# Charging a 6volt battery with a 12volt charger - circuit help please.

#### Huw

Jul 13, 2010
8
Hello all,
another nooby with no clue. anyone that attended physics lessons with me will testify that i have out done myself being able to type (albeit one fingered).

electrics, circuitry and anyother of the black arts escape me - could you help?

i have a 12volt car charger and need to charge a 6v battery. is this possible and more importantly if so - could you please explain it as you would to a monkey that doesn't speak english (i'm almost certain there are english speaking monkeys) - god knows there are enough french ones.

cheers,

Huw

#### Resqueline

Jul 31, 2009
2,848
This would usually be done by building an electronic circuit and this sounds like it's not for you, but there are two other ways.
One way is to just hook up an old headlight bulb (where the low-beam has burned out) and hook the high-beam in series between the charger and the battery.
This will give you a constant current, and you won't be able to see on the Amp-meter when the battery is fully charged, so pay attention to gassing & heating.
The third way is to reduce the mains input to the charger (by means of an auto-transformer, i.e. a 220 to 110V transformer). This'll make the Amp-meter work as usual.

#### Huw

Jul 13, 2010
8
hi Resqueline,

i've been told there's a simple circuit with a few resistors (2x 5W 100ohm) but you make me think there's more to it than this. (the circuit was on a ehow.com website).

many thanks for your help. with the bulb idea are we hoping to reduce the voltage across the battery and dissapate some of the energy?

energy laws i get - just didn't like physics teachers when it came to electronics.

and to be fair it wasn't paying me to pay attention either.

cheers,

huw

#### Resqueline

Jul 31, 2009
2,848
Bulbs, like resistors, will drop the excess voltage (and thereby limit the overcurrent that would otherwise flow) going to the battery, dissipating power in the process (not exactly energy laws related, more like throttling & braking a car at the same time).
The resistors (or the bulb) needs to be dimensioned to suit the charger & battery in question. You haven't said which, so I just presumed a motorcycle or an SLA battery.
A 100 Ohm 5W resistor can drop & pass up to 22V & 0.22A. I don't think those alone would be suitable, but if you add a zener diode & a transistor too it could work better.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
A great advantage with bulbs is that if you operate them from no more than their rated voltage, and don't do something silly like wrap them in insulation, then they won't overheat.

Resistors in such an application need to be correctly rated to ensure that you don't overheat them.

The disadvantage with bulbs is that you don't know exactly what resistance they will offer at any particular voltage.

#### Huw

Jul 13, 2010
8
thanks for the replies people. if i've learnt anything from my missadventures its that not knowing anything isn't nearly as dangerous as thinking you know a little. now, if i can grab a zener diode and a transistor aparently i'm business - ladies and gents do these need to be in any specific order, or like the missus' cooking is it the done thing just to throw it all in together and hope for the best.

at the moment my method of cooking is

(charger - positive charger clip) - "something" - "battery" - "possibly something else" - negative (battery charger clip - charger).

many thanks gents - alternatively i'll throw a resitor in series, sit it on a bathroom tile to keep it from over heating and watch the battery. if it \$hits itself and the battery melts through the floor i'll take this as another sign that "electronics" are part of the black arts and unless you lot are heavier than swans, should be burned at the steak.

yes resqueline its an SLA 6v bike battery. fairly new.

again thank you !!!

#### Resqueline

Jul 31, 2009
2,848
Hehe, there are plenty of opportunities to make a mistake and "let the smoke out"..
The series-bulb is by far the easiest, cheapest, quickest, fool-proof way there is, just check up on the battery every (half) hour or so.
The classic transistor zener regulator (Google those last three words) consists of just three parts (plus a heatsink) and is quite cheap & easy to get right.
If you go that route I'd suggest using a 160 Ohm 0.5W resistor, an 8.2V zener, a darlington power NPN transistor, and a fairly big (25W) heatsink.
Your cooking method is correct (so far), and there are plenty of recipes for the details in the "something" to be found with Google.
Oh, and if the missus always burns the steak (black art) then I guess she might just as well burn on the stake..

#### pappy

Jul 15, 2010
2
battery charger

An old trick is to use a light bulb in series with the battery to be charged. The light bulb should be rated to the charger voltage for a minimum and the light bulb current can be calculated with Ohm's law. Divide the rated wattage of the light bulb by the voltage to get the DC current. Most NiCad's, and gelcells are recharged at the 10 hour rate. So if your battery is rated at 6 Volts and 10 Amp hours, the ten hour rate is 1 Amp charge for 10 hours.

Good Luck,

Pappy

#### Huw

Jul 13, 2010
8
Hey gents owing to the fact that i had a long lunch break today i'm now the proud owner of:
8.2v zener diode
160ohm .5watt resistor
darlington power NPN transistor

length of wire
crocodile clips.

could anyone suggest a suitable order to connect these in?
its actually easier for me to get a hold of the above than it is to get an old headlight bulb.

cheers

Huw

#### Huw

Jul 13, 2010
8
also the incase you were wondering the batter is an SLA 6v bike battery for a 1974 Honda XL 350 motorbike. the bike never had a good alternator hence i'd like to set up a good, reliable way of charging the battery every now and then.

#### Resqueline

Jul 31, 2009
2,848
Yes, that'll extend the life of the battery considerably. I like the look of the 70's bikes.
I revamped one of the many diagrams available for you. How big is that charger [A] & the battery [Ah]? What number is that transistor? Did you get a heatsink & mounting details?

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#### Huw

Jul 13, 2010
8
took me a while to realise i wasn't required to make the left hand side of the diagram.
thank you for that.

charger is a red and black one...and more importantly a 12volt, 8amp charger with "charge" and "boost" settings.

#### Huw

Jul 13, 2010
8
ah - did i get a 25 watt heat sink? er no i don't think the shop keeper trusted me to leave with what i had let alone more gear. he actually asked me if i was buying it to give to someone who knew what they were doing... he wasn't that far off really.

mounting details? as in get your mrs plastered, when she's gone to bed... moun - oh wait you mean something to mount the work on?

yes - a piece of ply wood.

#### Resqueline

Jul 31, 2009
2,848
Good thinking, I forgot to state that the charger was the left-hand side.
Hehe, you have a way with words..
When talking transistor mounting it's referring to using a screw & a nut plus a special insulating washer & a mica sheet - to prevent the heatsink becoming "live".
Oh, yes, almost forgot, one often (usually) applies a white heatsink goo on both sides of the mica sheet. Very messy stuff..
You'll need a good piece of aluminum to sink the heat away from the transistor, there's no way around that. Then you can fasten a connector block onto the heatsink.
Hardly need for any plywood here, It's such a simple circuit. Still missing the transistor number (& the battery size) btw..

#### Huw

Jul 13, 2010
8
Hi there,

right battery - GN6 other than that absolutely no other markings.

transistor - (on) 921
BD681G

they are the only markings.

i tested the transistor and presumed that if it were a reverse facing letter "E" that the top lead is the base, middle is the collector and the bottom is the emitter.

I wired up the circuit and have only managed to heat the transistor. the battery charger indicated that the battery is full, which it really isn't..

i had the transistor mounted on a stainless steel piece of metal as a heat sink. making sure that it did not short out. when i say mount i mean insulation tape. and when i say piece of metal i mean lid from a cocktail shaker - the only thing in the kitchen the mrs wont miss.

the insulation tape started to melt so experiment aborted.

using my newly purchased multi meter i can tell you that the battery says its putting out 5.8v (i think that's *******s).
and for some reason the multi meter wont give me the current.

as usual - help gratefully received.

#### Resqueline

Jul 31, 2009
2,848
I figure that's a 6Ah battery. 1A for 6 hours should fill it up.

Here is the datasheet for the BD681.
The transistor is a bit on the small side for comfort. It's only a TO-225 type rated at 4A (& 40W). A bigger TO-220 device with a rating of at least 10A would be my choice.
Remember it's a very simple circuit with no safety features of any kind so it needs to be dimensioned fairly well to survive (but the 4A might survive if cooled well enough).

Steel (or brass for that matter) is a very poor heat conductor and will not do at all as a heatsink material. Aluminum and copper are the only common metals that'll do.
An aluminum "sheet" would also need to be at least 3 mm thick and 20 x 20 cm to come close to being up to the task. Copper is much better and can be thinner (1mm).
You might even consider water cooling (like a small pan, if you dare)..
PVC tape is also a very poor heat conductor, as you noticed. If the heatsink is well protected from touching anything else then you can do without transistor insulation.
A small drop of (cooking) oil between the transistor & the heatsink/ underside of the pan could make do as a heat transfer improver.

Test the output voltage of the circuit first without connecting the battery. The battery, being small, will not draw as much current as a car battery does, fooling the indicator.
A battery resting voltage of 5.8V means it's very flat indeed. 6.4V would mean it was full however (not much of a difference).
For measuring currents you usually have to move the red test lead to a separate socket on the meter (and remember to move it back before measuring Volts again).

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