Rainwater toilet- don't laugh ( too much)

[pontiff]

So if say, 50Ah worth of brand new, fully charged batteries were used with the APC unit above, what would a rough estimate of the backup time, bearing in mind the combi seems to average about 140W when fired up?

[knighty]

50ah @ 12v is 600watts (50x12)

so a 120watt system would run for (600/140) just short of 4.5 hours

but.... the ups is most likely 24v (most are) so you'd need 2 batteries

so

50ah @ 24v is 1200watts (50x24) or just short of 9 hours run time :-)



not exactly on topic... I bought myself a new UPS for the cctv system at work, don't really think anyone will be cutting the power wires, but it'll keep it running in a power cut, and help smooth out the power to the cctv computer/cameras etc... then someone gave me another for free which he found in a skip.... so I plugged my ups into it, so it goes CCTV-UPS1-UPS2-Wall..... so far.... all works great :-)

[SimonHobson]

So if say, 50Ah worth of brand new, fully charged batteries were used with the APC unit above, what would a rough estimate of the backup time, bearing in mind the combi seems to average about 140W when fired up?

As a very rough estimate, 50Ah x 24V is about 1200Wh. 1200/140 is about 8 1/2. That doesn't account for losses and conversion efficiency etc, but you are still looking at several hours - probably 4+, maybe even 6+. It's likely to be fairly high since you are discharging the batteries at quite a low rate - so you actually get a lot more out of them that you would if the load was larger.

I've also remembered another reason not to put large batteries on a UPS not designed for them - and it's to do with the recharge profile. A good UPS will have a 3 stage charge regime :
1) Bulk mode - the UPS supplies as much current as the charger can produce (constant current mode), and the battery absorbs it. This is the initial bulk charge and will provide around 80% of the stored energy (charge).
2) Absorption mode. When the battery voltage reaches a certain level, the charger switches to constant voltage mode and the charge current drops off as the battery fills up.
3) Float mode.When the charge current drops below a set current, the charge is complete and the charger will reduce it's output voltage - the battery is now being 'floated', it's not taking any additional charge in, the current going in is just correcting for the (small) amount of self discharge that happens in all batteries.

The problem is that with large batteries, the threshold current to switch from stage 2 to stage 3 may not be reached. At the higher charge voltage, once the charge is complete then the extra energy goes into splitting water into hydrogen and oxygen and the battery will always take some current. If the battery size and charger are very mismatched then it's possible that the residual current is higher than the switching threshold and the charger might never switch to float mode. Depending on conditions (hotter is worse), this may lead to batteries drying out - "dry cell" AGM batteries have an incredibly small quantity of water in them so they are easily killed by overcharging. Wet batteries would not be a problem since they can easily be topped up.

Hmm, it's not getting any simpler is it

[Other-Power]

Further to my PM pontiff, you dont want to take more then 20% out of the batteries without killing them, they would be able to do 80% dis charge but they will only do that a handfull of times before dying.

With regards the APC UPS, the max power out of them is around 70 watts, a pair of of 24 amp hour batteries work well with the 1000VA model but your really dont want to run them to hard.  Using bigger batteries is ok, the APC UPS will charge them and go to float mode, I know as I have measured it.

If you have worries about reaching the charge rate required then its simple, use a second charger that is plugged into the mains, when the mains dies the UPS will cut in, when the mains comes on, the UPS will turn off and the charger will come on.

the other option you have is to make your own UPS from a power inverter, a battery, a charger and a relay.

The relay holds the inverter off and the charger on, when the mains goes off the inverter comes on and the mains goes off.

The advantage of this is better sizing of the inverter to the load making the inverter more efficent, also, the inverter would be a high frequency power inverter unlike the ups which is a low frequency transformer based inverter making it more efficent further.  You could use any size and type of battery you can get a charger for and you have as much run time as you like.

Just my obsavations and thoughts on the matter.

Regards

Jonathan

[knighty]

I would agree with you Jonathan, but if it's just for the one off very rare power cut I don't think it's such a big deal

if power cuts are more regular in your area, you'd be better off with one of those little petrol suitcase generators to keep you going....
(even if you only use it to charge the ups)

[pontiff]

Thanks for all the great replies. Time to start making my post Christmas present list! Have been looking around for a little genny for a while so it might be time to buy.