5 ms·

The key issue is that lead acid at these low capacities may be able to take one or two amps above 50% SoC. Especially at 80%+ if the panel can do 4 amps but you

The key issue is that lead acid at these low capacities may be able to take one or two amps above 50% SoC. Especially at 80%+ if the panel can do 4 amps but you can charge only with 2, you are wasting precious sunlight. Lead-acid can charge at 0.3C but only when empty until 50% or so. In the absorption phase, current drops (dramatically). Also lead-acid means 50% usable capacity for longevity so the 12v 12Ah lithium is 12v 24Ah lead-acid (minimum). Which also means you can never charge with 0.3C. And even then is longevity of lifepo4 so much better it’s not funny. Please spend the extra money on lifepo4, save yourself some headaches.

dragontamer 3y ago
> Also lead-acid means 50% usable capacity for longevity so the 12v 12Ah lithium is 12v 24Ah lead-acid (minimum). That's not a concern, is it? We're talking about battery backup, meaning the "expected use case" is that the Lead-Acid battery is sitting at near 100% capacity for almost its entire life, without significant amounts of discharge. Sure, you've got to discharge when the sun goes down but I don't expect a properly-sized battery to be deeply discharged (especially when we're aiming for multiple days worth of "worst-case" power, like multiple days of clouds/rain to reduce our power-collection). Lets just be dumb with napkin math for a second: 12-hours of charge, 12-hours of discharge on a "typical" day, with a 3-day / 72-hour period of worst-case charge. Where does this leave us? 12-hours of typical discharge / 72-hours of worst-case discharge == DoD of 100% to 83% on your typical day-to-day basis. Hardly anything that considers a "deep discharge". And if we really need deep-discharges, most Lead-Acids can survive something on the order of 300x to 500x deep discharges right? So its not like the 5 to 10 times a year where you need all 72-hours of battery are significantly hampering your lifespan? > And even then is longevity of lifepo4 so much better it’s not funny. Measured in deep cycle-counts (ie: cell phones or laptop usage), sure. (like 3000 cycles on LiFePo4 vs 500 on Lead Acid). But years of sitting on a float-charge, I don't think its that much better than Lead-Acid. ---------- That being said, LiFePo4 is a "superior" battery for sure, and its much cheaper than I remember. But Lead Acid remains significantly lower-priced than LiFePo4 as far as I can tell (2-to-1 or less) I think you have a point about Lead-Acid charging slower when its near full (and keeping it full is key to keeping Lead-Acid long-lasting). So I admit to not recognizing that (LiFePo4 also charges slower when near full, its just that it charges like 3x to 4x faster than Lead Acid so its less of a concern) > Please spend the extra money on lifepo4, save yourself some headaches. We're at the hobbyist level where the original post decided that MPPT solar charge controllers weren't needed (at least, in an earlier version of this project) Clearly no one here is talking about the strictly most optimal design, but are instead talking about simpler and cheaper solutions. -------- Simplicity here is key. LiFePo4 does _NOT_ like topping charges or staying at 100% charge for long periods of time. Lead Acid however, prefers 100% charge and UPS use Lead Acids for this reason. LiFePo4, in UPS/Standby like systems, need to stop charging, and wait for the battery to be used before engaging in a new charge cycle. Lead-Acid on the other hand, can just have a constant voltage applied (ex: 13.5V or so) and you can just hold that indefinitely, for incredibly simple standby circuits.
- daotoad 3y ago
  I may be full of crap, it's been over 15 years since I worked on this stuff. You have been warned. The systems I worked on used multiple deep discharge, cold weather hardened lead acid batteries. Deep discharge batteries have an important distinction from automotive batteries. They tolerate significantly more deep discharge cycles but can't produce as many amps. If you're just trickling a more or less steady amount of power into some electronics, you don't need massive amounts of cranking amps that an auto battery provide. Solar PV and battery tech have definitely improved quite a bit since then, so were I now building these same installations, I might do something entirely different. But "back in the day" your advice of "buy another battery" was more or less exactly how we handled charge rate limitations.
- shkkmo 3y ago
  LifePO4 was about the same as lead acid in terms of lifetime costs (i.e. cycles * usable capacity / $) several years ago and have gotten significantly cheaper. Pretty much the only reason to buy lead acid for a solar generator that faces daily partial discharge is because you can't afford sufficient LifePO4 capacity in the short term but don't care that you'll pay more long term.
  - dragontamer 3y ago
    Or maybe you want to run the Lead Acid battery off a simple constant voltage float charge circuit. Which for a hobbyist: simplicity is everything. As far as I know, Lead-Acid still is very popular in UPS, which is basically what this SolarPi is. > (i.e. cycles * usable capacity / $) That's not really how UPS systems work. UPS systems sit at 100% state of charge for years at a time getting constantly fed power. As far as I'm aware: LiFePo4 still doesn't like being stored at 100% charge, like any other Lithium chemistry.
    - shkkmo 3y ago
      > As far as I know, Lead-Acid still is very popular in UPS, which is basically what this SolarPi is. No, SolarPi is a solar powered server. The application here is a "solar generator" type setup that has no grid connection and is very different from a UPS style setup.

dragontamer 3y ago

> Also lead-acid means 50% usable capacity for longevity so the 12v 12Ah lithium is 12v 24Ah lead-acid (minimum). That's not a concern, is it? We're talking about battery backup, meaning the "expected use case" is that the Lead-Acid battery is sitting at near 100% capacity for almost its entire life, without significant amounts of discharge. Sure, you've got to discharge when the sun goes down but I don't expect a properly-sized battery to be deeply discharged (especially when we're aiming for multiple days worth of "worst-case" power, like multiple days of clouds/rain to reduce our power-collection). Lets just be dumb with napkin math for a second: 12-hours of charge, 12-hours of discharge on a "typical" day, with a 3-day / 72-hour period of worst-case charge. Where does this leave us? 12-hours of typical discharge / 72-hours of worst-case discharge == DoD of 100% to 83% on your typical day-to-day basis. Hardly anything that considers a "deep discharge". And if we really need deep-discharges, most Lead-Acids can survive something on the order of 300x to 500x deep discharges right? So its not like the 5 to 10 times a year where you need all 72-hours of battery are significantly hampering your lifespan? > And even then is longevity of lifepo4 so much better it’s not funny. Measured in deep cycle-counts (ie: cell phones or laptop usage), sure. (like 3000 cycles on LiFePo4 vs 500 on Lead Acid). But years of sitting on a float-charge, I don't think its that much better than Lead-Acid. ---------- That being said, LiFePo4 is a "superior" battery for sure, and its much cheaper than I remember. But Lead Acid remains significantly lower-priced than LiFePo4 as far as I can tell (2-to-1 or less) I think you have a point about Lead-Acid charging slower when its near full (and keeping it full is key to keeping Lead-Acid long-lasting). So I admit to not recognizing that (LiFePo4 also charges slower when near full, its just that it charges like 3x to 4x faster than Lead Acid so its less of a concern) > Please spend the extra money on lifepo4, save yourself some headaches. We're at the hobbyist level where the original post decided that MPPT solar charge controllers weren't needed (at least, in an earlier version of this project) Clearly no one here is talking about the strictly most optimal design, but are instead talking about simpler and cheaper solutions. -------- Simplicity here is key. LiFePo4 does _NOT_ like topping charges or staying at 100% charge for long periods of time. Lead Acid however, prefers 100% charge and UPS use Lead Acids for this reason. LiFePo4, in UPS/Standby like systems, need to stop charging, and wait for the battery to be used before engaging in a new charge cycle. Lead-Acid on the other hand, can just have a constant voltage applied (ex: 13.5V or so) and you can just hold that indefinitely, for incredibly simple standby circuits.

daotoad 3y ago
I may be full of crap, it's been over 15 years since I worked on this stuff. You have been warned. The systems I worked on used multiple deep discharge, cold weather hardened lead acid batteries. Deep discharge batteries have an important distinction from automotive batteries. They tolerate significantly more deep discharge cycles but can't produce as many amps. If you're just trickling a more or less steady amount of power into some electronics, you don't need massive amounts of cranking amps that an auto battery provide. Solar PV and battery tech have definitely improved quite a bit since then, so were I now building these same installations, I might do something entirely different. But "back in the day" your advice of "buy another battery" was more or less exactly how we handled charge rate limitations.
shkkmo 3y ago
LifePO4 was about the same as lead acid in terms of lifetime costs (i.e. cycles * usable capacity / $) several years ago and have gotten significantly cheaper. Pretty much the only reason to buy lead acid for a solar generator that faces daily partial discharge is because you can't afford sufficient LifePO4 capacity in the short term but don't care that you'll pay more long term.
- dragontamer 3y ago
  Or maybe you want to run the Lead Acid battery off a simple constant voltage float charge circuit. Which for a hobbyist: simplicity is everything. As far as I know, Lead-Acid still is very popular in UPS, which is basically what this SolarPi is. > (i.e. cycles * usable capacity / $) That's not really how UPS systems work. UPS systems sit at 100% state of charge for years at a time getting constantly fed power. As far as I'm aware: LiFePo4 still doesn't like being stored at 100% charge, like any other Lithium chemistry.
  - shkkmo 3y ago
    > As far as I know, Lead-Acid still is very popular in UPS, which is basically what this SolarPi is. No, SolarPi is a solar powered server. The application here is a "solar generator" type setup that has no grid connection and is very different from a UPS style setup.