Disconnect 12v charger while motor is running?

Discussion in 'General Electronics Chat' started by CollieJim, Mar 26, 2016.

  1. CollieJim

    Thread Starter Member

    Nov 27, 2004
    29
    0
    I depend on Solar PV and a big battery for mains power, with a diesel generator for backup. I bought the generator new 6 months ago.

    The generator is wired for automatic start when the State of Charge drops to 82%. Twice now it has failed to start due to a flat battery. The SLA battery is the size one would find in a compact car, and the drain on it appears to be about 10ma when the generator is not running.

    I tried a small pv panel, but with no sun for several days the generator's battery was not up to starting this morning.

    If I connect a small automatic charger, can it be left connected while the generator is running?
    Alternatively, I suppose I could use the 12v output to activate a relay to automatically disconnect the charger.

    Thanks
     
  2. mcasale

    Member

    Jul 18, 2011
    210
    12
    I depend on Solar PV and a big battery for mains power, with a diesel generator for backup. I bought the generator new 6 months ago.

    The generator is wired for automatic start when the State of Charge drops to 82%.
    *** State Of Charge of the BIG battery, right?

    Twice now it has failed to start due to a flat battery. The SLA battery is the size one would find in a compact car, and the drain on it appears to be about 10ma when the generator is not running.
    *** Is this the battery that starts the generator.
    *** Is it a standard 12V automotive, lead-acid type?


    I tried a small pv panel, but with no sun for several days the generator's battery was not up to starting this morning.
    *** So you're using another PV to charge the generator battery?

    If I connect a small automatic charger, can it be left connected while the generator is running?
    *** That depends on the charger.
    *** Does the generator charge the battery when it's running?
    *** If so, I'd guess it should be disconnected.


    Alternatively, I suppose I could use the 12v output to activate a relay to automatically disconnect the charger.
    *** Yes. Just be sure the timing is correct.

    Thanks
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,837
    1,337
    At 10 mA, 10 A-h SLA battery (smaller than a paving brick) would last 1000 hours, or 41 days. ish. Something else is running down the battery, or the battery has a problem holding a charge.

    ak
     
    BR-549 likes this.
  4. BR-549

    Well-Known Member

    Sep 22, 2013
    2,312
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    What is the amp hr rating of generator battery? How often does generator come on in winter?

    Perhaps a larger auto battery.

    Perhaps changing 82% to 88% in winter.
     
  5. BR-549

    Well-Known Member

    Sep 22, 2013
    2,312
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    Can you monitor battery voltage while cranking diesel?
     
  6. WBahn

    Moderator

    Mar 31, 2012
    18,295
    4,967
    Possibly -- depends on the charger.

    One simple thought (that may or may not work) would be to put a diode in series between the charger and the battery. If you use a silicon diode then the battery will only be charged to ~0.6 V below the what the charger is set for, but that may be sufficient to hold the state of charge high enough to start the generator. If not, then you could use a Schottky diode, but be sure to consider the reverse leakage current since some of them can be pretty high.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,913
    3,519
    Since the generator charging voltage should not be significantly higher then the normal end charge voltage from the charger then I don't think there will be a problem keeping the charger connected when the generator runs.
    It does sound like the battery is abnormally discharging with only that small current load.
    Can you temporarily disconnect the battery and see if it maintains the charge with no load? You may have a bad battery.
     
  8. CollieJim

    Thread Starter Member

    Nov 27, 2004
    29
    0
    Thanks for the replies.
    MCASALE: SLA = sealed lead acid. Except for the small terminals with holes, the generator's battery is the same as for a small car. The generator is supposed to charge its own battery, but at present this is not happening. I have contacted the supplier regarding this problem.

    AnalogKid: While your figures represent the total capacity of the battery, far less discharge is tolerated before there is not enough left to start a diesel motor.

    BR-549: I have no idea what the capacity of the generator's battery is. The only markings are 5E08C9B090738 and 5E14D90E0673. No indication of who made it. The voltage during starting can be monitored, but I have not done that yet. Long-term, the generator is on an average about 15 minutes per day supplying 2.3kW. This past week we have racked up 14 hours due to unusual weather.

    WBahn: The diode sounds good, but would it interfere with an automatic charger's sensing?

    Long-term (summer, no rain) the 8.3ma drain is enough to discharge the battery to the point starting is a problem.

    Short-term, I need to get the generator fixed so it will charge its own battery. I checked while it was running yesterday and the meter read 12.35vdc. This was after running for 2 hours. Once that is fixed a simple trickle charge should be sufficient.
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,218
    How do you know it is not charging?

    Do you know if the generator is suppose to charge its starting battery?

    The generator I have in my RV does not charge the battery it starts from. The generator has no charging capability itself. The battery charging comes from the power management system that is external to the generator.
     
    Last edited: Mar 27, 2016
  10. CollieJim

    Thread Starter Member

    Nov 27, 2004
    29
    0
    The normal charging voltage in a vehicle with a 12v system is about 14.4v. Two days ago I had to charge the starting battery to start the generator. After 2 hours running, with the generator stopped, the meter read 12.16v. No charge. Until this last bad weather there has been no problem, so I assume (incorrectly?) that the generator charges its starting battery. The generator has a 12vdc,8a output as well as 240vac.
     
  11. Tonyr1084

    Active Member

    Sep 24, 2015
    754
    147
    A little battery experience under my belt: A 12 volt Lead Acid battery (nominal 12 volts) should be charged to a FLOAT voltage of 13.6 volts, NOT 14.4 volts. Charging a battery to 14.4 volts will dry out the cells of the battery, leaving them useless. It should also be noted that a common Lead Acid battery is considered dead when the charge reaches 80% of nominal (9.6 volts). If your generator is not recharging the battery until it drops to "Dead" then it's no surprise the battery is too weak to start the generator.

    When checking a battery in a car I look for these things: When the battery has sat over night with no charge or drain the battery voltage should be resting ideally at 12.2 volts. However, if it's resting at 11.6 volts I consider this to be a strong and healthy battery. If the resting charge is 11.4 or lower the battery is weak. Could be due to insufficient fluid in the battery cell reservoir(s) or could be extreme calcification on the plates. If the resting voltage is in the 10 volt range - the battery is done. Time to replace it.

    With regards the charging system on the generator (focusing on an automotive alternator to recharge the battery) When the engine starts it's normal to see the battery voltage shoot up into the 14 volt range, not uncommon is 14.4 volts. This - like I said - is normal. However, it's also important that within a few minutes of starting up the voltage should have dropped down into the 13 volt range and should settle down at 13.6 or 13.8 volts. This is a sign of a healthy automotive regenerative system. The same should be true of your diesel generator (assumably so).

    Something many people miss is a lose or worn belt. IF the alternator is allowed to slip then it won't charge the battery properly, especially if there are other loads running at the same time, things like ignition, car radio, headlights and other electronics (not to be found on a generator).

    Lead Acid batteries are generally good for about 5 years - depending on the manufacturer. A cheap battery is cheap because it's made poorly, and likely won't last the five years. The better battery is the better choice.

    Whatever you're running and however you are able to set the parameters on the battery sensing circuitry will make a difference on whether the battery is ready and charged for service or not. Running an external charger - well, I'd suggest you diode protect the circuitry just to prevent any back rush of currents unintended. If properly protected the charger could hold the battery at a float voltage of 13.6 volts and the battery will always be ready to start the generator. And when the gen kicks in the battery should already be close to float charge and not need any charging. BUT: If exercising the generator depends on an ever dropping battery voltage then with a charger attached it won't ever exercise. And exercising a generator is advisable. You MIGHT want to install a timer to control the exercise as opposed to a diminishing battery voltage.

    Well, that's about it for what I know about batteries and charging circuitry. Hope this is useful.
     
  12. CollieJim

    Thread Starter Member

    Nov 27, 2004
    29
    0
    Thanks.

    Regarding the charging voltage, I have a Suzuki SUV with a second battery installed with a meter on it. While the motor is running and charging, I always see 14.4v so that's why I used that figure.

    The wiring diagram for the diesel shows a "flywheel generator" connected to "regulators", the output of which go through a 30a fuse to the motor's battery. I can only find one 30a fuse and it is not blown. The motor's 12v and the generator's 12v, 8.3a supply are independent.

    The motor's battery has been isolated for 26 hours and has dropped only 0.06v.
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    In order to fully charge and desulfate a battery you have to hold the voltage at 14.4 to 14.7 voltage until the charge current drops below a certain threshold. After the battery is full charged, the float voltage should be between 13.2 to 13.6 volts. The time the battery is held at 14.7 volts is called the absorption phase and is essential for long battery life.
     
  14. CollieJim

    Thread Starter Member

    Nov 27, 2004
    29
    0
    It finally occurred to me today that I can use the signal from the solar charge controller that starts the generator to energise a relay to disconnect a smart charger.
     
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