Current Draw, Charging batteries

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

Try here for more info

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

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

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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

Most 4WD (SUV) shops will have them

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

here somewhere, but I don't know quite how to apply

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