T1 Terry wrote:

---

Warning, long post with lots of explaining stuff. I'll put the numbers in red so if you want to just get the settings without the explanation why I'm suggesting them, then you can skip the boring bit.

Mamil wrote:

---

At the risk of interrupting a good argument with some sensible questions, Terry can I pick your brain for a moment?

I have a 100Ah LiFePO4 battery with built-in BMS and a Redarc charger with Li profile.

Firstly, what is a good cut-off voltage to set the load shedding on the Redarc charger at? The battery's internal BMS will shut down at 9.2V so obviously it should be above that, but how much above?
Second, the Redarc also has an SOC alarm, what do you suggest I set this at?
Lastly, the datasheet on my battery says minimum charge current of 10A. Now I don't really understand why there a minimum charge current at all, surely even a trickle going in is fine - that's all its getting at the end of it's charge cycle anyway?!? Sometimes on a cloudy day or early or late in the day my solar is generating a lot less than 10A, so is this bad for the battery in some way, and I should switch the solar off if it's producing less than 10A?
Thanks.

---

A few things you have mentioned are well within the description of an oxymoron. Not what you have said but what the manufacturer has claimed. First off, a lithium BMS that can't measure cell voltages and act on those voltages is not a lithium BMS at all. The fact both the battery manufacturer and the charge controller manufacturer both claim their product is a lithium battery BMS shows they are both either very naïve or they are telling major porkies.

 

Back to you questions:

Cut-off voltage.

A safe minimum cell voltage is 2.8v in any cell. The problem is you can't measure each cell voltage, only the 4 cells added together, this means 4 x 2.8v = 11.2v .... but 3 cells at 3v + 1 cell at  2.2v also equals 11.2v and a damaged battery. The batteries BMS cutting off at 9.2v even with all the cells balance perfectly at this low voltage (doesn't happen with a top balanced battery)  would have each cell at 2.3v and haemorrhaging themselves to death. The more likely scenario is one cell going very low voltage while the others weren't damaged quite as much. Because you can't change one cell, it is still a stuffed or seriously damaged battery.

With that background knowledge in hand, I'd say 12v is as low as you would want to discharge the battery to be sure no cell went much below 2.8v.

 

Low end SOC cut off.

We have no idea what discharge rate the battery manufacturer used to determine it had 100Ah capacity, it could have been a C100 rate (discharged over 100hrs) a C20 rate (discharged over 20hrs, the accepted rate for AGM batteries) or a C2 rate, (discharged over 2hrs and the rate used for Winston lithium cell testing) A cell rated at C100 gives a lot higher capacity than one rated at C20. This link to a top quality AGM battery shows their 120Ah battery is measured at the C100 rate, at the accepted C20 rate it is only a 100Ah battery  https://www.solaronline.com.au/120ah-concorde-12v-agm-deep-cycle-battery.html, at a C2 rate it would be around the 60Ah, half of the name plate capacity. Lithium batteries can multiply this effect by probably double, a 100Ah battery rated at a C2 rate but discharged at a C100 rate would be deliver over 200Ah, or reversed, the claimed 100Ah at a C100 rate would mean the battery tested at a C2 rate would only be a 50Ah battery.

So, with all of that in mind, probably a 5% SOC warning, if it is only a true 50Ah battery that will leave 2.5ah in the battery and that should stop a cell going low voltage, yet still give you the majority of the capacity useable. If it really is a genuine 100Ah you will have 5% remaining and have 95% useable.

 

Minimum charge current or trickle charging

The problem they are trying to avoid is cell voltage run away due to the high charging voltage they have claimed the battery can handle. At 10 amps the charge rate is either 10% of the 100Ah capacity or 20% of the 50Ah capacity. The idea is the full cell with rapidly climb to a high voltage and reach the 14.6v or what ever cut off voltage they claim the battery can handle. 3 cells at 3.6v safe max leaves one cell hitting 3.8v for a short time once the high voltage cut off is reached. But 3 cells at 3.35v means one cell will reach 4.1v before the 14.6v limit is reached. Again, very short term if the charging rate is high, but it could be at the 4.1v or more for quite a long time if the charge rate was 1 or 2 amps and that would damage that cell. This is the reason cell voltage monitoring and cut off control is so important, even charging at 0.5 amps, the 3.6v in any single cell safety cut off is maintained so the cells are always protected from high or low voltage damage.

About all you can do is limit the max charging voltage to 13.8v so you have a big safety margin between the high cell and lower cell voltages. That way the minimal balancing circuit inside that battery will get the high cell back down over a few days and the lower cells can catch up. This does mean you might sometimes not have all the advertised capacity available because the capacity is only what the lowest cell is holding at the time, but better than wrecking a battery. 

 

T1 Terry

---

Hi Terry, thanks for your detailed reply. At first reading what you said makes a lot of sense (even despite your usual tendency to assume the worst of 'drop-in' Lithium systems ) but I'll need to take some time to digest it all properly!

By the way, the 100Ah rating of my battery is at C/5, and don't get too hung up over Redarc's description of their charger as being a "Battery Management System" as they don't mean it in the sense you and I understand a BMS, and do say clearly in the manual that, "When using the Battery Charger to charge a Lithium Iron Phosphate battery, only batteries that feature an inbuilt battery management system featuring inbuilt under and over voltage protection and cell balancing are suitable." 

Lastly, to those that have criticised you for writing too much technobabble, I can only say that in this case I asked the question, and your answer was at the right level of detail and technical depth for me, and is much appreciated, so if they don't like it then I suggest they just pass on over it...

:lol: You only have to use the coded TMFM (The Man From Mannum) over on the Caravaner forum because T1 Terry is a dirt name over there, it actually gets changed to *******   

Someone really got their knickers in twist over there, it wasn't enough to just ban me, you one could mention my name either because it might sully their sacred site or something :LOL:

No doubt there are few here that would like to see the same thing happen on this forum

T1 Terry

Dncwell wrote:

---

Hello T1 Terry
I am looking to create a supplementary 12 volt in our motor home, I have a 5000w inverter coupled with 1000 watts of solar panel and a 160ah agm battery, not good enough.
What capacity lithium battery should I purchase to replace the agm?
Thank you,
Best regards,
Neil.

---

Hi Neil, this post almost got buried in the avalanche of off topic posts and replies, some seem to have a lot of time on their hands and a lot of anger issues.

I'll give a bit of background context so you can get an idea how much a combination of solar capacity and battery capacity can achieve. The battery capacity I'll put in red for those who don't want to read through the long post.

We have 2 motorhomes we use that are totally electric, no gas except for one of those gas can stoves because they cook an egg or a steak that can't be cooked in the air fryer.

On one motorhome, the Mazda 3500, we have 700w of solar and 360Ah of lithium batteries at 12vdc, the system is about 8 yrs old now but still does the job just fine. We have a 12v compressor fridge, a 3000/9000w W7 type inverter that powers all the big stuff, and a 400w inverter for the electric blankets, my wife's CPAP machine and device chargers as required. The W7 has a terrible standby/overhead consumption so leaving it on 24/7 isn't really practical, but the little 400w inverter is on 24/7.

The other motorhome is a Hino RB145 and is 24v. It has 1620w of solar, just because that was how many Tindo 24v 275w panel fitted from front to back and made a sort of summer roof for our trip to WA in the middle of the hottest summer for a long while. This motorhome has a 180Ah @ 24v lithium battery, so the same capacity as the other motorhome, just more than double the solar. We run a split system air conditioner as well as a 240vac fridge and a 5kVa inverter that runs 24/7 to power the fridge and everything else so no need for the small inverter as well.

That gives you an idea of what can be achieved with a solar set up smaller than you have and one larger. The 1600w is over kill in summer, but it does power the air conditioner and the water heater. In a South Australian winter it really comes into its own though, no need to watch the Victron BMV like a hawk because it charges something even when it's raining.

So, depending on just how much storage you need for over night or just how much poor weather you want to be able to endure before you need to look at an alternate charging method will determine just how much battery capacity you need.

100Ah @ 0.5CA rating (need to know how the capacity advertised was measured) would give you a bit more than you have now.

200Ah would be enough to last a moderate user for a few days, even a few hrs of air conditioning while the sun was out if you wanted, yet still power a coffee machine etc quite a few time between sun down and sun up. It might be a bit tight if an air fryer was adding the over night use pattern along with all the other appliances and the inverter on 24/7. 

400Ah would give a system a step up from the Mazda capability and allow for virtually worry free use over night, an electric fan heater on all night might be a bit over the top though

T1 Terry

Kebbin wrote:

---

I often wonder how EVO Lithium Power Solutions feel about the constant free advertising that Terry gets on this forum, seen as they pay for advertising and Terry doesn't?

---

 I may not be the brightest star in the sky,but I fail to understand how Terry gets free advertising when I,for one,do not even know the name of his business,or from where he hails.Perhaps you could enlighten me? All Terry seems to do is to help others,yet he seems to get constant bombardment from all angles.Please explain.Cheers

Kebbin wrote:

---

I often wonder how EVO Lithium Power Solutions feel about the constant free advertising that Terry gets on this forum, seen as they pay for advertising and Terry doesn't?

---

 I seemed to have missed the flashing glossy adds mate, what pages are they on ...???

vik... 

Kebbin wrote:

---

It must be the sunnies.

---

 We eagerly await your explanation of how we can identify Terrys business,and its location.Cheer

Thank you very much Terry, had lost this post somewhere below the surface and the forum would not let me reply after I read it first.

Have posted as a new post last night if you can find it.

Best regards,

Neil Creswell.

yobarr wrote:

---

Kebbin wrote:

---

It must be the sunnies.

---

 We eagerly await your explanation of how we can identify Terrys business,and its location.Cheer

---

 Think about it all you have to do to contact anyone on this forum is to pm them ,cant be any simpler can it...

Thanks Terry for the great information that you post.   As for the knockers, they have not provided too much info that could be used.

I am yet to see Terry`s address and business details in any of these posts.

Keep up the good work Terry.

Many thanks for 

the information Yobar. Trying to fathom some realistic scenarios using lithium to provide another system.

There seems to be a lot of malace to wade through to obtain some simple advice.

sorry for the delay in response - freecamping with no signal.

Best, Neil Creswell 

Dncwell wrote:

---

Thank you so much Terry.

 I now have to analyse the lithium batteries on the market. Most have very scanty specs etc.

Apologies for the tardy reply, am freecamping with no radio signal, but am leaning towards 200ah.

Really dont know how you tolerate the aggression show on here.

Best regards,

Neil Creswell.

---

It's pathetic isn't it Neil?

The drums have been banging against Terry for years, without letup.

The protagonists can't even explain their hate.

Acting like spoilt children screaming out at the top of their lungs because some other child touched their toy.

I have been through a few members post history and honestly those that continue to post the hate against Terry have had very minimal at best to offer on this forum.

We would be better off without them all.

Hi Neil....Dncwell wrote:

---

Thank you so much Terry.

 I now have to analyse the lithium batteries on the market. Most have very scanty specs etc.

Apologies for the tardy reply, am freecamping with no radio signal, but am leaning towards 200ah.

Really dont know how you tolerate the aggression show on here.

Best regards,

Neil Creswell.

---

 Hi Neil...After much research I went with Victron,and I have 6x90ah in the van,through a 5kw Victron inverter.In the car there is 1x90ah through a 2kw Victron inverter.Everything thing works well,but I suspect that the 90ah in the car will struggle to run frypan,microwave etc as well as charge all my batteries for drills,grinders,blower,reciprocating saw etc etc.Time will tell,but I always can just move to the van to charge them if necessary.Cheers

Thank you Yobar for the specs on your system.

Still wading through the positive information.

Best regards, Neil.