Paralleling DC Batteries

It depends why you are doing this.

Also depends on how important the loss of power is to you.

The first reply is correct.

One other very important thing to consider is that most cheap non commercial continuous duty rated UPS units have limited a very run time while at their rated outputs and can not run for very long without burning themselves out.

I have a 1000 VA rated UPS that uses a transformer in its output stage that is roughly the size and working capacity of a 400 - 500 VA continuous duty rated one. The reason being that in a standby mode the battery capacity is typically sized to run out about the same time the under sized and over driven transformer starts to overheat.

The point is that if you add more battery capacity to extend run time you may have to greatly reduce your continuous load level to half or less of what the unit is rated for to prevent it from burning out.

If the internal resistances of the two batteries are different there will be circulating current.

All the above are true. I have experimented with this and found the following to work. You need to charge the larger battery from a separate source with a power diode inline between the battery positives. I have done this with a "car battery" to a ups with factory battery. Huge caution, the inverter in most ups don't dissipate heat well and will be damaged if you pull very much current or use for long periods. (especially really cheap ups).

you can add parrallel battery. problem will be charging it. if ups has it's own battery charger, then you have to check battery charger capacity. otherwise when main's supply restores you have to see that additional battery is disconnected from ups and you have to charge it separtaly. if you are able to replace the charger unit of your existing ups system by higher capacity then it will be better for you.

What is your total volts and amps or kilowatts requirement when you lose grid power?

is the need in one location, or spread over many racks of gear, or many rooms, or many buildings?

This sure points-up the need for a decent AC battery.

Will duration of charge retention or discharge time be a problem in AC batteries like ultra/super capacitors or flywheel?.

I have faced two problems from a similar experiment.

a. If you parallel two batteries, the system might give you an extended back time - in theory. In practice, the electronics will not survive the continual load being put on them. If you observe, the normal UPS with its battery would start getting hot and running it for extended periods on added batteries would only burn out the electronics. They are not designed for the extended hours, else, one electronics would be sufficient for any AH back up and there is no need to have multiple ratings of the UPS (800 VA / 1200 / 2000 VA etc.)

b. As others have rightly pointed out, the charging currents from the normal UPS would be too low to charge the higher AH (read added batteries) and will not be able to charge them sufficiently to sustain the next power failure.

Simply put, invest in a new higher capacity UPS.

Beside all the other issues mentioned here, always keep in mind that paralleling batteries is not as simple as it seems. They are supposed to be absolutely identical (in order -e.g.- to be able to give more current to a load). In practice this never happens. The following figures that I made show this issue.

In the above figure, -e.g.- V_A is somewhat higher than V_B. In such case, a current will flow from battery A to battery B (i.e. battery B will behave as a "load"). In order to avoid such a thing you should do the following.

In this way both batteries will supply current to the load. (However, battery A will supply somewhat more current than battery B.)