if there is one).

    If the battery initial voltage is too low (a deeply discharged battery),
    then the initial charging current may be too high and may damage the battery.

    charger1.JPG
     
  6. HellasTechn

    HellasTechn

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    It could be due to the fact that lead accid batteryes that stay over discharged for too long may become what i call "decapitated" messed up internally.

    Im not sure. just guessing.

    Dorke's explanation makes more sence though.
     
  7. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Unless specifically designed for "deep discharge," such a certain marine batteries, lead-acid batteries are irrevocably damaged by discharging to zero terminal voltage. Not sure why this happens, but I know from several personal experiences that it is true. Even if you do get it to accept a "charging current," it will never be the same and its capacity will be severely diminished. Best to buy a new one, and not waste your time trying to charge a lost cause. Life is short: eat dessert first.
     
  8. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Thanks for the explanations.
    Yes, I understand the relationship between voltage and current during the charging cycle and I agree there are certainly limitations of the battery.
    But, I have experienced this problem many times with this type chargers and the batteries almost always perform fine just by boosting them a few volts. These same batteries can then be used again and again on this same charger and will have a good run time.
    These batteries are not totally dead, just ran down more than normal.

    I don't know the exact cut-on and off voltage of this charger, but I think it will only begin a charge at >20v and will stop once they reach ~28v for a 24v battery.
    I'm sure I'm just below the threshold of ~20v, (nowhere near zero) but I would expect this cut-on voltage to be much lower and still not present a problem.

    Even with a extremely dead battery, the starting current should still not be higher than the rating of the charger and if it is, the battery is damaged already (shorted cells) Besides, the charger has additional protection like a thermal and a current limiting fuse, as well as a timer feature that will turn the charger off after multiple hours in case the voltage never rises to it's cut-off (fully charged) voltage.

    So yes, I see the point that this cut-on voltage could be a safety feature but it sure seems high to me.

    Regards, John
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Well, the nice thing about it is you are free to change it. How hard can that be?:D
     
  10. dorke

    dorke

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    Jun 20, 2015
    John,
    What is the model and make of the charger?
    Are you talking about Lead-Acid batteries?

    In general,
    the issue is protecting the battery,the operator and mostly the charger manufacture from lawsuits,
    not so much the charger itself(which may probably have all the protection it needs internally).

    Batteries are potentially risky and dangerous items,some may explode,catch fire etc,
    unfortunately people died because of batteries faults.

    If you are talking about Lead-Acid batteries,
    they are less risky(don't explode or catch fire),but they can spill/"spit" their acidic electrolyte liquid and release flammable gases which may cause harm in many ways.

    The above unwanted and potentially damaging things may be caused by charging a Lead-Acid battery with too high current.
    This high current causes over heating of the battery and the electrolyte may gas and boil.

    So,
    yes a deeply discharged battery may still be good and may be revived.
    but since they are "out of spec" some commercial charger manufactures prevent their charging for protection as explained above.

    Dealing with this situation requires the operator to understand what he does,take precautions
    and as they say "at is own risk".
     
    Tha fios agaibh likes this.
  11. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Screenshot_2017-09-10-11-13-01-1.png I don't know the model number off the top of my head but it's made by Lester and looks like this one.
    Yes, it's for lead acid battery use.
    I believe its max is 30amps.

    I understand the manufacture wanting to cover their arse, but it's a little frustrating to have to find a backdoor solution every time.

    I agree, but the harmful high current side effect of gassing, boiling over of electrolyte and overheating can happen even if the battery voltage is not low. A battery can have one bad cell while its overall voltage is within specs. It's not until the charge cycle is midway that you find it's not accepting a charge and these things are evident.

    I don't own the equipment (my employer) so I don't want to start modify things. Besides, it doesn't happen all that often.
    I do feel better complaining about it though.
    Lol.

    Migh be fair to say the cut-on voltage option has good intentions but this company went overboard. Or, should I say overvoltage?
     
  12. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    I know it's more complex than the first question but; Anyone want to take a stab at Q2 and Q3?
     
  13. Terry01

    Terry01

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    Jul 5, 2017
    If you wanna know about batteries and stuff take a look at wheelchairdriver,.com website. You'll find everything(I mean everything) there. I use it for wheelchair stuff. Some of the guys there are very clever and into the tech stuff.
     
  14. Jamesla

    Jamesla

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    Nov 9, 2017
    Hi there, I think if your use is more you need to have a strong battery with higher voltage. My grandpa who uses a power chair for his mobility he uses a high voltage battery from Spinlife. I think they can provide you the best stuff.
     
  15. Terry01

    Terry01

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    Jul 5, 2017
    One of the things with wheelchair batteries is say someone who uses their 4mph chair for dodging around a flat smooth care home with vinyl floors all day may charge up overnight and still be full at the end of the day so "think" their batteries are fine. Stick those same batteries in my 6mph chair and use it in the house with carpeted floors then out for a few hours up some hills and a few 4 inch kerbs from a standing start and they'll be dead within the hour. Same even then once the chair konks out dead you could leave the LED lights on for a day or there about till its properly dead.

    That may be where you "think" some batteries are fine. Just depends what you use them for how fine they really are.

    Same too you can buy cheap Chinese batteries that will do for an easy days use but even the more expensive MK Gel or Oddesy deep cycle ones will lag soon enough if your a heavy user. Once I say my batteries are done and need to buy new ones they would do for running lights etc fine.

    One guy uses his old wheelchair batteries for running his house,just keeps them on a trickle charge from his solar set up. Gets most of his elecy from it.
     
  16. kellys_eye

    kellys_eye

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    Jun 25, 2010
    It doesn't. Just because there are only two wires going to/from the battery doesn't mean there aren't multiple connections to the two wires from the point at which they may terminate (i.e. on a circuit board).

    This pdf http://www.ti.com/lit/an/snva557/snva557.pdf is very good at explaining the various methods for detecting battery state and adjusting the charge rates.

    One 'key' sentence from that paper states the following:

    .... it must be remembered that every real cell contains an internal ESR (Equivalent Series Resistance), and the voltage that the charger senses across the battery is influenced by the ESR.
     
  17. Terry01

    Terry01

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    Jul 5, 2017
    Another answer for the Q2. Question. The chargers work on a simple BMS module detecting how much the battery is pulling from the charger. It just cuts off as the charging A gets near zero whether it's 10ah or 100ah.
     
  18. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Like I said, There aren't any connections or modules on the battery, Only two power leads.
    Thanks for you post. I will read your pdf link in its entirely when I get a chance.
    Yes, I believe Esr is key to understand how it determines what type of battery it is. (like I asked in post 4)
    So I'm thinking basically, a certain voltage and esr resistance of a battery will give the charger a current value range that it correlate as being a certain number of cells in the battery.
    I'm sure its more complicated than that, but I wanted to get a basic understanding of how it worked.
     
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