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rAs bad as 60%? I didn't know that. The only time I've used/measured battery capacity was in rapid charging of NiCad's. And there the efficiency was pretty good.
George H.
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13 years ago
Formula for charging time of 12v battery
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13 years ago
local
-- http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf
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13 years ago
akes 17
solar
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dThat's interesting, Thanks JF. Since the lead acid's loose efficiency above an 80% state of charge (SOC). Could you just operate them up to 80% SOC in a photo voltaic system where you might want maximum efficiency? Or are the batteries 'damaged' by not being completly charged?
George H.
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13 years ago
Regarding your question, both 15 and 20 amp branch circuits are permissible in the US, and some of those branch circuits _must_ be 20 amps. (That doesn't change your correct observation that the 20 amp outlet information is useless with respect to the op's post.) Those branch circuits provide power to "normal" receptacles (often referred to as "outlets".)
Ed
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13 years ago
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dI don't get it. The article suggests battery efficiency drops off near full charge, however figure 1 shows about 68 AH input for about the same out, while an input of 116 yields only about 97 out. That would indicate less efficiency at deeper discharges, and higher efficiency near full charge.
What did I miss?
-Bill
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13 years ago
17
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-- Let's say we have a 12V lead-acid battery which has been discharged to the point where its terminal voltage is 10.5V Then let's say the battery has a capacity of 97Ah, but to get that out of it we have to put in 116. Since efficiency is equal to output divided by input, that gives us an efficiency of about 0.84, or 84%. Now, with the battery discharged to 10.5V, (since we took 97Ah out of it) let's pump 68Ah back into it. We find that in this case we can get, say, 65Ah out of it until it gets discharged to 10.5V, so thats's an efficiency of 65Ah/68Ah, or about 96%, so efficiency decreases as the battery approaches full charge.
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13 years ago
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Yes, I was looking at it from the top down rather than the bottom up. So, it appears efficiency is better when the state of charge (SOC) is less than maximum. But that brings up another issue of battery life operating at lower SOCs. Maybe it's incorrect, but I thought lead acid battery life is extended by keeping the battery fully charged. So, the trade-offs might be better efficiency with reduced life, or visa versa?
-Bill
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13 years ago
takes 17
solar
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Yes, like that. It's very hard to push the top 20% of capacity back into the battery, that's why car batteries quickly fail when the car is used for mostly short trips, they're never fully charged.
If you look at voltage (or energy) while charging the battery, you need to push 30 to 50% more into the thing than what you got on discharge. The industry talks in AH and expect about 1:1 AH charge/discharge for a good battery, the failure modes for LA batteries is a race between corrosion and sulphation, difficult to get right at times.
Other day I noticed a 12V 5AH SLA had split it's case. They do that sometimes, all by themselves, just crap out -- that battery was on standby float charge for several years, had no useful capacity, output voltage dropped to about 8V on load, which is 2 of the 6 cells shorted? I floated them at the low end of the range too (13.5V).
Grant.
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13 years ago
takes 17
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-- It appears from: http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery#Sulfation that battery life will be decreased by keeping it less than fully charged due to the action of sulfation. That being the case, it'll cost more, in electricity, to keep a battery fully charged than it would to keep it partially charged, but it'll cost more, in terms of $ spent on batteries, if less electricity is used than is required to keep the battery fully charged. I don't know where the crossover point is, but I'd guess that the electricity used to keep a battery fully charged costs less than replacing the battery would.