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Comment by Siecje | Hacker News Reader

I'm still thinking about this. What is the benefit of adding lead if you don't fully discharge the lead? Or do you?

If you are only using the lead until the LiFePO4 are dead then what is the benefit? Or do you have something to disconnect the batteries with they are at 50% (or what 55% to be safe) charge?

So you can increase the battery capacity by 50% of the lead you buy instead of 100% LiFePO4. Which means the lead needs to be half the price or less (excluding the cost of the DC-DC charger) for it to make sense.

Am I missing something?

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In a setup where you only move between 80-100% charge for most of the time, dip to 50% maybe once a month, and to 10% once a year.

It's kind of like tiered storage on a file server: most capacity is rarely accessed but still valuable to have, so instead of using a lot of mediocre storage devices you use a few really good ones and a lot of really cheap ones. The good ones will stem most of the read load, and buffer writes to the slower storage if necessary.

> What is the benefit of adding lead if you don't fully discharge the lead? Or do you?

You do - but not always. If you only discharge the lead 1/10th of the time, your lead batteries will last incredibly long (if otherwise properly maintained).

Additionally, lead doesn't take up charge quickly. Putting in Lithium allows you to take full advantage of your solar. This can be mitigated in other ways, but this makes it "easy" and a not complex system with many charge controllers, many batteries, etc...

Additionally, it allows your input voltage to be much higher (from the solar) meaning you can have smaller wires (but amps are amps!).

> If you are only using the lead until the LiFePO4 are dead then what is the benefit? Or do you have something to disconnect the batteries with they are at 50% (or what 55% to be safe) charge?

I'm not sure what the question is, sorry. Yes you have equipment to make sure you are safely drawing from the lithium and safely charging the lead.

> So you can increase the battery capacity by 50% of the lead you buy instead of 100% LiFePO4. Which means the lead needs to be half the price or less (excluding the cost of the DC-DC charger) for it to make sense.

Well, if cost is not a problem, it would be better and a more simple system if you bought 100% LiFePO4. But that is really expensive.

Insurance.

You want your LiFePO4 to absorb the day to day brownouts/outages as well as sunlight variation and charge/discharge, and you want your lead acid there for when you get taken off the grid for 3 days and maybe have your solar ripped off your roof or partially degraded, once every ten years.

You also can afford to capture more energy faster and charge your lead acid slower - vs just dissipating the extra the lead acid can't use as heat.

>get taken off the grid for 3 days and maybe have your solar ripped off your roof or partially degraded, once every ten years.

Depending on location, the "taken off grid for 3 days and having your solar ripped off" could be from the same storm situation (hurricanes, tornados, etc)

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