Battery Charging

Discussion in 'General Electronics Chat' started by RodneyB, Jul 13, 2015.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I am trying to understand something about battery charging. I have a 7 Ah battery sealed lead acid. 12 Volts. The load is a 35mA tracking unit. I have a small charger with a maximum current of 130mA.

    Will this charger ever be able to fully charge a 7Ah battery.
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    I would say no. A good rule is to charge at 10% of the AH rating. So, a 3/4 amp charger would be ideal. A 1/2 amp would be minimum.

    A 130 ma charger would be a good float charger if you start with a full charged battery, but 130 ma. would probably not be strong enough to take a discharged battery to full charge.
     
  3. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Thank you for the answer
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Hmmm.. I would think C/50 would be enough to raise the dead. It would take a loooong time, but I don't recall a minimum charging current for an SLA; that might be because I've never needed to know.

    ak
     
  5. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Thank you for the answer
    what is C/50
     
  6. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I am trying to actually understand this charging circuit understanding that its going to take a long time to charge a 7Ah battery if at all. I have built the circuit and I decided to try it on a battery pack from a solar Lantern using this charger.

    I started charging the battery pack from this charger and it seemed to be charging. I just don't know what current its charging at but the voltage seems to gradually be increasing and the batteries not getting hot at all.

    I then connected the 35mA tracking unit and the battery, after half an hour dropped to 5 volts. It was like it was not even charging.

    I thought that the battery would only be drained when there was a loss of power and the unit was drawing from the batteries only. I had hoped that the charger would trickle charge the batteries and run the unit but it looks like this is not happening. The batteries are connected in series there are 12 x 1.2 Volt batteries.
     
  7. Lestraveled

    Well-Known Member

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    The current you draw from a battery it is due to a chemical reaction. That chemical reaction causes sulfates to form on the plates from the sulfuric acid. When you charge a battery you are causing a chemical reaction and those sulfates are removed from the plates and reform to make sulfuric acid. If you leave a discharged battery, uncharged for a long period of time, those sulfates will harden, forming an insulation on the surface of the battery. Once the sulfates have hardened it is very difficult to get them to reform into sulfuric acid through charging. The battery will get weaker and weaker. The same thing occurs when the battery is charged with too little current. Too little charging current does an inadequate job of getting the sulfates to reform into sulfuric acid. Sulfates will continue to build up over time, thus killing the battery.

    In other words, charging with too little current will significantly shorted the life of the battery.
     
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  8. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Does a battery limit the amount of current it draws? As the voltage rises the capacity increase and the current flowing nto the battery decreases is that correct. I am trying to understand the circuit I posted earlier and why the need for current limiting
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Nice explanation. To answer the TS question -

    Rechargeable batteries are rated in ampere-hours. This frequently is abbreviated A-h or Ah, although the correct form is A h. The idea here is that the battery output voltage is essentially flat over its discharge cycle, and the time it takes to discharge it is related to the amount of current it is supplying. So in theory a 10 ah battery can supply 1 amp for 10 hours, 2 amps for 5 hours, or 10 amps for 1 hour. The battery's energy capacity is its max A h rating, and this is abbreviated C (for capacity). Based on this number, the charging current is talked about as some percentage of C. For typical sealed lead acid batteries, the usual charging rate is C/10. With your battery and wall wart, it works out to a charging rate of about C/53. Les thinks this is too low to be effective, and I have no reason to doubt him now that I've read his explanation.

    ak
     
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  10. Lestraveled

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    There are good chargers and not so good chargers. The good chargers will have three modes; bulk, absorption and float.
    - Bulk charge is a constant current charge at C/10 amps until the battery voltage reaches a set voltage. (The charger will adjust the voltage output to keep the current at C/10 amps.)
    - When this set voltage is reached, the charger will go into absorption mode. This is where the voltage is held constant until the current drops below a set point. When the current drops to the set point, charging is complete.
    - Float is there the voltage is held constant at just above the normal battery voltage, keeping the battery from discharging.
     
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  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I dispute this statement! I have a lot of experience maintaining both flooded-cell and sealed-lead acid batteries at near full charge where there is no external load current draw; only self-discharge. In the steady-state, to prevent the battery from self-discharging, the battery terminal voltage must be held constant at a value determined by battery chemistry and ambient temperature. Let us use an example:

    12V 100Ah Sealed Lead Acid. To keep this battery fully charged, the terminal voltage is held at 13.65V, and in the steady state, the charger only has to deliver <20mA at 25deg C to keep this battery from self-discharging. As this type of battery ages, the self-discharge current increases, so a constant-voltage float charger automatically delivers the higher current to keep the terminal voltage constant... The self discharge rate is also a strong function of ambient temperature. At 35degC, this same battery will require about 40mA to overcome internal leakage.

    If this huge battery was fed with say a constant-current of 50% higher than its self-discharge current, say 30mA instead of 20mA, then the battery would be accumulating charge because the excess current (10mA over and above what it takes to overcome the internal leakage) is driving the terminal voltage ever higher above the ideal float voltage (at a given temperature), and the chemical reactions that produce charging are happening... At such a low net charging current, it would take a long time to add a significant number of Ah to the battery, but charging is happening. It is the voltage that drives the reactions; not current. Excess current drives the voltage...

    Les's implication is that the magnitude of the charging current somehow contributes the "quality" of charging just doesn't hold water. If during charging, the current is sufficient for the battery terminal voltage to reach the absorbtion voltage of ~14.4V for a while, then it matters not what the current magnitude was to get it there. Once that voltage is reached, then the charger has to back off, and let the battery stabilize at its long term float voltage...

    Here is what Powerstream says:

    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.
     
    Last edited: Jul 14, 2015
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  12. AnalogKid

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    Mike, ai think you took Les's comment out of context. He is talking about a very low charging current and a dead, sulfated battery, not float charging a healthy, charged battery. Also, the Powerstream statement also does not address the current available at 2.15 or whatever voltage. I checked with our battery wiz downstairs (used to work for a UPS company), and while he has no numbers for a minimum useful charging current (such as C/20 or C/50), he agrees with Les's characterization of the processes involved.

    For the TS question in post #8 - lead acid batteries are stupid. They don't know what's good for them, they agree with Mae West ("Too much of a good thing is just right."), and they drink way too fast for their own good; you must limit their intake. This is why all battery chargers have some form of current limiting. Sometimes it's just the size of a transformer or a big resistor, sometimes it's a more complex circuit.

    ak
     
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  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That suggests there's a problem with the battery. If those cells are NiMh or NiCd some could have died.
     
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  14. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I took out the current limiting and connected the charger as a constant voltage using the LM317T. The current into the charger is 315mA and seems to be charging. I am worried about no current limiting

    The batteries I am testing on are 600mAh do I just divide the consumed current by the capacity to get the standby time I should get
     
  15. Lestraveled

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    May 19, 2014
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    Mike, Peace
    In post #2:
    I think this is consistent with what you said.
     
  16. Lestraveled

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    One of the messages I try to convey, is that a good charger reconditions a battery as much as it recharges it. During the absorption phase the battery is being gently over charged. So, more desulfating is being done in this phase, than charging.

    In other words, besides putting energy back into a battery, the charge process is fixing/repairing things in the battery.
     
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