Opinions on this? http://www.qsl.net/wb3gck/charger.htm
@Bill,
As far as I know it has never been built, but I trust it a lot more than the design I'm seeing on the 1st few posts. You need a constant current that is limited to a set voltage, which I don't see in your original design. All I see is a voltage regulator with a back current prevention diode, and the lack of any current regulation is a serious design flaw, and the sense when charged is not automatic, which means if you forget to check on it goodbye battery. It is also depending on the overload protection on the LM317, which is not a good idea. I do not always have to build circuits to see their weaknesses, that is a very flawed design.@Bill,
Thanks. I had looked at this, but had a few questions.
Is the final design in post 12? What are the wattages required for the resistors? And did anyone ever build it or is it a simulation only project? I would like to build something that has been proven in actual use.
Schematic from post 12:@Bill,
Thanks. I had looked at this, but had a few questions.
Is the final design in post 12?
R4 needs to be calculated as described above. For higher currents, Nichrome wire wound on a ceramic cylinder works quite well if you want fairly precise control over the charge current. You could also use power resistors in a series/parallel configuration, or simply select one that's close to the charge current you need. 1/4 Watt would be fine for the remaining resistors .What are the wattages required for the resistors?
I've built a very similar circuit to charge a cell phone battery at a low current, since I didn't have a method to monitor the battery temp at the time. It worked just fine.And did anyone ever build it or is it a simulation only project?
It should work just fine, if the cautions about power dissipation are followed.I would like to build something that has been proven in actual use.
The base-emitter junction of Q2 will have a lower Vf as the temperature increases, which will cause Vce to decrease, which will cause the regulator's output voltage to decrease. This negative temperature coefficient is just what's needed for lead-acid batteries.I was thinking of asking you to look at this thread, because I have a question.
Where does the temperature compensation come from?
R4 and Q1 fold the current back, this I can see. But how does Q2 sense temperature and adjust for the battery? I'm thinking thermal runaway (or something similar) but you have the appropriate feedback on the base resistors R5 and R3.
Yep, it isn't. Basically, you want roughly C/5-C/10 (C=AH capacity) current flow into the battery until it's nearly at the float charge level. This design:You mentioned the current limiting on this design wasn't linear.
...starts decreasing the current right away, even if the battery is nearly dead.
Take a look at the attached schematic/simulation. The green trace is the current going into the battery, and the blue trace the battery voltage. I had to adjust R13 downwards in order to get a reasonable amount of current into the battery, and that caused the "knee" to go away completely. If you only wanted a few mA (I'm talking 10mA-20mA current) into the battery, then you could use a larger R13 - but that would not be sufficient for a gel cel or other SLA battery.I don't see why it would need to be linear on current, the idea is to trickle charge a battery in use. I accept temperature compensation is something different, but as a base charger I vaguely remember you recommending the configuration for another float charger on another thread.
You can make your own power resistors from Nichrome wire and a suitable form; preferably ceramic or other heat-resistant media. I did that for a charger I whipped up for my father-in-law's cell phone on his last visit; he'd forgotten his charger and needed his phone available on his arrival back home.
I just started in electronics a few weeks before you did.Your amazing!
I picked up some ceramic tubes that have tinned ends from Skycraft Parts & Surplus here in Orlando. They were just perfect for what I was wanting to do.What are some good common items to use for a core?
If you use a 0.62 Ohm resistor for R4, Q1 will keep the voltage across R4 from rising above ~0.62V nominally; I(R4) is 1A; P=EI, so P=0.62x1 = 620mW. You double that for reliability's sake, so 1.24 Watts. 1 Watt is ~81% of 1.24 Watts, or ~19% less - so you'd really want to go to 1.5W or 2W.How do you calculate the wattage? I know P=I*E but what voltage and current do I use? Do I use.62 volts * the rated current?
I sure envy you guys that have resources like this.I picked up some ceramic tubes that have tinned ends from Skycraft Parts & Surplus here in Orlando. They were just perfect for what I was wanting to do.
If you use a 0.62 Ohm resistor for R4, Q1 will keep the voltage across R4 from rising above ~0.62V nominally; I(R4) is 1A; P=EI, so P=0.62x1 = 620mW. You double that for reliability's sake, so 1.24 Watts. 1 Watt is ~81% of 1.24 Watts, or ~19% less - so you'd really want to go to 1.5W or 2W.
You don't have to fiddle around with making your own resistor.
Digikey stocks these:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P0.62W-2BK-ND
That's just what you need. $0.35+ USPS 1st Class shipping, probably less than a buck total.
Because in #15 you said the LM317 was only capable of 800mA. So I figure I would dial back my expectations.If you were thinking more like 0.5 amps, then why did you say 1 amp in post #14?
Since you already know how to get the output that you want (it's been explained several times already), I see no point in continuing this discussion.
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