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Hello I have the following circuit that I'd like to use to charge a battery. The battery is specified to charge up to 12.6V max and so I have implemented a voltage regulator that will produce this voltage, such that I cannot overcharge. The issue I have is that if the battery is drained to say 10V there will be a huge inrush of current which will likely damage the battery. I need to limit this current to about 300mA.

Can I add circuitry to the outlet of my voltage regulator to limit the current? The portion in red is the main circuitry for the voltage regulator. The non circled portion is an RC delay circuit that ensures the regulator is only active when my engine is on. It sees a square wave oscillating from 0-12V at a frequency = (rpm*4)/60 so at 600rpm 40Hz. After a few seconds the voltage will rise above a minimum voltage for IC1 at 3.08V and 200ms later the shutdown pin will no longer be pulled low, activating the regulator.

LT3029IDE#PBF

TLV803EA30DCKR

battery

circuit

This article seems to suggest I should add either a constant current limiting circuit or a fold-back limiting circuit. Not sure which is best for this circumstance...

article

I'd like to replicate the circuit shown below. And have attempted to do so by limiting current through selecting R2 = Vf/Imax = 1.1V/0.35A = 3.14 rounded up to 4 ohm to provide extra protection. Chose R1 assuming a gain of 50 on the npn transistor 350mA/50 = 7mA. At lower end of voltage 11V/0.007A = 1571 Ohm took down to 1250 Ohm to ensure had enough current.

Have I sized my circuit with the correct methodology for the problem of max 12.6V and 350mA?

reference circuit

attempt

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  • \$\begingroup\$ Not only should you, you are probably required to do so. What chemistry are we talking about? \$\endgroup\$
    – winny
    Commented Jul 16, 2022 at 15:43
  • \$\begingroup\$ Running the two outputs in parallel like that probably won't get you the results you desire -- it's more likely that because of slight circuit differences, one regulator section will hog all the current. \$\endgroup\$
    – TimWescott
    Commented Jul 16, 2022 at 16:26
  • \$\begingroup\$ What battery chemistry? If this is for lead-acid, your charge voltage is probably too low unless it's a really mild trickle-charger circuit. If this is for Li-Ion or LiPo batteries, then you can't just charge at 4.2V forever -- you need to have a managed cutoff. Keeping Lithium-whatever cells happy is not trivial, nor is squeezing the most life out of good old fashioned lead-acid cells. \$\endgroup\$
    – TimWescott
    Commented Jul 16, 2022 at 16:32
  • \$\begingroup\$ @TimWescott added the datasheet for the battery. I believe you are correct about adding the cut-off as well. I imagine this would mean additional circuitry after the current limiting circuit. Also I do see that I have connected the two voltage outputs but the only reason I did this is because I don't need two independent output voltages just one. Maybe it is better that I only use one voltage output channel, I am not sure. \$\endgroup\$
    – Feynman137
    Commented Jul 16, 2022 at 16:52
  • \$\begingroup\$ The fact that it's a battery pack with it's own internal charging circuitry changes everything -- fortunately, it changes it for the better. \$\endgroup\$
    – TimWescott
    Commented Jul 16, 2022 at 18:24

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