# 6 Volt charger from 12 volts DC

Discussion in 'Electronic Design' started by CampinGazz, Apr 26, 2004.

1. ### CampinGazzGuest

We have a motorhome, 12 volt leisure battery in the back to run all the
loads we use while living in it,

Bought a battery powered hoover for the van at the weekend, it's got a 6
volt 4.5 ah sealed lead acid battery in it for power, and comes with a simpe
transformer for charging from the mains, this puts out 9 volts DC at 150
milliamps,

It says to leave the hoover in the charging stand all the time, and it'll
take 10 hours to charge a flat battery in the hoover,
No mention of there being any sensing circuitry in the hoover to cut the
charge off when the battery is full.

anyway i dont want to have to have the inverter powered up all the time just
to charge this battery.

What i want to do, i make up a sensed DC battery charger for the hoover that
runs on 12 volts DC (well, 10.5 volts to 14.4 volts)

i want something that i can leave connected all the time, if it senses the
battery is low when it's put back on the charging stand, it will charge
untill the battery is full, then cut off, if the battery self discharges
over time, it'll charge again untill the battery is full.. basicaly
maintaining the battery,

But idealy i'd like to be able to charge the battery up faster from flat
than the 10 hours it takes with a 150 milliamp 9 volt supply it has now.

i'll obviousely take the hoover appart and remove the diode from the
charging socket, to allow the charger i make up read the battery's voltage,

And i'd imaging i'd be better off charging at a higher rate at 7 or so
volts, rather than 9 volts?

What circuit would people suggest to charge this thing from a 12 volt DC
supply?

i was thinking very basic, drop the 12 volts to 9 volts, and have a 10 hour
timer on it, so when the hoover is placed in the stand, it applied power for
10 hours at 150 milliamps max,
but idealy i'd like slightly better than that.

2. ### Active8Guest

Look back over the past week's (or so) posts here in SED for the
latest discussions on battery charging where you'll find links to
specs and a FAQ. Check the specs for the hoover batt if you can get
it. Then google for constant current power supply and maybe charger.
That should return an article on a charger that can be turned on and
off remotely (by your timer) and features an LM317 in constant
current config as well as a pass transistor for higher current
needs, IIRC. I think the pass transistor was there for fast charge.
I think it also had a temperature sense circuit.

From there it's just a matter of changing resistor values to give
you the desired charge rates.

3. ### budgieGuest

Do a web search for the Unitrode (TI) UC3906 SL:A charge controller chip. The
documents may appear with filenames like SLUS186A.PDF or SLUA115.PDF, or as
appnotes like U-104.pdf etc. These describe the care&feeding of SLA's fairly
well and (obviously) desribe the UC3906 controller. There was (and may still
be) a kit using this chip for 6V SLA charging from a 12V nominal source.

4. ### JoergGuest

Look which brand of rechargeable battery it is and then check the mfg site for
the max allowed continuous charge rate. Mostly batteries in devices such as
portable phones are charged all the time the set is in the cradle. And most
batteries allow, for example, to be charged at 1/20th "amp capacity" (which
really is a misnomer) without much danger of overcharging.

What this 1/20th would mean: If it was a 4.5 Ah battery then you could charge it
with about 200mA pretty much forever. But consult the battery manufacturer
before. You could play it safe and go with the 150mA the wall charger gives. A
big resistor from 12V down to 6V would do. But keep in mind that when the van's
engine runs it goes above 14 volts.

Regards, Joerg