Monday, August 23, 2021

EE Question

12VDC lead-acid battery. The terminals connect to a cigarette lighter outlet to provide 12VDC power.  A 15VDC wall wart connects to a cigarette lighter plug to recharge the battery through that socket (power flows from 15V to 12V).  There is no real need for a battery minder; I just want to know when the battery is back to 12VDC so I can disconnect the charger and when it drops below say 10VDC and is therefore not producing enough current for feeding the telescope mount.  My guess that an LED on the line to one of the terminals with appropriate resistor will show when the battery is outputting 12VDC (green LED) and something lights a red LED when output drops below 10VDC. (This number does not to be exact: 9V or 9.5V would do.) Similarly, I intuit that when the charger has charged the battery to 12VDC, a green LED identifies fully charged.

But what does the circuit for the red and green LEDs identifying low and full voltage connect?  One terminal to the other with LED on that circuit? What about the fully charged LED?

Yes I am using the salvaged  battery from the telescope power back and two very inexpensive 12VDC cigarette lighter connectors.   I am sure they are Chinese, but not so much ammo for them as an entirely new unit.  It also makes an interesting do it yourself project. 

One of you suggested these adorable digital voltmeters.  (Chinese of course.)  Simpler than connecting diode and resistor.  They draw 30mA, so with my 9Ah battery they should take 300 hours to drain.  I can also disconnect the battery if not in use.

The female connectors for the battery turn out to be Taiwanese!  Not only not from an enemy, but from a nation that needs to be working on nukes and IRBMs, right now, before the Big Guy sells them out.


  1. Why not just add a volt meter with a momentary switch, so you can check the current voltage without draining the battery? Something like MCIGICM Voltage Tester, 2.5V-30V Mini Digital DC Voltmeter with 0.28 inch LED Display (5 Pcs) would probably work.

    1. Boy those are cute! Just need a switch to put it in circuit.

  2. You can analyze this simple circuit by using Kirchhof’s First and Second laws.
    1. Sum of currents at a node is zero.
    2. Sum of voltages around a circuit is zero.

    I think that you’re going to need a resistor in that circuit or things will burn up.

  3. "12V" lead acid batteries consist of six 2.1V cells in series, yielding 12.6V. To charge them, the standard approach is to do so to 14.4V, then drop to a float of 13.6VDC. If you only charge to 12V, the battery will barely have any charge at all. If all else fails, a constant 13.6V supply will do without needing any other circuitry.