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Re (6): "which will effectively double the size of our battery store (Li batteries can be fully discharged)" Not strictly true. You size your batteries for the

Re (6): "which will effectively double the size of our battery store (Li batteries can be fully discharged)" Not strictly true. You size your batteries for the rare times when you have long periods of no sun. By definition these rare occurrences shouldn't greatly impact the life of a lead acid battery. Especially flooded lead acid cells. Conversely if you discharge your Li-ion cells 100% on a daily basis then you have no capacity for longer periods of no sun. The "100% discharge" myth comes from grid-tied installations that are working their batteries hard each day to claim the surplus power rebate. Re (7): "They will (correctly) limit the current to the batteries based on their current charge level, tapering off as the batteries near full". The better regulators keep track of the state of charge (rather than just voltage), so are able to charge at the maximum safe current until the batteries are full. They don't "taper off" towards the end of charge. It is true that most Li-iron batts can safely absorb greater current than lead-acid, but that is not the fault of the charge controller.

PaulDavisThe1st 6y ago
It is strictly true. 400aH of lithium allows me to use 400aH, over any period I choose, without deleterious effects on the battery. 400aH of lead-acid allows me to use 200aH if I want to avoid deleterious effects on the battery. If I'm entering a period of no sun, 400aH will last me twice as long as 200aH. The only way in which this is not true is if I'm prepared to use 100% of a lead acid battery's store, which negatively impacts the battery's life. In a fixed home situation, it is true that you might have battery storage that so far exceeds your normal requirements that it is only necessary to go below 50% SOC twice a year. In such a scenario, I'd agree with you that you could do this and not worry about it. But it also means that you've got far more battery capacity than you need, just to cover the two days a year when you need to do this. Using Li would allow you to reduce the capacity, dip down below 50% SOC more often, without impacting battery life. It seems that you're suggesting that my Morningstar charge controller might not be one of the "better regulators". I'd beg to differ. It is precisely "charge at the maximum safe current" that is what I mean by "tapering off". You cannot dump 18A @ 13V into lead-acids that are at 90% SOC without harming the batteries. The Morningstar (an MPPT controller) will modulate the current as the battery approaches 100% SOC, which as I said is precisely what you want for battery life, but precisely the opposite of what you want for "gather as much power as quickly as possible". But yes, perhaps I did make it sound as if the charge controller was at fault for doing this, which is not true. I was describing it more as a kind of unexpected behavior that can oddly impact your battery use. For example, if you're somewhere that is very sunny in the mornings and then routinely clouds up in the afternoon, starting the day at say 85% SOC with lead-acids may not get you back to 100% even though there is sufficient power to do so - the charge controller will limit the current delivered to the batteries during the morning (correctly!), then the afternoon comes around and the power levels drop (this happened to me a lot in southern Spain for example). It's a somewhat surprising behavior if your mental model is "I've got 500W of panels on the roof, these babies should be recharged by 10:30am"

PaulDavisThe1st 6y ago

It is strictly true. 400aH of lithium allows me to use 400aH, over any period I choose, without deleterious effects on the battery. 400aH of lead-acid allows me to use 200aH if I want to avoid deleterious effects on the battery. If I'm entering a period of no sun, 400aH will last me twice as long as 200aH. The only way in which this is not true is if I'm prepared to use 100% of a lead acid battery's store, which negatively impacts the battery's life. In a fixed home situation, it is true that you might have battery storage that so far exceeds your normal requirements that it is only necessary to go below 50% SOC twice a year. In such a scenario, I'd agree with you that you could do this and not worry about it. But it also means that you've got far more battery capacity than you need, just to cover the two days a year when you need to do this. Using Li would allow you to reduce the capacity, dip down below 50% SOC more often, without impacting battery life. It seems that you're suggesting that my Morningstar charge controller might not be one of the "better regulators". I'd beg to differ. It is precisely "charge at the maximum safe current" that is what I mean by "tapering off". You cannot dump 18A @ 13V into lead-acids that are at 90% SOC without harming the batteries. The Morningstar (an MPPT controller) will modulate the current as the battery approaches 100% SOC, which as I said is precisely what you want for battery life, but precisely the opposite of what you want for "gather as much power as quickly as possible". But yes, perhaps I did make it sound as if the charge controller was at fault for doing this, which is not true. I was describing it more as a kind of unexpected behavior that can oddly impact your battery use. For example, if you're somewhere that is very sunny in the mornings and then routinely clouds up in the afternoon, starting the day at say 85% SOC with lead-acids may not get you back to 100% even though there is sufficient power to do so - the charge controller will limit the current delivered to the batteries during the morning (correctly!), then the afternoon comes around and the power levels drop (this happened to me a lot in southern Spain for example). It's a somewhat surprising behavior if your mental model is "I've got 500W of panels on the roof, these babies should be recharged by 10:30am"