Paralleling power supply and battery - Page 1
Forum section: electronics / projects

author: Manoel
author_rank: Newbie
timestamp: November 27, 2015, 10:13:43 pm
content: Hi guys,Long time reader of the forums but this is my first post here. I'm an EE and I do control loops, digital circuits and mA. Sorry if my question seems weird or dummy, but power electronics isn't really my area - amperes can kill!I currently have a motor application where we use two different power supplies, a 36 V 500 W (Chinese specs, don't know if it holds) switching power supply and a bank with three lead acid batteries.The motor is not very powerful and the Chinese power supply can drive it most of the time - it shuts off when we demand more from the motor, as its peak current is around 80 A.The battery solution is quite good, but since we need long autonomy (around 3 days), we are currently using big 50 Ah batteries. That works fine, except now we are looking for a solution that we can transport as baggage on a plane, so I want to replace the bank with smaller, lighter batteries.Basically, I want the dependability of the power supply (works as long as there's AC) and the peak current capability of the battery pack. Gimme long autonomy, low weight, and cheap. Easy, eh?So, my question is, what is the recommended solution for this problem? The power supply is semi smart, with PFC and all, so maybe paralleling it with the battery bank might work, though I haven't tried it. I could design a switching circuit that uses the power supply most of the time but resorts to the batteries when necessary, but maybe there's an easier way. I even thought of using smallish 9 Ah batteries with a permanently attached charger (with diodes to prevent the motor's back EMF from blowing it up), but that might quickly ruin the batteries.Am I completely off base here? What is the "standard, proven, industrial" solution to this problem? Surely it must have been tackled a million times before, right?

author: qwaarjet
author_rank: Regular Contributor
timestamp: November 28, 2015, 12:31:03 am
content: The standard solution is get a power supply/ motor that can handle the load all the time.With that being said I wouldn't just wire the power supply in parallel with the battery as long term solution, I have done for short term testing though. Since you will basically be charging the battery at all times this can damages many battery types. If the battery and power supply were paralleled though diodes a separate battery charger could be used without over charging the battery. you could even use a relay to connect the battery to the 36V supply to recharge it and then disconnect after the correct state of charge is reached. I don't see a problem with using a smaller battery as long as the discharge current is within the specs of the battery.

author: johansen
author_rank: Super Contributor
timestamp: November 28, 2015, 01:55:30 am
content: lead acid batteries have a rather high impedance particularly when they are fully charged.whether or not you can get sensible current sharing between the power supply and the batteries is up to the voltage regulation of the supply. don't be surprised if the batteries do basically nothing at all.80 amps is a 10C discharge for a 9 ah battery (its an 8 ah at best btw).a 9 ah battery will have a short circuit current of 320 amps.* at an 80 amp discharge its terminal volts will be on the order of 8 or 9.*this is a fundamental property of most lead acid cells. amp hours times 30-40 equals short circuit current.automobile cranking amps are one fourth of the short circuit current, at which the terminal volts must be more than 9 vdc.

author: Manoel
author_rank: Newbie
timestamp: November 29, 2015, 11:24:15 pm
content: Quote from: qwaarjet on November 28, 2015, 12:31:03 amThat's in the ideal world, in the real world, unfortunately, you gotta make it with what you have.Quote from: johansen on November 28, 2015, 01:55:30 amQuote from: johansen on November 28, 2015, 01:55:30 amThose two statements directly contradict each other. The first one is patently false.With further research, it seems a good solution to this problem is a MOSFET ORing circuit. Can anyone point me in the general direction for a circuit that can handle the aforementioned current levels?

author: johansen
author_rank: Super Contributor
timestamp: November 30, 2015, 02:28:41 pm
content: perhaps you don't understand that you only need half an amp or so of current to make the terminal volts of a 9 amp hour lead acid battery swing between 12.56 and 16 volts?Secondly, using a 10 amp hour battery as a reference, knowing its short circuit amps are 300-400 we can simply take 12.56 volts divide by 350 amps and you get 36 milliohms.Most folks here would be pretty offended at a 12v 30 amp power supply if its output dropped 1 volt under a 30 amp load.

author: KL27x
author_rank: Super Contributor
timestamp: December 01, 2015, 12:34:42 am
content: QuoteThis is the norm. Take any computer PSU and test a rail at max rating. No surprise. So no, I don't find that offensive, anymore than a "12V" battery gives only 9V under a heavy load, or a 3.6V li ion battery gives only 2.75V under a heavy load. It's reality. If a power supply can produce exactly 12V at 30A, then it could in all probability provide a heck of a lot more output at a slightly lower voltage. Why stop at 30A? If you don't expect to see any sag, then you're too used to playing with your HP bench meter. Ideal power supplies don't exist, although with enough active circuitry, you can get pretty close (circuitry that is wasteful for running a motor).36mohms of ESR... it's relative. And it's not a lot, in this case. Hook up 10 of those in parallel, and you have 4mohms ESR... is that still a lot to drive a motor? OP is running with a PSU that is doing 90% of the work, so this is the effective case, and voltage sag on a motor affects the speed... but it will still turn.If OP's PSU is typical, and it does have some sag, this is good for his battery boost plan. Using a 12V SLA battery, the PSU will do most/all of the work until the rail sags a bit and the battery will kick in to help.Just rectify the outputs with some diodes and put a 13.8V current-limited supply on the battery side that can run 100% duty cycle (it will only really charge the battery when the load is off). OP just needs a little help to prevent his PSU from shutting off during the peaks, and even a small battery could be enough. OP will figure that out once he gets there.

author: johansen
author_rank: Super Contributor
timestamp: December 01, 2015, 08:50:23 pm
content: Quote from: KL27x on December 01, 2015, 12:34:42 ammy 17 amp, 12v astec drops 0.1 volts under 8.5 amps.how much do you figure that 1 volt drop is the resistance of the 18 awg wires?

author: Christopher
author_rank: Frequent Contributor
timestamp: December 01, 2015, 09:07:01 pm
content: Most PSUs are specced at the output terminal (e.g the point on the PCB where the feedback divider is) and have a load regulation figure.E.g "The output voltage (nominally 5V+-1%) will change +-1% at the output terminals over the operating current range (0-10A)"I'm sure you know what this means.Also, some PSUs have cable drop sensing which is most handy in some applications although are quite limited in some exotic applications (1V or so).

author: KL27x
author_rank: Super Contributor
timestamp: December 01, 2015, 09:25:26 pm
content: It is not uncommon at all for a regulated power supply to sag about 10% at sustained full-rated output current. In some instances, that may even be one way the max output is calculated.USB rail is allowed to sag to 4.4V, for instance. So the average ATX PSU manufacturer, for instance, is not going to rate their PSU for 4A because output drops to 4.9V at that point. They will push it until it drops to 4.4V and put the bigger number on the spec sheet.Same for 12V rail. It is not uncommon to have to buy more PSU than should technically be needed on paper, if your specific gear/graphics card can't handle the voltage drop that often occurs at full rated output. Part of that voltage drop is from the output wires, yeah. That's one part of the equation.Quote8.5 amps is only 50% of the rated output of this PSU. I would be "offended" if a regulated power supply sagged 10% from rated voltage at 50% rated output, yeah.Quote30A is 100% of the rated output of this example.I fully expect a PSU to sag ~10% at full rated output, unless explicitly tested, myself. That would be more of the norm than the exception, AFAIC.

author: johansen
author_rank: Super Contributor
timestamp: December 03, 2015, 01:56:19 am
content: Quote from: KL27x on December 01, 2015, 09:25:26 pmso there is a seller of LLC resonant, Unregulated powersupplies on the diy audio forums, honestly i forget who it is but iirc in his signature he has a link to his Murton-Pike Systems circuit board design.anyhow... unregulated 117/230 vac in, 340vdc feeds the LLC topology.. +/-50 volts out.he claims 90% or better efficiency and 10% voltage drop under full load.i'm not sure what you're trying to get at, but 1% volt drop under full load would be good.. 2% is ok, as my semi expensive astek delivers... 10% is achievable from an unregulated system..

author: KL27x
author_rank: Super Contributor
timestamp: December 03, 2015, 05:02:34 am
content: There are a wide range of psu designs of various cost and quality, and +-a fraction of a percent load regulation isn't a priority for all of them?Anyhow, at 36mohm of ESR, the battery could be fine. You have no idea how much MORE power he needs in addition to his current power supply. And we don't even know if his power supply is regulated. There are more than a few assumptions in your first post. You were dissuading what could be a perfectly good solution for the OP. For no reason.That's all I was getting at.

author: johansen
author_rank: Super Contributor
timestamp: December 04, 2015, 01:03:46 am
content: Quote from: Manoel on November 27, 2015, 10:13:43 pmQuote from: KL27x on December 03, 2015, 05:02:34 amyes, the assumptions that are covered in his first post.80 amp discharge will drop 9 ah batteries to around 9 out of 12 terminal volts.. his power supply, even if its an unregulated, "dumb" rectifier.. will have a lower impedance than this, and attempt to supply most of it. (500 watts at 36 volts is 14 amps)as i said, " don't be surprised if the batteries do basically nothing at all."