Power supplies

[BrianG]

Originally Posted by magman

Any others you recommend? I am only looking to get up to 8 amps through the charger, that's the max my batts are rated at.

Just because the batts are 8A doesn't mean that's all the charger will pull from the power supply. It also depends on the pack voltage as well. A charger has a switching power supply to convert the 12v input to whatever output voltage needed for the pack you're charging. This is done via a switching power supply similar to how a UBEC functions. If it wasn't for this power supply, you would be limited to charging batteries at no higher than 12v, or just under 3s lipo (3 cells X 4.2v/cell = 12.6v).

Here's an example:

Let's say you have a 5s 8Ah pack. The max charging voltage will be 21v (4.2v X 5 cells). Then, you are charging at 8 amps. That's 168 watts (21v X 8A). The charger has to supply this power to charge your batteries.

Since your external power supply outputs 12v, you would need 14A. Actually, most chargers are about 80% efficient, so you really need 210w, or 17.5A @ 12v.

BTW: That 168w is not all the time. When you start charging, the pack may only be at 15v (3v X 5 cells). At 8A, that's 120w. At once the CC/CV threshold is reached at 4.2v/cell, the current drops steadily, and so does the power. That 168w is worst case.

You might wonder "why not just use higher voltage?". Well, some chargers can take up to 27v input, but most use 12v presumably to make them usable in the field running off your car battery.