Can a 12v 70ah agm deep cycle battery’s current become weak, while the voltage remains normal?

Discussion in 'Power Electronics' started by cj_crafted, Aug 5, 2018.

  1. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    I ask this because last year i purchased an AGM 12v 70ah deep cycle battery to use as a power source for magnetic relays (tattoo machines) that usually operate on 5-7v and a 25 watt 35ohm rheostat to control the current. I kept the battery in a plastic housing with the top off so it never touched the floor and I don’t think the battery ever really got too low, yet I have maybe charged it 5-8 times since last summer with a ctek MXS .5.

    Last year the relays would kick on and run even in low current, albeit not very strong. Now in med to low current i see the springs look like they are going to activate. They move just a hair but never kick on unless i give them a nudge or turn the rheostat up very high. Even after this they are easy to stop with my finger. There seems to be a lack of strength or resistance somewhere.

    I have tried:

    • Reconditioning the battery in large battery, AGM, recondition mode on my CTEK.

    •Checking the voltage with a mm through the rheostat, it was fine.

    •Replacing mildly corroded clamps from rheostat housing to battery.

    •Purchased new 35 ohm 50w rheostat and replaced the old one correctly.

    •Sanded some very minimal oxidation from terminals.

    All this and I’m having the same problem. I am totally confounded. I think I may have to return the battery. Or if anyone can help me solve this issue I’ll make you a small A5 watercolor painting of an electric tiger face with lightning and mail it to you. Thanks for taking the time to read this.
     
  2. Audioguru

    Expert

    Dec 20, 2007
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    Maybe the water level inside the battery has become too low.
     
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  3. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    Interesting, I thought you weren’t supposed to water AGM. But read you can do 1oz per cell. I will try this tomorrow and see if it works. Thanks Audio Guru.
     
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Does the battery voltage drop when you are trying to activate the relays or does it stay around 12V?
    If it stays at 12V then the problem isn't the battery.
     
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  5. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    G
    Well, I tried to measure the voltage through the relay and seemed to be like 4-6v or something but the battery through the rheostat remains 12.9ish v. Im using a cheapo Uni-T Ut120C but its brand new. I tried to measure it in the ohm setting and it didnt do anything.
     
  6. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    Oops wrong setting. I realized after re-reading the manual. I’ll measure current tomorrow.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    Don't do that. You might end up killing your multi-meter, if you haven't done that already.
    Yes, a battery may show ample voltage and still fail to deliver sufficient current. That is the common sign of a failing battery.

    Get a 12v automotive headlight bulb and mating connector. Connect the bulb to the battery while shielding the light from your eyes with an aluminium pie plate or something similar. Measure the voltage across the battery with and without the bulb connected. If the voltage drops more than 1V then you have a failing battery.

    Caution: The bulb will also get very hot. Don't touch it with bare fingers.
     
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  8. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    Oh no. I hope i didn’t ruin it. But the manual says you can so I might be ok. I may just go with a different and smaller 12v battery this time. I may be able to return it also.
     
  9. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hello,

    To the point of the main title question, the answer is "no".

    A battery can not control the current to a load, only the voltage. The current is whatever the load draws with the given battery voltage. So if the battery is 12v and the load takes 2 amps, the current will always be 2 amps UNLESS the battery voltage falls. If the voltage falls too much then the battery is at fault unless the load is really high current.

    Not sure, did you mention your actual load current? That would tell us more.
    For a battery like that even a 1 amp load would not drop the voltage too much unless the battery was faulty or needed a good charge. Make sure the battery is fully charged before doing any tests.

    You can test your meter somewhat by measuring a brand new 9v alkaline battery. It must be brand new. The voltage should read around 9.5v approximately. You can test a small 1.5v battery too but the 9v battery is closer to the 12v you really want to read with the lead acid battery. Keep in mind also that the lead acid battery after charging might read higher like 13.5v but will probably drop down to around 12.5v or so after a while with no load.
     
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  10. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    The relays usually read no more than .4 amps. After a charge the batteries are around 13v and eventually settle to 12.7 on my meter.
     
  11. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    What is the voltage across the battery terminals when you are operating the relays?
     
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  12. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    Not quite sure. I tried to measure it by clamping to the relay terminals and it said something like 6v but i have a dongle that i plug my ctek into thats attached to the battery terminals and i will try to measure from there in like 45-60 minutes and see. Ill report as soon as i get a reading. The help is much appreciated to btw.
     
  13. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    When running a relay, measuring straight out of the battery from the ctek dongle Im getting 12.76, and off the positive and negative terminals on the relay it’s really low. Between 7 and .6 even .1v. These were both taken while the relay was running. I may just try to rewire the housing and see what happens.
     
  14. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    From that measurement the battery is fine.
    Now you coud measure between the positive battery terminal and the positive relay terminal with the relay running. Those two should be connected together so the voltage should be very low, <1V. If it is higher then you know that connection is not good.

    If it is good you can repeat that for the negative connection.
     
  15. BillB3857

    AAC Fanatic!

    Feb 28, 2009
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    Doesn't a tattoo machine act like a buzzer to move a needle up and down? Your problem may be just dirty contacts that open when the "relay" picks up and close when the "relay" drops out. Try cleaning the contacts.
     
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  16. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    I clean them every month but still checked and they’re clean. Im not really that messy and they stay relatively clean for the most part aside from dust and what-not.
     
  17. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    I tried the + battery terminal with my + from the DMM and held the negative on the + terminal on the relay. For some reason it jumped to 122.7v- 223.5v DC I have no idea why or how that could be. I measured the negatives the same way and got 11v.
     
  18. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    12.27V ??
     
  19. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    No the decimal was after 122. IDK why. I edited my Last post too. When I did the same thing for the -batt and -relay terms and got only 11v.
     
  20. cj_crafted

    Thread Starter New Member

    Jun 15, 2017
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    To be honest man I get a different ridiculous measurement everytime i do from +batt to +relay. But like a steady 11.89 when I do -batt to -relay. Theres definitely something weird about the +batt to +relay connection. I am going to change out all the wire I think and re wire the housing. That 11.89v is pretty weird though. This battery is only a year old too btw.
     
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