Read this first to understand why.
I have looked (albeit quickly) through some of the previous postings re battery charging. I am looking for a bit of explanation as to why these battery chargers go from super simple to super complex. (I am guessing just features, ie charging rate, charging shut off, although I struggle with this one...later coment) I have a few old battery chargers, typical automotive ones, 6 Amp and 10 Amp. The circuit board in one (10 Amp, 6v/12v) was removed several years ago (burned...given to me this way) It consists of a center tapped transformer and the primary is switched (to accomodate the slow current charge or 6v or 12 volt, I assume)
I have bypassed the cct board for testing so essentially, full wave CT rectifier and measured 12.6 volts with the primary taps I selected. I then connected a small lead acid battery (measured 9.6 volts before any charging) and the output then measured 15 volts. I expected it to increase gradually as battery charged....? Also, the ammeter in the charger indicated only about 2 amps....I expected more charging current due to differential so can someone give a brief explanation as to why voltage across battery increased immediately (meter averaging? ) and current is so low (is there a higher impedance in small batteries that limit current?)
The original cct bd was simple...cannot recall components and some were burnt beyond recognition but I am guessing a large SCR or transistor and a few small discrete components. Some of the circuits I see posted here ie Guleph cct (http://www.sentex.ca/~mec1995/gadgets/labc2.htm), look quite complex. Any suggestions for a simple charging cct? Can I just switch (via Mosfet or transistor) the rectifier output to the battery till battery voltage comes up, assuming I have to stop charging to monitor voltage. This as a result of my tests today indicate that the battery voltage appears higher during charging than the actual battery voltage is? So, it becomes charge, stop and check voltage, if necessary, charge...repeat. If not necessary, just monitor voltage.....?
I have ventured into uControllers a little so thinking of using one but it seems overkill considering the original cct in this box.....and leads to including a +5 supply for it, etc, so simple gets complicated. I guess it is true that "simple problems bring complicated solutions" ....;-)
Why is overcharging protection needed?....I would think that if a charger put out say 13.8 volts, then the battery charging current would gradually decrease until virtually zero as the battery voltage approached the max of 13.8 that the charger put out....why does it overcharge? I know it does/can with out protection, but why? or is this only in the case of simpler chargers which may put out more voltage?
Looking forward to the comments.
Last edited by ShawnR; 25th April 2012 at 02:38 PM. Reason: punctuation and additonal comments
Read this first to understand why.
Last edited by nsaspook; 25th April 2012 at 04:09 PM.
My comments are my own and don't represent the views of any company.
In GOD we trust, all others we monitor.
1. The previous cct had only one diode - you have two.
2. If the voltage rose immediatley, the battery is faulty.
3. Read-up about sealed lead-acid batteries and the "gassing voltage. "
Thanks nsaspook for the articles. They did help.
Colin, what cct do you refer to? "1. The previous cct had only one diode - you have two. "
As for faulty battery, maybe...I will put it on my "fancy" charger and see what it tells me about it. Being that it is an old ATV battery, was so low in storage voltage and reacted to the charger as indicated, sounds like it is defective. I will try some other batteries around here to see what happens. Kind of making this into a Battery Charging 101 course, which I could use. Thanks
I am now thinking that if I am to take the time to repair these old chargers anyways, maybe a little uController and set some charge levels would be a worthy project....?
Interestingly, I put another (larger lawn tractor battery on the "charger", which right now is just a transformer and 2 diodes. The battery voltage before connection was 12.5 volts or so and as soon as I connected, it became 16 volts so it looks like the battery acts like a capacitor? in that it is showing a higher voltage, and the current flow into the battery was about 6amps so,.....A battery with a higher cca and higher starting voltage drew more charging current....not what I would have expected. I would have thought that with a higher initial charge on the battery, the charging current would have been less. So the size of the battery contributes to the actual charging current?
When I connected the first battery mentioned in this thread to the "smart" charger, http://www.canadiantire.ca/AST/brows....jsp?locale=en, the voltage displayed was 11.9 so quite different than what I was measuring with my meter on the "dumb" charger. :-)
So if I am to build a circuit to replace the old one in the charger, it does need to limit the output voltage during charging? I would have thought the battery would take care of that. Would you put a capacitor in the output cct similar to a power supply? I would not have, thinking that I do not see them in chargers as I do in power supplies but perhaps that is what the circuit board did in there, limiting the peak voltage out? One of the articles touched on peak values but I have to go back to look at it again.
Am I getting there? ;-)
I think you are getting fooled by the measurement method. While your output is DC it is pulsating DC. Take some measurements with and without the battery on both AC and DC. And before and after the bridge.
Last edited by ronv; 25th April 2012 at 07:36 PM.
The 16v battery is faulty.
Try a known GOOD battery
You said your circuit had a bridge - this is 2 diode-drops and will deliver less current. A centre-tapaped arrangement is one-diode-drop and will deliver a higher current.
A capacitor is not needed.
It was only neeeded in SCR circuit to provide DC for the controlling circuit.
The only way to measure current is via the voltage across a 0.1 ohm resistor.
Ronv, that is what I was wondering too so I checked again and with different meters...I measured the battery voltage to be 12.8 volts and charger output (no battery connected) to be 13.8 volts....all sounds good. Then when I connect the battery to the charger, the voltage as measured across the battery terminals (or charging clamps) is 16.1 (and it sounds high (audibly) from the battery so I never leave it more than a few seconds.
Would a "good" charger put out a smoother waveform? akin to a power supply output? I measured the output on the smart charger with a DVM and it was about 13.7 whereas the display indicated 13.5 so well within what I would expect. Anyone know what these smart chargers do to the output with regards to how much filtering?
I have another (a third one) similar to the dumb one discussed above http://www.canadiantire.ca/AST/brows....jsp?locale=en. I put it on another battery (bat volts =12.9) and the measured charging voltage was over 16 as well. Either that charger is screwy too, although I have considered it a back up (till now) or that is what these chargers charge at, which I find hard to believe.They would have cooked a lot of batteries, no? Maybe I am chasing my tail but as I mentioned, the battery sounds like it is bubbling inside so it backs up the high voltage readings.
These old chargers (meaning this type of charger) have been around for years and I think worked fine, so I want to know how they work....or are supposed to work
Last edited by ShawnR; 25th April 2012 at 08:33 PM.
Colin, I guess I was typing same time as you.....no bridge in my charger, just the two diodes and center tapped transformer. As mentioned in my posts above, I have tried a different battery and same result. I also used a 2nd charger on a 3rd battery and similar measured results (unless that one is defective too, in some way, although IIRC, it has the circuit board in it and has worked in the past. Totally different battery too.
1. Remove the filler caps and if you see one cell bubbling more than the others, it is faulty.
2. Some smart chargers put out very high pulses to get rid of sulphating, so AC pulses are not a problem.
3. Most simple charger must be turned off as they still deliver a current when the battery is charged and it will eventually dry-out.
4. Read up on gassing voltage for the batteries you are charging to see what type of charger you need if you want to keep the charger connected.
5. Are you measuring the voltage with a $10.00 analogue meter? You should.
Last edited by colin55; 25th April 2012 at 08:44 PM.
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