Goodday all

I am off grid in the Northern Cape , looking for a suggestion for simple straightforward LiFePo4 battery. Distance from technical help and me not being computer savvy or electrician, makes it imperative for a easy fault find or fix and preferably no hassle.

Have been using a Victron 3000/48 ( a inverter that I won on this excellent forum) with 8 Trojan batteries . The lead acid have really surprised my expectations ! 6 years old now ( had the batteries before I won the inverter) but batteries are done. The Victron Phoenix is still running strong !

I have a new Victron Multiplus 2 (5000/48) and would like suggestions for around 10 kwh LiFePo battery or batteries.

Is it worthwhile looking at second hand batteries?

Thanks for your time reading.

@Meerkat roll your own, would be my suggestion, that's what I did, 4 X JK BMS' here and each of those has 16 X 300-odd Ah EVE cells connected and running our home totally off grid near Grootdrink, I'd be wary of 2nd hand batteries and even new ones, what BMS do they use etc? Personally I reckon that most BMS' out there are not something I'd not be happy with, the JK BMS' are the 1st ones that had active balancers of up to 2A, where initially most BMS would balance at 100mA or less, by running excess energy into a resistor and heating up the place :-)

@Kalahari Meerkat I am not comfortable with my limited electronic knowledge , am lost just with what you wrote.

But I agree 100% with your statements.

The reason I ask on this forum is because I don’t have the knowledge or expertise or money too make a mistake…

One method that has been suggested to me is to look at what's being sold second hand. The theory behind this being that if people are getting rid of them it's because they are unhappy with them. You could refine this by finding what brand of inverter the seller has. I used to have Revovs, they didn't get on well with my inverter, so it's a good bet that Goodwe owners are more likely to dump Revovs than Victron owners are.

This isn't scientific, but it may reflect real world experience.

Thanks @Bobster.

Good suggestion 👍🏻

Opinion on 2 hand ( 5 years old - 500 cycles ) Solar MD batt 7,4kw x 2 with logger? for R20 000.00. So end off warranty…

Will get confirmation Monday on condition and other information as far as issues. Apparently batteries were not worked hard , but so , some cars were only driven by old ladies …

I am not sure on the inverter being used, but can find out.

I wouldn't buy 2 x 7.4kw 2nd hand for R20K if one can get a new 16.4kw for about R30K on the Powerforumstore

50 minutes ago, dax021 said:

I wouldn't buy 2 x 7.4kw 2nd hand for R20K if one can get a new 16.4kw for about R30K on the Powerforumstore

Thanks @dax021

Will go and have a look.

Money is a issue , but one has too be sensible.

2 hours ago, Meerkat said:

Apparently batteries were not worked hard , but so , some cars were only driven by old ladies …

Exactly...

Even on this Forum there is disagreement on the definition of "hard" - some people consider it totally normal to fully discharge their (lithium) batteries, or to drop them to below 20%, 10%, etc. SOC whilst others (and I fall in this category...) would consider a battery to have been worked hard if it is regularly discharged lower than 50% SOC.

A second, and just as important, question would be at what temperature were these batteries "driven" - there is a "Goldilocks" operating temperature zone for maximum battery life (for LiFePO4 batteries this is between 15°C and 25°C). Outside this range battery life expectancy is reduced by lithium plating and/or electrolyte oxidation. It is rather unlikely that the seller of the second hand batteries will be able to supply internal temperature records for the time these batteries were used.

A third point to consider (and rarely disclosed by the secondhand reseller) is the rate of charge and/or discharge during the battery's working life. As with a car, the harder you drive it, the less distance (or life...) one gets out of it...

Just for interest sake, here's a graph that summarize the first and third points above (from https://gwl-power.tumblr.com/post/130701906811/faq-lifepo4-cycle-life-based-on-dod-the-graph?):

Edited March 28Mar 28 by HennieL

49 minutes ago, HennieL said:

Exactly...

Even on this Forum there is disagreement on the definition of "hard" - some people consider it totally normal to fully discharge their (lithium) batteries, or to drop them to below 20%, 10%, etc. SOC whilst others (and I fall in this category...) would consider a battery to have been worked hard if it is regularly discharged lower than 50% SOC.

A second, and just as important, question would be at what temperature were these batteries "driven" - there is a "Goldilocks" operating temperature zone for maximum battery life (for LiFePO4 batteries this is between 15°C and 25°C). Outside this range battery life expectancy is reduced by lithium plating and/or electrolyte oxidation. It is rather unlikely that the seller of the second hand batteries will be able to supply internal temperature records for the time these batteries were used.

A third point to consider (and rarely disclosed by the secondhand reseller) is the rate of charge and/or discharge during the battery's working life. As with a car, the harder you drive it, the less distance (or life...) one gets out of it...

Just for interest sake, here's a graph that summarize the first and third points above (from https://gwl-power.tumblr.com/post/130701906811/faq-lifepo4-cycle-life-based-on-dod-the-graph?):

Thanks for your valid points on 2hand batteries too consider.

21 hours ago, HennieL said:

Exactly...

Even on this Forum there is disagreement on the definition of "hard" - some people consider it totally normal to fully discharge their (lithium) batteries, or to drop them to below 20%, 10%, etc. SOC whilst others (and I fall in this category...) would consider a battery to have been worked hard if it is regularly discharged lower than 50% SOC.

A second, and just as important, question would be at what temperature were these batteries "driven" - there is a "Goldilocks" operating temperature zone for maximum battery life (for LiFePO4 batteries this is between 15°C and 25°C). Outside this range battery life expectancy is reduced by lithium plating and/or electrolyte oxidation. It is rather unlikely that the seller of the second hand batteries will be able to supply internal temperature records for the time these batteries were used.

A third point to consider (and rarely disclosed by the secondhand reseller) is the rate of charge and/or discharge during the battery's working life. As with a car, the harder you drive it, the less distance (or life...) one gets out of it...

Just for interest sake, here's a graph that summarize the first and third points above (from https://gwl-power.tumblr.com/post/130701906811/faq-lifepo4-cycle-life-based-on-dod-the-graph?):

That diagram goes to a point that I recently disputed with Goodwe. I'd ask them to look at my inverter. They said they wanted to change some settings, I said OK.

Ha!

They set the maximum charge current very low. It was taking 7 to 8 hours to charge the battery on a sunny day. On an overcast day (and we were having those at the time in JHB) no chance. I am not used to this.

They then said that they set the charge current limit as per an instruction from FreedomWon. This is related to the two tier warranty on batteries being sold now. My 10/8 does not qualify for this. But the two tiers are based on how the battery is charged. If you charge at 0.25C then they will extend the warranty - more charge cycles. Otherwise the original warranty that my battery has (lower number of cycles) applies.

37 minutes ago, Bobster. said:

If you charge at 0.25C then they will extend the warranty - more charge cycles. Otherwise the original warranty that my battery has (lower number of cycles) applies.

That's interesting to hear - but wow, 7-8 hours of charging means they set the charging current way too low, or you are seriously discharging your batteries.

If your battery capacity is considered to be 8kWh, then 0.25C equates to 2kW (roughly 40A), and at 10kWh capacity that would be roughly 50A (not sure how the 10/8 naming convention works...)

My 20kWh batteries are normally discharged to between 70% and 80% SoC at sunrise, and on a "normal" March day takes roughly 3.5 hours to charge back to 100% SoC at 50A charge (0.125C) - the "missing" 1.2kWh is presumably wasted "system losses" - so the real-life efficiency is around 85%. I could of course charge at double this current, but there's no need to charge more rapidly as the 3.5 hours (roughly from 07:30 to 11:00 still leaves me with ample time to run the geyser for 3 hours, and the oven for another 2 hours before the sun loses power. Of course, during cloudy weather there's less chance to use the oven... but in the roughly 1.5 years that I've had my system I've only needed to revert to grid power during the third really cloudy day, and that does not normally happen all that often (although I must admit that we are currently having an abnormally wet season, and I went back "on-grid" for the last 3 days...)

Edited March 29Mar 29 by HennieL

34 minutes ago, HennieL said:

If your battery capacity is considered to be 8kWh, then 0.25C equates to 2kW (roughly 40A), and at 10kWh capacity that would be roughly 50A (not sure how the 10/8 naming convention works...)

We went through a couple of rounds. The situation originally had been that the BMS set the limits regarding charging. Since it's a 1C 10kW/h battery, and the hardware in the inverter can't go past 100A, the inverter can never charge that battery at 1C. Looking back through the history, the charging seldom went past about 80A, which turns out to be the absolute hard limit that FreedomWon allow - on a 1C 10kWh battery, go figure. But Goodwe showed me the documentation from FreedomWon. Since 80A pretty much matched what had been observed in real life we eventually agreed on that. They had initially set it to 20A (they thought I had Pylontechs, and they have a document from Pylontech recommending no more than 20A). They then set it, as you suggest, to 50A. They dug their heels in at 80A, but as I said, this pretty much matched the most I got in real life.

They are under pressure from battery manufacturers to adhere to their recommendations, but I also felt that this should be my problem: They could caution me, but if I told them that they should let this be between me and FreedomWon then surely that allowed them to wash their hands of the matter.

34 minutes ago, HennieL said:

My 20kWh batteries are normally discharged to between 70% and 80% SoC at sunrise, and on a "normal" March day takes roughly 3.5 hours to charge back to 100% SoC at 50A charge (0.125C) - the "missing" 1.2kWh is presumably wasted "system losses" - so the real-life efficiency is around 85%. I could of course charge at double this current, but there's no need to charge more rapidly as the 3.5 hours (roughly from 07:30 to 11:00 still leaves me with ample time to run the geyser for 3 hours, and the oven for another 2 hours before the sun loses power. Of course, during cloudy weather there's less chance to use the oven... but in the roughly 1.5 years that I've had my system I've only needed to revert to grid power during the third really cloudy day, and that does not normally happen all that often (although I must admit that we are currently having an abnormally wet season, and I went back "on-grid" for the last 3 days...)

I usually have about 50% at sunrise, that's after the heat pump has made it's early morning run. I would start getting grumpy if I didn't have 100% SOC by midday (and usually would have it by 11:00), to not be fully charged by 15:00 would mean pretty foul weather or unusually high loads. To have to wait until past 15:00 on a day with good sunshine was way beyond what I was used to.

25 minutes ago, Bobster. said:

I would start getting grumpy if I didn't have 100% SOC by midday (and usually would have it by 11:00), to not be fully charged by 15:00 would mean pretty foul weather or unusually high loads. To have to wait until past 15:00 on a day with good sunshine was way beyond what I was used to.

I hear you - and fully agree 😊 Glad they corrected it for you.