October 2016 Archives

UPS Battery Management

Over the years, I've looked at a lot of UPSes and battery-backed power systems. These are my recommendations for power systems where backup is desired, or "clean power" is needed.

1. A 48 VDC sine wave inverter in the 1500 - 2200 - 3000 VA class with a low-voltage shutdown feature
2. a battery bank larger than 25 Ah at 48VDC
3. (2) Schottky diodes large enough to handle the maximum current the inverter can sink, rated for 60VDC
4. A battery charger capable of reaching the equalization voltage (14.4x4), but not used for equalizing.
5. (6) fuses: one from the batteries to the load, one from the batteries to the charger, and four connecting the batteries to each other. 
6. 1F worth of super caps at 60VDC.

The preferred embodiment is such that the charger is connected to the battery pack through a fuse, and the battery pack connected to the anode of the series protection diode through a fuse. The cathode of the diode is connected to the cathode of another diode as well as the positive terminal of the series of supercaps. The anode of the second diode connects to ground, so that any counter-emf current is clamped.

If an APC SmartUPS 2200VA or 3000VA is substituted for the inverter, the supercaps will simulate a battery for the purposes of the UPS's charge and self-test cycles, along with providing a power bypass in the case of restored utility power, or generator power that matches the expectations of the UPS. The batteries may be automatically charged to "full" (13.8x4) once a week by the charger and allowed to float down the remaining six days, unless a UPS self-test cycle occurs. If the battery bank is in the 50+ Ah range at 48V, the effects of the self-test cycles can be ignored.

The advantage of this implementation is that smaller power supplies can be used to put a fraction of the total power required into the battery pack, which can slow the rate of energy reserve depletion. In the first implementation, a 3000 VA (output) power supply made up of four 750 VA modules was used, allowing four 1000VA car inverters to be used to power individual sections or combinations. Additionally, the entire power supply could be used to clean up "dirty power" since the power supply sections were universal can could run from 100 to 250 VAC. More recently built power supplies may also support 380VDC inputs, as the peak-to-peak voltage of 240VAC is 339 V.

The reason for the recommendations are many, and it's mostly addressing the issues present in the APC UPS products.

1) The batteries are severely taxed by the charging, discharging, and self-test cycles of the UPS. The reason why APC's warranty is so short and the cost of replacement so high is that the battery manufacturer will not warranty the batteries, period. You cannot reasonably expect a 7Ah battery to regularly withstand cycles of 30+ A loads being attached and then recovering.

2) The above manifesting in battery failure, it is not unusual for a single cell to short, resulting in the entire chain going up in voltage spread across the remaining cell. Large numbers of batteries in packs mitigates a shorted cell failure very effectively in this way.

3) Individual cell voltages will change over time. In the case of 12V lead-acid batteries, the smallest units are six-cell groups of 12V, so you are stuck with this basic commodity. The voltages in cell differences can add up over time and groups, resulting in less than favorable life expectancy.

4) Cooked, swollen, or leaking batteries do not a happy funtime for electronics make.

On the Care of Batteries:

1) If possible, discharge individually and charge the batteries in parallel or independently. There are limits to this, one gets into the thousands of amps quickly at 1.2VDC for NiMH and NiCd chemistries, or 2.0V when dealing with lead-acid.

2) Charge once a week, don't float batteries. That's old hat. This advice best matches chemistries with high temperatures at the end of charging and high self-discharge, such as NiMH. No surprise, I learned it from the Panasonic NiMH Battery Handbook.

3) If you're using LiPos, use a BMS. LiPos are very much their own animal and the chief reason for letting the battery voltage be what the battery voltage will be and the load voltage being something else entirely (i.e.: 13.8VDC, 120VAC).

4) Not too cold: Batteries will warm due to resistance during discharging events. However, current available will be limited if the batteries are below 60 degrees F.

5) Not too warm: Batteries should not be charged above 100 degrees F unless the manufacturer has provided you with special guidance. More of an issue for people in the American Southwest, freshly discharged batteries, or large packs of NiMH batteries.

6) Control the rate of recharge: It's not a good idea just to dump 1000A into the battery bank after a 1000A discharge unless they are rated for that charging rate. Usually not more than 1/10 C for lead-acid batteries. NiMH and NiCd have different requirements during charging due to specific cell behavior.

7) It's not if batteries fail, it is when. Always assume that they will leak at the most inopportune time and cause the most possible damage at that time. If you put batteries in the same cabinet with the electronics, when the batteries fail, the electronics will corrode in the gasses. It is best to put batteries in a separate, ventilated enclosure away from expensive electronics. NiMH, NiCd, and lead-acid batteries give off different corrosive vapors that are not compatible with each other. These chemistries should be separated from each other. In the case of flooded or other unsealed lead-acid batteries, a spill kit should be nearby, along with a source of distilled water for regular topping of batteries. A battery box or tray should be able to contain any electrolyte that might spill from a broken battery and resistant to the chemicals used in the electrolyte.

This is a lot of information, but applied correctly will result in a backup power plant that achieves or exceeds the warranty of the batteries.

In the case of a large battery plant, it may be desirable to break the plant into battery strings, and individually switch those strings to the load bus, or a charging bus as needed to equalize charging intervals. This can be done with Schottky diodes and MOSFETs in today's world without too much cost since the MOSFETs are either on or off, and the MOSFET's body diode can perform the function of a one-way diode. On the other hand, the MOSFET's body diode may require the use of additional Schottky diodes to prevent the reverse flow of current. Schottky diodes are fast diodes, while the MOSFET's body diode is not known for fast recovery times however that should not be a factor for the reader unless there are large inductances present in the implementation.

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