Current Draw, Charging batteries

Ohm's law is in here somewhere, but I don't know quite how to apply
it....

I use a deep cycle battery, ~80AH @ 5hr rate.

I would like to tote it along on trips, but it needs to be somewhat
portable, so I am looking to wire up something in the trunk, or back
of my truck, to juice it up when I am driving.

Where I get lost is, if I the battery is basically fully discharged,
how do I know what size wire I need to use, to get it back up to
snuff? Do I need some sort of control ckt?

It would be great if I could just plug it into the cigarette lighter,
but that is fused at 10A, so I am assuming a deeply discharged battery
of that size, would pop the fuse. Right? Same thing for a 14Awg set of
connections, they would overload on a dead battery?

Don't really want to run 8Awg wire to the bed, or the trunk, so how do
I figure out what the max current is, that a "dead" battery of this
type, will draw, so I can keep it simple?

I know there are other issues, like safety, starting the vehicle
first, and then connecting the battery, and such, but the basic
charging issue is where I would like to get some advice, please.

Thanks a lot,

John

 

First, it is Really Bad to fully discharge a battery.

Try here for more info http://www.batteryuniversity.com/
This should answer many questions


martin

 

Actually, it is quite okay to discharge a true deep cycle battery down
to 20% of capacity, 50% being better, but that wasn't the question
now, was it?

I don't need a lesson on batteries, I was asking about a specific
method of charging from an alternator, for the occasional need.......

 

Alternators are built to supply uncontrolled, but finite, current to
shorted battery so the best you can do when traveling connect this
battery in parrallel with the main one by cables and contactor of rather
convenctional sizes, just in case. Melting insulation on battery
connections is not worth even trying.

HTH
Stanislaw.

 

If you're going to run a wire anyway, you might as well run a fat
wire. But it wouldn't have to be anywhere near as fat as starter
cables or jumper cables (which are meant to handle starter current).

#10 or #8 should be fine to just keep a deep-cycle topped up, as
long as it's not literally "fully" discharged.

And the frame should be negative - if not, it needs some bonding
straps.

Good Luck!
Rich

 

If its not fully discharged and the bettery terminal voltage is above ~9V,
I'd charge it through a 1 ohm resistor. That will limit the current to
about 4.5 amps (asuming the alternator voltage is 13.5V) and will not blow
the cigarette lighter fuse. The resistor will dissipate close ot 25 watts
at first (hot) so get at least a 40W resistor. Two packs of those 10W 1 ohm
radioshack resistors should do the trick. Put two in parallel and them put
the two parallel pair in series with each other to get back to 1 ohm. All
of $3.50 and some solder and your done.

As the battery voltage increases, the current will decreasewhich means that
you'll be getting very little current at hte end of the charge so that last
few % of capacity will take a LONG time.

You might want to consider a diode in the path so when you turn off the
vehicle the deep cycle doesn't keep your electronics lit up. In addition to
draining the battery it may damage something.


This a a very crude battery charger and is only used for "special cases" and
not for regular use.

 

Could it much higer than that.

30 to 50 Watts of wasted power will be coming out of your gas tank
anyway.

Battery will peak at 13V but Alternator will not.

PWM switched MOSFET might be better.

 

I need one of these myself, but haven't got around to designing/
building one. It can be a simple linear circuit You should not be
draining your battery to lower than 10V. So the drop across the
circuit would be a 10A and 4V, or 40 watts. Toasty, but not the end
of the world. [Mine would be linear because I want to listen to the
radio and won't put up with the switcher hash. Your mileage may vary.]

The circuit needs to current limit at whatever is safe for your
outlet, which is why I picked 10A, though it could be 12A. I wouldn't
go for the rated load. The circuit would use a PMOS pass device,
perhaps two PMOS devices so that the body diode of one fet doesn't
provide a current path should the battery voltage be less than the
charging voltage. If you had a gel cell, you would want some
overvoltage protection since some cars put out over 14V.

Incidentally, if by deep discharge you mean a marine battery that has
vents on it, I would not charge this in the back of a SUV or car. The
vapors will cause corrosion. You need a a gel cell.

You can get black boxes at autostore that allow this two battery
scheme. I think all they do is prevent the reverse current flow.

I have done "expeditions" with 5 serious size gel cells. They
certainly are heavy. ;-) If you are just camping, you might consider a
very small generator plus one or two of these batteries. You can get
decent chargers for about $100.

I use
http://www.battery-chargers.com/catalog/22.pdf

 

Mook's idea is simplest and easiest to implement.
Use it to keep the battery topped off. I would
suggest a modification to the cigarrete lighter
socket, if it is always live:


------ +---[1R]---[Fuse]---+
---o-----| | |+ |
+ -----o--^ | | | |+
Acc --[Relay]--+ |Socket|=[PLUG] [Battery]
| | | | |
Gnd -----------+---| | | |
------ +-------------------+

The relay is energized by the accessory line, so the circuit
to the socket and the deep cycle battery is interrupted when
the vechicle is off. Use an automotive relay rated for 30 or
40 amps. If your vehicle kills power to the cigarrette
lighter when off, no modification is needed, nor is the diode
Mook mentioned.

To _charge_ the battery when it is fully discharged (meaning
to 50% capacity)vs keeping it topped off, you want something
different. You need to put 40 Ah into the battery - that's
*well* over 10 hours of driving when the current is limited
to ~ 4.5 amps and the thing is plugged into the cigarette
lighter. As the charge builds, the voltage in the battery
rises and the current drops, so the charge rate tapers off,
thus > 10 hours for full charge.

I would use Mook's idea, making darn sure that the resistors
were not in a place where they could burn anything. If you
find that insufficient to keep the battery charged, you have
two choices: either charge it with jumper cables with the
car running in a parking area, or build/buy a setup for
dual batteries.

Ed

 

The major problem of connecting different batteries having varying
states of charge in parallel without any form of isolation is that of
heavy current flowing from the battery having the higher charge into
the battery having the lower charge.

You are best advised to install a battery isolator (intelligent is
best) thus avoiding any problems.

Here is an example http://www.perfectswitch.com/isolatorcombiner.htm

Most 4WD (SUV) shops will have them
http://www.hellroaring.com/4wheel.htm

 

That is about what I would do. If you plan to discharge the battery below 9
V, consider using a 2 Ohm resistor instead. At any rate, even with 1 Ohm,
there is no point to using wire bigger than #16 or #18, anything smaller
gets fragile. I would also put an 8 amp fuse in the line, and consider
adding a 10 Amp relay to disconnect the battery when the ignition is off. I
have seen these both ways, but the cigarette lighter in my car does not turn
off with the key out.

Tam

 

If you put the battery in the trunk, you should be aware of the dangers of
possible explosion, and corrosion. To be legal, you will probably have to
vent it to the outside.

Tam

 

Thanks Rich. *That is pretty well what I was figuring. I appreciate
you addressing the question I asked, rather than sending me off to
learn all about batteries, or second guessing what I am trying to do.

A friend camps with me maybe three times a year, and he uses a cpap
machine at night, so all I want to do is tote the battery for those
few times, and, just wanna 'refill' it if we decide to extend the trip
a few days. Don't need anything more than that.

I think everyone knows that you don't drain a battery to zero.... I
sure hate being told stuff like that.....

Much obliged for the common sense and logic.

John

 

This is why I suggest a gel cell, though they are not tolerant of over-
voltage. [The meter on my car peaks at 18V, though it never goes
there, but Yikes! It is not all that unusual for auto voltage to be
well above 14V.] Maybe I'll get around to prototyping a circuit. BTW,
when I did my seat of the pants analysis that the charger might have
to dissipate 40W (i.e. 10V on the battery, 14V on the car generator,
and 10A), I didn't consider the battery voltage rising as soon as the
current is applied. For instance, if it jumps to 12V immediately, then
the power dissipation is only 20W.

 

Also, you can't ignore the resistance of the wires. My previous car had the
battery under the rear seat. The built in voltmeter on the instrument panel
often displayed 15.4 V. A digital VOM plugged into the cigarette lighter
showed the same. I can't believe the battery voltage was actually 15.4,
since the battery was still good after 4 years, and the 15.4 was probably
the alternator voltage.

I used to charge gel cells with an HP lab power supply. It generally took
about 14.3 V to get a couple of hundred ma charging current into the ~7AH
battery.

Tam