Bench PSU 30V/10A as a car battery charger - Page 1
Forum section: electronics / beginners

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 12, 2019, 12:20:29 pm
content: 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,

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 12, 2019, 12:40:33 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2019, 12:50:40 pm
content: 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

author: soldar
author_rank: Super Contributor
timestamp: June 12, 2019, 01:03:13 pm
content: Quote from: sdancer75 on June 12, 2019, 12:20:29 pmWhat seems to be wrong is that you do not understand how a battery and a charger work. Everything else seems to be fine.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2019, 01:04:23 pm
content: Quote from: SparkyFX on June 12, 2019, 12:40:33 pmAny 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.QuoteNiMH 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

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 12, 2019, 01:26:09 pm
content: Quote from: soldar on June 12, 2019, 01:03:13 pmYes, thank you!So please answer to my question: Does this mean that a car charger 12V/6A (i suppose my PSU in the time depth will do the same thing) always adjust the voltage/current (high voltage - lower current or lower voltage - higher current) according to the charging capacity of the battery?I mean, does this mean that my PSU when my battery will get enough charge, will gradually drop down the current demand and raise the voltage to the initial settings ~14.2 ?

author: soldar
author_rank: Super Contributor
timestamp: June 12, 2019, 01:49:20 pm
content: Charging a car battery is somewhat like filling a toilet tank. At first the flow of water is constant and the bulk of the "charge" is taking place. As the level reaches the top the float valve begins to close and the charging slows down until the tank is full.One thing to realize is that you never have direct control of the level (voltage) except by controlling the flow going in or out. That you can control but the voltage is determined by the chemistry.It is an analogy that falls apart if you take it too far but it should help you understand how a battery charges.If you want to raise the water level of lake Superior all you have to do is put enough water in it but you cannot change the water level without putting the water in.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2019, 02:07:18 pm
content: Yes; leave it for a few hours, and when you come back it should be around 14.something volts. Some hours more, and you should see it at the same voltage as the setting, and current dropping quite low.Tim

author: ArthurDent
author_rank: Super Contributor
timestamp: June 12, 2019, 02:13:05 pm
content: One warning on using a bench power supply as a battery charger is that some supplies have over voltage protection in the form of a crowbar, an SCR circuit can be set to short the supply output if there is an over voltage that could damage your load. The Astron RS-20 and others supplies by Astron with a fixed 13.8VDC output are like that. A place I worked had two of these supplies charging car batteries (that were used for backup) die violently in about a week so I looked at what was happening. It turned out there were some spikes on the lines that triggered the crowbar to give a direct short across the output and the output was the large battery with huge fault current feeding into the supplies to vaporize runs, wire, and SCR.There may be other circuitry in other bench supplies that don't like voltage being fed into the supply so a blocking diode or a low resistance MOSFET configured as a low drop diode would be a wise idea. The circuit below describes a simple method of using a MOSFET. The simplest chargers you can get may just have a transformer and diodes inside and don't have this problem and other chargers have this reverse protection built in.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 12, 2019, 02:30:11 pm
content: Quote from: sdancer75 on June 12, 2019, 01:26:09 pmA lead acid battery will draw whatever current its chemical composition allows it to, so as long as the source voltage is higher than the battery voltage, it will draw current, otherwise try to supply current. The 12V tag is obviously just the nominal battery voltage (under nominal load).QuoteYes, which is why the initial setting should not exceed the voltage a fully charged cell can maintain safely - unless you are able to monitor constantly or use a charger with a builtin detection to shut supply off when charging needs to end.Quote from: T3sl4co1l on June 12, 2019, 01:04:23 pmI know cars that are designed to have up to 15.5V onboard system voltage, but they utilize charge counting to the battery, therefore apply this only with an empty battery and call such a system microhybrid (situational use of the generator). I haven´t heard of exceptionally high battery failures with them, seems to work fine. Besides that, there are always failures rooted in a suboptimal driving profile.QuoteMight be wrong on the NiMH and NiCD, haven´t messed around with those...I thought that the LiPo/LiIon types have no easily detectable deltaV charging end and require thermal monitoring throughout, therefore need charge counting to really utilitze the capacity. I am sure one can get away with keeping it in between 20-80% level, i heard this is done to increase the lifetime/amount of cycles as well.This is why one really needs a seperate charge logic for every type of battery, it could end catastrophically.

author: radiolistener
author_rank: Super Contributor
timestamp: June 12, 2019, 05:46:35 pm
content: Quote from: sdancer75 on June 12, 2019, 12:20:29 pmthat is correct behavior for accumulator charge, if your accumulator is 50 A/h.Quote from: sdancer75 on June 12, 2019, 12:20:29 pmthat's is your mistake, you can damage your accumulator in such way.Quote from: sdancer75 on June 12, 2019, 12:20:29 pmand you're right. At first step you're start to charge accumulator in the correct way (by limiting current and voltage).But when you decided to increase volatage and current limit, you're go to the wrong way.

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 12, 2019, 05:51:05 pm
content: Quote from: T3sl4co1l on June 12, 2019, 02:07:18 pmThank you Tim !

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 12, 2019, 05:53:01 pm
content: Quote from: radiolistener on June 12, 2019, 05:46:35 pmThank you for pointing me about my mistakes.Best Regards,

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 12, 2019, 05:54:00 pm
content: Excellent! Thanks.

author: mzzj
author_rank: Super Contributor
timestamp: June 12, 2019, 06:42:10 pm
content: Quote from: radiolistener on June 12, 2019, 05:46:35 pmNo big deal even if you charge the car battery at 9 amps.After all they are charged with up to 120 amp(or even more) alternator with 13.8 to nearly 16 volts depending on temperature.

author: mzzj
author_rank: Super Contributor
timestamp: June 12, 2019, 06:50:03 pm
content: Quote from: SparkyFX on June 12, 2019, 02:30:11 pmLi-ion is really easy to charge compared to Ni-Cd/Nimh but when things go wrong they go wrong in a big bang.Ni-chemistry is real pain in the *** to charge properly and you can't for example parallel them. But lot more forgiving, even if you charge and abuse them badly they are pretty hard to make explode.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 12, 2019, 07:59:16 pm
content: Quote from: SparkyFX on June 12, 2019, 02:30:11 pmNope. Li-Ion cased cells work fine with a simple current cutoff. You charge in constant voltage mode until the current drops to some threshold and then stop. The only really tricky part about LiIon is that the charge voltage tolerance is pretty tight. Set it too high, battery goes boom (or has greatly diminished lifetime). Set it too low and you will be losing a large chunk of your capacity. The main point of charge counting is to display a battery gauge and to keep track of the cell capacity degredation.NiMH/NiCd cells are trickier which is why there have been a number of ways to charge them. Simple trickle chargers are really slow. Older cheap medium speed chargers used a timer designed to get "mostly charged" and then dropped to a trickle charge to finish off. Other fast chargers do look at the delta-V or the temperature rise to detect end-of-charge. But at least they are less likely to catch fire when overcharged although they don't like it at all.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 13, 2019, 03:35:47 pm
content: Quote from: ejeffrey on June 12, 2019, 07:59:16 pmWhich requires the charger to know the nominal battery capacity, right?

author: mzzj
author_rank: Super Contributor
timestamp: June 13, 2019, 04:07:16 pm
content: Quote from: SparkyFX on June 13, 2019, 03:35:47 pmSort of.But it's not overly picky about cut-off current, you can cut off early and you would only lose couple of % capacity or you can cut-off at lower than specified current with longer charging time and very slightly increased risk of something going wrong.3000mAh battery terminated at 500mA charge current might be at 95% charge and if you terminate at <50mA it might take couple of hours extra.You could leave li-ion battery to 4.1 or 4.2v power supply infinitely butifwhen something goes wrong at some point after 10 years or 1000 cycles the battery can overheat, explode or start WW3.