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Switch from flat to charged battery while under load
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Psi:
Reading though your post a bit more i have a few more thoughts.

- Be careful trying to utilize a UPS for on time longer than it's original batteries.  They are usually not engineered for always on operation and overheat easily.  When i modified a UPS intended for small SLA to have a big SLA external battery i had to add some case fans to prevent it shutting down from overheating after a few hours.  This point is more of an issue with home UPS, business grade 'always on' UPS's are fine.


- Maybe have a system like this..
   * Supercap bank attached to the UPS through a resistor that limits current flow to whatever the max charge/discharge current is for the supercaps.
   * A relay that connects battery 1 across the supercap
   * A relay that connects battery 2 across the supercap. (these 2 relays could be interlocked to prevent both connecting)
   * A timer system to disconnect one, wait a few seconds and connect the other.

The current limit resistor is there so that the battery banks can only charge/discharge the supercap within its spec.  Since the supercap is quite small this will happen pretty fast.  The bank will charge/discharge the supercap to match its voltage within seconds.


- A diode-OR solution has some losses. Unless you do it using fets.  There are ICs that will drive two fets in a diode OR type config that may work
gnif:
Yes, this is a HP Enterprise UPS designed for data center usage and is provided with the means to connect a large external SLA pack to it to allow for extended run time, so I doub't I will see issues. It's also rated for 2200W but I intend to only draw about 500W max from it.
langwadt:

--- Quote from: gnif on August 11, 2019, 01:44:53 am ---Diodes alone wouldn't work as the UPS needs to be able to charge the pack, however it would allow the switch over and then a bypass on the diode to allow the reverse flow after the switch occurs.

:D

Edit: I even have some 35A rectifiers around here I could use simpy as a diode pack

--- End quote ---

does the ups have to charge both batteries? if not maybe could split the connection to the a UPS battery so the charger is directly connected but the inverter after the diode
gnif:
No, the UPS is intended to only keep its own set float charged. The other pack will be charged from solar. The intent is to completely disconnect the UPS batteries while there is sufficiant power in the pack charged from solar. This way they are not used unless there is an actual power outage and still available as a backup.
NiHaoMike:

--- Quote from: gnif on August 11, 2019, 01:15:09 am ---I think you don't understand what I have tried to explain, or I didn't explain it very well..

I have a UPS feeding a server already with a battery bank, I do not want to use this battery bank for solar, i'd like it to continue to remain charged incase of a power outage while a secondary (solar chaged) bank is also flat. This way I wont loose redundency which is the entire purpose of a UPS.

--- End quote ---
Just connect the batteries in parallel and don't use the full range for solar. For example, switch to solar when the charge level goes to 95% and rising, back to mains at 80% or when the sun goes down. You can even tap into the CPU frequency controls through software and vary it according to system power state, maybe even run a cryptocurrency miner if there's plenty of solar and nothing better to use it on.
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