Can I charge my car battery with 75 amps?

Discussion in 'Power Electronics' started by earthtodan, Nov 30, 2010.

  1. earthtodan

    Thread Starter Member

    Dec 3, 2008
    24
    0
    I have my car in the garage while I do a pretty extensive stereo install, probably two weeks. I'd like to tend the battery, since once in a while I use some function or other. I have a 12V 75A power supply in the garage. Will it damage the battery to hook up a high current supply like this? How long should I leave it if I do?

    Thanks,

    Dan
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    315
  3. timrobbins

    Active Member

    Aug 29, 2009
    318
    18
    Does the power supply have a variable current limit? Does the power supply have a variable voltage limit limit? Does the power supply have a reverse current protection mechanism or circuit scheme to stop the battery discharging back in to the power supply?
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,851
    859
    12V psu is not enough to charge a lead-acid battery, you need about 14V.
     
  5. tom66

    Senior Member

    May 9, 2009
    2,613
    215
    You could however try wiring the power supply in to the car and running it off the power supply. It should be fine to run the accessories. Remember to disconnect the actual battery. Do not try and start the engine though; you are likely to damage the supply due to the hundreds of amps pulled by the starter.
     
    tranzz4md likes this.
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,194
    1,761
    You really do need a charger designed for automotive batteries. A plain 12v power supply won't do the job.

    Fully-charged automotive batteries will read 12.7v-12.8v when their internal temperature is 25°C/77°F. If they are allowed to discharge down to 12v, they will be over 50% discharged, and will have a very short service life.

    Get a decent automatic charger capable of between 5A and 12A output, and use it at least once a week to keep your auto batteries fully charged. The money you save will be your own.
     
  7. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,422
    69
    +1 Sgt.
    Despite 12V not being enough to charge the battery, using a 75A power supply will probably spell disaster for your battery, especially wilthout temperature monitoring...etc.

    iONic
     
  8. timrobbins

    Active Member

    Aug 29, 2009
    318
    18
    I very much doubt that anything like 75A would flow though a discharged car battery at 12.0V, depending on battery chemistry, and if the SOC was high then you probably wouldn't get any charger current at all.

    Over-current damage is really only a concern if SOC is quite low (given that charge voltage is at manufacturer recommended level).
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,422
    69
    The assumption would be if the voltage was higher, 14V - 15V. It may not reach 75A, but probably more than the reccomended charge current.
     
  10. earthtodan

    Thread Starter Member

    Dec 3, 2008
    24
    0
    Thanks for the replies guys. I've been meaning to read up on Battery University but I haven't had time yet.

    The PS is a Cascade (made by Iota I believe), with reverse current protection but no variable voltage. It's nominally 12V but I measured it at 13V. The car's charging system is 13.8V. The battery is fine, it's not discharged, but I'm worried it will get drained over two weeks of working on the car, and opening and closing the doors, since can't find all the fuses that control the dash lights. So I'm not trying to charge it, I'm trying to tend it.
     
  11. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,422
    69
    I'd still rqther use a 12V+ wall wart at 500mA. Will you be removing the car battery or leaving it in the car?


    iONik
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,194
    1,761
    Disconnect the negative (ground) cable from the battery terminal until you are ready to test it.

    Your power supply was not designed to function as a battery charger. Don't try to use it as such.
     
  13. earthtodan

    Thread Starter Member

    Dec 3, 2008
    24
    0
    That's advice I can take.
     
  14. cparke

    New Member

    Aug 28, 2017
    8
    1
    I know this is a old, long abandoned thread. However, it appears at the top of my Google Search results, and it covers a timeless topic, so I'm going to revive it now with new questions.

    I notice that large professional car battery charger units on wheels (ones which support starting a car with a dead battery and which also have testing functions, typically sold in places like AutoZone as high-end models) also boast high charge currents like 60-amps, so I'm wondering how is that considered safe for a battery? Most people seem to recommend 10-amps max. for charging car starter batteries.

    Furthermore, I have a 12V 100-amp Converter/Charger (PowerMax PM3 Series is the unit if anyone is curious). Like the OP, I'm wondering if this can be used to charge a car battery? The device is supposed to support 3-stage charging for lead acid batteries, but that max. charge current seems so high for the battery! The unit is also quite small compared to the professional units, how can this little guy put out so much current? The unit also somehow functions as 13.6V power supply when not connected to a battery, does that make any sense?

    Thanks everyone for your thoughts!
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    7,023
    1,453
    Welcome to AAC!
    Depends on the battery. The battery label (or manufacturer) should tell you the recommended charging current.
    Bear in mind that those are Chinese Amps :D.
     
  16. bwilliams60

    Active Member

    Nov 18, 2012
    841
    136
    Charging batteries at high amperage or high voltage will result in boiling batteries and excessive gassing. The result is a lot of hydrogen gas in the air and a volatile situation. Batteries should be charged at low amperage (2-10) for optimum charging and voltage should be monitored. Most people believe that their batteries are being charged in the 13-14 volt range, but if it is sulphated, you can find them up around 17-18 volts which also results in gassing so you really need to be aware of how you charge a battery.
     
  17. MrAl

    Distinguished Member

    Jun 17, 2014
    3,612
    756
    Hi,

    If you think about this just a little you will get some insight. There are a few different issues to consider.

    First, charging at 60 amps isnt a good idea except for maybe up to 10 seconds when trying to start the car.

    Second, even if you could charge at 60 amps that would require at least 15 volts and as you know DC power is volts times amps so we have 15*60 which equals 900 watts, which is just under a regular room space heater which might be rated 1000 watts for example.
    So you can see right away a problem comes up and that is one of power dissipation. It's a little more complicated than that because we have what is called the charge acceptance factor which tells us how much energy the battery is absorbing as a charge increment, but in simpler terms the battery is not perfect and so it will get hot. As it gets hot it becomes dangerous due to the type of chemicals inside which can start to actually boil. You would not want to boil a can of gasoline for example.

    So 60 amps isnt a good idea. Maybe 10 amps or 20 amps max if you watch the case temperature. 10 amps is a good charge rate that isnt too slow and isnt too fast.

    Another issue is that you need the voltage level to be high enough to push a charge though the battery in the first place. It should be obvious that if your battery was 12v and your charger was only 11v, it would not be able to charge the battery at all. The charger voltage must be above the existing battery voltage and it must be significantly above it. For example, if the battery is 12v then the charger has to be at least 13.5v as a quick estimate, but that will probably just push the minimum charge current through the battery. More typical values are 13.8 to 14.4 and that upper end there usually does it. It all depends on how good the battery is too though. If the battery is aged it may take a little more to push significant current like 10 amps through it. The equivalent internal resistance is what limits this current with a given charger voltage.
    The whole idea really is to adjust the voltage of the charger so that you get a charge current that is resonable, such as 10 amps. A little below that is ok too, like 5 amps, it will just take longer to charge.

    Batteries do explode due to escaping gases and a spark, so it's a good idea to connect up before turning the charger on, if that is ok with the manufacturer of the charger.

    I use a power supply to charge my lead acid batteries. The power supply is a current limit adjustable and voltage adjustable power supply, so i adjust the voltage up to some level where i get a reasonable current flow and then that's it, i leave it for a while and calculate the ampere hours that went into the battery over time and if that seems like a reasonable amount i halt the charge. As the battery charges with constant voltage the current level drops little by little so if i start out at 5 amps it could drop as low as 1/2 amp and then it is probably done when using a drive voltage of 14.4 volts. The wires i use are about 4 feet long so there is a little drop in voltage there, which causes the charge to be tapered, which means the current drops as time goes on with the voltage constant.
     
    Last edited: Sep 3, 2017
    tranzz4md likes this.
  18. jayanthd

    Active Member

    Jul 4, 2015
    635
    60
    A 75 Ah battery required 7.5A charging current.

    A 50 Ah battery required 5A charging current.
     
  19. cparke

    New Member

    Aug 28, 2017
    8
    1
    eteresting responses - however many of you seem to not be familiar with how 3-stage lead acid battery charging works:

    1) In the first stage, called the bulk stage, the charger applies a constant current. It does this by applying an initial voltage just above the battery's current voltage to push some current through and then slowly raises the voltage until the current reaches the charger's maximum current or maximum voltage (whichever comes first). It monitors the current flow, and as the current starts to drop, it raises the voltage to maintain the constant current. This continues until a maximum pre-set voltage limit is reached, typically 13.7V-13.9V for a 12V battery. A very good charger has a battery temperature monitor and adjusts that maximum voltage based on the battery's varying temperature. Typically, 75%-80% of the battery capacity will be filled during in this stage at the maximum speed.

    2) In the second stage, called the absorption stage, the charger maintains the maximum pre-set voltage, and watches as the current decrease until it tapers off to a very low value. When this is done, the battery is considered to be filled to its maximum capacity.

    3) In the final stage, called the float stage, the voltage is reduced to a low value just above the battery's voltage, to allow a trickle current to offset the battery's typical self-discharge and stay fully charged. Ideally, the battery would be kept under float charge at all times. Lead-acid batteries should be kept at full charge to prevent irreversible sulfation and permanent loss of capacity.

    My understanding is the maximum in-rush current level is based on the battery's design, should be adjusted for battery temperature, and typically is rated as some percentage of the battery's amp-hour compacity C. A recent poster suggested to use .1C as a rule of thumb for a car starter battery which has many thin lead plates in parallel to achieve low resistance with high surface area to allow for high CCA.

    Knowing all this - back to the original question - Can a high amperage charger (like 60 or 100 amps) be used on an automotive starter battery? Because the battery is essentially a variable resistor, and the charger has a maximum voltage, the battery might not actually take the maximum amps available from the charger. However, that doesn't mean battery won't take a current that is higher than appropriate during the bulk stage. Overheating, gassing, explosion, etc. have been mentioned as risks, assuming of course there is no temperature monitoring on the charger to lower the maximum voltage during bulk phase as the battery temperature rises. (during absorption, I assume a smart charger would notice the amperage increasing improperly, and lower the voltage accordingly to stop the current rise by implied battery heating?).


    Ok, smart as I may seem to be, I'm still confused! Even if the charger has temperature monitoring and 3-stage programming, could the battery still get overcharged or damaged by the higher current? Is plate damage (corrosion, shedding, etc.) still possible? Why do manufacturers and repair shops make and use larger, higher amp chargers for automotive batteries? I've read that higher charge current and/or pulsing is sometimes used to restore lost capacity caused by sulfation that hasn't fully hardened? (called " conditioning") And a new question, does the type of DC charger/power supply (switched mode vs. linear) make any difference to the battery (I notice it does for DC-AC inverters)?
     
    Last edited: Aug 29, 2017
  20. bwilliams60

    Active Member

    Nov 18, 2012
    841
    136
    I am confused. First you come in and pose a question on an old thread for reasons unknown, and then you are a self proclaimed expert on battery chargers. What gives? If you know the answer, share it. If not, I thank you for the education on 3 phase charging, which has been around for a while, and yes, some of us do know how they work, and tell us why you really joined the forum. There are a lot of good and knowledgeable people here who know what they are talking about. Be humble and share what you know. Ask questions for what you don't and you will find some really good answers.
    Cheers.
     
    recklessrog likes this.
Loading...