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OT: Charging a car battery

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  • OT: Charging a car battery

    So I've got an old standard car battery. It's sufficiently worn out that my smart charger says it's bad and won't charge it. But it works just fine for running my chain saw sharpener.

    I've got a nice DC variable power supply. Electronics is a weak area for me. Seems to me that I should be able to hook up the DC power supply to the battery, set it for 13.5 volts and just let it charge it up as best it can. Is there some non-obvious to me reason why this is bad for the power supply or the battery?

    Thanks.
    Steve

  • #2
    Originally posted by SteveF View Post
    So I've got an old standard car battery. It's sufficiently worn out that my smart charger says it's bad and won't charge it. But it works just fine for running my chain saw sharpener.

    I've got a nice DC variable power supply. Electronics is a weak area for me. Seems to me I should be able to hook up the DC power supply to the battery, set it for 13.5 volts and just let it charge it up as best it can. Is there some non-obvious to me reason why this is bad for the power supply or the battery?

    Thanks.
    Steve
    Your sharpener doesn't require anywhere near the amperage the starter motor does. 2-3 amps versus 300-500 amps. The battery is probably mechanically worn out- over time the lead on the plates degrades and literally falls off (there is a dead space at the bottom of the battery case to collect the debris). Since the plates are now smaller (less surface area), they cannot provide that huge surge of current to the starter motor, no matter how charged up the battery is.
    Replace the battery.

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    • #3
      Yup plus all the stuff that falls off can be still somewhat conductive and collect at the bottom high enough to get to the plates internally shorting them out, it's toast and time for a new one...

      Comment


      • #4
        If your smart charger won't charge it then you may have low volts and bad cells. Measure the "charged" battery voltage after you take a few minutes of load on it. If it's not say 12.6 (2.1 volts per cell), don't leave it on a charger at 13.5

        13.5 to 13.7 is ok IF the battery has all cells functioning. If one cell is shorted (one of many possibilities) it raises the possibility you will get to gassing voltage on the good cells. Also.. you don't say what capacity your power supply has - if large limit your current while charging, and then particularly when "floating".

        Float charge is 2.35 per cell and gassing voltage is 2.35-2.41. (depends on cell type and temperature).
        Last edited by lakeside53; 01-28-2017, 04:33 PM.

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        • #5
          I have an automatic charger that won't kick in if the voltage is too low. I had a battery that I charged a little bit with a non-auto charger then put it on the auto charger and it charged up OK. Can't speculate as to the condition of your particular battery, but I would check the electrolyte, charge it for awhile and see if it can be made to work acceptably. Just in case you didn't know, running a battery down really hurts its life.
          Location: North Central Texas

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          • #6
            Nobody is answering the question.

            The power supply as a charger CAN charge it. But the difference in voltage between the battery and the power supply will draw a lot of current.

            If you can set a safe (for the charger) limit on current, sure, you can charge the battery.

            I would do a few things.... I'd set a max voltage, I'd set a max current, and I think I would put a suitably rated diode between the battery and the supply, just as a safety measure. The output voltage would need to be higher by the diode drop.

            Lacking a current limit, a high power resistor with a low resistance value would limit current, and allow you to charge without a lot of problems. There would be plenty of "losses", the resistor would get hot. Resistor value would depend on the power supply capability.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              Besides the usual issues with sulphation and battery plates shorting out with age, a battery needs sufficient charge to bring all the plates up to voltage so it can hold a charge and be able to deliver a high current when needed. To do this, a higher voltage of say 14 volts should be applied for long enough. After resting off charge for say 1/2 hour or so, the battery voltage should stabilize at around 13.6 volts or so, which is a full charge. Then you could apply a maintenance charge, which could be that level of voltage from your power supply.

              Simply giving that voltage to the battery all the time doesn't mean it will come to full charge. You might find that after a rest period off charge the voltage will drop to perhaps 13 or so, maybe less.

              Ideally you'd want an intermittent fast charge 'pump up', then the maintenance charge. You should also subject the battery to a high discharge current on a regular basis, just to help keep the plates clear of buildups. A lot depends on battery type.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                A good limiting resistor would be an incandescent lamp, perhaps a headlight bulb. It will probably draw about 10-15 amps with 12V across it (about 1 ohm), but will be about 0.1 ohm when cold. So you could set the power supply to about 14 volts and the charge current will be well under 10 amps into a discharged battery reading about 11-12 volts. If your power supply is current limited you can just set the current to a safe value and set the voltage to 13.5 or 14 volts and it should charge at the maximum current and then taper off to a float charge of perhaps 100 mA. If your smart charger sees a low battery voltage you might be able to fool it by putting a diode in series to boost the reading by 0.7 volts, which might be enough. But it might expect a certain voltage from the battery before it will try.
                http://pauleschoen.com/pix/PM08_P76_P54.png
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030

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                • #9
                  Bulb is a very decent idea. I like it.

                  You can select one that will not pull more than the supply can provide, even if the battery shorts completely. And you get a warning light if that happens!
                  1601

                  Keep eye on ball.
                  Hashim Khan

                  Comment


                  • #10
                    I used 2 x 120v 150 watt light bulbs in series as a "fuse replacement" when I was working on my washing machine 3 phase motor controller. Sure saved a bunch of fuses and fried diodes.

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                    • #11
                      I remember working on a speaker system in a local watering hole once. The speakers had inline bulbs for protection. It was interesting to power it up and watch the light come through the speaker cone when you cranked it. Never was a decent sounding system.

                      Need to know more about the power supply. Maybe it has everything you need on it already, but maybe not. If it doesn't have enough current capability then it could be working too hard constantly just to put a reasonable charge into the battery. If it has lots of current capability, then you might need to protect it in case of a shorted battery- the headlight bulb as resistor might be fine for this. If it has current limiting you can set that as a safety against a shorted battery, but you also have to be aware that it will probably overheat if left in that mode too long. It's possible that all you need is a voltmeter and some common sense. If the battery voltage comes up to say 11 volts or more when under charge, then it's not shorted. If the voltage comes up to whatever you can dial in and no current flows, then it's a dry battery. If lots of current flows but the voltage doesn't quickly rise above about 11, then you have at least one shorted cell.

                      Tell us more about the power supply-
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                      • #12
                        Generally, if the battery voltage after charging is some multiple of 2.1 you can infer a few things. For instance, 10.5 volts indicates that you have lost one cell, and it is shorted. 5 cells are still working. 10 volts is sometimes useful too.

                        Some of the smart chargers have an "anti sulfation" mode that is supposed to reduce or reverse the impact of overcharging. I've had success with that once.
                        At the end of the project, there is a profound difference between spare parts and extra parts.

                        Location: SF East Bay.

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                        • #13
                          Originally posted by SteveF View Post
                          So I've got an old standard car battery. It's sufficiently worn out that my smart charger says it's bad and won't charge it.

                          I've got a nice DC variable power supply. Electronics is a weak area for me. Seems to me that I should be able to hook up the DC power supply to the battery,
                          Thanks.
                          Steve
                          I also dont know much about electronics eiether Steve, worse at spelling

                          The "smart charger" is just a gimic. The best battery chargers are like old school welders. The bigger the transformer the better. Problem is, copper is expensive.


                          The electronic chargers see a short. So they fault out as they should. An unregulated power supply should not be used. JR
                          My old yahoo group. Bridgeport Mill Group

                          https://groups.yahoo.com/neo/groups/...port_mill/info

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                          • #14
                            Originally posted by J Tiers View Post
                            ...Nobody is answering the question...
                            Perhaps not specifically but they were providing the "correct" answer. The battery is toast, throw it away and get another one. Messing around with power supplies and resistors and capacitors is just a waste of time.

                            Originally posted by Joel View Post
                            ...The "smart charger" is just a gimic...
                            I dunno; we've had a Black & Decker "smart" charger at the shop for about a year now and it's the best charger we've ever had. It's load sensing; charges at 2, 10, 20 and 40 amps; has a "boost" mode as well as the ability to test and desulphate. It works like a charm--I'll never buy another "conventional" charger again. Oh, and it's very light for its size--easy to carry around...
                            Keith
                            __________________________
                            Just one project too many--that's what finally got him...

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                            • #15
                              Originally posted by LKeithR View Post
                              Perhaps not specifically but they were providing the "correct" answer. The battery is toast, throw it away and get another one. Messing around with power supplies and resistors and capacitors is just a waste of time.

                              .......................
                              No, tossing it isn't the right answer. This battery became questionable for vehicle usage (voltage would drop down to about 12.5v after a few days) about two years ago. It sits in my shop, gets topped up with the smart charger periodically (desulfation didn't help) and does a perfect job of running my chain saw sharpener every couple of months. The last time I tried to charge it the smart charger didn't like it and I just got the DC power supply.

                              So thankfully Jerry is an engineer and knows how to stay focused on the question (as well as some of the others).

                              Thanks for the answers. Sounds like I'm good now. May just decide the run the sharpener off the power supply directly.

                              Steve

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