Redundant Power Supplies
Food for thought. If your server has dual power supplies, the power pulled from each supply will be 50% of the total power needed. Therefore if you have battery backups that you’re plugging each of the power supplies into, each backup will have 50% of the total load on it.
Now for the tricky part that will hopefully save you a power failure. For simplicity sake I’m using round numbers. If you have 10 servers that all draw 200 watts, you have a total demand of 2000 watts. Each battery backup will have a 1000 watt demand on it. Say you have a 1200 watt limit on each power supply. You’re fine during normal operation. You’re even fine if one or two servers have a single power supply fail and the other supply has to provide all 100% of the load. The issue is that if you have one of the battery backups fail, or switch off for some reason, the full load of all 10 servers has to go to one battery backup. This means that you’ll have a load of 2000 watts, far more than the 1200 watt limit.
To solve this, when you’re setting up your power redundancy, just remember never to exceed 50% load on any battery backup that’s powering a dual power supply server. For best results, try to keep the load closer to 40% max so you have room to wiggle.