Effect of Charge Current on Undersized Battery Bank

Edsimon

Please I need advice on the risks I am taking if I use a 24V, 15kVA 600ADC UPS to charge 24V, 200Ah battery bank comprising 12pcs of 2V 200Ah AGM. Obviously this unit should have a higher capacity bank but I already have this 200Ah batteries that I was using with a 5kVA system. Thanks in anticipation.

Estragon

The main risk is in overheating the batteries, particularly if the UPS doesn't have a temperature sensor that measures the battery temp directly. As the batteries heat, the chemical reaction speeds up so charge faster. Temp compensation measuring battery temp directly should reduce target voltages accordingly. If not compensated for, the batteries could vent and be permanently damaged.

Even if the UPS gas a remote temp sensor, I would monitor battery temps manually as a precaution anyway, and stop charging if they get to 95-100°f or so.

Edsimon

Thanks a lot Estragon. The UPS is a Gutor industrial type with temperature sensor as option but not equipped. Worst still, system is not monitored, only inspected like once a week unless a failure occurs.

BB.

What is the charging current and the absorb / float voltage set points.

You might find the ups is relatively gentle when charging flooded cell batteries.

- Bill

Edsimon

It's a DC UPS with similar characteristics as those below:
DC output Voltage:  24/48/110/125/220 VDC
Setting range:
Float voltage at -10/+10% line power voltage : 100 - 120%
Float voltage at 0/+10% line power voltage : 100 - 130%
Boost voltage at nominal line power voltage : 100 - 130%
Battery operating range :150% 

Estragon

I assume the % numbers are versus nominal? So float at 100% would be 24v in your case?

If this is correct, you may be able to charge using very conservative settings, like 107% for float and maybe 114% for boost.

I'm guessing ambient temps are fairly high, so if the UPS corrects for temperature based on an internal sensor in the absence of an external one, at least the compensation shouldn't be huge.

If the UPS has a current limiting setting, this would be useful. Limiting current to, say, 25-35a would reduce the risk of overheating a lot, and allow for voltages that would charge the batteries more fully in a reasonable time. Are you charging with solar? If so, it would help to know the array size.

Also, it would be useful to know if the UPS compensates for temp based on an internal sensor. A UPS may not, as it may be assumed that it lives in conditioned space at around 20-25C. Does it?

jonr

You can connect it to your batteries (briefly) and measure the charging current.   Less than 50A should be OK.  Even better if you have specs from your exact batteries.

Edsimon

Estragon, UPS is in a conditioned room but batteries are in a naturally ventilated space. I am not charging with solar, solely generator.
I need to also mention that the this is standby duty which means lots of float hours! That makes it more worrying for me.
Here is what I'm considering: The requirement is for twin units; the batteries on ground all put together equals 600Ah. Client would prefer no battery purchase at all. But I want to advise client to procure just another bank of 600Ah or 800Ah for one unit while I use the existing batteries 600Ah for the second unit.
What is a likely issue with having different battery types and capacity on a twin charger DC UPS/Rectifier system?

mvas

Edsimon said:

Estragon, UPS is in a conditioned room but batteries are in a naturally ventilated space. I am not charging with solar, solely generator.
I need to also mention that the this is standby duty which means lots of float hours! That makes it more worrying for me.
Here is what I'm considering: The requirement is for twin units; the batteries on ground all put together equals 600Ah. Client would prefer no battery purchase at all. But I want to advise client to procure just another bank of 600Ah or 800Ah for one unit while I use the existing batteries 600Ah for the second unit.
What is a likely issue with having different battery types and capacity on a twin charger DC UPS/Rectifier system?

If your UPS was designed for AGM type batteries then "... lots of of float hours ..." should not worry you.
Float is Float.

What should worry you is ....
The day you deeply discharge your battery bank and then the generator tries to recharges them - maybe too quickly or not???
 AGM batteries can withstand huge amps during Bulk Recharge, but you ( the charge controller ) 
must cut-back the amps as soon as +/-14.4 volts is achieved, to prevent out-gassing.
This should happen automatically, if this system was originally designed for AGM type batteries - WAS IT?
So, this smaller battery bank with hit 14.4 Volt much sooner ( in time and % SOC ) vs a larger battery bank.
Therefore, your smaller Battery Bank will need to stay in Absorb Mode longer, to compensate.

So this could work, if the smaller battery bank can handle the Charging Amps from the Charge Controller.
That is what YOU need to figure out.
And the only way I know is ... YOU must actually try it.
Put the UPS in Battery Mode, drain the AGM battery bank ( say 20% ) and then recharge the battery bank
During charging measure the Amps, the Volts and the Temperature.
You need to learn what will actually happen.

There is a very good chance this will work, if the Charge Controller was originally configured for AGM type batteries.

Estragon

If each twin bank has separate charge settings, having different types could work. The problem with mixing types is you may not be able to set up a float/charging regime that properly charges both without overcharging one.

The temperature issue also has to be determined. If the UPS is compensating, it will adjust voltages based on the (usually cooler?) conditioned space temp and may supply too high voltage to warmer batteries. As mvas points out, this may not appear to be a problem until a "boost" charge is needed after a discharge.

It may help if you could give us a link to documentation on the UPS.