Will a battery bank 'pull down' the voltage of an AC-DC charger (for instance a laptop power brick) in the same way a PV panels voltage is pulled down

[Dzl]

I'm looking into solutions for cheap/easy charging of a DIY power station.

I'm wondering if using a ~19.5V laptop power brick could be a workable solution. They are cheap and readily available. Obviously there would need to be a high voltage disconnect device that cuts charging at ~14V or so, but if that is taken care of, I'm wondering if this would be a workable solution.

Will a battery bank 'pull down' the voltage of a simple AC-DC charger (for instance a laptop power brick or meanwell type PSU) in the same way a PV panels voltage is pulled down to the voltage of the battery bank?

Any downsides?

 

[schmism]

typically they are regulated to maintain voltage at their rated output current. So no I would not expect much voltage sag between a noload condtion and full load condition. (maybe a 1 volt tops?)

Typically their only issue is with current output. Most are only 250w or so. so ~10 to 12a. Not a lot for 12v high capacity LFP cells.

 

[Dzl]

typically they are regulated to maintain voltage at their rated output current.
So no I would not expect much voltage sag between a noload condition and full load condition. (maybe a 1 volt tops?)

In this case sounds like this solution would not work well. But without a background in electronics, I'm struggling to understand how this would work, if the battery bank was at say 13V and the chargers output is normally 19V, and voltage is a difference in potential between two points, how could the charger output and the battery bank voltage be at different potentials, one or the other would have to rise or fall wouldn't it? Or would the charger just fail to function properly?

Typically their only issue is with current output. Most are only 250w or so. so ~10 to 12a. Not a lot for 12v high capacity LFP cells.

Not a lot of current, but virtually free if you search or dumpster dive at E-recycling places, ubiquitous, and still more current than the power brick style chargers made for LFP I have come across. I haven't searched much, for this project I'm trying to use commonly available and flexible components as much as possible and keep costs down. A laptop charging option would check almost all of these boxes but I'm not tied to it. Portability is also much more important than high C rate AC charging so that shapes my priorities.

 

[schmism]

In this case sounds like this solution would not work well. But without a background in electronics, I'm struggling to understand how this would work, if the battery bank was at say 13V and the chargers output is normally 19V, and voltage is a difference in potential between two points, how could the charger output and the battery bank voltage be at different potentials, one or the other would have to rise or fall wouldn't it? Or would the charger just fail to function properly?

I don't know that I can do a full electronics explanation for it.

I can make an attempt at a EL5 version....Reverting back to water analogy.
One body of water sits higher (19v) the other body of water sits lower (13v) There always has to be an elevation difference if you want water to flow (current flow). If you instantly lowered the high bucket to pond level, then no water flows out of the bucket anymore. (you effectivly stop charging at 3.65 constant voltage charge because both buckets are at the same level) So in order for there to be water movement (current flow) between the two bodys they HAVE to be at different potentials.

A dead short is like an infinitely tall water fall. You seen the video of angle falls... huge amounts of water fall off the top and at the bottom its a mist... Kind like that. Dead batteries that can absorb huge amounts of current can look like dead shorts to electronics. This is why you cant get fractional volt car batteries to charge off the new fangled electronic battery chargers.

My truck battery has been dead for a while. Volt meter on it says about 6v. Once I jump it and the alternator is running volt meter at the terminal says 14.4 volts. But that doesnt mean the battery instally filled with electrons. (I did not instantly fill the bucket with water). If i let it run for 20 min and shut it off and check the voltage it says something like 10v. (typically if i drive for about a half hour I can restart the truck when I come out after my errand)

 

[Johncfii]

No, the battery will not pull down the 19.5-volt laptop supply. The supply will regulate output voltage to 19.5 volts, up to its rated capacity. If you connect it to a low impedance load that would draw more than the rated capacity of the supply, the power supply will go into a self-protect mode, and shut down the output.

An unregulated supply will behave differently. How differently? It depends ....

But there is a $10 solution .... you can go on eBay and buy a 20-amp-rated DC-DC step-down “buck“ converter that will accept the 19.5 volts and step it down to your desired charge voltage. You limit the current by adjusting the converter output voltage.

Downside? This isn’t automatic! You will have to monitor it to assure that you don’t overcharge any of your Lithium cells. But this may be a good way to learn a little more about electronics, if interested.

 

[Mike Jordan]

A laptop power supply is my plan for charging the power station, beyond solar, that I am building. I got the idea from this guy. He starts off talking about his NOCO charger. But then somewhere just before 5 minutes in, he goes into using his power supply through his Victron SCC

 

[Dzl]

Thank you all for the feedback / explanations.

However there is still something I am not fully grasping. Setting aside the solar panel comparison, what would be the difference between a laptop style power brick and the type of benchtop power supply (or this type of basic power supply sometimes used in its place) used for top balancing. Is the issue that the laptop charger is not current limiting? Or that the difference in voltage is smaller? Something else? or Something I'm fundamentally understanding?

A laptop power supply is my plan for charging the power station, beyond solar, that I am building. I got the idea from this guy.

But there is a $10 solution .... you can go on eBay and buy a 20-amp-rated DC-DC step-down “buck“ converter that will accept the 19.5 volts and step it down to your desired charge voltage. You limit the current by adjusting the converter output voltage.

Actually, I have been considering both of these solutions, which led me to consider whether the DC-DC converter component was even necessary or whether the charger would act like a PV panel and be pulled down to the battery bank voltage. Sounds like it won't, so I'll circle back to one of my original ideas. Unless you have any other interesting and creative ideas up your sleeves? This is as much a creative/learning project for me as something practical so I'm interested in creative solutions.

Downside? This isn’t automatic! You will have to monitor it to assure that you don’t overcharge any of your Lithium cells. But this may be a good way to learn a little more about electronics, if interested

This (if I am understanding correctly that you are referring to high voltage disconnect?) would be covered by the BMS, and optionally by another layer (TBD)

You are alluding to something like this right?

 

[Mike Jordan]

Thank you all for the feedback / explanations.

However there is still something I am not fully grasping. Setting aside the solar panel comparison, what would be the difference between a laptop style power brick and the type of benchtop power supply (or this type of basic power supply sometimes used in its place) used for top balancing. Is the issue that the laptop charger is not current limiting? Or that the difference in voltage is smaller? Something else? or Something I'm fundamentally understanding?

I have a meanwell power supply also. So I will try it for kicks. But I will probably stay with the laptop power supply. The difference for me is, I have several old laptop power supplies, they are much smaller, and don't need to be vented. So they will be much easier to squeeze into a small space in a tight solar gen box

Speaking of several laptop power supplies.... just made something else pop into my head. Which might not be a good thing. Can I parallel them, or series them, to feed the SCC? It would still be about the same amps or volts as 2 solar panels

 

[Dzl]

I'm curious if anyone could explain how a laptop power supply functions, or point me in the direction where to look. I have tried searching before but don't know enough to know what search terms will get relevant results. I think understanding this, will help me answer some of my own what if questions on this topic.

 

[Johncfii]

I'm curious if anyone could explain how a laptop power supply functions, or point me in the direction where to look. I have tried searching before but don't know enough to know what search terms will get relevant results. I think understanding this, will help me answer some of my own what if questions on this topic.

Ahhhh .... complicated subject, not knowing your background, so where to begin?
Most such power supplies are “switching” or “switch-mode” power supplies.
Google either of those terms for lots of good explanations.
Those are in contrast to the other broad category of “linear” power supplies.

 

[Just John]

I'm curious if anyone could explain how a laptop power supply functions, or point me in the direction where to look. I have tried searching before but don't know enough to know what search terms will get relevant results. I think understanding this, will help me answer some of my own what if questions on this topic.

If you are wanting to use a laptop power supply through a solar charge controller, that should work fine for 12 volts. I'd have to look, but most should be marked, I think 19 volts is a common value that would work fine through a charge controller. I'm not sure how the laptop power supply will handle it, probably be putting out maximum power depending on the solar charge controller capacity. I see a lot of 90 watt 19 and 20 volt laptop supplies on amazon, that would mirror a 100 watt panel nicely.

 

[Johncfii]

Speaking of several laptop power supplies.... just made something else pop into my head. Which might not be a good thing. Can I parallel them, or series them, to feed the SCC? It would still be about the same amps or volts as 2 solar panels

An unusual but interesting idea.

Yes, you could do that. The SCC work work just the same when supplied by any DC source that is within acceptable voltage and current parameters. Certain power supplies might get fussy when connected in series or parallel, but many times that will work.

 

[Johncfii]

Thank you all for the feedback / explanations.

However there is still something I am not fully grasping. Setting aside the solar panel comparison, what would be the difference between a laptop style power brick and the type of benchtop power supply (or this type of basic power supply sometimes used in its place) used for top balancing. Is the issue that the laptop charger is not current limiting? Or that the difference in voltage is smaller? Something else? or Something I'm fundamentally understanding?


Actually, I have been considering both of these solutions, which led me to consider whether the DC-DC converter component was even necessary or whether the charger would act like a PV panel and be pulled down to the battery bank voltage. Sounds like it won't, so I'll circle back to one of my original ideas. Unless you have any other interesting and creative ideas up your sleeves? This is as much a creative/learning project for me as something practical so I'm interested in creative solutions.


This (if I am understanding correctly that you are referring to high voltage disconnect?) would be covered by the BMS, and optionally by another layer (TBD)

You are alluding to something like this right?

The bench top supply is not vastly different, but it gives easy control over the output voltage. That is, if you want to feed 3.65 volts into your cells, turn the knob until the output is set to 3.65 volts (as confirmed with your pretty-accurate digital volt meter). Most bench top supplies also let you set (limit) the maximum current output by turning another knob. The issue is not that this bench top supply is current limiting. The issue is that it is versatile and convenient.

Yes, that is the buck converter I was referencing.

 

[Dzl]

Ahhhh .... complicated subject, not knowing your background, so where to begin?
Most such power supplies are “switching” or “switch-mode” power supplies.
Google either of those terms for lots of good explanations.
Those are in contrast to the other broad category of “linear” power supplies.

Coincidentally, this (switching power supplies) is what I have been googling for the last half hour or so. Haven't found a good conceptual level explainer yet.

But I wonder if this may not be precisely targeted to the point I'm trying to understand.

I believe this, is a switching power supply as well, and I believe it can be used in the way I would like, (though maybe not with such a high difference in voltage?)

If I understand CC / CV (and I'm not sure that I do beyond a basic practical level), constant current limits the output to a certain maximum current, CV sets the voltage to a certain target. When the difference in voltage is high, the 'load' maxes out the CC rating and the charger operates in CC mode, as the battery is charged its voltage rises and at some point the difference in voltage will be small enough that current (I = V/R) will drop below the max CC at which point the charger is operating in CV mode current tapers and voltage flattens at its set point. Is this roughly correct in your understanding? Or am I fundamentally misunderstanding things?

Going back to the solar panel example, an ~18V solar panel hooked up to a ~12V battery works without a DC-DC converter (MPPT) to convert the voltage and even without a PWM controller if you are not concerned with 3-stage charging so long as there is a means to disconnect the PV array from the battery before the voltage of the battery gets too high. Does this sound right to you?

Likewise with a benchtop power supply, we would see something somewhat similar, right? Say you have a fully discharged lithium battery ~10V, you set your power supply to 15V (I know too high) and your current to some value. Immediately upon connecting your power supply you would see current rise to the max but voltage will be pulled down to the voltage of the battery bank, right? Your BMS is set to disconnect at 14V and does once your batter bank reaches that voltage.

In both of these examples, I believe, voltage difference between the source and the battery bank can be some amount higher than the battery bank voltage, and no harm will occur to the cells or the charger/pv panel so long as the cells are disconnected before the battery bank voltage exceeds its limit.

So now I'm trying to understand the above, in the context of the aforementioned hypothetical 19V laptop power supply, with nothing between it and the battery apart from a switch that disconnects automatically at 3.6V per cell? What separates this situation from the above? It still seems to me like it has to either be an inability to limit current or maybe just the higher difference in voltage?

I really want to understand what I'm missing here.

 

[MattiFin]

An unusual but interesting idea.

Yes, you could do that. The SCC work work just the same when supplied by any DC source that is within acceptable voltage and current parameters. Certain power supplies might get fussy when connected in series or parallel, but many times that will work.k

Coincidentally, this (switching power supplies) is what I have been googling for the last half hour or so. Haven't found a good conceptual level explainer yet.

But I wonder if this may not be precisely targeted to the point I'm trying to understand.

I believe this, is a switching power supply as well, and I believe it can be used in the way I would like, (though maybe not with such a high difference in voltage?)

If I understand CC / CV (and I'm not sure that I do beyond a basic practical level), constant current limits the output to a certain maximum current, CV sets the voltage to a certain target. When the difference in voltage is high, the 'load' maxes out the CC rating and the charger operates in CC mode, as the battery is charged its voltage rises and at some point the difference in voltage will be small enough that current (I = V/R) will drop below the max CC at which point the charger is operating in CV mode current tapers and voltage flattens at its set point. Is this roughly correct in your understanding? Or am I fundamentally misunderstanding things?

Going back to the solar panel example, an ~18V solar panel hooked up to a ~12V battery works without a DC-DC converter (MPPT) to convert the voltage and even without a PWM controller if you are not concerned with 3-stage charging so long as there is a means to disconnect the PV array from the battery before the voltage of the battery gets too high. Does this sound right to you?

Likewise with a benchtop power supply, we would see something somewhat similar, right. Say you have a fully discharged lithium battery ~10V, you set your power supply to 15V and your current to some value. Immediately upon connecting your power supply you would see the voltage drop to that of the battery bank and current rise to the max value. Your BMS is set to disconnect at 14V and does once your batter bank reaches that voltage.

In both of these examples, I believe, voltage difference between the source and the battery bank can be some amount higher than the battery bank voltage, and no harm will occur to the cells or the charger/pv panel so long as the cells are disconnected before the battery bank voltage exceeds its limit.

So now I'm trying to understand the above, in the context of the aforementioned hypothetical 19V laptop power supply, with nothing between it and the battery apart from a switch that disconnects automatically at 3.6V per cell? What separates this situation from the above? It still seems to me like it has to either be an inability to limit current or maybe just the higher difference in voltage?

I really want to understand what I'm missing here.

Laptop power supply is missing the current limitation or the CC-part. It will attempt to maintain steady 19 volts and if current limit is exeeded it will shut down or enter ”hiccup mode” and attempt to restary once per second.

Proper CV-CC charger lowers the output voltage to match the current limit setting.

 

[MattiFin]

An unusual but interesting idea.

Yes, you could do that. The SCC work work just the same when supplied by any DC source that is within acceptable voltage and current parameters. Certain power supplies might get fussy when connected in series or parallel, but many times that will work.

Laptop power supply and solar charge controller probably wont play nicely together.
Solar panel is essentially current limited source. Pwm-based controller would be really nasty with laptop power supply(constant voltage without active current limiting)

Very small mptt controller might work if the laptop power supply has enough power to ”overload” the mptt controller.
large mptt controller would attempt to track for maximum power and shut down the laptop power supply.

 

[Dzl]

Laptop power supply and solar charge controller probably wont play nicely together.
Solar panel is essentially current limited source. Pwm-based controller would be really nasty with laptop power supply(constant voltage without active current limiting)

Very small mptt controller might work if the laptop power supply has enough power to ”overload” the mptt controller.
large mptt controller would attempt to track for maximum power and shut down the laptop power supply.

You predicted my next set of questions before I asked. This was the logical conclusion I was coming to.

Would this same principle apply to using a laptop power supply through a DC-DC converter or is that a different story?


In regards to the second factor I pondered above (whether a large difference in voltage between the charge source and battery bank voltage) is harmful or not (if the battery is disconnected before it exceeds its upper voltage limit). Do you have any thoughts/input?

To use an extreme example, say you have a 19V cc/cv charger, a 12V battery and a BMS set to disconnect at some safe limit (say 14V). What would be the impact of such a large difference in voltage, could it harm the charger or the batteries (assuming the cells are disconnected at 14V).

Thank you for entertaining all my hypothetical and 'what-ifs'

 

[MattiFin]

You predicted my next set of questions before I asked. This was the logical conclusion I was coming to.

Would this same principle apply to using a laptop power supply through a DC-DC converter or is that a different story?


In regards to the second factor I pondered above (whether a large difference in voltage between the charge source and battery bank voltage) is harmful or not (if the battery is disconnected before it exceeds its upper voltage limit). Do you have any thoughts/input?

To use an extreme example, say you have a 19V cc/cv charger, a 12V battery and a BMS set to disconnect at some safe limit (say 14V). What would be the impact of such a large difference in voltage, could it harm the charger or the batteries (assuming the cells are disconnected at 14V).

Thank you for entertaining all my hypothetical and 'what-ifs'

If you use laptop power supply together with dc-dc power supply that has constant current mode it works just fine. Suitable dc-dc modules start from couple of bucks on aliexpress. Or get one with display, like the popular Riden.

Disconnecting 19v cv-cc supply wouln’t be any problem. 190v would be different matter and most probably toast the bms. ?

 

[Dzl]

If you use laptop power supply together with dc-dc power supply that has constant current mode it works just fine. Suitable dc-dc modules start from couple of bucks on aliexpress. Or get one with display, like the popular Riden.

So in principle the DC-DC converter would need to be limited to some number <100% of the chargers maximum output current (and some sensible voltage)? Or am I still misunderstanding.

I like the Riden and will probably buy the RD6024W when it comes out, but that would be overkill for this project.

Disconnecting 19v cv-cc supply wouln’t be any problem. 190v would be different matter and most probably toast the bms. ?

 

[MattiFin]

For example this should work

https://m.aliexpress.com/item/1005001639883304.html?

300 chinese watts should be good match to ~100w power supply. set the current limit so that the laptop power supply is running maybe 70% max load. Or if you use this really occasionally you could probably use all 100%

 

[Dzl]

For example this should work

https://m.aliexpress.com/item/1005001639883304.html?

300 chinese watts should be good match to ~100w power supply. set the current limit so that the laptop power supply is running maybe 70% max load. Or if you use this really occasionally you could probably use all 100%

Funny enough, i think i was looking at the same exact little device on ebay

 

[Johncfii]

So in principle the DC-DC converter would need to be limited to some number <100% of the chargers maximum output current (and some sensible voltage)? Or am I still misunderstanding.

No, the DC-DC converter does not need to be limited in capacity. A converter rated at 20 amps maximum would be fine. That is just the maximum amperage cab be drawn from the device without damage. It is a bit like driving a muscle car with a big engine. You have that potential power under the hood, but you don’t need to use it.

The current that is drawn by the battery, from power supply and converter “system“, will be determined by the state-of-charge of the battery, and the difference in voltage between the battery and the charger. With this regulated laptop supply connected to the DC-DC converter, the battery will be charging in constant voltage mode, since you will not have a constant current source. As the battery charges, the current will taper off unless you intervene by bumping up the converter output voltage.

 

[John Frum]

A constant current constant voltage power supply lowers its voltage to control the current flow to the battery.
Said another way its the difference in voltage between the power supply and the battery that determines the flow.
Garden variety power supplies don't have this feature.
If the voltage differential between a non cc/cv power supply and battery dictates a current flow higher than the psu can supply then the psu will "hiccup".
The remedy is to put a buck converter or boost converter or buck/boost converter between the psu and the battery.

I will put this buck converter between a pc psu and each of my lifepo4 cells to top them off. For those who are wondering, there is a diode and wago connector off the positive lead to protect the converter from back feed from the cell.

 

[HRTKD]

Interesting discussion. I have access to hundreds of those laptop power supply "bricks". I passed on a few to my brother that were exact fits for his laptop. At first he thought, "WTF am I supposed to do with all these?" It didn't take him long to see how nice it was to not have to carry a power supply everywhere. Just leave a power supply where he commonly used his laptop (van, desktop, garage, etc).

I hadn't really thought about using those power supply devices for anything other than laptops. If I were to use them with my LiFePO4 battery bank, I think I would want to funnel the power through something that (intelligently) understands the LiFePO4 charge profile.

The question of powering a solar charge controller with something other than a solar panel came up within the past few months and the reaction at first was that it was not going to work. Based on the few threads I've seen, it does work. It may not be ideal, but the solar charge controllers seem to tolerate it.

 

[Johncfii]

Interesting discussion.

I hadn't really thought about using those power supply devices for anything other than laptops. If I were to use them with my LiFePO4 battery bank, I think I would want to funnel the power through something that (intelligently) understands the LiFePO4 charge profile.

As usual, the clever Chinese engineers have you covered. Search on eBay, or AliExpress, for “Lithium Battery Charge Controller”. There are dozens/hundreds offered there for sale. They are brilliant, and very low cost, but limited to about 10 amps output. You connect your own “dumb” power supply as the input, For higher amperage charging, I would try using my solar charge controller, as mentioned above.