LiFePO4 is great - especially as an addition to solar systems currently using lead acid.
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).
This is nice idea that I watched some video about it few months ago, but will prowse (do you know him?) didn't like the idea as he prefers all lithium (he thinks it's cheap enough), I'm living in 3rd world country with a LOT of power outages (basically no power), I have a solar array with tubular lead acid but I'm soon switching to LiFePo4 when my package come from China, I feel tempted to implement your idea instead of selling my acid batteries. Do you think I can use MPPT controller instead of DC-to-DC charger? Technically it works like that I think
Yeah he knows way more than I do - and I agree that all lithium is probably the best way to go if you can afford it and are starting from scratch.
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.
Thanks for the advice I will consider posting a question on the forum when the package arrive. Will's opinion was a comment on this video btw, I can see now it's not totally negative though [1]
For the problem #1 I think the BMS can solve the issue as I already ordered a JBD BMS so it can cut off whenever it reaches the critical voltage.
For #2 you're right I think I should check, my MPPT is good one (Epever) but not sure it could lift the voltage properly from such a narrow difference, unless maybe I use higher voltage (such as 48) than my pack 's voltage (24v) so it will be guaranteed to only reduce the voltage to the lead acid levels.
Thanks so much!
Yes - I have watched this video. Emily & Clark have a different setup, as he uses a different device (or even, no device) to connect the two different banks (lithium and lead acid).
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!)
You're right, I think my situation is similar to yours. My solar array is not so big (for now at least) so I have lot of power in the noon but less in other times, so it will be very helpful to have a lithium pack that absorb all that power quickly and then feed it to the lazy lead pack over the next few hours.
I have 1540W and it's enough to cover more than 16hrs of power outages (light fridge, fans etc...), but I'm planning to expand as things are going wild in the near future probably. I'm afraid at the current rate of wars and energy insecurity even some rich countries will face the issues we're facing and we will be even worse!
I used to have an Epever MPPT. Really nice unit. However, as I recall, the manual emphasized that you should only use solar panels for the input. This is because when the battery is full the MPPT will stop charging by shorting the input. This is fine when the input is solar. But if you are doing LiFePO4 -> MPPT -> lead, it's going to short out your LiFePO4 battery when the lead battery is full, which would be very very bad.
Woah, you can get a 10kWH LiFePO4 Powerwall for $2600. I didn't realize heavy battery tech is that far ahead. Seems rather compact too. That just seems entirely reasonable to install in your garage.
This is why I'm always mystified by the hoards of people that say that batteries will never be a significant grid resource. They are getting incredibly cheap already, even as our demand is exceeding supply.
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.
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.
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.
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)
If you are draining the LiFePO4 batteries faster than you can charge them and the lead batteries are charging the LiFePO4 batteries than wouldn't you have the same problem that will reduce the lifespan of the lead batteries?
You would probably need switch (battery isolator, Dc-DC charger, etc ) of some sort to isolate the two systems to fully eliminate this, but part of the answer is the Lifepo4 keeps its voltage higher through most of it's capacity than lead does. Thus the voltage of a Lifepo4 at ~20% is the same as a lead acid at ~100%.
This is true, but in my case I use a DC-DC charger that does not allow the lead to charge the lithium (I don't want this or want to think about it, even if it was beneficial, in my case the complexity is not worth it) - the only way the voltage can travel is from the lithium to the lead.
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.
Sorry if I was unclear - it is the other way around. The LiFePO4 is before the Lead. So it is:
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.
Not sure - but very easy to make with off the shelf Victron products. I use their DC-DC charger and their charge controllers and have had great success so far.
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).