A couple power supply theorys - RC-Monster Forums

Metallover · **A couple power supply theorys -** 03.15.2009, 01:54 AM

I have been thinking about a couple things that could be used or modified to be used as a powersupply. First off is a power inverter. The ones used in cars that run 12v input into a standard 120v home outlet. I thought the inverter could be used backwards somehow.

The other one is a car battery charger. Can it be hooked up in any way to where it could supply power to my charger?

At the moment I'm using a sla battery and it works fine, but I am going to start using those for my electric bike when that's finished so I'll be needing a power supply. If all else fails, I might be adding a power supply to my next purchase at hobby city.

BrianG · 03.15.2009, 12:55 PM

Answer to your two theories:

1) Inverter. This is also a switching power supply, but it boosts voltage. Simply wiring it backwards will not work.

2) Car batter charger. Well, I kinda wonder the same thing, but I don't think it would work well for higher currents because their duty cycle at their max rating is small - in other words; they can't run at the max capacity for very long before they overheat. So, you'd be limited to lower currents. Then, you'd also want to regulate the output, which means a regulator IC, several pass transistors, large heatsink and other components.

Metallover · 03.15.2009, 01:00 PM

Hmmm.. Thanks for the answers. Is there a good place to get an old car battery? That would be good enough for me. If you can get one for free then there's no need for an ac power supply.

Jabe · 03.15.2009, 01:39 PM

I beleave that you can use your car battery charger, if you modify it. Or you could just make it charge battery, then hook your charger to that battery.
But why not, just modify your PC powersupply, you can find many instructions about it, i did that, used two pc powersupplys in parrel 350W+350W, well the other one was behind switch, so i could apply it, when needed, never needed it thought.

Metallover · 03.15.2009, 01:56 PM

Is there a place I can get a really cheap used/salvaged pc power supply? I don't have any laying around.

BrianG · 03.15.2009, 02:17 PM

Quote:

Originally Posted by Jabe

I beleave that you can use your car battery charger, if you modify it. Or you could just make it charge battery, then hook your charger to that battery.
But why not, just modify your PC powersupply, you can find many instructions about it, i did that, used two pc powersupplys in parrel 350W+350W, well the other one was behind switch, so i could apply it, when needed, never needed it thought.

Just be aware that there is some "fighting" when using two seperate power supplies in parallel. The current isn't simply additive, it's more like 80% of PS1 + 80% of PS2. And if the voltages aren't very close, there will be more losses. Generally, it's best not to parallel power supplies, but to get a single one that has the total outout you need.

Quote:

Originally Posted by Metallover

Is there a place I can get a really cheap used/salvaged pc power supply? I don't have any laying around.

The problem with "salvage" power supplies is that they are generally older models that don't have as high currents on the 12v rails as newer ones do.

The PS you get will be determined by your power needs. What is the max cell count and current you will charge? Better yet, what is the power limit of your charger? Just take that power, divide by 0.8 to account for typical charger efficiency, then divide that by the voltage. So if you have a charger with a 120w power limit, that's 150w. Divide that by 12v = 12.5A. And since you generally don't want to push anything to its max, I like to figure in another 25% for safety (and to account for some voltage drop under load), which would be ~16.5A.