ok electronic engineers, heres a ?? for you

[cammeddrz]

whether or not it is lead acid, lithium ion, lithium polymer or or anything else......it is irrelevant because that was not the question i asked. i know what it is, and i know the requirements are. i did my homework on that part. i did not just pull that number out of my ass. i have found that if i give too much information when asking a question the questions of "why would you want to do that" pop up, along with "instead you should........" on any given forum thread asking a question half or more of the responses are just a waste of time for everyone involved. for example of those who responed......are any of you really electronic engineers? thank you those who provided real and valuable information, and especially thanks to gribly who gave me exactly what i needed above and beyond what i asked for

and btw just in case you're still not satisfied it is a 6ah, valve regulated, lead acid battery which can handle .6 amps for 10hours. and before you ask, naturally i am running a regulator on the battery side of the diode

 

[robertc1024]

Whoa dude, I'm not dissing you. You know as well as I that there are a lot of people who ask questions on here who don't know WTF they're talking about. And you never said that "Naturally" your're running a current regulator on the charger. That's one of the key points which Gribley's suggested charger addresses. And the battery type is important.

You wan't me to .pdf you a copy of my degree?

 

[cammeddrz]

You wan't me to .pdf you a copy of my degree?

no its not a big deal. the battery info is listed in my previous post

 

[gribly]

Whoa dude, I'm not dissing you. You know as well as I that there are a lot of people who ask questions on here who don't know WTF they're talking about. And you never said that "Naturally" your're running a current regulator on the charger. That's one of the key points which Gribley's suggested charger addresses. And the battery type is important.

You wan't me to .pdf you a copy of my degree?

It's gribly, all lowercase, no E. It's more of a description of myself in one word. As in - "He is a gribly lookin' feller, ain't he?"

I know it isn't a real word, but it sounds like it would describe a fellow with a distinct lack of class.

 

[gwaii]

to keep a constant amperage you'll need to do more than add a simple component or two across the feed.the amperage going to the battery will be determined by the voltage drop between the source and the battery being charged-in other words,when the smaller battery has been discharged it will want to draw a higher amperage than when it is nearly fully charged.
this means you'll need a regulated circuit to maintain a constant voltage difference between the small battery and the source power,so when the small battery is at,say 8 volts,the supply voltage may be 10 volts-rather than the 14.7 the truck is operating at.
this can be accomplished using a negative feedback system that has a zener diode regulating a transistor in a circuit,where the source voltage becomes irrelavant,as long as it's not lower than the battery you want to charge,or so high it exceeds the transistor's ability to hold it back(thus burning out the transistor).in this way the voltage going to the small battery will increase gradually as the battery charges,thus maintaining a set amperage.

having said that,this circuit may be too complex for someone not totally familliar with electronic design to make,but it is something that can be found commercially for a reasonable price.

ask for an electronically regulated dc-dc charger at your radio shack.

if they don't have it,an electronics supply store will.

 

[Legend]

cammeddrz asked for answers from electronics engineers. I am an electronics engineer and thought I would try to be helpful. But this seems to be turning into a big deal.

The original post didn't supply very much information and the question sounded like the poster wanted something simple and didn't know much about electronics.

I agree entirely with what gwaii wrote, however it doesn't provide a simple answer to cammeddrz.

If you simply put a battery (charge state unknown - may be completely discharged) across 12 volt system of the truck, the battery MAY draw a huge amount of current and blow up or cause a fire. The current does need to be limited. [Good thinking on the part of cammeddrz.] The simplest form of limiter would be to put a resistor in series with the battery. A 24 ohm resistor will limit the current to .5 amp (or actually .6125 amp if the truck is running and the 12 volt system is at 14.7 volts).

Here it starts to get really strange.....
adsm08, who usually has very good sensible advice posted:

Quote:
Originally Posted by Legend View Post
If you hook a 24 ohm resistor across the 12v battery, .5 amp of current would flow.
If you run a 24 ohm resistor across a 12v battery .5 VOLT will flow. The amount of amperage that can get through will be higher because car batteries and alternators are rated in hundreds of amps.

which is completely incorrect. The actual equation is voltage / resistance = current 12 / 24 = 0.5 The 24 ohm resistor in series with the battery to be charged will limit the current to .5 amps maximum. As the battery is charged and the voltage across its terminals rises, the voltage drop across the resistor will drop and the current will be reduced. When the battery being charged reaches 11 volts and only 1 volt is being dropped across the resistor the current will be 1 / 24 = 0.04167 amps and the charging will be verrrrry slow - but that's the kind of compromise you get for a simple solution.

BTW about the fuse.... power = current x current x resistance. So if the battery is completely discharged and the current starts out at maximum the power dissipated in the resistor will be .5 x .5 x 24 = 6 watts. To be on the safe side, probably should use a 10 watt resistor. A .5 watt resistor will likely act as a fuse and handle a slightly discharged battery just fine, but will burn out if you connect a fully discharged battery.

The value of a diode would be to protect your truck from a battery to be charged being hooked in backwards.

 

[gwaii]

The original post didn't supply very much information and the question sounded like the poster wanted something simple and didn't know much about electronics.

this is the problem i had...so i concentrated on the information that was given,that being the amperage values.

As the battery is charged and the voltage across its terminals rises, the voltage drop across the resistor will drop and the current will be reduced. When the battery being charged reaches 11 volts and only 1 volt is being dropped across the resistor the current will be 1 / 24 = 0.04167 amps and the charging will be verrrrry slow - but that's the kind of compromise you get for a simple solution.

and this,as you've noted is where those amperage values cause problems using a simple solution.
while a resistor will allow an amperage in the rated range when the battery is fully discharged,as it charges the values will fall out of the specified range-meaning that for the majority of the time charging will not be in the specified design parameters.this is why i rejected this solution.

the only way i can see maintaining the design parameters with the information given,that being a specified amperage range and voltage limit,a feedback system is the only thing that will perform in a manner that will stay in these limits.

and i understand that my solution may not be practical to someone that wants to design a system by putting a few components together to meet these requirements-purchasing a battery charger,hooking two wires to the truck's system and two to the auxillary battery is a simple way to achieve the design parameters.

 

[gribly]

gwaii - there is a reason I gave a simple answer and an option to build or buy; If he were able to completely understand what you are saying about a feedback circuit - he wouldn't be asking us what to do. I ain't no expert either, I've got a few solid state engineering courses under my belt though - from my HVAC degree.

 

[gwaii]

gwaii - there is a reason I gave a simple answer and an option to build or buy; If he were able to completely understand what you are saying about a feedback circuit - he wouldn't be asking us what to do. I ain't no expert either, I've got a few solid state engineering courses under my belt though - from my HVAC degree.

my thoughts exactly.

this is why i explained the feedback in basic terms,then suggested puchasing an over-the-counter part

i think you,i,and legend are all on the same page here.there are different ways to get that battery charged without blowing it up,which to use depends on the comfort level the person doing it has with the components involved.

 

[gribly]

I'll agree with that.

I have a $200 dc to dc charger myself, but that's for radio control stuff. 13v to 9v charging.

 

[feellnfroggy]

there is a formula, a wheel of sorts for voltage, amps, and ohms. I dont remember what it is. But youll need more than a resistor, a resistor resists the flow of current, hence lowering the ohms, what you need is a diode i think. Here is a helpful link, maybe

http://uk.answers.yahoo.com/question/index?qid=20081204202510AAa3pPB

 

[adsm08]

Here it starts to get really strange.....
adsm08, who usually has very good sensible advice posted:

I will admit that electronics is more of a hobby than a profession for me, and I'm not and engineer in the academic sense. I am familiar with Ohm's Law, but sometimes the more abstract ends of electrical theory get away from me. Particularly the mathy stuff. Thanks for the corrections.

 

[cammeddrz]

there is a formula, a wheel of sorts for voltage, amps, and ohms. I dont remember what it is.

as i recall from my A.P. physics class it was a "triangle" type formula

 

[robertc1024]

You got it exactly right

V
_
I R

Volts = Current * Resistance
Current = Volts / Resistance
Resistance = Volts / Current.

 

[gwaii]

You got it exactly right

V
_
I R

Volts = Current * Resistance
Current = Volts / Resistance
Resistance = Volts / Current.

yup...it's called 'ohm's law'.