AT power conversion

I know it ain't a bike. But the experts here could probably turn this idea into a cakewalk.

Computer power supplies provide 5v and 12v DC. I was thinking that I could find a use for some old AT boxes if I could get a couple clues from the wizkids that frequent this board. I want to build a battery charger, and maybe swapping out some resistors or something might allow me to charge my digital camera batteries and bike battery as needed, without a lot of work to do so.

Boosting the 12v to around 13.5 is probably not really out of line, and would recycle hardware that makes economical /ecological sense. I think the PS can handle the amperage requirements, and if they don't, well, this stuff is pretty much useless anyway otherwise. I can get piles of 'em for the cost to pick them up.

What say ye?

Been done, after a fashion:

Text of conversion and linked photos...

"Floating" means the battery is allowed to be at whatever voltage it needs to be at based on it's condition, charge, temp, etc. Typical float chargers put out pulses of DC and the instantaneous voltage is set by the battery. The pulses are important so the average DC voltage can be below the peak voltage of the supply. In a regulated supply, the peak and average are the same.

A regulated DC voltage can potentially force too much current into the battery. To be safe, it would have to be set low, like around 12.8 or 12.9v. But then it may not exactly charge very well.

Wire's link is for a device to prevent the battery from draining while running accesories with the car off. That is a slightly different purpose. He doesn't care about charging the battery, he just doesn't want it to drain. However, he is hitting the battery with a pretty high sustained voltage without letting it float. Car batteries can take a lot more abuse than a bike battery, though.

You can make a regulated supply float, but only to an extent. You just need to put the right resistor in series with it to make the output voltage vary with the current. The problem is, as the current reduces due to the battery being charged, the output voltage approaches that of the regulated supply. It's a tricky balance, but can probably be done as long as you are seeking a trickle charge and not a fast charge.

Personally, I think a better idea would be to yank the supply apart and use just the transformer and rectifier. Then use a resistor (it may have to be big like 50 watts) to set the average current to 1 amp or so (average current = average voltage on the resistor divided by the resistance of the resistor). It will be a trial and error affair. The wattage power of the resistor will be the average current times the voltage on the resistor times a safety factor of 2 or 3.

Post edited by: loudhvx, at: 2007/03/26 15:10

Hi,

I have a different interpretation of the "float charge" concept.

Typically a "float charge" is a constant voltage charge placed on a battery, usually in a standby application. This allows the battery to "float" in a ready state until it is ready for use (in a UPS for example).

For a 12V battery, a typical float charge voltage is about 13.5V.

A float charge will NOT charge a discharged battery (well, it might, depending on the condition of the battery, but it would take days).

The AT power supply is a good start to a simple battery charger because it is a regulated DC power supply. Typically about 250W, this is more than enough power to charge your small motorcycle battery (which only really needs 20W to charge).

IMHO, the best thing to do with an AT power supply and a motorcycle battery would be to up the 12V circuit of the supply to about 14.4V (2.4V/cell) and current limit the circuit to 1A. This basically turns it into a regulated taper charger limited to 10% of the battery's capacity (most of our batteries are 10-15Ah). You could consider the battery to be charged when the current falls to about 300mA, then apply your float charge overnight to "top off" the battery.

Some cautions:
- the current must not rise too much above 2A, this will start to damage the battery.
- the 14.4V must be shut off before the battery becomes overcharged and starts boiling excessively. This entails monitoring the charge or creating an automatic current monitoring cut-off circuit


Another AT power supply, without current limiting, and regulated to 13.5V would be useful as a "12V system power supply" as in the link. You could run your electrics with the battery removed. At 250W you would be able to draw some 18A, enough for the standard loads (not the starter!) of our bikes. However, caution here that on turn-on, the inductive loads of the lamps could cause a large current spike, pulling the voltage way down. Actually, on our bikes I don't see why that voltage dip would be a problem - there's no electronics!

Good luck, and if you ever go nuts and decide to build an electric car or motorbike, I can recommend a very good battery charger w/ DC-DC converter.

Inline79, the cautions you raise are the reasons why you don't want to put a regulated supply directly to a battery. You don't want to have to monitor the voltage, current etc.

The old fashioned float chargers are just a transformer and rectifier. They naturally have to put out pulses since they have no filter and no regulator. If you put an oscilloscope on the output of one of those chargers, you'll see the pulses. Also if you put a dc voltmeter on it, you'll see the voltage is way below 14 volts. It'll probably be like 9 or 10 volts, when not connected to anything. That is because meters read average-voltage and float chargers put out very low average-voltage. But they can still charge a battery because their peaks are around 15 or 16 volts. When you connect it to a battery, the meter is reading the average voltage of the pulses in addition to the contributions of the charger pulses, which will raise the DC level of the battery to around 14 volts.


Float chargers charge batteries usually with 10, 6, or 2 amp selections. The selection is made by applying 120vac to different primary windings on the transformer, (or tapping into different secondary windings, but that would be wasteful). They work because as the battery voltage increases, a smaller portion of the pulse is above the battery's ambient voltage. This means the battery gets a smaller and smaller pulse of current as it's voltage increases (as it's charge increases). It inherently does what you want, naturally.

The problem with using a regulated supply is current-limiting it at the beginning of a charge cycle, and shutting it down at the end of the charge cycle. To do that you need current detectors, voltage detectors, and really, a temperature detector as well. Car regulators used to have temperature sensors built in because they regulated the voltage to the battery based on voltage. At different temps, the battery would get different levels of charge (because it's ambient voltage is different at different temps). (Not an issue on bikes because you rarely ride in subzero temps.) Also, cars and bikes have a similar behavior to float chargers because the output from the alternator/rectifier is also in pulses (usually peaking near 16v, and troughing near 13v, giving an average at a little over 14v).

(BTW, I certainly am not trying to be a smartass or anything like that, but this is a topic I've studied quite a bit.)

Post edited by: loudhvx, at: 2007/03/27 00:21

loudhvx,
Ok, what you were saying before makes sense now. If these old style "float chargers" are just a transformer and rectifier, I understand the line-frequency waveform you describe. I never put a scope on a KZ rectifier/regulator, but I suppose there's no reason they would be filtered either, huh?

I come from the world of microprocessor-controlled switching DC power conversion. This includes battery charging, so I hope you don't mind me sticking a post in here and there about batteries and charging.

From reading your posts, I won't dispute your vast experience with motorcycle electrics though. That I'll bow down to. :laugh:

No bowing necessary, Inline, I know you know your electrics.

I think it's more of an issue that terminology is changing. To me "float chargers" were all you could get, unless you had big bucks, back in the 60's and 70's. They were the simple little devices I described above. But now, after doing some quick internet searches, I see "float chargers" described as the trickle-charge devices you mentioned. These are a different animal, with far more developed "brains". But I think their purpose is slightly different. They seem to be designed for the last phase in a three part charging scheme for industrial batteries. The first two blast the battery with higher current and charge rates, then the float comes in to top them off with a trickle.

So the old-fashioned floaters are basically working as trickle chargers if the battery is mostly charged, even though they were able to provide larger currents if the battery is dead, like 6 or 10 amps.

Post edited by: loudhvx, at: 2007/03/28 01:57

Thanks for the responses. Now, If I can get one of these things to actually do what I need it to do, I will post results here.


Louhvx when you indicated concerns about monitoring the battery, does that mean mostly checking it for heat buildup?

KaZooCruiser wrote:

Thanks for the responses. Now, If I can get one of these things to actually do what I need it to do, I will post results here.


Louhvx when you indicated concerns about monitoring the battery, does that mean mostly checking it for heat buildup?

Heat, bubbling, water level etc. would be a good idea, but I meant voltage and current.

The simplest thing would be to get a 1 ohm, 10 watt resistor. Simply putting a voltmeter on the resistor will show the current through it. (Current = the voltage on the resistor divided by its resistance.) Then start out by cranking the regulator to 13 or 13.5 volts, and connect it to the battery with the resistor in series. This will basically be a trickle charger, but should be safe if you left it connected for couple days. When the voltage on the resistor drops to about .01v, then the current is .01a, and the battery is charging very slowly.

The problem is that I don't think this method will really work if you want to charge the battery faster, unless you are willing to monitor the voltage and current diligently. Definitley worth tinkering with if you have the power supplies laying around.

Post edited by: loudhvx, at: 2007/04/05 04:26

You probably already know this but... make sure you are using AT and not ATX supplies. An AT supply is safe to turn on without being under load but an ATX will smoke if there is no load (plus you need to ground the green wire to turn it on). I use AT supplies for all kinds of stuff but have never tired to modify them. I have one powering a car stereo out in the garage right now. Not sure it's worth the effort for battery charging, the black and decker motorcycle charger is only like $17 at Walmart
Oh one other caution. Be careful not to short the thing out while experimenting. Super sensitive and let the magic smoke out instantly . Same thing for over current draw.. I kind of assumed they had some kind of protection but if they do it isnt very good.

Ron