(One) trouble with lead acid is that deep discharges are really bad for the longevity of the battery. So you can use only a small fraction of the capacity if you want to run regular cycles form it.
If you add a couple of LiFePO4 batteries before your lead acid (you should do some basic math where you have enough LiFePO4 to last about 1-2 days, depending on your location, without having to hit the lead) using a DC-DC charger (which does reduce efficiency - but that's ok) you increase the size of your pack, really help pull ALL the power out of your solar that you can, add a very safe battery (LiFePO4 is really safe compared to traditional lithium), and best of all, you really, really, really extend the lifespan of your lead.
I mean like, you probably will never be able to kill your lead doing this, for multiple reasons: primarily, you're just not using it that much, but you're also really keeping it topped off when you do use it.
The key is that lead acid isn't great at sucking up charge when it is 80-100% charged. So it is really hard to "catch up" charge with solar, even if you have a ton of it. Solar is not going to put out more than it draws. So if your batteries will only accept some X of AMPs to charge, that is all you're getting (well, you can use extra power for other stuff when this is happening, if you're set up that way, like powering appliances, so you can get some benefit still).
However, with LiFePO4 - that bad boy will take all the power you can throw at it (well, not technically, but for the purposes of a solar array, sure) and top off really quick. Then, when the sun is down, it charges your lead.
I think the benefits of this is more important for smaller arrays with fewer lead acid batteries, and where the LiFePO4 battery will be a single $500 100ah battery (rather than 10 of those), but I imagine even in large large setups this could really help.
LiFePO4 is amazing but I think the cheapest you can go right now by buying used server rack equipment is like $250 for $100ah? So 2.5x or more the price of lead, and you're comparing a used battery vs a new battery (and with the lead, $100 will probably get you a reputable battery, not some no-name used LiFePO4 battery).
Solar -> Charge Controller -> Lithium -> DC-DC Charger -> Lead Acid -> Load
The load is always pulled from the lead acid, but in reality if the load increases your DC-DC charger will just put out more juice (until it hits its limits) and not really drawn down the lead. If you max out the DC-DC charger yeah you'll start to draw from the lead but that is ok - that is what it is there for.
For a really large bank that you need to draw down lots of amps you may need to rework things, but likely lead acid doesn't work for you in those cases anyway. This works for banks where your load is like 50 amps or less. For my case, I have 1kAH including the lithium and I do not ever draw more than 20 amps. Works great.
Solar -> Charge Controllers -> LiFePO4 -> DC-DC Charger -> Lead -> Load
In this configuration, yes, the lithium is always charging the Lead, and the DC-DC charger is creating heat. But the DC-DC charger is not two ways. So the lithium does not get charged.
But the benefits are above, and also you can make the lithium bank a higher voltage bank (so 24v, or 48v, or whatever) which has benefits for your solar: you can use smaller wire.
This guy also has a lot of educational videos: https://www.youtube.com/c/WillProwse
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?
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.
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)
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.
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.
In my case, similar to yours, I had a ton of lead already - replacing all of it would have cost a lot of money, and would have been wasteful as the lead was only a few years old.
> Do you think I can use MPPT controller instead of DC-to-DC charger?
Only two issues I can think of, but definitely do your own research: 1) unsure if you can set up something for the MPPT to cut off charge if your source battery drops below a certain voltage, which you can do with at least some DC-DC chargers, and 2) it is common for MPPT charge controllers to lack the capability to boost voltage, just convert it lower.
#2 can be mitigated by setting up your lithium bank to provide higher current, which has its own dangers, but has added benefits as well such as requiring smaller diameter wire.
#1 is a serious concern and can also give you safety problems outside of maybe killing your lithium - so please make sure you're being safe and doing your research. maybe ask in diysolarforum.com ? I haven't posted there myself but have gotten great info from threads there.
I didn't see Will's comment tho, thank you for sending that, very thought provoking! The comment by him "Trickle charging lead acid all night with lithium seems very inefficient" is 100% true, but in my case I consider this effect a bonus b/c the lithium acts like a capacitor for the solar array (quickly charges, then slowly charges the lead!)
There's some great modeling by Christopher Clack about how deploying tons of small storage and solar on the grid edge (at homes and businesses, next to the meter), and doing it right now, will enable far far more penetration of utility scale solar later.
Because, contra utility talking points, distributed solar and storage is actually a massive grid asset that lessens the transmission requirements and greatly lowers the overall cost of our electricity system.
We have the technology to get to 80%-90% renewable energy today, and at today's prices it will be cheaper than our current system. And by the time we get to 80-90% renewable power, other tech will have advanced far enough to go the rest of the way.
We just need to reshape regulations and markets so that the cheapest grid can be built, and that grid will be carbon free, and cause massive amounts of wealth generation. The only losers in this transition will be the corporations that fail to make the right bets on the future.
Let me introduce you to the Optima Blue Top[0] marine battery which is designed specifically for draining to 0 and recovering to full charge. Doing this to a typical SLA battery found in cars is definitely not recommended. These are commonly used in fishing boats to run trolling motors and pumps in the boat's live wells. Being the hacker nerd type, I built a DIT cart around these with an inverter to power my gear in remote locations without needing puttputts.
[0]https://www.optimabatteries.com/optima-product-categories/ba...
I've been running Yellows in my Jeep due to the heavy electrical load from the winch--I've nearly drained the battery a few times having to winch myself out with the motor off--without issue.
From a quick glance, they seem to be the same construction?
There are 2 blue tops, and just realized the one I linked previosly is not the same version (dark gray) as the one I have being the light gray.
Essentially, the differences are in use cases. Lots of short but heavy loads vs long sustained loads. Looks like the blue tops are still Optima's best "drain to 0 frequently" type of battery where the yellows look like an occassional drain to 0 is survivable but not the recommended use case. I had forgotten the differences after making the decision years ago. Just looked them up again for a quick refresher