You can't run that from batteries!

Recently, I have managed to acquire a used, APC 3000 VA uninterruptible power supply – minus batteries.  (APC branded battery packs cost almost as much as a new UPS.  Generic sealed lead-acid batteries of the same capacity can be bought from the likes of http://www.tayna.co.uk/ for a fraction of that.)  The UPS is basically a self-contained battery pack, charger and inverter.  It converts DC from a bank of batteries, which are ordinarily kept charged from the mains, to AC when the mains fails.  I plan to use this, in conjunction with a bank of large batteries, to implement a solar energy storage system.

Now I have managed to pick up some batteries.  Although they are not the right ones for this UPS, they are "just about" compatible – the right voltage, but the wrong capacity  (there would normally be two series chains of four 12 V, 5.5 Ah batteries, in parallel for 48 V / 11 Ah; I have just one series chain of four 12 V, 7 Ah batteries, for 48 V / 7 Ah).

These batteries are rather used, and have a remaining usable capacity somewhat lower than advertised – but they cost nothing, which is always a point in favour.

And of course, having some batteries, we can test out the UPS!

You can see clearly that these aren't the right batteries -- I have had to stand the battery tray on top of the UPS because these are too tall.  The lamp was from an earlier test; look out for the black extension lead heading off the bottom left corner.  The 4-way extension lead is fitted with a "C20" plug for the UPS output (which can be up to 3000 VA, which is more than the usual "C14" / kettle-type can handle.)  The other end goes to .....

This microwave oven!  It's cooking just the burger from a microwave cheeseburger.  I toasted the cob separately, and added my own special tomato and herb sauce.

I was worried that the microwave would pull down the battery voltage far enough to trip the undervoltage cut-out in the UPS –this was what happened with a 2 kW kettle.  Fortunately, the slow-start action of the magnetron filament heating up was enough to allow the UPS batteries to recover.

This is a good sign.  And the nice people at http://www.tayna.co.uk/ are very helpful.  They certainly know their batteries.