Electronics > Beginners
Bench PSU 30V/10A as a car battery charger
sdancer75:
Hi,
I own a bench power supply 30V/10A. I tried today to charge my car battery and I adjusted output voltage ~14.2V.
When I connected the battery, the output current adjusted automatically to ~5A and dropped the voltage about ~13V. I tried to raise the voltage but nothing has happened. I then regulated the output current about ~9A and voltage raised about ~13.5V.
I am thinking that ~9A is too much for charging, am I wrong? Some Chinese advertise that they work with 12V/6A. So, what seems to be wrong here?
Regards,
SparkyFX:
You can charge a lead acid (or lead acid glas mat or gel) battery with pretty high currents (there is no charging circuit in a conventional vehicle other than generator voltage regulation), as long as you reduce the voltage to 2,4V per cell (14,4V) once it reaches full capacity and it does not overheat while charging. If you exceed 2,4V on a charged cell it will start to release hydrogen, which is explosive.
Also keep in mind that single battery cells might be damaged, shorted and therefore might overheat.
The bench PSU approach has no builtin limitation for a charging process other than the voltage and current limit settings, which is good enough for lead acid and NiMH/NiCd, but not for Lithium cells.
T3sl4co1l:
So, what, you set it to do charging, and got worried that it was charging exactly as it was supposed to..? :)
You can only have one or the other at a time -- current or voltage. A typical CC/CV charger (works nicely for lead acid and lipo batteries) will deliver CC into the battery (voltage pulls down to whatever the battery terminal voltage is), until the voltage reaches CV, at which point the current decays slowly. Eventually current drops to some low margin, and you're either in the float charging regime (which is fine for maintaining lead acid, albeit at a slightly lower voltage), or that's your cutoff to stop, and you're done done.
Tim
soldar:
--- Quote from: sdancer75 on June 12, 2019, 12:20:29 pm ---So, what seems to be wrong here?
--- End quote ---
What seems to be wrong is that you do not understand how a battery and a charger work. Everything else seems to be fine.
T3sl4co1l:
--- Quote from: SparkyFX on June 12, 2019, 12:40:33 pm ---If you exceed 2,4V on a charged cell it will start to release hydrogen, which is explosive.
Also keep in mind that single battery cells might be damaged, shorted and therefore might overheat.
--- End quote ---
Any cell -- applying extra voltage will force electrolysis of water as an alternative reaction, worsening charge efficiency and generating gas. Worse still, the gas bubbles block electrolyte, raising resistance and allowing the voltage to rise even further (if otherwise unlimited). This sets the maximum charge rate on a typical lead acid cell.
Setting the maximum charge voltage to 14.4 or thereabouts (give or take exact ratings of the battery in question) gives the best chance of avoiding electrolysis. Charging rate could still be high, say if the terminal voltage is low (nearly or fully discharged) and therefore a lot of current is sunk pulling it up to 12-14V. Hmm, I'm not sure offhand if electrolysis occurs for any overpotential (in which case, it would be more about just current density), or the absolute potential.
Indeed, shorted cells obviously will drag down the total, and trying to charge that at nominal levels will not have a nice outcome. Over-charging and -discharging carries a greater risk of disturbing/sloughing off the electrodes, and of dendrite growth through the separator, causing shorts.
--- Quote ---The bench PSU approach has no builtin limitation for a charging process other than the voltage and current limit settings, which is good enough for lead acid and NiMH/NiCd, but not for Lithium cells.
--- End quote ---
NiMH and NiCd are the ones with weird charging curves (voltage goes up, then curls over slightly as temp rises). ??? Lithium depends on type, but ordinary LiPo/ion types are CC/CV to cutoff, AFAIK, very easy to use.
Tim
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