Nickel Iron batteries

[Deanforest]

Hi All. Has anyone actually changed the electrolyte in Ni Fe batteries which is potassium hydroxide solution, (Chemical formula KOH). I had a malfunction of my automatic topping up system, resulting in electrolyte overflow, and considerable dilution.The batteries are only 2 years old, so I am intending to add a concentrated solution of KOH to the batteries most affected, to bring the Specific gravity up to the required 1.15 -1.2. The KOH flakes or pellets are readily available in the U.K., but no one seems to know what purity level to use in the batteries. Commercial grade flakes are only 80% pure, and 90% is also readily available. 99% purity is available, but much more costly and is called chemical grade by some suppliers. I have come across some American info that refers to ‘Battery Grade’ KOH, but that’s meaningless to U.K. suppliers, without a figure for % purity.
The batteries came from Ukraine, (Just got them in before Brexit put paid to importing), and the suppliers have a website in the Ukrainian language, and I have tried contacting them, but I guess they have more pressing concerns right now. Someone out there must know the answer, and I would really appreciate some help on this one. My electrical system is solar, and completely off grid, and this has really pulled down the battery capacity. The U.K. suppler I purchased the batteries from was no help at all, saying they only supply and don’t get involved with operation.

[Krill]

...wouldn't it depend on what the impurity is? If its NaOH then I'd don't think I'd be concerned, the issue is what other reactants are being added to the chemistry.

[Oldgreybeard]

Yes, I've done this a couple of times with my old NiFe packs. Not hard to do, but I would definitely not try doing it piecemeal, as it's very difficult to get the electrolyte right if there has been a loss. When I've done it I've emptied all the cells, rinsed them out with distilled water, cleaned the outer cases and terminals and then refilled them with the correct concentration of potassium hydroxide solution. This needs to be as pure as you can get it, these cells do not like any impurities in the electrolyte, and as potassium hydroxide is dirt cheap, when compared to the cost of the cells, it makes sense to just buy the really pure stuff.

FWIW, my mention of having a couple of decades of experience with running incredibly durable NiFe cells is the reason I got kicked off this forum's predecessor! They are fantastically robust and long-lived cells, mine were in wooden crates and had been aircraft engine ground starter packs, made in 1944. They were still as good as new (same capacity as when they were first made) when I sold them about ten years ago. The chap that has them now hasn't needed to do anything to them, other than top them up with distilled water, and runs his whole off-grid house from them. He does have a big wind turbine though, so the poor efficiency and high self-discharge rate doesn't bother him.

[openspaceman]

so the poor efficiency and high self-discharge rate doesn't bother him.

Given the main use of a battery storing energy is just to tide a household till the following day is high self discharge much of an issue, how much charge would be lost in 20 hours?

[Oldgreybeard]

so the poor efficiency and high self-discharge rate doesn't bother him.

Given the main use of a battery storing energy is just to tide a household toll the following day is high self discharge much of an issue, how much charge would be lost in 20 hours?

Typical self-discharge rates for my 65 year old NiFe cells when I had them was about 5% per week, so not massive. Those cells are now getting on for 80 years old and I'm pretty sure they are still in use every day.

[Krill]

Purely out of interest, what is the typical discharge rate for LiFePO? I wonder what it would be like to try and use 2 battery arrays and what such a system would even look like...

[nowty]

Purely out of interest, what is the typical discharge rate for LiFePO? I wonder what it would be like to try and use 2 battery arrays and what such a system would even look like...

Feck all, but its the BMS which will be the dominant drain factor.

[Stinsy]

Purely out of interest, what is the typical discharge rate for LiFePO? I wonder what it would be like to try and use 2 battery arrays and what such a system would even look like...

Self discharge rate of LiFePO4 is 1-2% per month.

Self discharge rate of NiFe is 1-2% per day.

[Oldgreybeard]

Purely out of interest, what is the typical discharge rate for LiFePO? I wonder what it would be like to try and use 2 battery arrays and what such a system would even look like...

Feck all, but its the BMS which will be the dominant drain factor.

A couple of years ago I dug out a Headway LiFePO4 pack that I'd built in 2008, and used for a couple of years on an electric motorcycle. I removed it from the bike and stored it, without the BMS connected. It had been sitting for around ten years, and was still showing about 3.5V per cell. I still have it here, sat on the floor by my desk waiting for me to either bin it or do something with it. I suspect the cells are well past it, as they were never that great in the first place, but I might do some tests before binning them, just to see what they are like. The snag is that these cells had about a 20% failure rate when new, so it will be hard to separate out that degradation from the age related degradation.

[Krill]

I wonder how efficient a large NiFe battery could be when linked to WT farm or a massively oversized household solar array, given cost implications of lithium. What is the C value anyway, 0.2 like with lead acid?

Although looking at those calls you'd need to design an space efficient and ergonomic access arrangemt which would not be simple...