LA versus lithium battery sizing

Dave Angelini · February 2021

It also depends on the battery design along with best practices!

Many can go to zero from 100% as they save some charge and shut down the battery until charge begins. They close the loop and protect them self at 100% Soc and run loads.

Probably the biggest loss of warranty is high heat and charging below 32. Quality designs take that into account and log temperature extremes, cycles, and save data in non volatile memory.

Many have 10 year power warranties and specify how much capacity is left after the 10 years. A few can be closed loop with your solar, grid, or genset to really save energy when charging. A few actually output their BMS data into inverter based power systems. This then can go out to your phone, system monitor, or the cloud. There is no external Battery Monitor required and installation is simple.

Horsefly · February 2021

MichaelK said:

I've often heard the claim that Li batteries can be drained down to 0%, but is that really true? I've seen recommendations from BattleBorn that their battery's lives are severely shorten once drained past 80% (I read a 5 fold reduction in life expectancy), and I've also read that Li batteries don't like routine charging all the way to 100%, and 90% is a recommended target.

So, in the real world, if people should be managing their Li batteries between 20-90% charge, it seems you are comparing a 70% battery to a traditional 50% lead-acid battery. Is that conclusion off-base? It appears to me that totally draining a Li battery shortens it life as much as 100% draining shortens lead life?

I think what you find when you look at the base cell physics (vs the stuff from the companies that package them into 12V cells) is that there isn't much harm to taking LFP cells up to 100%, or down to 0%, but you don't want to go above 100% or below 0%. It won't kill the cells, but it does shorten their life to something less than the 3,000 cycles you would get otherwise. So I think the valid comparison to FLA is more like 95%-10% vs the 50% SoC for FLA.

The only reason that it is wise to keep the SoC on LFP between 95% and 10% is really that it is between the "knees" of the very flat voltage curves of FLP. You can make sure all the cells in the battery remain between 0% and 100% if the pack stays between 10% and 95%, because the voltage curve for LFP is so flat. When charging, it is almost completely flat until you get to about 98%-99% SoC, then it rises dramatically. Similarly, it is very flat on discharge until you get to about 10% SoC, then it drops dramatically.

I have read (but can't confirm) that the reason Battleborn says you can go to 0% is really that they size their battery for something over (e.g. 110%) what is spec'd. So if you run it down to 0% SoC it is really still at 10%. If you have a good BMS - and I'm sure Battleborn does - it does no harm to the battery.

KenMorgan · May 2021

mike95490 said:

I see no thermal sensors, air gaps and no thermal separators in the pack, everything is all cuddled together, with the cells in the middle, likely 5 degrees warmer than the outer.

This sounds interesting....even promising... if I can get an extra 5 degrees in the center of the pack then my battery warmer will not need to be on as much. I might just investigate setting my cells in a pack like that.... originally I was going to just do a horseshoe layout which would have been long and narrower than the pack that Raj174's built it a shorter rectangle (not quite square) where as mine would be less square and a longer rectangle ... being that I live in a colder climate that would be good for me in the winter I think...do you really think it would add 5 degrees? and is that °F or °C

edited for clarity: mine would be a 2 cells x 16 rectangle, approximately 1136 x 364 where as Ricks is a 4 cells x 8 rectangle and if our cells are the same size that would give him roughly a 568 x 728 footprint... much more compact and heat dense possibly which is good for me. even in the summer months where it does it a bit hotter (hottest recorded was 30°C I could just simply pull the top off of my battery box and it should be fine. we get down to -14°C regularly but inside my old battery box it rarely got below °5C but when it does it drops as low as -5°C hence me planning on a battery warmer for the morning charge cycle. and extra 5 degrees would mean less energy being used to heat the batteries. plan was to hold them between 8 and 10°C

mike95490 · May 2021

Your thermal gain is mostly going to be cell internal resistance, If used gently, and charged slowly, you may not see any thermal gain,

KenMorgan · May 2021

Oh well i had some hope!

Dave Angelini · May 2021

LFP can be a PITA in cold climates if you want the costing pay out over a long lifetime. Many of my clients in Alaska and Canada keep them in a controlled environment and even then have triple redundancy for heating.
If they have to leave home in winter, they use fossile fuel heating as a failsafe.
Many just use LA and keep their life simple in cold climates. Less is often more Offgrid.

Raj174 · May 2021

@KenMorgan

Yeah, there is not much temperature rise charging at common solar rates. My tests have shown only 1°F higher temp at the center of the bank while charging at 60 amps, which for my bank, is .17C. So I wouldn't count on that being a significant source of heating.

Rick

BB. · May 2021

Yep... Lithium Ion batteries are very efficient. Yea! >98% efficient

But that means they will not "self heat" like lead acid batteries which can keep warm in an insulated box during normal cycling.

As flooded cell lead acid batteries approach 90%+ state of charge--gassing--they get down to 80% to 50% efficiency in terms of self heating--Gassing is also a loss, but not to heat.

Plus Lead Acid batteries are just fine when cycling below 0F (-18C) as long as reasonably charged (i.e., electrolyte is not turning to slush/ice).

One quick chart I could find:
https://heartlandowners.org/showthread.php/43227-Battery-electrolyte-freezing-temperature-vs-battery-charge-and-electrolyte-specific-gravity

Specific Gravity of Electrolyte	Freezing Point (degrees F)
1.265	-75 F
1.225	-35 F
1.200	-17 F
1.150	+5 F
1.100	+18 F
1.050	+27 F

Since we suggest keeping your lead acid battery >50% State of Charge during normal operation, then you are looking at "good down to" -10F or so.

Nice FLA manual:

http://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf

-Bill

jtdiesel65 · May 2021

Horsefly said:

. So I think the valid comparison to FLA is more like 95%-10% vs the 50% SoC for FLA.

The only reason that it is wise to keep the SoC on LFP between 95% and 10% is really that it is between the "knees" of the very flat voltage curves of FLP. You can make sure all the cells in the battery remain between 0% and 100% if the pack stays between 10% and 95%, because the voltage curve for LFP is so flat. When charging, it is almost completely flat until you get to about 98%-99% SoC, then it rises dramatically. Similarly, it is very flat on discharge until you get to about 10% SoC, then it drops dramatically.

The ~10 to ~90% seems common to avoid the knees where cell voltage diverges and for longevity, but I think in a realistic off-grid setting it is more likely to be 15 or 20 to 90 in order to have some capacity in case generator doesn't start, etc.

KenMorgan · May 2021

Awe you guys are just breaking my itsy bitsy teenie weenie little heart

(j/k) so its back to heater pads in a depth in defense layered approach as well as the 4" thick foam insulation with charge and discharge settings designed to keep her off her knees (and that really sucks) ha ha ha. good thing is when its cold here... its cold here. power goes out and food will not spoil so as long as the BMS stops cold/over discharge and prevents cold/overcharging ... worse case is the magnum does not fire up the genset and I have to manually do it.

So I have not asked this particular question yet to avoid being clowned... Us Marines have issues with being made fun of (just keep your booger hooks away from my crayons ya hear?)

just spitballing with no actual experience other than spending the money and currently top balancing the batteries... so no actual experience with the BMS yet.... What would happen, if I took two identical BMS units and wired the common ports in series. IE one BMS fails (for whatever reason does not disconnect the batteries) since they are in series as long as the second unit is functioning it should (theoretically) disconnect as long as its battery sense leads are all hooked up properly... yes...no... overly complicated? sorry not much of an artist in the colloquial use of the word, but I would think that since the BMS (Daly common port in this scenario) so the battery plus and the P - (minus) wires in series. if either unit fails to disconnect the second one in line reading the same readings from the battery should disconnect giving me defense in depth as I can monitor via Bluetooth and if one is power goes down and one BMS is reading wonky I can remove and replace

mcgivor · May 2021

KenMorgan said:

Awe you guys are just breaking my itsy bitsy teenie weenie little heart (j/k) so its back to heater pads in a depth in defense layered approach as well as the 4" thick foam insulation with charge and discharge settings designed to keep her off her knees (and that really sucks) ha ha ha. good thing is when its cold here... its cold here. power goes out and food will not spoil so as long as the BMS stops cold/over discharge and prevents cold/overcharging ... worse case is the magnum does not fire up the genset and I have to manually do it.

So I have not asked this particular question yet to avoid being clowned... Us Marines have issues with being made fun of (just keep your booger hooks away from my crayons ya hear?)

just spitballing with no actual experience other than spending the money and currently top balancing the batteries... so no actual experience with the BMS yet.... What would happen, if I took two identical BMS units and wired the common ports in series. IE one BMS fails (for whatever reason does not disconnect the batteries) since they are in series as long as the second unit is functioning it should (theoretically) disconnect as long as its battery sense leads are all hooked up properly... yes...no... overly complicated? sorry not much of an artist in the colloquial use of the word, but I would think that since the BMS (Daly common port in this scenario) so the battery plus and the P - (minus) wires in series. if either unit fails to disconnect the second one in line reading the same readings from the battery should disconnect giving me defense in depth as I can monitor via Bluetooth and if one is power goes down and one BMS is reading wonky I can remove and replace

There is another trick which can be utilized, this is to cut the Morningstar controller from charging when temperatures drop below a given value, the method is as follows.

Use a digital temperature sensor to power a relay, over a normally open contact of the relay run one leg of the remote temperature sensor, when a low temperature is detected by the temperature sensor it cuts power to the relay, this in turn breaks the RTS sensor input which faults the controller in an invalid input, thus terminating charging.

Multiple temperature sensors in series powering the relay would add to redundancy, the controller may need to be rebooted after such a fault, as is typical on other faults, Morningstar support endorses this as a method of charge cut-off, they will tell you if a reboot is required, or you could experiment by removing a RTS wire from the input even with the current LA batteries.

Hint. When wiring relays or temperature sensors as safeguards, always use normally open contacts (open without being powered) to safeguard against outside influences, such as a broken wire from rendering the safety null and void.

LA versus lithium battery sizing

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