6 ms·

Don’t do lead acid. I say this from experience. Lead acid charges too slow. You want a battery that can capture every ray of sunshine - especially with intermit

Don’t do lead acid. I say this from experience. Lead acid charges too slow. You want a battery that can capture every ray of sunshine - especially with intermittent sunlight - and charge at full speed. Lifepo4 can do this. Lead acid can’t.

dragontamer 3y ago
A 12V 12Ah battery is 144Wh. I'm fairly certain that Lead-acid can accept 0.3C, and you're right in that a 60W panel is slightly more than the 43W that Lead-acid can accept. But sizing 12V 20Ah (ie: 240 Wh), and now 0.3C is 72W or 6A of safe charging (less than 0.3C). ------------- See #4: just buy a bigger battery if you need a bigger battery. Bigger battery provides more power and energy proportionally for the chemistry. And a 12V 20Ah lead-acid is just $40 on Amazon. --------- I guess your overall point is that "Trickle-charge" isn't enough for Solar? Which is something I admit I didn't think about earlier (trickle-charge just doesn't send as much current to the battery due to the lower voltage). But I guess there's a "simplicity of circuit" advantage of trickle-charging. A more complex state-of-charge circuit (likely a microcontroller keeping tabs on the battery) is needed to safely send 0.3C down to the Lead Acid. EDIT: On the other hand, having your "charge controller" just be like, two power diodes, is a gross benefit to simplicity. Its something you can do with Lead-Acid that's fully impossible with LiFePo (and is why LiFePo4 needs expensive charge controllers to work). For hobbyist purposes, there's something to be said about using simpler technologies, even if their specs are worse (and I'm not entirely sure if Lead-Acid has worse specs than LiFePo4 in this use case).
- louwrentius 3y ago
  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.
deleted 3y ago
[deleted]
beachy 3y ago
We have an off-grid cabin with 4 x trojan lead acid batteries and they have worked flawlessly for 8 years so far and still going strong, despite regular over-draining by solar-ignorant guests (who would bring their own bar heater to plug into a solar system??) They get water topped up every few months and fizzed every couple of years. Speed of charge is not everything.
- dragontamer 3y ago
  To be fair, Trojan is an unusually durable lead acid battery. Their specs are far far beyond 'normal'. It seems like some serious chemists are at work there.

dragontamer 3y ago

A 12V 12Ah battery is 144Wh. I'm fairly certain that Lead-acid can accept 0.3C, and you're right in that a 60W panel is slightly more than the 43W that Lead-acid can accept. But sizing 12V 20Ah (ie: 240 Wh), and now 0.3C is 72W or 6A of safe charging (less than 0.3C). ------------- See #4: just buy a bigger battery if you need a bigger battery. Bigger battery provides more power and energy proportionally for the chemistry. And a 12V 20Ah lead-acid is just $40 on Amazon. --------- I guess your overall point is that "Trickle-charge" isn't enough for Solar? Which is something I admit I didn't think about earlier (trickle-charge just doesn't send as much current to the battery due to the lower voltage). But I guess there's a "simplicity of circuit" advantage of trickle-charging. A more complex state-of-charge circuit (likely a microcontroller keeping tabs on the battery) is needed to safely send 0.3C down to the Lead Acid. EDIT: On the other hand, having your "charge controller" just be like, two power diodes, is a gross benefit to simplicity. Its something you can do with Lead-Acid that's fully impossible with LiFePo (and is why LiFePo4 needs expensive charge controllers to work). For hobbyist purposes, there's something to be said about using simpler technologies, even if their specs are worse (and I'm not entirely sure if Lead-Acid has worse specs than LiFePo4 in this use case).

louwrentius 3y ago
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.

deleted 3y ago

[deleted]

beachy 3y ago

We have an off-grid cabin with 4 x trojan lead acid batteries and they have worked flawlessly for 8 years so far and still going strong, despite regular over-draining by solar-ignorant guests (who would bring their own bar heater to plug into a solar system??) They get water topped up every few months and fizzed every couple of years. Speed of charge is not everything.

dragontamer 3y ago
To be fair, Trojan is an unusually durable lead acid battery. Their specs are far far beyond 'normal'. It seems like some serious chemists are at work there.