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Author Topic: Lithium batteries, the care and feeding of  (Read 399 times)

mikenash

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Re: Lithium batteries, the care and feeding of
« Reply #15 on: October 03, 2019, 01:09:59 AM »
An adjustable power supply capable of at least 3 amps at 15V will do your equalization after the battery has already been charged to full via PV. 

I use this:
https://www.amazon.com/gp/product/B07JCRG6G6/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

It can also be used current limited, to charge any battery to a set voltage at a current rate that is safe for that battery.  I use 7-15 AH  sealed batteries in 12 and 6 volt for electronics development work and it's nice for those.  It also works nicely on 120VDC which is handy for me. I have other 12, 24V chargers but none have equalization capability.  Apparently good consumers are supposed to just buy new batteries more often.  I have one AGM battery that is still in service at 12 years.  It was my house/shop 12V battery, demoted at 8 yrs to House of Lister battery, which is low capacity, mostly float service, and it doesn't get annual equalization any more.  It was a 110AH Universal Battery AGM, a bargain on sale.

The percent of charge shown by your controller is utterly bogus.  If it's 10.8V, it's done, 0.
Being left in that state is sure to cause serious sulphation.  A careful equalization charge might save it for a while.

Hi Bruce

yes, I figured anything under an actual 12.2 was probably toast

Last time I was up there was just for a few hours, and without a multimeter so I just figured I'll worry about it later

That PWM "controller" is a piece of Chinese junk I suspect.  But I bought the whole lot for not much more than the value of the panel, so what the hell.  LKearning & making mistakes on some expensive "proper" stuff - that would be a mistake . . .

Cheers

BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #16 on: October 03, 2019, 05:51:34 PM »
The standard minimum off grid setup here is 8 Trojan L16's (about $5000) set up as a 48V system.  Newbies usually destroy the first set the first year, then wise up about power management.   

I don't see anything that makes me want to switch from my lead-calcium "marine" batteries so far. 




starfire

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Re: Lithium batteries, the care and feeding of
« Reply #17 on: October 08, 2019, 03:43:03 AM »
OK, it seems this "top balancing" is a fudge. Research tells me that under high charging currents, the cell chemistry is unstable, changing cell resistance and therefore cell voltage. This settles down after charging, but then cell voltage drops,  rendering these dissipative cell balancers useless. By cutting back the completed charge rate, keeping the cell voltage high, then these BMS systems have time to work. But not at the 100mA these things used originally.
So, I have modded a  Chinese  balancer by using the dump FETs to switch the base current of four PNP power transistors. These bang a 1 ohm resistor across that cell..... Current drain of around 3 amps differential. Now, at the time these activate, the solar controller has detected its max battery voltage at 14.6 and has shut down, supplying only the voltage required to supply the low cells and the additional shunt currents from the balancing drains. This seems to be working well, although I'm still not convinced its required, all cells are very close without. But, it will do no harm I guess. The biggest change I have noticed with lithium is the much shorter charge time , a direct result of higher chemical efficiency..... Energy not wasted producing oxygen and hydrogen gas, as you get with lead acid.

starfire

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mike90045

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Re: Lithium batteries, the care and feeding of
« Reply #19 on: October 08, 2019, 05:44:23 AM »
http://nordkyndesign.com/protection-and-management-of-marine-lithium-battery-banks/
I found this useful.

That's a very good article. It covers many points that destroy batteries.

 I only wished it delved more into the situation that arises when the BMS or battery fails and begins to heat rapidly.  Having a flame resistant container for the battery, arranged so you can quickly extract the flaming box and put it somewhere where it's not going to cause more damage, is important.  I've seen the results of 2 burned shells of early Li banks and am glad I was not involved in their production.

BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #20 on: October 08, 2019, 06:37:37 AM »
Most interesting...  lead acid charge methods suck for Lithium. I stand corrected (and thank you); balancing during charging seems to be a poor choice.  It seems you likely could do manual balancing via cell shunt load only as needed, perhaps as the cells age.

The voltage drop after the full state of charge confuses me on charge management.  Since loads are variable, it's seems challenging to maintain the full state of charge.  What I could not find was a chart that shows the current if a fixed voltage is maintained indefinitely.

Most cheap commercial lithium battery gear uses fixed voltage charging, so I'm missing something tonight.


starfire

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Re: Lithium batteries, the care and feeding of
« Reply #21 on: October 09, 2019, 07:28:40 AM »
Yes, with only the terminal voltage to actually gauge battery SOC, it's not very accurate.  By manually boosting each cell with a separate supply, it's possible to get them all at the same level  and then let the balancer take over. It's fascinating to watch the indicator LEDs flashing as the dumps turn on and off.  I will perservere with the resistor ladder network and comparator method to monitor cell voltage during a charge cycle ...this just compares each cell to the fixed ratio of a resistor network.... I know, schematics would help... I'm working on it..  Another scheme would be a sigma delta detector, ....DvDt.  where the rate of voltage rise is the indicator for end of charge, and not the absolute voltage itself, this method was successful in the old fast charge NiCad days. I have noticed this effect also seems true for lithium.
It's fun learning this stuff, and Bruce, I don't envy your 120 volt system when it comes to battery management. With 600amp hours in each cell, it's very slow going to see any change occurring, and time consuming too. If my electronics gets any more complicated, it may be better to just have dpdt relays  between cells to force parallel connection of all cells occasionally?
I'm still wondering if I'm over thinking this.

starfire

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Re: Lithium batteries, the care and feeding of
« Reply #22 on: October 09, 2019, 07:43:45 AM »
Oh, and Bruce, the fixed constant voltage once batteries are full apparently is a no no..... something about the lithium actually electroplating the separator? There seems to be a very defined upper voltage plateau where charging must end completely. Now, this really can't happen with an off grid situation, the battery is also being discharged  concurrently. There really is no way of knowing if the current in is exceeding the current out at the milliwatt levels alluded to when full charge is reached.  I guess the easy way here is simply lower the terminal charging cutoff voltage  at some safe level and forego a few ah of capacity.

starfire

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Re: Lithium batteries, the care and feeding of
« Reply #23 on: October 09, 2019, 12:04:58 PM »
OK tried registrating myself on the image gallery to send it schematics, not happening.???? Im a total failure.
I used the  LEF2017 thing as a global password.... is that correct to do that.?
Anyhow it didnt work, if some kind soul can walk me through?
'

BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #24 on: October 09, 2019, 07:01:35 PM »
Lithium sure is an interesting battery to care for.  I enjoy a chance to learn something new and greatly appreciate your sharing your project, Starfire.

Your stated full cell voltages are on the low side, so I suspect that is the solution your lithium battery vendor has chosen.  Then you can have a fixed "full" voltage, low enough that it won't cause damage.  Some loss of capacity but simple and good battery life, so a good trade off for an off grid home power bank.  With a microcontroller and coulomb counting via load and charge shunts, it would be possible to regulate charge for higher capacity while having variable loads and variable charge (PV) rate, but it would be a heck of a project.  Fixed (lower) voltage charge regulation is a much more attractive solution given that it also extends lithium battery life.

For an off grid lithium bank, cell balancing might be best done at night (switched on by low PV voltage), by very modest shunt loads switched to slowly discharge high cells -  comparing to the average cell voltage (divided total) with some hysteresis to allow modest variation.  Readily doable in analog or via any small microcontroller.  Overvoltage monitoring on charging would then be for insurance only, with charge disconnect and latched warning piezo buzzer.  It should never happen unless there is a serious failure.

Alas, I can't help on the image posting, it always works for me.



 

starfire

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Re: Lithium batteries, the care and feeding of
« Reply #25 on: October 10, 2019, 12:28:27 AM »
schematics in the gallery

Car alternator cutout simply latches a relay and applies field current until Vsense reaches 14 volts.

cell monitor is a series of differential comparators detecting cell voltage variations compared to the fixed relationship from the left hand resistor network. Absolute voltages are ignored. Outputs ORed to give indication of an error. Additional 100 ohm resistors in that network give a few mV offset to prevent mindless chattering of outputs.

modded BMS is simply additional sinking capability added to an existing Chinese BMS balancing PCB, section with minimal count fan controller.

A bonus that someone may find useful, a diesel autostart control i recently designed for remote water pumping.... easily adapted to any diesel generator.
 The "float switch"  when grounded initiates a 10 second delay then cranks engine. Oil pressure rises activating fuel solenoid. Engine starts and runs until float switch opens, shutting down engine.
Oil pressure loss causes instant shutdown. After 10 seconds, engine will crank and  attempt a start, if still no oil pressure, engine latches in a permanent fault condition and cannot be started until reset.
Low fuel causes permanent shutdown until reset.
maximum cranking time 10 seconds, if no start, permanent shutdown.

BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #26 on: October 10, 2019, 01:42:11 AM »
Your cell monitor is interesting...always good to learn how someone else solves a problem.   12V does make cell monitoring much simpler than with a 350V system (Tesla). 









starfire

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Re: Lithium batteries, the care and feeding of
« Reply #27 on: October 10, 2019, 08:28:10 AM »
The real killer i found with lead acid is that final 20 percent or so charge, where the battery will only accept a fraction of the bulk charging current,without excess bubbling so this finishing charge takes many many  hours to complete.
 In real life, the bank is being continuously cycled, charging and discharging at the same time, the final top up never quite happens.... so the things will eventually sulphate regardless..... i know, pessimism right there.
.Forcing wont work, bubbles and fizzing is what sheds plate material. This i think is lithium's big advantage. over lead acid..... or at least they promise this.


BruceM

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Re: Lithium batteries, the care and feeding of
« Reply #28 on: October 10, 2019, 03:48:22 PM »
The lithium capability of operating happily without a full state of charge would be a marvelous thing.  I can hardly imagine it as my own PV/battery system was designed for lead acid's need for being fully charged, and when I think about batteries, I immediately think of how to keep them topped off to extend life!

For wet lead-acid, the true deep cycle batteries are a horror show of inefficiency. For good life they must be fully charged, but charge efficiency for the last 10% is near or below 50%, and they eat power like crazy even when full. The self discharge rate is terrible. The floor polisher battery (6V) I got recently for my welder sucks 2 amps continuously when topped off, and this is within the manufacturer's specs.  I was shocked.

A much better lead acid technology is the AGM batteries, which don't continue to eat power when full, and have high charge efficiency. They are what I designed for.  The cheap semi-deep cycle, lead-calcium batteries I'm using don't have performance as good as AGM, but vastly better than wet deep cycle lead acid in that the charging cycle current does  taper to nearly nothing (50ma).  Lightly loaded and charged at moderate currents, the Lead-calcium batteries are a decent value.  By using 120V instead of 12V, keeping actual battery loads to a few amps, and DOD down to 15% on average it's a different situation.  My batteries are fully charged every day long before noon, but I don't have to wait for that, I have enough excess PV power during the morning for big loads.  In fall, my batteries are nearly full but not in float by 10AM.  Direct DC use and propane refrigerator/freezer is a big plus, there is zero load at night when I turn off my lights and computer.  The inverter is only on when needed, though it only draws 15W when idling.  A house heating cycle draws 20 Watts for about 4 hours from my 12V AGM battery for the circ pump, daily in near zero F weather, every other day for most of the winter due to thermal mass and super insulation.  Yes, I am a fiend about low power design.

If/when lithium ever gets down in price to compete with my lead-calcium batteries for ongoing replacement cost, I'll have quite a project to design a new BMS for a 120V series string of cells.  It's good to learn that I can do cell balancing slowly at night, as I'd need nearly 40 cell regulators instead of your 4.

Thanks for sharing your pioneering project on lithium batteries! 
« Last Edit: October 10, 2019, 04:13:11 PM by BruceM »

mikenash

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Re: Lithium batteries, the care and feeding of
« Reply #29 on: October 10, 2019, 07:33:27 PM »
Fascinating stuff Bruce & Starfire

Much of what you postulate is completely out of the realm of possibility for me.  Where possible I like to keep the tech as simple as I can - that's why I have old Toyota Camrys and why my last diesel truck was a non-turbo, pre-common-rail and why my last four motorcycles all had carburettors.  If possible, I like to run things I can understand in operation

I still have a few years to retirement (if there is such a thing) and I hope you guys will have sorted out all the potential wrinkles in any decision-making I need to do at that time

As I have no need for a water pump, have solid-fuel heating, and a gas hob plus direct solar or wetback water heating (and, more importantly, as any women up at my shed are visitors rather than residents) my electricity needs are modest and I still lean towards a keep-it-simple-stupid approach to power generation:  12VDC + solar + AGM batteries (although I could be talked into 24 or 48VDC if the case for the benefits was strong enough to outweigh what i see as the advantage of being able to use cheap automotive stuff in the system . . .)

Keep it up.  I am being both entertained and educated here.  Thanks

On a semi-related note, current wisdom says the LIFEPOs will intantly evaporate into a mist of expensive, radioactive steam if charging current is applied at sub-zero temperatures.  I wonder how the Teslas et al do in Canada/Alaska?  I figure they are "charging" with regenerative braking while being driven?  Perhaps they are "smart" enough to have a work-around

Cheers