Battery Charging Expert?

[Lectrician]

I am not too familiar with battery charging as such.

I have a small 450watt UPS that is undersized really, feeding a network switch, router, MS SBS Server and a phone system PBX. The loading is fine, it's not more than 450watt, but it leaves the run time at around a minute max.

The battery is now shot, running out in seconds.

I was going to order a new 1500watt APC UPS, but then thought about replacing the battery with a larger one (there is some room), and even paralleling a second external battery.

Concerns?

Will the charging circuit cope with larger/multiple batteries?
Will the battery monitoring circuit take exception to different batteries? (there is no battery percentage monitor, but there must be something, as the orange "battery error" led is lit.

Large sealed lead acid batteries are available pretty cheaply, as used in intruder and fire alarms, and compared to a new UPS, it would save hundreds.

 

[bernardgreen]

Separating the system into three modules, invertor, battery, charger will give a lot more flexiblity removing the restriction on batery size that the invertors charger will impose.

The existing invertor can be used in this three module approach if it's charger can be disabled or prevented from charging the battery.

An other approach is to connect an external battery and trickle charger to the existing internal battery with a diode in series. While the internal battery is fully charged it voltage will ( should ) be equal to that of the external battery so the diode will not conduct. As the internal battery starts to discharge and lower its voltage the diode will start to conduct and the external battery will then be supplying the invertor. The problem with this is when the internal battery is discharged the external battery may force excessive current through the internal battery

 

[davelx]

An other approach is to connect an external battery and trickle charger to the existing internal battery with a diode in series. While the internal battery is fully charged it voltage will ( should ) be equal to that of the external battery so the diode will not conduct. As the internal battery starts to discharge and lower its voltage the diode will start to conduct and the external battery will then be supplying the invertor. The problem with this is when the internal battery is discharged the external battery may force excessive current through the internal battery

unless you put a blocking diode in line with the internal battery too. But note that the current is pretty high - probably about 46A at 80% efficiency - and a normal silicon diode is probably not suitable due its forward voltage drop at those currents (1.4-1.6V) which will result in the inverter cutting out earlier than it should as the battery voltage falls.

Something like this may do the job:

http://uk.rs-online.com/web/p/rectifier-schottky-diodes/0192685/

but even that has a Vf of 0.74V @ 60A, and you'll nee to put it on a chunky heat sink (it will dissipate about 35W @ 46A)

 

[Lectrician]

Should of said UPS, not inverter, not sure where that came from

It is a cheapy crappy UPS designed for running a couple network switches, not really meant for a server. Was hoping to either increase the run time on it slightly, or just ditch and replace with new. If swapping the battery for a larger one was just a matter of that, then I would do it. If it needs anything above just swapping the battery and paralleling another, I wouldn't bother.

 

[ban-all-sheds]

It is a cheapy crappy UPS designed for running a couple network switches, not really meant for a server. Was hoping to either increase the run time on it slightly, or just ditch and replace with new.

http://www.ebay.co.uk/itm/PowerWAVE...erruptible_Power_Supplies&hash=item53f3b0788b

 

[ericmark]

I would guess a UPS gives a float charge and as such battery size should not be a problem. With set battery chargers there would be a problem as it may not switch stages if the battery is too big.

In the main the UPS has solid state devices which switch from mains to battery and have to syn with the mains for a seamless transfer.

Using a charger and inverter however there is no attempt to syn the mains and inverter outputs. Net result may be the same but battery or inverter failure will interupt the supply.

The problem where true UPS is not used is earthing. Each UPS should either have an independent earth or only supply one item. With inverter and charger an earth needs to be provided as one can't rely on the mains earth.

 

[Adam_151]

We had a cheap forgein made no-brand UPS around the house a few years ago, and it ended up with external batteries fitted, it seemed to work well and the run-time seemed to be much better though that was before too much was asked off it.

However when the run time was tested a bit further by having the power off to do a bit of work the UPS burnt out before the batteries ran out, it seemed that the increased battery capacity allowed the inverter to exceed its duty cycle...

 

[Lectrician]

That doesn't sound great!

May give it a go and see what occurs!

 

[SimonH2]

... it seemed that the increased battery capacity allowed the inverter to exceed its duty cycle...

That could well be an issue.
Many small UPSs don't have any forced cooling, and I'm sure a lot of the designs will rely on a limited energy input limiting the heat that needs to be dispersed. But a bigger battery on the system, and you remove that limitation.

Unfortunately, you seem to need to get to a fair size of UPS before external batteries are an option.

But for the price of a couple of batteries, it's worth a try. If it fails, then you just need to buy a new unit - and you've put off having to pay for it in the meantime. It's most likely that you'd be able to add the batteries you've bought to the new unit* - so they won't be wasted.

* Larger small ones tend to be 24V, so buy two matched batteries if you are thinking ahead. Or just use them for something else if this happens.

 

[Lectrician]

Ok.

I swapped the battery for a 12v 14Ah one (the one in it was a 4.5Ah).

The UPS itself is a "280" model, showing a rating a rating of 280VA (180watts), with an "average backup time" of 12 minutes.

With the 4.5Ah battery connected, the UPS would fail as soon as the power was removed (seconds).

With the new 14Ah battery connected, it sustains a load. Measuring the current from the battery on power fail, the draw is 12amps.

12amps x 12amps = 144VA.

So I am below the 280VA(180watt) rating of the UPS.

Do we think this is safe to leave like this? Part of me just doesn't want to leave it unattended!

In theory, this battery should last around an hour (likely less). The old 4.5Ah battery at 12amps (144VA) should of lasted around 20 minutes. I guess the battery was just really past it.

 

[SimonH2]

I'd just try it and check that it doesn't get too hot. After something like 20 minutes, it should be at a stable temp and you'll be able to see if it's going to melt or carry on.

Your runtime will be a lot longer than 14/4.5 times as long as a new 4.5AH battery would have given you. The current drawn, in relation to the AH capacity of the battery is about 1/3, and because the effective usable capacity is influenced by discharge rate, it'll probably give something more like 5 times the runtime rather than 3x like you might expect.

 

[dhutch]

...the UPS burnt out before the batteries ran out, it seemed that the increased battery capacity allowed the inverter to exceed its duty cycle...

Nice