Modifying a RCT2000VA UPS with an External charger

[heartbroken]

OK, this is how I ran a similar setup at work for a couple of years without any problems. The extra battery pack (12V or 24V depending on your UPS) is connected in parallel to the UPS internal pack via the normally open contacts of a 220 Volt relay. The relay is fed from the mains supply, thus as long as there is mains the extra batteries are disconnected from the UPS internal battery pack. The ups chargers its own battery pack and the external pack is connected to a separate charger. When the mains fails the relay drops out and connects the external pack to the internal pack and Bob's your Auntie.

I like this, think I am going to do this, I have 2x35ah batteries that I can charge from solar - I can just pop in the relay and connect to the UPS, then the UPS will still have the small 7ah one to cope with power surges and what not.

 

[The_Unbeliever]

Got a Mecer 2kVa UPS sitting around doing sweet FA.

It have an external connector. Its batteries are FUBAR. Will open it this weekend and take a gefingerpoken around.

 

[Joe_Moer]

Hi Spiff,
I am not quite sure I see your need for charging multiple batteries via the relay and timer setup. How are your external batteries connected. In series to give you 24V or more? Or are they connected in parallel to give you 12V with a higher capacity?
Charging via a timer gets messy, because you have no control of how long the mains power is off, thus you are not always going to charge your batteries properly.
A good charger will charge the batteries even when they are connected in parallel and go over to trickle charge when they are full.
If you are running on 24V (2 in series) then a good 24V charger will do the same.
No messing with addition relays and timers. Just connect and almost forget!

 

[Gnome]

Just some input on this.

I've gone down this road before and in my opinion when you start doing this level of modification I'm not sure it is worth it anymore because your costs will escalate quickly.

Just to be clear, it is entirely possible that you cannot disable the charging circuit.

Disconnecting your battery pack so you can charge it using a relay may be your only choice. Keeping in mind you are dealing with a huge amount of current so you need a heavy duty relay which costs quite a bit of money. Then you start to realize it is much cheaper to just get an inverter and use a transfer switch.

If you are interested in the technical details of how it works, this guy explains it well: https://www.youtube.com/watch?v=Fj7e3WGUKO8
He then inspects an APC UPS and explain how the charger and interverter are a single circuit. He also links to patent which explains how the H-Bridge runs in reverse with the transformer as an inductor to create a saw-tooth charger.

RCT I suspect simply copied this design looking at some of the pictures posted. But if I had good pictures of the circuit board I would be able to tell.

 

[Gnome]

Got a Mecer 2kVa UPS sitting around doing sweet FA.

It have an external connector. Its batteries are FUBAR. Will open it this weekend and take a gefingerpoken around.

Yep, not worth doing these backyard setups IMO. You spend as much and more money and pretty much end up with a hackjob setup that is less reliable than what an Inverter + battery + charger setup would have cost in the first place which is just far more reliable.

I'm still amazed at how brazenly people do these hack jobs. If your house burns down you are in a situation where you had a dangerous electrical setup which means no insurance and possible criminal charges. Not really worth it imo.

 

[KOPITE]

As long as the external charger will not overcharge the batteries, but switch into trickle mode, then you should be OK.

I don't know about whether the UPS manufacturer designs their UPS'es to stop charging/trickle charge their batteries once full, but common sense says it should trickle charge once full, otherwise you'll get a lot of unhappy batteries which'll tend to do Funny Things, and that is Things You Most Certainly Do Not Want....

If someone can confirm that.

 

[KOPITE]

My apc ups was happy with my external charger conected
I found id add the ups charging current and external charging current . But both stoped when battery full

How do you know when the ups charger stops charging

 

[thehuman]

If someone can confirm that.

The ups I use does the above.

 

[KOPITE]

The ups I use does the above.

So you have a rct2000va charger. Do you charge the batteries via ups charger and external charger at the same time.

 

[Gnome]

How do you know when the ups charger stops charging

Simple but not accurate test: Connect a multi-meter to the battery terminals.

The cycle voltage for a lead acid AGM battery is typically ~14.4v (28.8v for 24v)
The standby voltage is ~ 13.5v (27v for 24v)

(Notice that every battery is different and the actual voltages are usually on the battery itself)

Standby voltage is to prevent self discharge. Cycle voltage is to charge the battery to full.

APC UPS for example would look ok when you look at it with a multi-meter. But when you view it with an oscilloscope you would see it has very high ripple voltage (due to saw tooth charger). That does shorten battery life a lot (hence APC batteries only last 3 years instead of the usual 5-8 years when using CSB AGM batteries).

At least do the multi-meter check, doesn't cost much or require much special skills.

 

[thehuman]

So you have a rct2000va charger. Do you charge the batteries via ups charger and external charger at the same time.

No mine not an rct-2000 but an APC make
Should still be similar in charging
Most ups have a basic current limited float charger

 

[spiff]

Hi Spiff,
I am not quite sure I see your need for charging multiple batteries via the relay and timer setup. How are your external batteries connected. In series to give you 24V or more? Or are they connected in parallel to give you 12V with a higher capacity?
Charging via a timer gets messy, because you have no control of how long the mains power is off, thus you are not always going to charge your batteries properly.
A good charger will charge the batteries even when they are connected in parallel and go over to trickle charge when they are full.
If you are running on 24V (2 in series) then a good 24V charger will do the same.
No messing with addition relays and timers. Just connect and almost forget!

I have a charger I bought with the battery I got from First National Battery - it's a 7Ah smart charger and does the job in -+6hrs then goes over to trickle charging. I do not know if it can charge all three batteries in parallel at a time? or if it can then great problem solved - question is how long would it take to charge 3 x 12v 100Ah batteries?

I'll add batteries in parallel to give more capacity @ 12v - since the UPS appears to be a 12v type.

true the timer is not an elegant solution - a solar charger would be much better but my setup is done on a shoe string budget. I could go all out and install a R150k solar power system to run the entire house, but that is a bit extreme to spend that kind of money just to save R400 a month, I just want a light / tv / dstv on during load shedding.

 

[Joe_Moer]

Theoretically charging a completely flat 100Ah battery with a 12V 10Amp charger should take 10 hrs. At a thumb suck you should get at least 2 hrs use from a 100Ah battery running at 600W draw. Thus with 3 batteries in parallel you should have used 33Ah in 2 hrs. So to put back 33Ah at 7A will be less than 5 hrs.
Best is to try it. You cannot damage your smart charger by charging all 3 batteries at the same time. Worst case is that it may take very long or if the batteries are to flat they will take very long to charge.
Some general info at http://www.sinetech.co.za/battchrgselect.htm

 

[spiff]

Theoretically charging a completely flat 100Ah battery with a 12V 10Amp charger should take 10 hrs. At a thumb suck you should get at least 2 hrs use from a 100Ah battery running at 600W draw. Thus with 3 batteries in parallel you should have used 33Ah in 2 hrs. So to put back 33Ah at 7A will be less than 5 hrs.
Best is to try it. You cannot damage your smart charger by charging all 3 batteries at the same time. Worst case is that it may take very long or if the batteries are to flat they will take very long to charge.
Some general info at http://www.sinetech.co.za/battchrgselect.htm

thanks for the info.

I have had my CRT tv / dstv / 14w LED light running off the battery for 3hrs the other night (+-300w) - without any problems - I can see the way things are going with eskum I'll be breaking that time soon best I buy some more 100ah batteries soon

 

[BigEars]

Most ups have a basic current limited float charger

Popular choice for these things is a LM317T regulator that does the charge control. It limits the current @ MAX 2.2A and then throttles quickly to 1.5A. Suits 7Ah and 9Ah batteries. Like you say only float charges @13.6 - 13.8V.

Never takes the batteries up to +-14.4V and then floats them like a three stage charger does (very basically explained).

 

[KOPITE]

Popular choice for these things is a LM317T regulator that does the charge control. It limits the current @ MAX 2.2A and then throttles quickly to 1.5A. Suits 7Ah and 9Ah batteries. Like you say only float charges @13.6 - 13.8V.

Never takes the batteries up to +-14.4V and then floats them like a three stage charger does (very basically explained).

So what you saying I can use both at the same time

 

[thehuman]

So what you saying I can use both at the same time

Should be fine

 

[BigEars]

So what you saying I can use both at the same time

No. I am not saying that. What I am saying is that what I described above is a typical charging method for internal UPS 7Ah and 9Ah SLA batteries. I do not know your UPS. You need more research on your unit. I wish I could find a circuit diagram on the Net for it.

 

[KOPITE]

Or can I plug the power of the ups out, but it's still connected to the battery terminals. The external charger will be permanently connected and switched on. Can this work perhaps.

 

[thehuman]

Or can I plug the power of the ups out, but it's still connected to the battery terminals. The external charger will be permanently connected and switched on. Can this work perhaps.

How many AH bateries each ?
How strong charger in amps ?