Charging LIFePo4

[tripper]

Have you got a good AH-counting BM?

No I do not, I have a pretty good feel for the amp hour draws of the devices I use. I wouldn't mind a nice BM but it is low on my priority list of items I need and a good one is pricey.  I've been successful in just monitoring voltages for a ballpark SOC for the last few years. 
The Victron android monitors software  in float mode gives me a good representation of current draw during the daytime, as I have enough solar normally to cover all loads without drawing from the battery at all, so i get a live picture of current drawn. I can look at the watthours used overnight to get a good idea of amperage usage overnight and subtract that from my battery capacity to see how much I used. For example:
[img=270x272]https://vanlivingforum.com/attachme...ivingforum.com/attachment.php?aid=15105[/img]

[img=160x90]https://vanlivingforum.com/attachment.php?aid=16152[/img]

 

[John61CT]

I suppose an accurate volt reading plus "learned intuition" can get you pretty far, as long as

your loads are fairly low current, and

you err on the conservative side as you get lower down the SoC curve.

Personally I'd save for a 702-BMV for the long term, especially if my raw bank was worth over a grand.

 

[ZacLee]

This is only part of the equation.

Any battery will last 'longer' (measured in number of years) if used less, but, are you actually getting more 'use' out of it?

Probably not. 

If you discharge a battery only 25% for 600 times, or 50% for 300 times, its the same amount of 'life', measured in total number of amp hours or watt hours derived (not counting peukert's derating curves).

If you never discharge the battery at all, ever, then it will last a VERY long time....but you will have gotten little use out of it.

One obtains more amp-hours cycled over the life of the battery if LiFePo4 batteries do not exceed 75% depth-of-discharge.  On the other hand, limiting DOD to less than 75% does not provide significant benefit for LiFePo4 batteries.  I posted the data for this in a previous post:

Looking at the total amp-hour yield over the cycle life of the battery provides some insight. The 24V Telecel brand LiFePo4 batteries I use have this cycle life rating. For simplicity, the calculations for ah over lifetime below are for a 1 amp-hour rated battery. These calculations assume the capacity remains constant over the life of the battery though it would be slowly declining over that time. End of life is defined by most manufacturers as the point when the battery loses half of its rated capacity. If I get a chance, I redo the calculation on a spreadsheet to account for the diminishing capacity.

80% depth-of-discharge, 2000 cycles (1600 ah over lifetime)
70% depth-of-discharge, 3000 cycles (2100 ah over lifetime)
25% depth-of-discharge, 8000 cycles (2000 ah over lifetime)

There is minimal (5%) difference in ah yield over the lifetime of the battery for 25-75% depth-of-discharge, but the yield drops by 25% when the depth-of-discharge is extended to 80% and likely drops even further with deeper discharge. It would be interesting to see cycle life data from other LiFePo4 batteries for comparison. In a nutshell, this data indicates discharge depth between 25% and 75% yield roughly the same cycle life while extended discharge depth to 80% reduces life by 25%.

 

[tx2sturgis]

70% depth-of-discharge, 3000 cycles (2100 ah over lifetime)
25% depth-of-discharge, 8000 cycles (2000 ah over lifetime)

These two numbers are in the range of normal use that we were referring to, (my numbers were 25% and 50%) and your data supports my statement.

Between these two extremes, 25% and 70%, there is only a very small difference in accumulated ah that is able to be used.

 

[John61CT]

And remember these cycle ratings have been found to be very lowball numbers when charged according to the guidelines I've been posting.

So if you follw them, and have a good reason to bring DoD down, long as you don't hit that bottom voltage curve "shoulder" for your discharge rate, don't worry about it.

The higher your discharge rate, the lower that shoulder point. At very very low rates, need to stop at 12.7V or so.

With an accurate AH-counting BM I'd cut off no lower than 85% DoD in normal usage, maybe 88-89% in an emergency, recognizing taking chances.

 

[ZacLee]

These two numbers are in the range of normal use that we were referring to, (my numbers were 25% and 50%) and your data supports my statement.

Between these two extremes, 25% and 70%, there is only a very small difference in accumulated ah that is able to be used.

My point is that discharging LiFePo4 batteries below 75% depth-of-discharge will significantly reduce battery life.

 

[tx2sturgis]

My point is that discharging LiFePo4 batteries below 75% depth-of-discharge will significantly reduce battery life.

I think we are saying the same thing, but when you state it this way: "75% depth-of-discharge"  to me that means, only using 25% of the battery capacity, leaving: 75% remaining. 

100% is full. 

50% is half full.

25% is nearly discharged. 

I read your statement and interpreted it to mean, 75% charged. 

All battery monitors display percentage of remaining capacity in this way, as far as I know.

What I think you are saying is, 25% remaining. 

If you had said, 75% discharged, or 75% depleted, or 75% used up,  then I would have clearly understood your meaning.

It's this darn thing called English that keeps getting in the way of clearly communicating!

 

[ZacLee]

What I think you are saying is, 25% remaining. 

If you had said, 75% discharged, or 75% depleted, or 75% used up,  then I would have clearly understood your meaning.

It's this darn thing called English that keeps getting in the way of clearly communicating!

75% depth-of-discharge is the same as 25% remaining.  I'm sorry for any confusion.

 

[John61CT]

Yes, 90% SoC is 10% DoD. 80% DoD is 20% SoC.

Most BMS will "define a" floor voltage that isolates the batt from loads, in effect to the user that becomes 0% SoC / 100% DoD even if technically say 7% is left you never get in that range.

Similarly setting all charge sources to stop at your top voltage setpoint (in my case 13.8V) defines "your 100%" Full, which should not be exceeded in normal usage. Usually this might be 5-8% down from the **vendor's** definition of 100% charging at a higher (IMO harmfully so) voltage.

Those with SoC Battery Monitors can set these how they like, to me making the "harmful do not enter" top and bottom ranges 0% and 100% makes the most sense.

Since quality units ship with more than rated AH, can be 10-15% after proper commissioning protocols and breaking in, these precautions rarely cause much effective loss of capacity.