NeilP wrote:Almost Cor..the insulating mat is to stop all the bare wires from shorting out on the metal bench.
Ah, you already took care of the cases=ground. Good.
NeilP wrote:So yes, a pair of PC supplies would give you about 15 amps...jsut depends on the output the 12v line would put out. if you can get more, go for 5 of them on the 5 volt line
That would likely give you 25V, 30A or something like that.
NeilP wrote:You can sometimes getaway with not having a current limiting source. I have had limited success doing it ..but often you end up chasing the supplies along the line..as you power one up, another drops out...I have a huge stack of dead PC PSU..30 or more..and have probably zapped a good few..10 or so at least.
Yes, PC supplies are not so well regulated, so the voltage varies quite a bit with load - this "soft" behavior helps in cases of overload as in battery charging.
However, the simplest way to use PC supplies (or other fixed power supply) and charge a pack with them is to set the supplies to the maximum charge voltage by tuning the output voltage if possible or by combining different voltages - for PC supplies you can choose from 12V, 5V and 3.3V and with multiple supplies you can make a lot of combinations, for example to charge a 4s LiPoly pack (up to 17V max if it is well balanced and each cell goes to 4.25V) you can add the 12V from one supply to the 5V from another supply (usually you cannot combine the voltages from the *same* supply as they are tied together at the negative side) or use 3 supplies for more current: place 2 12V supplies in parallel to double their current and add the 5V from the third supply.
Note that if you can't get the right voltage then you can create a voltage a little higher and add heavy duty low voltage diodes in series to drop the voltage by about 0.8V for each (silicium) diode.
Note also that the 12V supplies might not be the exact same voltage and so they might not share the current evenly.
Also, to charge a pack that is quite empty and has a low internal resistance, the voltage of the charger will need to drop. Power supplies do not like this, they try to deliver all they have, overheat and die.
One simple way to solve this if the voltage difference between your pack and power supplies is no more than 15V is to place a few car headlights between the supply and the battery. Incandescent bulbs are pretty good current limiters and a typical 55W headlight will give you a current between 4 and 5A over a wide range of voltages, probably from lower than 8V to above 15V. So, to limit a PC supply setup to a max of 15A you wire 3 headlights in parallel between the string of supplies and the pack. No more blown supplies and the headlights give a visual indication of the charging progress: first bright, then slowly extinguishing while the pack comes up in voltage (charge). When the current drops below 1 or 1/2A then the charging is done.
If you can scrounge a couple headlights from a junkyard (or from your garage) then you only need to wire them in and off you go. No modern electronics required