Lead acid battery charging circuit

Discussion in 'General Electronics Chat' started by donaldm444, Nov 22, 2011.

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  1. donaldm444

    Thread Starter New Member

    Jul 16, 2010
    I am going to be using the circuit (see attached file) to charge a 12 V lead acid battery. I found the circuit on this forum, it is a current limiting voltage regulator (In simulations I am using a LT1083 instead of LM 350). I will be placing my battery in place of R8 (I was just using the resistor for simulation purposes). My question is if it is ok to ground the negative terminal of the battery (where I have R8 grounded in the diagram).

    I want to ground the negative terminal of the battery because the circuit is part of a larger system I am making, which contains voltage sensors and bidirectional current sensors, microprocessor, etc. (I am trying to make sure that I have a correct ground point for all of these components). Any advice would be greatly appreciated, thanks.
  2. jimkeith

    Well-Known Member

    Oct 26, 2011
    Looks good to me--that is how I would do it too.
    Tying the battery neg terminal to the left of R4 would negate the function of the current limit circuit. There is a shunt voltage translator (current mirror) available that allows R4 to go into the positive rail--that would enable all voltage sources to be grounded to the same rail--off hand, I do not know who makes it--perhaps another forum participant can make a recommendation.
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  3. cravenhaven

    Active Member

    Nov 17, 2011
    I think it would depend on whether the "-" on your V1 is grounded separately somewhere. If it is floating as per your diagram I dont see that you would have any problems.
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  4. SgtWookie


    Jul 17, 2007
    12v SLA Float Charger temp compensated.png Sure, it's OK to ground it there - as long as your input supply's ground is not common with any other circuit ground. If it is, then you will have problems.

    There is no reason for R5 or R6; they should be removed.

    You really need provisions for adjustment at R2.

    Adding a diode in the output will keep the charger from discharging the battery when the mains power is disconnected/off.

    Have a look at the attached; it's quite similar, but it also has temperature compensation; that keeps the battery charge voltage correct whatever the temperature happens to be. Q2 in conjunction with R3 and R5 form a voltage multiplier with a negative temperature coefficient which is roughly -18mV/°C; just right for 12v lead-acid batteries. Chargers that aren't temperature compensated will overcharge batteries when it's hot, and undercharge them when it's cold.

    For best results, Q2 should be thermally coupled to the battery positive terminal.
    Last edited: Sep 15, 2014
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  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    I used to use a regulator with remote sense line feature so I can put the blocking diode inside the feedback loop and the battery voltage is regulated directly.
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  6. SgtWookie


    Jul 17, 2007
    The remote sense of the actual battery voltage is certainly preferable.

    The charger schematic that I attached was drawn up with the idea that just about anyone, anywhere can obtain an LM317 regulator, a 2N3904 transistor, and a suitable Schottky diode between the regulator and the battery - even by salvaging them from junked electronic equipment.

    There are much better regulators available for something like this than the ancient LM317; but many people in emerging countries simply cannot obtain the newer models; they can only get what local vendors have on hand at the market or what they can salvage. They frequently can't even order components online, as the companies won't ship to their country.

    What I posted is basically a "lowest common denominator" circuit. If it were an automobile, it would be a 1966 Ford Fairlane with patched vinyl seats, a straight 6 engine with an automatic transmission and air conditioning - but it'll overheat if you're not careful (use a heat sink on the LM317).
  7. Philogic

    New Member

    Mar 31, 2015
    What would you recommend? If you were to design your ideal 12v SLA float charger, would you use the same circuit with a different regulator, or would you design a completely different circuit?
  8. WBahn


    Mar 31, 2012
    You might have noticed (since you had to click a box specifically telling you) that you are responding to a thread that is more than a year old (and more like 3.5 years old in this case).

    If you want to discuss something related to this thread, please start a thread of your own and include a link to this one.
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