Regarding SLA Battery

Discussion in 'General Electronics Chat' started by R!f@@, Nov 26, 2015.

  1. R!f@@

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    I need to place an access controller in my home front door.
    All the components are on it's way and I went to the Wiegand ones which is secure.

    It's necessary to see that these should be on a back up type supply.
    I have seen a lot of the PSU with 12V7AH SLA charger ones. Basically same theory as UPS but not that good.
    I am saying not that good as even thought the seller says it has battery protection features I doubt it is but a basic 12V charger which can be adjusted to 14V if desired. I guess it's nothing but a 3A to 5A PSU.

    An APC UPS is better in all sense than these power supplies. I know. I can tap of 12V from battery to run the access controller, no problem.

    One problem as power cut is once in a blue moon the battery as always on charge. As like all my UPS. I always discharge the battery in the UPS once a month to keep them cycled. But I cannot do that in an access controller, I know I can't so I am looking for a solution. I need back up as my access controller is fail secure type. No power no one goes in or out, you know.

    The controller only draws a noticeable current like 1Amp during unlocking and all the other times it is just idling, that is the control board draws at around 100mA and a few more for the card reader.

    Method 1.
    My plan is to use the 12V7AH SLA to power the system always and use a SLA charger to charge the battery. Charger is not always on line but switches on and off after monitoring the battery voltage. I can make the charger and the monitoring circuit, no problem.

    Method 2.
    Use a UPS, tap of from battery or use a PSU which is powered from the UPS.

    Method 2 is how UPS is actually used but as far as I have seen the continues usage of UPS without monthly discharge seems to bulge the battery with in a year or so.
    And it is heavier and takes more space.

    Question....what do you suggest.

    Method 1 will always cycle the battery. As the system is powered from battery putting a continues drain on it and will charge if below the discharge thresh hold. Of course during charging the charger will supply the access controller.
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
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    Build a charger that has a regulated 13.7 -13.8volts, with enough current to run the system + 0.10C of the battery.
    If you are using a 7A battery this relates to .7A or 700mA. If the the battery is charged at13.7 to13.8V it will never be overcharged.
    This is the reasonable charge current at this voltage. The battery would normally be fully charged when the charge current drops to 0.01% of the 7A capacity(70mA) The only other thing is I would have a disconnect circuit if the battery were
    to discharge to about 11.9V.
     
    Last edited: Nov 26, 2015
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  3. MikeML

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    I run my Arduino-based motorized drive-through gate off a 12V 7Ah SLA. The standby current is a few mA. Motor start/stall current is ~3A, motor run current is ~1A.

    I run everything directly off the battery. The chargers are two; a whimpy unregulated AC-powered wall-wart, and a small solar panel. The Arduino controls both chargers, measuring the battery voltage, and either turning on or off the respective chargers. Since the charging current is small (<<1A), the rate of voltage increase across the battery is quite small, so the charger control acts like a house thermostat, the battery voltage is either always slowly increasing, or decreasing...

    It has been running for a couple of years. Battery seems to like the cycling between about 13.5V and about `13.8V.
     
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  4. R!f@@

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    Battery cycling is good then. Eh!
    I am assuming that you guys are agreeing that I power the system by Battery and automatically connect charger when the Vbat is like 11VDC
     
  5. MikeML

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    Never, ever discharge a SLA t0 11V! It is stone dead at a terminal voltage of 11V

    I advocate cycling it from 13.8V to 13.5V as long as a charging source is available. If there is is no sun, no AC power shut off the load when the battery voltage discharges below 12.0V
     
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  6. spinnaker

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    I push an SLA down to 11.5. It was been running fine now for more than two years. It is charged by a solar panel. At night the battery provides power for an LED array to light our communities front sign. Seems to stay lit most of the night even after two years of use. And the LEDs in the array are very old. I am sure there are much better selections now but it works!

    I use an off the shelf charge controller from Sunforce.
     
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  7. R!f@@

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    Isn't 10.6V low threshold of 12V SLA?
     
  8. AnalogKid

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    It is on many units from PowerSonic, my favorite brand. Also, to your original question - Like you I like to build stuff, so I recommend building the charger and monitor circuits rather than tapping into a UPS.

    ak
     
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  9. ian field

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    The microcontroller smart charger I have bulk charges to about 14.4V then backs it off to 13.6V to minimise gassing the distilled water out of the electrolyte.

    I left a spare motorcycle battery on it for about 3 years with electrolyte top up about every 6 months - the battery got lazy, but recovered well with cranking current and a few charge/discharge cycles.
     
  10. k7elp60

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    No! both MikeML and I say keep the charger on all the time but make sure the charger is between 13.5 and 13.8V. This is really easy to do
    with a LM317T., plus disconnect the load from the battery if the battery voltage is below 12.0V. A P-channel power mosfet works good for this.
    I can help you with actual schematic for the charger and disconnect circuit if necessary.
     
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  11. ian field

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    Charging to a voltage that won't gas it dry, won't recharge a depleted battery.

    Charging to a voltage that fills it up will gas it dry if left continuously.

    Float charging requires some smarts - it needs to sense full charge and then back the voltage off.

    Probably the simplest solution is a window comparator driving an SR flip-flop. Switch to constant current at 1/3 the Ah capacity for bulk charging, and switch to about 13.6V for float/maintenance.
     
  12. R!f@@

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    I haven't yet started designing the circuit but a schematic will help to speed things up.
    I am going to use a sealed maintenance free SLA that are found in UPS. I can buy the battery here easily.
    And I have LM317T. I was thinking about using that chip to make the charger.

    Still, what if I power with the battery keeping the charger off. When Vbat drops to the specified limit charger switches on, charges the battery and shuts off.
    That way I can keep the whole unit temperature low. The Transformer will be off, less heat in the box and all that
     
  13. R!f@@

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    What do you think of this circuit
     
  14. ian field

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    If you charge the battery with constant current - adding a series feed diode means that your window comparator/SR latch can simply switch in a shunt regulator set so the voltage after the diode never exceeds 13.6V in that setting.

    Yuasa published a "Little Red Book of Batteries" that had all sort of useful info, there were some sample schematics, but AFAICR: no component values - I can't find the one they sent me, and I don't know whether its still available.
     
  15. ian field

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    A Google search found 3 immediately.
     
  16. k7elp60

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    I agree with ian field on the battery will not get fully charged if the charge voltage is limited to approximately 13.8 volts. This method of charging
    is called the standby and it does work well. I have built a number of projects that used a 7ah Gel-Cel in the standby mode. They were all charged with a solar panel and they lasted 2 years or more. They all had a load disconnect switch in case the battery voltage went below 12.0V. Ian Field's method is called cyclic and most auto's use that method. the diode in the schematic posted by R!f@@ has another purpose, that is to prevent the battery being discharged if the line power is lost. I am going to upload a pdf file by powersonic about Gel-Cell batteries. I hope this helps.
     
    Last edited: Nov 26, 2015
  17. MikeML

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    This is an incorrect statement. The correct statement that applies to Lead-acid chemistry batteries is below: (Comes from this reference)

    Minimum voltage
    Anything above 2.15 volts per cell will charge a lead acid battery, this is the voltage of the basic chemistry. This also means than nothing below 2.15 volts per cell will do any charging (12.9V for a 12V battery) However, most of the time a higher voltage is used because it forces the charging reaction at a higher rate. Charging at the miminum voltage will take a long long time. As you increase the voltage to get faster charging, the voltage to avoid is the gassing voltage, which limits how high the voltage can go before undesirable chemical reactions take place. The typical charging voltage is between 2.15 volts per cell (12.9 volts for a 6 cell battery) and 2.35 volts per cell (14.1 volts for a 6 cell battery). These voltages are appropriate to apply to a fully charged battery without overcharging or damage. If the battery is not fully charged you can use much higher voltages without damage because the charging reaction takes precedence over any over-charge chemical reactions until the battery is fully charged. This is why a battery charger can operate at 14.4 to 15 volts during the bulk-charge phase of the charge cycle.


    This is also a wrong statement. Read the above.

    No it doesn't. Read the above.

    What does the switching? This is not consistent with the three-step charging method advocated by the battery manufactures.

    Read the charging methods described in the Powerstream reference. For a simple system like @R!f@@ requires can be handled by the section
    Taper chargers or Constant Voltage Chargers (Taper plus current limit).
     
    Last edited: Nov 27, 2015
  18. MikeML

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    It implements the three step charging algorithm described at Battery University or at PowerStream. It uses a current limiter, a two level voltage regulator, a low-current detector, all implemented in analog ICs.

    I have implemented this algorithm in a PIC and an Arduino.

    Unless the shorter recharge time is a requirement, the two-step (current limit, fixed voltage) is good enough.
     
  19. R!f@@

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    I don't think I would need shorter recharge time. Power interruption is very little here. Months can go by without a power cut.
    @MikeML
    Do you a the PIC one. If you don't mind can you share it with me.
    Like to learn that.
     
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