usable ah lithium

[GlennWest]

We all mention ah capacity. But most don't charge to max or discharge to minimum. Do we don't get rated ah in the real world. So say 400 ah rating what are you guys getting? I have my cutoff at 48v. 

Edited March 31, 2023 by GlennWest

[Randyretired]

There are charts for lithium battery SOC.  I use LifeP04 batteries and use a 12v cutoff equal to the 48v you use.  This is 9% SOC or 91% of capacity.  I charge to 14v which depending on absorption time is about 99% charged.  Totaled that means I can use 90%.  I don't use all of that most of the time but need all of that late in the year or during cloudy days.  I am planning an off grid system for our new place and it will require a lot of capacity for December and January and idle most of the rest of the year.  My planning is for 3 full days of storage and enough solar to fully recharge in 1 sunny day.  That will still require some generator time but hopefully not a lot.

[Payroll Person]

An interesting comparison is the Tesla model 3 lfp packs.

The first “change” is they are suggested to be 100% charged at least once a week. The information is there is little degradation reduction by trying to manage soc in the middle somewhere. One issue is range calcs are less accurate when there is no static 100% for a period of time.

It is interesting to see the different trains of thought on lfp when looking at the ev community vs mh community vs off grid community. Much is the same, but their are things some argue that others have deemed not important.

The tesla warranty on the batt is good, and it is reasonable to believe the batts will make it to as least the end of warranty. They base the warranty on no more than 30% degradation. For me, it ties nicely with charge and discharge for ease of use and for needs, with not so much time focusing on health of batt. 

Edited March 31, 2023 by Payroll Person

[Randyretired]

Some manufactures suggest fully charged and held at that state plus for a period of time for balancing.  There are a few things to consider for charging.  Some BMS's have active balancing and others passive balancing.  There is substantial differences in balancing power in BMS's.

Edited March 31, 2023 by Randyretired

[Payroll Person]

Indeed. And there are plenty of experts giving advice which no longer applies or never was accurate. One topic for rv is isolator, separate charger, etc. I got into “it” once with an expert who contradicts the manufacturer’s advice and ignored my own report (with images).

For tesla lfp, there are old timers who set flag on never charging to 100%. There seem to be few warranty claims as the actuaries would be shortening the warranty, yet there are still many never 100% and never SC when not absolutely needed types.

If one wants to try to reduce degradation and save the 2-5% capacity, great, but for most, over the 1500+ cycles or calendar life, a use as needed seems reasonable.

Seems reasonable not to get the the bms low cutoff is good, although there are ways to wake the batt up in many cases. It happens to us with our mini segways when we forget to keep them charged (I made a rig to essentially force power in past the bms).

Edited March 31, 2023 by Payroll Person

[Randyretired]

For most of us I wonder if cycles will even be the end of service determining factor.  The recent cells I purchased are rated at 6,000 cycles.  Most believe partial cycles are added to equal full cycles. That is a long time for solar applications and it would seem environmental concerns could come into play before cycle life ends.  Especially since cycle life is based on 80% of capacity and it would seem that the battery is still be useable beyond that. 

[GlennWest]

44v cut off as low as I can go with my inverters. Man I got mine from suggests 57.4 max. Cut off at 40.7. 

[hemsteadc]

I don't run my 48v 100ah pack down to 20% any more than I'd run my fuel tank that low.  About 50v or 50% capacity is where I get nervous.

[oldjohnt]

    While one reads and hears about how you can take Li all the way down until it shuts off HOWEVER Im NOT a fan of doing so. Based on my battery storage capacity and energy use I seldom (using my Shunt based battery monitor) draw down much over 50%, which tells me my design (based on use, solar capacity and battery AH) is adequate. Should I get down to 50% overnight (often much much less use) as long as there's decent sunlight I may achieve 100% SOC by mid day to afternoon. Of course, if its rainy cloudy or Im parked under a canopy I may have to fire up the genset and charger during the day grrrrrrrrrrrr

    I would have to look at my battery and its BMS specs to see exactly when it shuts down, be it 80%, 90% or 100% ????  but I've never depleted it near that much to observe ???

   John T

[hemsteadc]

1 hour ago, oldjohnt said:

    While one reads and hears about how you can take Li all the way down until it shuts off HOWEVER Im NOT a fan of doing so.

I've done that... inadvertently.  And then, my chargers would not work.  I had to 'prime the pump' by briefly attaching a 48v source (which was a spare Segway scooter charger) to the busbar to get the charger to wake up.  

[Randyretired]

Most BMS"s are set to cutout at 2.5v on any cell.  That is considered fully depleted.  I have my inverter cutoff at 12v to leave a little in the battery as I don't like to run it all of the way down.  Using the BMS to cutoff can cause problems.  We use a lot of power and buying extra batteries to maintain 50% would be expensive with little benefit. Our battery bank is 1400ah @12v and 50% or 700ah is a lot of power.  Our off grid place will have a substantially bigger battery and 50% would be pretty expensive. It will have enough to  last 3 days with zero solar so hopefully it won't deplete often.  Another problem we have with the RV is the 3,700 watts of solar will not fully recharge a depleted battery on one sunny day when the days are short.

[oldjohnt]

20 hours ago, Randyretired said:

I have my inverter cutoff at 12v to leave a little in the battery as I don't like to run it all of the way down. 

Neither do I !!!!!!!!!!!!!!!!!!!  Ive NEVER had occasion to see it get that low...

John T   White Springs Florida but headed North