How do I test the ups batteries?
I did the following:
I load the UPS with the 20% of it capacity and then I measure the voltage in the batteries every 2 minutes during 1 hour.
The problem is that I dont know if one of the batteries is bad because I have 30 batteries of 12V each one and the initial voltage that I have after shutdown the power supply is 372V (=12.4V * 30) and I think it would have to be at least 382V (=12.7V * 30) if everything is ok.
Then the discharge curve of the batteries was normal.
Thanks in advance, dialc
I am not sure how the newfangled battery testers work and I am not sure of their accuracy.
My experience with batteries was on a nuc submarine where I served as an electrician. That battery could be compared to a UPS because we floated it and only used it when we had no other source of power. That battery like several I have seen or worked on in civilian life was comprised of single cells ie any one cell could be removed.
I believe single cell is the best because
1. You are able to measure individual cell voltage (ICV)
2. You are able to measure each cell gravity.
3. This gives you the ability to evaluate the whole battery cell by cell.
Just voltage is a very poor indication of battery state of charge. Gravity is most accurate.
With many 6 cell ie 12V batteries you have no access to intercell connections and with sealed construction no ability to take gravities. These are tradoffs to gain lowered construction and maintenance costs. Batteries comprised of single cells are better but so is the price tag. The only place I have seen them is where maximum reliabilty was required ie submarine power plant large computer and phone company.
The numbers I will quote are from memory and from one specific make of battery. They will be close for any wet cell lead acid but will need confirmation from your factory.
The old rule was to install a battery and ideally size it to match load ie for the 4 hour discharge ie 25% capacity. Also with that rule the charge rate was to be at 4 hour rate - however depending on your charge methodology you will not be able to maintain a 4 hour current as the battery charges - you will be charging and holding voltage constant and current will drop as battery charges.
In my mind the best way to determine a battery capacity is to load it as it is used. So for a UPS setup you would start with a fully charged battery then you would turn off the AC and let UPS inverter and battery carry the load.
Ideally you would have a load bank where you can control the current but that gets a little complicated.
For a canned UPS ie big box where the factory put in the batteries and also the charger I would be very careful doing a test discharge. I would have a good idea of
1. The current load that is placed on the battery
2. The charger is really able to recharge a nearly full discharged battery.
Here is how a test discharge was done on submarine battery or on station battery
1. Fully charge the battery. Take full set of gravities and cell temperatures.
2. Be able to read each cell voltage
3. Be able to control and read current
4. Maintain current at 25% of battery rating ie 1250 amp for a 5000 Ah battery.
5. Take current and battery voltage reading every 15 minute.
6. Spot check each cell voltage and as you approach the end (around 3 hours) check cell voltages continuously. Do NOT let any cell voltage drop below 1.51 - if so stop the discharge. Cells at less than 1.51 may reverse - permanent damage - if discharged further at 1.51 or less.
7. If you get there stop the discharge at 4 hours.
8. Float battery and take pilot cell gravities and temperatures.
9. Recharge battery.
10 Calculate total Ah drawn - this value divided by battery Ah rating gives you percent capacity.
A correctly done test discharge takes a day when all is said and done charge discharge and recharge.
Dan Bentler
