nickandre said:
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 directlyand then other electronics could be run off of step-down buck converters for 12V or voltage regulatorsand 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
But its OK to have multiple sources of charging into the same battery.
>Do I get a 48V nominal power supply [size=small]/ 48v nominal Lthium-Ion charger will work just fine.
This would be a14s 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?
[/size]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.php?tid=8362&replyto=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.