Follow up on 6 volt battery question

[Edward McComas]

One of the electrical engineers I work with came up with an "interesting" suggestion. He said to try using a 6 volt headlight as a resistor when using a 12 volt battery charger on a 6 volt battery.

I normally would have just laughed and said "yeah, right, how stupid do I look?" However, this guy really 1) is one extremely knowledgeable electrical engineer, 2) has been doing this stuff for about 50 years (he is over 70), and 3) really doesn't play pratical jokes on anybody.

He said that the bulb did not have to glow, but the filament had to be intact to transfer the current. Just put the bulb in series between the charger and the battery.

Be very careful if you try this. I might suggest just wiring up the headlight to the charger, then using a voltmeter to check the voltage coming out.

[Mike M.]

Re: Follow up on 6 volt battery question

ed: wonder if the dipstick knows the difference between "in series" vs parallel? mike

[Dennis C.]

Re: Follow up on 6 volt battery question

Well, Duuhhh, anybode no's that:

Series is wen u arnt smiling or laffing at al.

Parallel is 2 lleels, butt i dont no what lleels r.

[Mike M.]

Re: Follow up on 6 volt battery question

can't wait to see your nashville presentation.

[John M.]

Re: Follow up on 6 volt battery question

Ed,
You were right to be skeptical! His assumption about splitting the voltage by using the Light bulb only works if the resistance of the bulb and the Battery are the exactly the same. The fact is that the resistance of the battery is constantly changing as the battery goes from discharged to charged, so the voltage would fluctuate wildly. The only way to do this properly is to use a REGULATED voltage supply. This series circut, I do not belive will do it.

Regards, John McGraw

[Craig S.]

Re: Follow up on 6 volt battery question

Simple votage divider circuit....I am one of those EE types but 49 not 70...Craig

[Craig S.]

Re: Follow up on 6 volt battery question

Nobody ever said engineers proofread for spelling errors....

[John M.]

Re: Follow up on 6 volt battery question

Craig,
Am I correct that for the voltage to be evenly split that the resistance must be about the same on both loads? It has been years since I studied the laws of series circuts, but my reccolection is that the Voltage drop at each load is the same ratio as the resistance ratios.

Regards, John McGraw

[Craig S.]

Re: Follow up on 6 volt battery question

John - you are absoluely correct. Same principle as s rheostat with the center wipe. Think of it as a 3 point circuit, both ends of a resistor and the center tap. If the resistance on each side of the tap is the same, and you have 12V at one end and 0V (gnd) at the other, the center tap will be 6V. Now, in your post, apparently the other engineer has done enough experimenting to find out that the headlight bulb is reasonably close so the voltage drop across it is near 6V so you are applying 6V to the battery. They only thing I question in this approach is the charger itself that he used vs the one you have, or others have. Many of the current chargers are electronically controlled with microprocessors (at least the Deltran I have is...the company that makes the battery tender, I managed to get their golf cart charger from someone on eBay for a song) and they may adapt the current via sensing and I honestly don't know how a battery in series with a headlight bulb will act in this case. Maybe a more basic charger will act better. But, even these have trickle down features etc. I guess if the bulb impedance doesn't vary dramatically as the current changes and the filament heats up, and, the battery impedance stays fairly constant, it will be reasonably close. If it were me, I would give it a try, and monitor the "output" terminal of the lightbulb to the battery, compared to the charger output terminal (the input terminal on the bulb). In other words, if the voltage across the bulb stays about 6V, then you have a balance between the bulb and battery impedance. Let us know!

My gut says if this fellow has done this over the years it probably works pretty well....Craig

[Craig S.]

Re: Follow up on 6 volt battery question

One other point, I mentioned the charger may apapt the current, which it will, but I also meant to say they adapt the voltage. The Deltran unit I have has a complex voltage mapping graph in the design, and compenstates depending on the load. I just don't know how that would act with a bulb in series. But, its certainly worth a shot, just watch the E drop across the bulb...Craig

[Jack H.]

Correct....

a number of the intelligent 'keep alive' chargers are microprocessor based. They send 'spurts' of charging current to the battery in a 'phase one' mode to get the battery fully charged. This is done by shooting current, STOPPING, and looking at the external battery voltage through the eyes of a window comparator....

With a passive load (6V headlamp) in series with the battery, there will be NO voltage contribution from the load when the charger STOPS shooting current and pauses to 'inspect' the battery. Expect the smart charger to go into an infinite loop trying to pump the 6V battery up to a fully charged 12V state and basically chase its tail and never enter the maintenance mode where it supplies 'just right' current bursts to compensate for the battery's internal self-discharge profile along with external load factors (electro mechanical clock sporadically 'sipping' to re-cock its winding spring).

The concept of putting a resistive load in series with the battery WORKS for a conventional (dumb) battery charger SO LONG AS you don't supply more current than the headlight's filament can handle and burn it up! I cannot see how the concept works with today's series of 'smart' battery chargers that cycle between charge and load inspection modes....