OK, finally got that IEEE whitepaper to load. Nothing really new to me but they did point out a few things I had not thought of for a long time. The main points to consider are as follows:
You need to have a rectifier/charger large enough to carry the load plus produce no more than 20% of the battery capacity charge current. For example, if you have a 100 Ah battery, you do not want to charge it more than 20 amps. It is recommended that 10% or less be used to preserve battery life. Too high of a charge current will shorten the battery life in a variety of ways. Sealed VLRA batts are much more susceptible due to a reduced amount of liquid electrolyte to serve as a heat sink. They also outgas thus losing electrolyte and capacity during deep discharge. Furthermore they have a chance of thermal runaway during heavy discharge or charge. Flooded cells do not have this same problem. So, say you have a 48 volt system, your load current is 10 amps. You want it to remain alive for 48 hours during a power outage. You need 24*4*10 = 480 Ah of battery. Pretty large battery. $3500 or so. Now, after an outage you need to charge that battery and run your load. So, 10% of 480Ah is 48 amps of charging current plus 10 amps for your load. 58 amps of charger/rectifier. But wait, you really need to do N+1 for redundancy so two 60 amp rectifiers would be needed. However then you have way too much recharge capacity after outages that could damage your batts. So, you need to have rectifiers that will limit the current. The ones I use allow you to set the whole shelf to limit the current. If you are paralleling units that do not talk to each other, set each one for load plus 5% of the battery. So in the above example, current limit the rectifiers to 34 amps each. If one rectifier dies, the other can still pull the 10 amp load plus have 25 amps for recharge. That will bring the batts back to fully charged in about 19 hours. However if both are working, and there is an outage, when the power comes back on there will be 68 amps of total current available. Take off 10 amps for the load and you have 58 amps going into the batts. 58/480=12% You are golden. No battery damage. N+1 operation. All is well. And you will recharge in about 10 hours.
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