UPS question (prompted by the leisure battery thread)

[SH]

I have a number of APC UPS around with dead batteries.

So to get these UPS'es back in order I need to buy some new batteries.

They typically have a run time of up to an hour in the event of a power cut and dependent upon the load it is supporting.

Given the risk of energy blackouts, I have been wondering if there is any technical reason why I could not use a much bigger battery to then give a longer UPS run time during a power cut?

I have no problem in extending the battery connections so that the leisure battery can sit outside the UPS and be electrically connected to the UPS.

I realise that there will be a limit to the charging current provided by the UPS to the now much bigger battery and accept it will take longer to recharge.

My thought was to put in a change over switch for all the house LED lighting, smoke alarms and CO detectors and house alarm.

I could also run aa laptop and 4g router or fibre router for communications.

Some of the UPS I have are 350 VA, 650 VA and 1kVA.

[SH]

Hmmmmmm....

[Andy Burns]

The inverter section might not be rated to run for hours on end without overheating, but you could add more fans ...

[Brian Gaff]

Its no doubt a suck it and see job. I think you could find that long duty cycles in the inverter if there is one might not be designed in. Brian

[Theo]

You may well find that the UPS is very inefficient at running an output of a handful of watts - it's sized for a hundreds of VA load for shutting down PCs in a power cut, not for running tiny loads for long periods. If there was say a 50W constant consumption and the load was 5W, that would be 10% efficient.

When I looked into this, I found this module:

a 24v version: there's a DRC-100 which is 100W that Farnell doesn't stock but available elsewhere.

This is basically a DIY UPS. You give it mains to charge whatever battery you install, and it gives you an (adjustable) 12-15v output up to 60W. When the power cuts out, it switches over to the battery and keeps the output going.

So you could power your 12V loads directly from that. For your mains stuff run a small inverter sized appropriate to the loads. For bonus points arrange switching so the loads are fed directly from mains when that's up rather than running the inverter doing AC-DC-AC all the time.

They have another module which is just the (24v) UPS part without the mains input:

it will take over the DC bus if it senses the voltage has gone - that's good for up to 40A.

Theo

[David]

APC sells expansion battery packs for extended run time for their larger UPSs.

[Martin Brown]

Path of least resistance is repopulate them with working batteries. I have the next largest size of SLAs in my emergency lights at home after realising that they would fit. Looks like they were designed that way and you could pay two different prices for 2Ah or 4.5Ah cells in.

The recharge time will be significantly longer and the stored energy in the bigger battery will be able to do a lot more damage in the event of a fault condition.

If you do that then you absolutely must have a slow blow automotive low voltage high current fuse inline near the battery so that in the event of a fault condition or short circuit power will be cut.

A big lead acid battery will have cables hot enough to melt insulation in a few seconds and then up to red heat if a dead short arises. I've seen it catch people out.

For things wanting small amounts of DC it is usually better to power them directly from a lead acid (or other) battery using a DC to DC converter to provide the right voltage and current at ~95% efficiency.

There is a price to pay for going round the larger loop.

SLA - UPS - mains - DC_PSU - device

This is a better solution

SLA - DC2DC - device

Various solar powered battery packs are handy if you spend some time intentionally off grid for hobby purposes. The ones with the largest area of solar panel being preferable. Less useful in mid-winter.

One thing to watch is that the components may only be rated to run at full power for the length of time that the original battery could support (which can be as little as 20 mins).