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48V DC UPS options
#1
I have been looking into DC UPSes of sorts -- basically a power system whose primary function is to supply power in the event of an outage but for a much longer time than a UPS.

The plan was to buy a large pack like a 48V Nissan Leaf battery and then connect it to the related UPS electronics.

There are a reasonable number of 48V appliances you can get (lights, networking gear). Some of them (like PoE units) have quite a large lattitude in voltage which could be run directly and then other electronics could be run off of step-down buck converters for 12V or voltage regulators and such.

Anybody have tips or reference design?
- Do I get a separate DC power supply and charger and then switch over to the battery when power fails?
- Do I get a 48V nominal power supply and use an MPPT to charge the pack at the higher voltage? Is there an all-in-one system that would handle this?

--Nick
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#2
(11-26-2019, 10:46 PM)nickandre Wrote: I have been looking into DC UPSes of sorts -- basically a power system whose primary function is to supply power in the event of an outage but for a much longer time than a UPS.

The plan was to buy a large pack like a 48V Nissan Leaf battery and then connect it to the related UPS electronics.

There are a reasonable number of 48V appliances you can get (lights, networking gear). Some of them (like PoE units) have quite a large lattitude in voltage which could be run directly and then other electronics could be run off of step-down buck converters for 12V or voltage regulators and such.

Anybody have tips or reference design?
- Do I get a separate DC power supply and charger and then switch over to the battery when power fails?
- Do I get a 48V nominal power supply and use an MPPT to charge the pack at the higher voltage? Is there an all-in-one system that would handle this?

--Nick

>basically a power system whose primary function is to supply power in the event of an outage but for a much longer time than a UPS.
OK - this is definitely a sane goal.   Like you, this was my first thought a couple of years ago as I have several APC UPSs.      An APC UPS 1500 charges up to 27.8v  and cuts off around 21v - which is compatible with 7s Lithium Ion - e.g. 27.8v = 3.97v/cell and 21v = 3.0v/cell.     And in fact I have built/use several 7s 18650 batteries that I'm running in my APCs right now.      NOTE: A senior member here as mentioned that server class APCs will even let you set custom voltages thru Powerchute.

You indicate your UPS is 48v - But the same principles apply.   You want to make sure that your UPS does not exceed 4.2*14 = 58.8v and have a cut-off before you get below 3.0*14 = 42v.   If this is a server class and customize able - even better as you probably don't want to 'push it' and charge to absolute max of 58.8v or drain down to minimum 42v.     Typically 4.1v/cell max and 3.3v/cell min is a bit more practical.


>Do I get a separate DC power supply and charger and then switch over to the battery when power fails?
The APC UPS charges at about 60watts - which is pretty slow charging. I assume your UPS is similar.  This might be OK for you if this is grid-fail only system...  as you can spend days charging since grid shouldn't fail all the time Smile   But its OK to have multiple sources of charging into the same battery.   

>Do I get a 48V nominal power supply / 48v nominal Lthium-Ion charger will work just fine.
This would be a 14s lithium-ion configuration - so normal 14s lithium-ion charging would apply.   You need to decide if you want to keep your cells at max (4.15v) - which is hard on them - or perhaps get a charger that you can customize to set max at 4.1v or 4.0v/cell for longer life. 

>and use an MPPT to charge the pack at the higher voltage?
Not sure what you mean by 'higher voltage'.   I see MPPT as simply a PV -> Charge Controller type of charging as apposed to grid powered charger.  Regardless of source of power to the charger - the same target top voltage would apply..   and as mentioned above you might want to have one you can customize.   

MPPT Charge Controllers typically let you customize the top (float) voltage.   

Grid powered chargers (and the UPS itself is a grid powered charger) may not be as flexible unless you go higher end.

**You'll also want a BMS. This is an excerpt from another very senior member on basic BMS from this thread today - https://secondlifestorage.com/newreply.p...lyto=57373
@daromer
A BMS is basically this in this order:

1. Critical fault protection unit
2. Notification on eventual issues and status
3. Capable of balancing.

The first 2 are the most important and balancing is what will make the battetery last a long time. You can go with lover balancing current but just DONT hide it with those cheap balancers. Its better to not have any balancer than hiding it in my world.

Whats important is that you get a unit that can disconnect the system if needed and that can inform you if so happened or will happen.

I vote for Batrium to and shared look require WM5.
With that said the others will work to.
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#3
Just to be clear, you don’t want to invert DC to AC, you want a UPS for direct to DC application?

If so, I would skip the transfer switch and run the load off battery at all times, just have a sufficient charger/supply to handle the load as well... if the charger ever looses power you are already on battery.
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#4
I agree with what OffGridInTheCity has stated, however to add, I would no use the UPS to charge the batteries at all. This would always be done by a secondary charger. As OTGITC said, they charge at a really slow rate compared with the size of battery you'd want to implement.
So even if the unit was used in a grid outage event only, if that downtime was a day or more, then the time it'd take for the UPS to recharge the battery skid would take a loooooooong time. You want to charge them back up as fast as possible (within reason/safety of the specs of the batteries themselves, of course)

As Crimp Daddy says, if this is only for battery usage, then forego the UPS altogether and get a decent charger. A standalone charger is usually a lot cheaper than a UPS would be, even off eBay or such (unless you get them almost free from the salvage/recycler yard like others have done [AveRage Joe for example] )
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#5
With the use of a UPS you will quickly realise how much standby power consumption the units have and over time the energy difference can be massive.

I'm currently running my inverter (off grid) in a forest with solar an wind grid tie inverters along with half of my battery packs (16kWh) and the one really little detail which I thought was logical at the time is quite significant.

My wind grid tie unit with a normally open relay which is closed when the inverter provides power at 230V as normally the low wind speed out put would go via a DC boost converter through a separate cable. The relay as it turns out is around 8W and given the inverter standby power consumption is around 22W that little relay is a massive overhead and adds up to 1.3kWh a week ! Massive in the sense of 100Wh on a cloudy day of generation from 3.3kW of solar panels.....

Go with them to start off with as they can be really cheap to learn with.

For most UPS units the topping (desulphate voltage) is the voltage to watch for on charging output.

Charging from the grid, buy a flatpack2 unit (2kW or 3kW) at less than £70 each, check my eltek serial numbering post before buying...

Running 12V kit from 48V buy some step down units off aliexpress, I will finish my post on them as they have turned up while I'm away. Typically with 48V packs you tend to need 12V and lower most times with some 24V.

Oh, and with UPS units if they are older they can have significant leakage if not earthed properly and your negative battery line can end up at half the AC output...... I found out the tingling what the f**** was that way (see post)
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#6
(11-27-2019, 01:56 PM)Korishan Wrote: I agree with what OffGridInTheCity has stated, however to add, I would no use the UPS to charge the batteries at all. This would always be done by a secondary charger. As OTGITC said, they charge at a really slow rate compared with the size of battery you'd want to implement.
So even if the unit was used in a grid outage event only, if that downtime was a day or more, then the time it'd take for the UPS to recharge the battery skid would take a loooooooong time. You want to charge them back up as fast as possible (within reason/safety of the specs of the batteries themselves, of course)

As Crimp Daddy says, if this is only for battery usage, then forego the UPS altogether and get a decent charger. A standalone charger is usually a lot cheaper than a UPS would be, even off eBay or such (unless you get them almost free from the salvage/recycler yard like others have done [AveRage Joe for example] )

Having a charger that can play double duty sounds intriguing but concerning. I need a charger that's capable of providing:
  1. The right CC/CV charging curve for my batteries.
  2. Enough current to handle my load and charge the batteries simultaneously.
  3. Not get bamboozled and do weird things.
I'm just a bit concerned about edge cases if I go this route. Referencing this article on lithium charging:
  1. In constant current mode, the batteries will see (Charging Current) - (Load Current). Sounds fine as long as Load Current isn't a large portion of Charging Current,
  2. In constant voltage mode, the charger should reasonably maintain the correct settings, but might get confused when terminating the charge. If its logic was to stop charging when the current reached 3% of the rated size, it might never stop.
  3. The charger would likely cycle on and off frequently once the charging stopped. This doesn't sound ideal :/
That's why the idea of a more intelligent charge controller sounded attractive -- one that could be aware of the charge current versus load current and make decisions based upon those two inputs.
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#7
I think you should first clarify to the group about what you mean when you say UPS. Some members here are under the impression you are inverting DC to AC. I understand that this is a direct DC application?
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#8
(11-27-2019, 08:45 PM)Crimp Daddy Wrote: I think you should first clarify to the group about what you mean when you say UPS.  Some members here are under the impression you are inverting DC to AC.  I understand that this is a direct DC application?

A DC UPS is a system capable of providing an unregulated 14S or so DC voltage from 38-54V regardless of grid power. If I didn't need the power to be uninterrupted, I could charge a battery using a charger and then connect a load when I wanted to use it. However, I want to keep the load connected and be able to simultaneously supply power and charge the battery, then continue using the battery when grid power fails. I wouldn't preclude ever using an inverter but basically:

1. Charge the battery when the grid power is available.
2. Provide power even if the grid power fails.
3. Not destroy the lithium batteries by confusing a charger with a combined charging + load.

The question is: what's the most straightforward way of doing this? I know there are solar charge controllers that can pull double duty and independently manage the power to the batteries versus multiple charging sources and a load but those seem a little overkill. On the other hand it doesn't seem like a lithium charger expecting only lithium cells would perform optimally if I were simultaneously loading the system while it was charging.
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