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Is manually setting a inverter made for lead acid a good idea???
#1
Hey Everybody,
I recently bought this inverter just to try it out:
https://www.photonicuniverse.com/en/cata...arger.html

And it works fine. I did some research on batteries and i found out that especially with used cells, the voltage is not directly correlated to the SOC. Up to this point i never fully understood the constant voltage part of the charging cycle, but battery university has some really good literature on li-ion cells and it really opened my eyes as to why there is a constant voltage scheme. 

Going back to the inverter, its made for lead acid but you can manually set the voltage range(as a lot of people here do) and thats what i did. 7s means setting it to 29.4 volt, i would prefer to go lower but the bms starts balancing at that voltage so i need to fully charge them. I discharge to about 3.4 volt per cell to maintain a long cell life(24 ish volt as limit on the inverter). I was under the impression that this is a legit way to go about it. 

The inverter is made for lead acid and has, cc, cv and float/tricke charge. I assumed that because my upperlimit was 29.4 the float wouldn't really matter at 29.4 volt, seeing as not much would happen. 

On battery university they discribe how it is actually really bad for a cell to keep a constant voltage on them indefinitly. When you charge a cell with a tp4056 it actually detects the current running in the cell, when this is about 7% of full current is stops the charging cycle. The problem is that you want the charging system to stop charging when a certain set current is running into the battery. 

I haven't found any hybrid inverters offering this feature. I also like to use quality stuff and was under the impression that my inverter from photonic universe was high quality since it came from the UK and was pretty expensive. I've been looking at inverters all day and noticed that they all look quite similar. There is a company on alibaba making them and i suspect dealers here in europe buy them from alibaba and rebrand them because they are exactly the same. I see Pete from HBPowerwall is using a hybrid inverter from aliexpress as well(or mpp solar is being faked really hardcore). 

My question now is, has anyone found an inverter that can actually just charge the li-ion cells and stop at the correct time? Does anyone have experience with inverters straight from alibaba and aliexpress? You can use a bms but since you charge and discharge with a with the same inverters you cannot cut it off from the inverter with a relay since you do want to be able to use it. I cannot imagine that there is not a specific type made for li-ion or that has the settings for li-ion because it just seems like software to me more than hardware. 

Hope someone can clear this up a bit!

https://batteryuniversity.com/learn/arti..._batteries
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#2
One way around this is to set the float voltage a bit below the fully charged voltage.
It's true holding Li cells at the top of their charge voltage (eg 4.2V for Li-Ion, typical 18650's) may not be good for them but as you back down from this becomes less of an issue.
So in addition to charging & discharging to reduced voltages, if your peak charge CC/CV cut off is the usually suggested 4.1V you can set the float to eg 4.0V and the cells will be happy long term.

If you set the float too low, you would end up with less effective pack capacity.
Korishan likes this post
Running off solar, DIY & electronics fan :-)
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#3
Hi Tom,

Still happy with that inverter 5 months later?
I'm considering something in that same range.
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#4
Yeah its actually fine, works like a charm!
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#5
I don't have this particular controller/charger - but as long as you can set the voltages for float etc... then you can taylor it for Li-ion.
In my case, in 7s terms, the max charge/float is set to 28.2v(4.03v/cell) pack and inverter cut-off at 24.0v(3.43v/cell). The battery bank is sized so this range (e.g. 60% max DOD) will allow the consumption of all possible PV power on brightest summer day. My design was also guided by Battery University - to try to get 10 or 15years out of my 18650 cells.

The inverter is allowed to go on (and automatic transfer switches provide power to house) at 26v(3.71v/cell). In a typical day, sun rises, inverter turns on, the excess power charges the battery bank... but it rarely gets up to max voltage because it takes a perfect (no clouds, cool weather) to reach max voltage.

Bottom line - as long as you have control over max voltage and min voltage - then you can adjust the system any way you wish.

I would also strongly suggest cell (pack) level BMS such as Batrium to monitor/protect against individual cell/pack voltages going too low (or hi).
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