External Charger for Online UPS

[minitt]

Lets start with the Objective


1.To increase the backup time of my Double conversion Online UPS
2.Ensure fast recharge for both internal & external batteries
3.Make sure external charger remains completely isolated from the UPS internal charging circuit.

A brief description:
The UPS internal battery is connected to a AC powered timer relay.
The AC powered timer relay also connects to the external battery bank and an external battery charger.

This is a DPDT( Double Pole Double Throw) Relay. Contact rating is 120A@28VDC. Relay Coil runs on 220 AC and directly powered by utility power.

So, when the relay is powered, it will connect the external batteries(75A Batt @ 24VDC) to the external battery charger.
But when power goes out the relay looses power and connects the external battery bank to the internal UPS battery in parallel.

There is a 60mv voltage drop due to relay contacts on both contacts (+ & -) Is this something I should be concerned of?

I've applied this with a Online UPS which has two 9A SLA(Yuasa) at 24VDC bus. Which roughly gives around 30 minutes@ 160 watt load. The internal charger can put put out max 1A.

The two 75A flooded lead acid batteries connected in series with an external SMPS charger which only charges the 75A batteries with a float voltage of 27.29V

A hand sketched design is attached which illustrates the whole modification.

Looking forward to your suggestions, tips to improve my mods.

Thanks in advance.

[stj]

Re: External Charger for Online UPS

cant you just double up the battery's in parallel?
2 chargers are going to be a pain to syncronise so they dont mess wih each other.

[minitt]

Re: External Charger for Online UPS

cant you just double up the battery's in parallel?
2 chargers are going to be a pain to syncronise so they dont mess wih each other.

The internal charging circuit will take around 5 continuous days to charge those two 75A batteries. Thats where the external charger comes in.

As for synchronization, which really isn't an issue here since the relay will connect the External batteries to the External charger after disconnecting the external batt from the UPS and vice versa.

This means the charger will be always isolated from the UPS internal charging circuit.


here C1 is External Batt +

and C2 is External Batt -


A1 & A2 is UPS (+ & - respectively)

B1 & B2 is External Charger (+ & - respectively)

So at NC position( relay coil has no power= Grid power failure) C1 & C2 will connect to A1 & A2 respectively

and when the relay coil is energized C1 & C2 will disconnect from A1&A2 and then connect to B1 & B2(External Charger) (grid power comes back)

[minitt]

Re: External Charger for Online UPS

[joshnz]

Re: External Charger for Online UPS

seems ok i have not look at diagrams though

[minitt]

Re: External Charger for Online UPS

seems ok i have not look at diagrams though

Could you have a look the attached picture which pretty much outlines the whole concept.

Concern:
But someone informed me that the smaller 9A (SLA) batteries will be damaged very soon by the bigger external batteries (Flooded Lead acid) due to circulating current.

IF the above scenario is a real fact, then how come APC or others ups manufacturer offer external battery banks that has much higher capacity than internal ups batteries, work in parallel with the internal ones but not suffering from this same effect?

[minitt]

Re: External Charger for Online UPS

bump....

[cheapie]

Re: External Charger for Online UPS

The main problem that I see is that if one set (internal or external) of batteries is near-charged while the other is near-discharged for some reason, then when the power goes out and the batteries are connected together, one set will start charging the other. While this shouldn't ever happen, placing a fuse (with a sane value) in the external string should prevent damage if it does.

[minitt]

Re: External Charger for Online UPS

The system has been running as expected. Had to change the dpdt power relay once.

[japlytic]

Re: External Charger for Online UPS

Beware, the battery can be at mains potential in some units, as I have seen.

[joshnz]

Re: External Charger for Online UPS

Beware, the battery can be at mains potential in some units, as I have seen.

Found that out the hard way on my old unit.

[minitt]

Re: External Charger for Online UPS

Beware, the battery can be at mains potential in some units, as I have seen.

could you please explain a bit as to what do you mean by mains potential?

tia

[eccerr0r]

Re: External Charger for Online UPS

Mains potential, meaning it's not isolated from the wall socket. This is an electrocution danger if you work on the batteries.