Most useful article I've ever found on LFP battery charging

Really interesting reading on memory effects of LFP in the Nordkyn articles. I am just starting with my BYD 24 v batteries which appear to have a memory effect in place. I use a MeanWell power supply feeding power into my only two charge controllers: ToolKitRC M8S and ISDT T8. They have very different high voltage protection functions. The M8S allows me to set a high voltage cutoff in a very limited range of (3.55 - 3.65). When the first cell reaching that voltage it disconnects itself from the battery. However, the MeanWell just keeps happily sending power to the M8S which just ignores it. For an automated system this would not work. The ISDT on the other hand uses the high voltage disconnect setting to keep the first cell at that voltage and then continues to charge up the other cells until all cells are at that same voltage. The current is indeed drastically reduced to about 0.5 - 3 amp (down from 10 amp bulk charge) during this time bringing the cells to the same voltage. So the T8 is a great top balancing charge controller whereas the M8S is NOT. Neither one actually STOP the charging process in the sense of turning off all charging power. They do however disconnect themselves from the battery, so for manual supervision like I am doing now it works fine.

Question: are there actually Grid powered chargers that will STOP charging and turn themselves off when a user set HVC value is reached? This question is germane to this thread because if we need twice per year to charge up to 3.6 VPC and at other times be more conservative and charge only up to a 3.4 - 3.5 VPC cutoff then we have to have a charger that will directly allow that.

Note I have not yet ever installed or used a BMS on my BYD batteries. But from Will's videos and other info my understanding is that a BMS will disconnect the battery from either charging or discharging upon user controlled HVC and LVC and H/L temperature cutoff. But a BMS cannot switch off inverters, chargers, or SCC except through relay connections to their on/off switches. Is this understanding correct?

Hi, I am planning a 10 kWh system for a tiny house that will operate off grid. The battery is large compared to the number of panels. So, the maximum charging current will be about only 0.05C. In that case there is not really a need for an absorption phase. You can just stop charging when the cut-off voltage is reached. So, it is probably okay to work with a cut-off voltage of 3.5 or 3.55V normally, and occasionally set it to 3.6 or 3.65V.
I am just not sure what would be the best strategy to decide when to start charging again. Using a lower voltage to start charging again is tricky because voltage will depend on loads (the inverter will be quite a load and could trigger the charger again).
Maybe counting coulombs until 5% of the charge has gone?
I will be building a solar charger based on an arduino, so I can implement any strategy that I can think of.

Zeerob said:

Hi, I am planning a 10 kWh system for a tiny house that will operate off grid. The battery is large compared to the number of panels.

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I'm curious what factors influenced your decision to build such a large battery bank relative to your solar array?

Well, the tiny house is for my daughter, who is building it now. They have a full-size washing machine, fridge, and she works on the laptop the whole day. She has calculated to use about 1800 Wh per day. In the winter they will live in Austria or Norway and there will be periods when there will not be much sun. The roof area for panels is very limited. So, the more capacity the better. And with some over capacity, you can run the cells with a low DOD which will be good for the lifetime of the battery.

Zeerob said:

Well, the tiny house is for my daughter, who is building it now. They have a full-size washing machine, fridge, and she works on the laptop the whole day. She has calculated to use about 1800 Wh per day. In the winter they will live in Austria or Norway and there will be periods when there will not be much sun. The roof area for panels is very limited. So, the more capacity the better.

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And with some over capacity, you can run the cells with a low DOD which will be good for the lifetime of the battery.

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I agree with you, but the benefit of overcapacity comes down to the ratio between battery capacity and consumption between charges, not battery capacity vs solar input.

If roof space is the limiting factor, there isn't much you can do to get around that, but there are vans with 1 kW of solar on the roof, so I would be surprised if there weren't a way to fit more than 500 watts. If I were in here shoes, I would be concerned with that small amount of solar. It may be just enough, but its cutting it really close. At the end of the day power in must exceed power out, a larger battery bank can extend the length of time you can run at a deficit, but it can't make it sustainable.

At 90% efficiency and 5 hours of full sun she is looking at generating 2250 Wh per day, At 70% efficiency she's looking at 1750 Wh per day. At 4 hours of sun those numbers drop to 1800 and 1400 respectively. How much sun can be counted on in a Norwegian Winter? Her array may be just enough to cover her needs, but its cutting it close, and leaves very little excess for recharging the batteries after prolonged periods of little or no sun. But maybe the 1800 Wh figure she calculated already has a big margin of safety built in.

Sorry I didn't intend to distract from your initial questions.

One way or another, I'm pretty envious of your daughter spending winters in a tinyhouse in Norway and/or Austria sounds pretty idyllic!

I would go with the Austrian locale, much warmer with fantastic solar gain and putting up with walabees is a small price to pay.

offgriddle said:

I would go with the Austrian locale, much warmer with fantastic solar gain and putting up with walabees is a small price to pay.

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I suspect she won't have a hard time avoiding Wallabees in the Central European country of Austria ?

Dzl said:

I suspect she won't have a hard time avoiding Wallabees in the Central European country of Austria ?

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Then again, there is a population of up to 100 feral hippos living in Colombia so I suppose you never know... ?

Dzl said:

I agree with you, but the benefit of overcapacity comes to the ratio between battery capacity and consumption between charges, not battery capacity vs solar input.

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Not quite sure what you mean exactly here. If you increase battery capacity, you will increase both ratio's and gain from lower DOD and you will be able to overcome a longer period of cloudy days with a deficit.

The panels will have a (peak) capacity of about 1800W. So, the 0.05C that I mentioned could be 0.18 in theory. But in reality it will probably be much lower. With sunny days she will have plenty capacity to recharge, but on winter days with no sun, it will not be enough. I think in the winter, she will only be able to recharge on days with some sun, and you can have periods of 1 or 2 weeks with hardly any sun.
The extra capacity does not really cost any money, as it will extend the lifetime of the battery:
If you double your capacity, the lifetime should more than double. Because of the lower DOD the lifetime in cycles increases and each cycle will be twice as long.

So regarding to charging strategy, I figured that an absorption phase is not really required. With expected winter charge-rates of around 0.05C stopping charging at a cut-off voltage is equal to charging with a higher current and an absorption phase. And at occasional higher currents (like 0.15), you will miss only a very small charge.

Zeerob said:

The panels will have a (peak) capacity of about 1800W. So, the 0.05C that I mentioned could be 0.18 in theory. But in reality it will probably be much lower. With sunny days she will have plenty capacity to recharge, but on winter days with no sun, it will not be enough. I think in the winter, she will only be able to recharge on days with some sun, and you can have periods of 1 or 2 weeks with hardly any sun.

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Oh okay now I get it, I assumed your array was 500w (0.05c x 10kWh) based on your earlier comment, but as I understand it now its an 1800W array, that you are estimating might output 500W on an average bad weather winter day? This makes much more sense, and seems like a much more reasonably sized solar array.

So regarding to charging strategy, I figured that an absorption phase is not really required. With expected winter charge-rates of around 0.05C stopping charging at a cut-off voltage is equal to charging with a higher current and an absorption phase. And at occasional higher currents (like 0.15), you will miss only a very small charge.

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This is an interesting point, I see your logic, I hadn't connected those dots before

The Nordkyn article is great. The commercial battery suppliers don't seem to like to talk about the memory effect, other than to encourage us to fully charge the battery once in a while. Their stated reason is top balancing.