Something I've wondered about is whether there's a /minimum /charge
current. If you have something like a Meanwell AD-155C then you have a
whopping 155Watts to work with. If you have 100W of load then you have
1amp of charge current. Then somebody maybe adds a couple of devices
and now you have 125W of load and maybe 1/2 amp of charge current.
I won't be concerned if the problem is that it took a month to fully
charge, but is there a point where the charger is too weak and doesn't
do anything?
I'm also not currently using a charger that small on such a large
battery, but in the past when I was doing everything on the cheap I've
definitely put a Meanwell AD-155C on a 100ah 48v battery to power a few
Canopy units. It worked fine, but I always wondered if I was hurting
something by having such a small charger.
I'll definitely keep that 10-20% range in mind for the future, and
thanks for sharing.
Thanks,
Adam
On 6/19/2018 1:11 PM, [email protected] wrote:
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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