Backyard Aquaponics
A place to discuss aquaponics

I have a 120Ah AGM battery that I hope to connect to my teeny tiny no-name chinese UPS.
The final charge voltage seems to be at 14.12v (its sitting here with the original little battery)

So the charge voltage is fine for the AGM, but for this UPS I think it is also the float voltage and AGM batteries prefer about 13.5v float for long life.

SO, I need a super gizmo to sit between the UPS and the battery to limit the max voltage on the battery but still allow the other UPS functions (charge/discharge)

This UPS will probably never really charge a battery that big, but If the power ever goes out for any length of time (rare in Brisvegas) then I will just put it on my something-stage AGM battery charger (not yet purchased, still looking)

A mate suggested a resistor across the battery to pull the float voltage down a little, but since I don't think the UPS actually has a float stage, not sure if this will work. I might work with charging, since the charger should deliver more current than what the resistor will be bleeding off.

Any ideas ?

What is the maximum charge current of the UPS?

Could you have a voltage divider? a small micro with a relay to take the resistor across the battery off when not floating?

I'd go the other way: trick the charging circuit in the UPS into thinking that there is more voltage on the battery than there really is. This would take some decent poking around with a multimeter but would give the best result I'd imagine.

There will probably be a trimpot for exactly this voltage control anyway...

Stuff all I'd imagine. Ill test when Im sure of the "float" level



A voltage dividor might be too much power. Your second choice could work but that is too much work !



Yeah I have not opened it up yet. It's very cheap looking (and really small for a 600VA) I'd imagine the were too tight for a trimpot
Ill have a poke around and check it out.

Are you absolutely sure that the UPS does not have a time out function on it rather than a battery sensing low voltage sensor? If it has the first, you are wasting your time putting a 120ah battery on it, it will just shut off anyway.
There are also a few UPS that have problems charging a large battery.

I am absolutely sure I know almost nothing about this UPS.
So it could have a time out... dunno.
It could just rely on the battery going flat, in which case I will be OK and I will just add a fan to stop it melting after prolonged use.

Yeah, I don't care too much about charging, I could always just add a battery charger to it later and use it as an inverter.
I also have to test my air pumps to see how hot they get on modified sine...

SuperVeg, I ran your initial post by a well credentialed mate of mine and here is his response:

The battery that the guy wants to use for the UPS will work fine with no modification at all. Putting large batteries on small-scale UPS is not uncommon and I have done it at remote radio retransmission sites. The 14.2v will drop back to 13.5v fairly quickly once connected and that is an appropriate float charge. There will not be a problem and if the power goes out it will offer considerable standby power. When the mains reconnects, it may take some time to recharge the battery, depending on discharge, but all will work correctly without drama. DO NOT DO THE RESISTOR THING, IT IS ASKING FOR PROBLEMS!

I trust his contribution helps.

I hear crickets.

This thread has gone very quiet since my post: was it something I said?

I like the sound of my own typing, so I'm chime in.

All the threads I'm subscribed to seem quiet.

When that happens, I turn on the news to see if something more important than aquaponics is happening.

How often does that happen, Bull? I mean, something more important happening than AP?

Well on rare occasions aquaponics is on the news and that's about it.

Nothing so far, but I'd count life on mars as worth a look

If only to see if there are some new fish species we can grow.

I'm hoping for large freshwater lobster like things with thousand island dressing and lime juice for bodily fluids, a head the size of a golf ball that's all mouth so they eat fast, no real body to speak of except just enough to hold the "mustard", thirty or so one foot long legs that are each 4 inches thick, and a tail that's a perfect sphere (just for presentation) roughly the size of a basketball.

Or similar, I'm not fussy.

I don't have any idea, but that wont stop me

Connect 20 capacitors (with your flux pen that should take no more time than a 40 pin chip... so 4 seconds or so and charge them to 0.62 volts, then disconnect from the charger for a zillionth of a second (by swapping to the other bank of 20 caps (4 more seconds soldering)), and discharge them into a 12v air pump (or a small motorbike lead acid battery that runs a 12v air pump)

Repeat ad-infinitum

Probably needs a 555 timer or something, and some chunky silicon things with 3 legs. ...so another 4-6 seconds of flux pen powered soldering

Total cost $4 (sans motorbike battery & international patents)

I told you it wouldn't stop me

Thanks for the info PLJ and everyone else. I have been flat out do if you don't like the quiet then too bad
I took a few days to test the "float" voltage on the small battery, it got to 14.13 do not the end if the world, didn't test current though. Stick three UPS on the big sucker tonight, room see how it goes.. I thought since it will take AGES to actually charge the batty with the UPS, if I just take done charges out of it every time it gets to say 14 volts or so then it Erik never over charge. I have a calliper that is supped to stop sulphation, I'll chuck that on it too. If the power ever actually goes out then I'll just do a proper charge cycle. Its a nice battery so I wanna look after her. Tipping in the browser on my phone is a nightmares x!!!