Help needed with battery charging circuitry please

Discussion in 'General Electronics Chat' started by Tony Pitt, Jul 1, 2015.

  1. Tony Pitt

    Thread Starter New Member

    Jul 1, 2015
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    I'm hoping someone might be able to steer me to fixing a lead acid battery charger that isn't working, please. (While I have a very basic grounding in electronics, most of it dates from 2 or more decades ago, and it's very rusty!)

    I have a portable amplifier (Bretford Entertaina Plus) that can operate on an integral 12V lead acid battery or on the mains via an external power supply. The basic problem is that the battery simply isn't charging. The amplifier works on battery when the battery is charged elsewhere, and it works on the mains. It would be really good to get the charging circuitry working again. (I bought the amplifier in this state and I do have some reservations as to its pedigree!) I have not managed to find a circuit diagram for it anywhere.

    I have replaced the battery, and so I don't believe that is at fault. Right now, the battery is showing just under 5V because it has been running the amplifier for a while. With the battery disconnected, the charging circuitry is delivering just under 15V on the battery leads. However, when I measure the charging current with the battery connected, it's a few mA which clearly isn't going to charge the battery!

    As far as I am aware, the circuitry includes overcharge protection, which would make sense given that the amplifier can be run from the mains.

    Without the circuit diagram, I'm shooting in the dark to fix it, I know, but I feel it's worth a try. Looking around the relevant part of the (fairly small) circuit board, I can identify the area responsible for the power, because of the thicker tracks around there, and because of where the battery leads are connected. In that area, there are 3 devices that suggest themselves to me - two voltage regulators (KA7809A and KA7812) and one MOSFET (SFP9540). There is no visible damage or scorching to the board or any of the components.

    Would it be a reasonable first assumption that the MOSFET will be responsible for the overcharge protection, and that it has probably died?

    Any suggestions would be most gratefully accepted.

    T
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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  3. dl324

    Distinguished Member

    Mar 30, 2015
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    We're even more in the dark than you because we have very little to go on...

    Try tracing the circuit, making your own schematic, and posting it; include voltages for some critical nodes, such as regulator input/output.

    BTW, you shouldn't let a 12V lead acid battery get to 5V and a charging voltage of just under 15V seems a bit high...
     
  4. Roderick Young

    Member

    Feb 22, 2015
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    It would be easier to diagnose with a traced-out circuit, but we can guess even with what we have (just won't be as good a guess). If the battery is several amp hours, there should be some heat-sinked element through which the charging current passes. It could be that MOSFET, it could be a bipolar transistor elsewhere. I'm making the assumption that it's a simple linear charger (if you see an inductor on the board, let us know - that suggests it may be a switching charger).

    There are two approaches I would take to debugging this. One, if I had a voltmeter on hand (a scope would be better), I would check the gate to source voltage on that MOSFET. If the voltage suggested that the MOSFET should be at least somewhat turned on, I would then check the drain to source voltage to get a feeling of whether the transistor was conducting.

    Low Vgs: check driving circuitry, or voltage sensing circuitry
    High Vgs, High Vds: possibly bad MOSFET
    High Vgs, Low Vds: possibly bad element in series with the MOSFET - look for a fuse, diode, or resistor open

    Approach Two, unsolder the MOSFET and test it on the bench. If that's not too much trouble. Take ordinary precautions against static discharge, if it's not a humid day.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Can you post a well-focussed close-up pic of the assumed charger circuitry? (but take pity on folk viewing this site on mobile devices, and keep the file size <300k ;)).
     
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  6. Tony Pitt

    Thread Starter New Member

    Jul 1, 2015
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    Thanks for the advice and suggestions so far. I've certainly learned a lot about SLA batteries! The first thing I need to do is to get a new SLA battery because the one I'm using at present is undoubtedly dead. I now have it on the bench hooked up to a PSU with 14.7V going into it (current-limited at 300ma) and it's taking just a few milliamps. Once I've done that, I'll try the charging circuitry again.

    After that, or maybe while I'm doing that, I'll set about sketching the circuitry.

    T
     
  7. Tony Pitt

    Thread Starter New Member

    Jul 1, 2015
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    I've just had another look at it. This is not a switched mode power supply - there is no inductor on the PCB.

    There's nothing with a heat sink near that area of the board - the only heat sink is on the amplifier itself, which is the other end of the PCB, and identifiable because the wires to the speaker go from there. The second voltage regulator - the KA7812 - is oddly on the back of the PCB, but does not appear to have been replaced. It's own heat sink - the metal "tab" - is soldered to the ground track of the PCB, perhaps to carry away a little excess heat ...

    At Alec_T's request, here's a picture of the power part of the PCB. Con 5 goes to the speaker, Con 4 to the power switch, Con 3 goes to the fuse and Con 1 goes to the socket for the external mains PSU. That is the order from left to right in the photo. Below that, with leads still attached, the left-hand connector takes power to the CD player, and the right-hand one goes to the battery.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The capacitor top-left of the pic seems to have a bulging end. If so, it's dead and needs to be replaced; as do any other bulging/leaking caps.
     
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