LiPo charging/low input voltage

[YonderGod]

I got this charger
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=74030

With these batteries
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=64439

I'm trying to use an old PC power supply.
It's rated for 18A on the 12v rail.

So I should be able to charge at 5 amps, right?

When I try to charge at 5A, I get a low input voltage error. 2A works.

Is the problem simply my power supply sucking, or am I misunderstanding something?

 

[Hillhater]

5 amps into a 6S pack is maxing out the charger at 120+ watts
most chargers need a powersuply at the mid-high end of the input voltage range ( say 20+ volts for yours) in order to work at the limit.
so yes,..its a power supply problem

 

[amberwolf]

Also, it's possible that your PSU's 12v rail is sagging if it's not also loaded down a little on the other voltages (3v, 5v, and if it has a second 12v rail, that too).

You can measure the voltage there to see.

Also remember that the "12V" these chargers typically expect to see is really 13-15V, because they were also designed to run off car batteries, which will be in that range (depending if hte car is running or not).

And as noted in the post above, some also may need even higher voltages to get the full rated output power from them.

 

[YonderGod]

5 amps into a 6S pack is maxing out the charger at 120+ watts
most chargers need a powersuply at the mid-high end of the input voltage range ( say 20+ volts for yours) in order to work at the limit.
so yes,..its a power supply problem

I bought this charger thinking it would charge twice as fast as my 50w ac charger that I already had.

So you're saying I have to buy a 20v+ power supply to use it to its full capability? It's not possible with any 12v pc power supply? I have a few more I was going to try.

 

[YonderGod]

Also, it's possible that your PSU's 12v rail is sagging if it's not also loaded down a little on the other voltages (3v, 5v, and if it has a second 12v rail, that too).

You can measure the voltage there to see.

Also remember that the "12V" these chargers typically expect to see is really 13-15V, because they were also designed to run off car batteries, which will be in that range (depending if hte car is running or not).

And as noted in the post above, some also may need even higher voltages to get the full rated output power from them.

hmmm, yes, I may try putting a load on the 5v rail and see if it improves.

I found I can charge at up to 3.5A with the PSU staying at over 10v.

 

[amberwolf]

If the PSU is dropping down to 10V, something is wrong, because it should be regulating at 12V +/- maybe half a volt or so, assuming a 5% tolerance unit (less than that if it's a precision unit).

Loading the 5V and 3V (especially the 3v, as it is the largest current-capability voltage on most even remotely modern units) to perhaps an amp or so might help, if it is a design with interdependent regulation.

(if you look at a lot of basic PSU testers, you'll find many have a high-wattage low-ohms resistor across every voltage, to ensure there is enough load to verify operation at correct voltage under a basic load. Some get hot pretty quick because of that, some don't (using a lesser load).

THere are instructables on how to turn a PC PSU into a basic lab PSU with fixed voltages, and those will help you on what kind of load you may need to apply.

There've been threads about this here, too, but may be easier to find on instructables.com.

 

[YonderGod]

Ok, I tried another PSU and it works at 4.5A, without any load on the 3v or 5v.

If the PSU is dropping down to 10V, something is wrong, because it should be regulating at 12V +/- maybe half a volt or so, assuming a 5% tolerance unit (less than that if it's a precision unit).

So if that's the case, it would just turn off entirely instead of dropping to 10-11v? These extra PSUs I have are really old, from the Pentium 4 era. The second PSU is working at 4.5A, staying at 11v. Is this unsafe?

 

[amberwolf]

I don't know. If they were designed to have a load and are operated without one, I don't know what conditions exist inside and if they will have any problems because of that (I'm not an SMPS expert).

The difference in output could be for a number of reasons--differences in PSU design, component aging, defects, etc. So...I don't know.

Best bet is what I said before.


Oh, also, you might want to measure actual current being drawn from the PSU, to see if the reason it's dropping is because it's actually the rating being exceeded, rather than my other speculations.

 

[wesnewell]

Read this.
https://sites.google.com/site/tjinguytech/my-projects/convert-pc-ps

Next came the load resistor. I knew I needed about a 1A load on the 5V bus in order to stabilize the 12V bus, and that more was better. I ended up using 2x 1.5ohm 10W in series to create a 3ohm resistor that would draw 5/3A. I made the connections and attached the resistor to the case to keep it out of the way.