Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

creeky

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

zoneblue wrote: »

My point two was a not so subtle swipe at gensets in RE systems (not going to get into a discussion about who or what is greener). However the fact is that in order to best utelise any kind of lithium storage setup, a generator is required. Cut the storage down to the bare bones, cycle the snot out of the bank, and use the genset to cover the weather vagaries. With lead you cant cycle the snot, so you end up with the reserve, so therefore you can dump the genset more easily. Sad but true.

I'm sorry but I don't understand this. If I buy a LFP battery pack I get more storage due to deeper dod without shortening the lifespan of the battery pack. And if I charge with a genny I get more efficient charging as there is 1) a lower voltage needed to reach a fullish state of charge; 2) more efficient acceptance of said charge; and 3) no absorb time required.

I don't care where you blow your nose. If you get 0 watts on solar you run a genny. no one where I live can have a solar system with battery and not have a genny. unless of course they don't use any power for weeks on end and the battery pack doesn't self discharge. Oh wait.

Photowhit

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

creeky wrote: »

I don't care where you blow your nose. If you get 0 watts on solar you run a genny. no one where I live can have a solar system with battery and not have a genny. unless of course they don't use any power for weeks on end and the battery pack doesn't self discharge. Oh wait.

You may not be in a good spot for solar. Some places just don't match a solar profile very well.

I and others including the US government have done analysis of the energy payback time involved, I think thin film is actually the winner in this, and most poly and mono crystalline panels come in in about 2 and a half years ( I think Evergreen's string ribbon cells use the least energy and they run around 18 months as I recall, and I don't know if they are made any more) this is for on grid systems, and add another year for balance of system.

Off grid systems generally take 4x the amount of array to produce the same amount of usable energy (perhaps a bit less with smart charge controllers now) toss in the manufacture and recycling of batteries and you get to null pretty quick and off grid inverters and charge controller make up a larger part of the system...
...and if you are going to have a generator that adds a good bit to both negative sides of the equation...

I live off grid and don't want to fool myself, I understand that the only way my system is 'green' is in my minimal use of electric compared to others. I believe I started or contributed to a thread early on here. I know it was a big deal in other forums and I got to argue the opposite point on grid tied systems with "Sunking" or "Sungod". Who believed, and perhaps still does, that grid tied systems never produce as much electric as it took to manufacture them.

I'll try to find the government link to energy payback for you. I typically do provide links, sorry I don't have this handy.

zoneblue

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Storage is now the single most expensive part of off grid RE systems. It is also the shortest lived. Hence many of the cost benefit studies ive seem show the best results where the storage is minimised. Lithium has a high upfront cost, and cost benefit modelling biases expenses in the future in favour of expenses now, due to the high cost of capital.

Lithium we know is happiest at lower SOCs, compared to lead which likes the opposite. This is my point.

It makes no sense to overinvest in lithium, and keep it at high SOC for long periods. So to use it like it wants to be used AND minimise your investment, hence your pay back time, im suggesting the use of small packs and "enhanced" generating sources, of whatever kind you can lay your hands on.

Im working my way through a paper kinda about this and its quite interesting.

"Cost-minimized combinations of wind power, solar power and electrochemical storage, powering the grid up to 99.9% of the time"
http://www.sciencedirect.com/science/article/pii/S0378775312014759

creeky wrote: »

I'm sorry but I don't understand this. If I buy a LFP battery pack I get more storage due to deeper dod without shortening the lifespan of the battery pack. And if I charge with a genny I get more efficient charging as there is 1) a lower voltage needed to reach a fullish state of charge; 2) more efficient acceptance of said charge; and 3) no absorb time required.

Photowhit

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Here's a link to energy payback from the government, for grid tied systems.

PNjunction

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Blackcherry04 wrote: »

How often do your FLA batteries come out looking like this after a .02 volt overcharge ??

What is your source reference for that pic of the swollen Winston cell? They will NOT do that with a mere .02v overcharge, like other chemistries like LiCo02 will. Your source may be misreporting his findings, or leaving out some crucial details like PRIOR abuse.

However, the pic does serve to show what happens when

1) You ABUSIVELY charge them, say higher than 4.0v per cell, with no more than 3.6v being the norm.

2) Just as importantly, this is what happens when you discharge them below about 90% DOD, and then attempt to recharge them with full current. When below 80% DOD, one should apply no more than about .01C, until a terminal voltage of 3.2v is reached, and THEN you may apply a full current charge. Thus, don't go much beyond 80% DOD in the first place. This is a common mistake for first-timers. They will fully discharge the cell, and recharge at full current, not knowing the damage they have caused.

3) Leaving them fully discharged below 80% DOD for long periods of time, and then do #2 with full charge current. This is why when you dispose / recycle these cells and discharge them fully, be kind to the dumpster divers and mark them as BAD / NO RECHARGE.

4) Drawing more than about 3C current continuously - basically heat damage.

Note: Strapping and banding is NOT a cure to prevent abusive charge techniques.

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

PNjunction wrote: »

What is your source reference for that pic of the swollen Winston cell? They will NOT do that with a mere .02v overcharge, like other chemistries like LiCo02 will. Your source may be misreporting his findings, or leaving out some crucial details like PRIOR abuse.

However, the pic does serve to show what happens when

1) You ABUSIVELY charge them, say higher than 4.0v per cell, with no more than 3.6v being the norm.

2) Just as importantly, this is what happens when you discharge them below about 90% DOD, and then attempt to recharge them with full current. When below 80% DOD, one should apply no more than about .01C, until a terminal voltage of 3.2v is reached, and THEN you may apply a full current charge. Thus, don't go much beyond 80% DOD in the first place. This is a common mistake for first-timers. They will fully discharge the cell, and recharge at full current, not knowing the damage they have caused.

3) Leaving them fully discharged below 80% DOD for long periods of time, and then do #2 with full charge current. This is why when you dispose / recycle these cells and discharge them fully, be kind to the dumpster divers and mark them as BAD / NO RECHARGE.

4) Drawing more than about 3C current continuously - basically heat damage.

Note: Strapping and banding is NOT a cure to prevent abusive charge techniques.

It came from the same place as your Information, The Internet. Didn't you forget about mentioning temperature of the battery before attempting to charge ??

zoneblue

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Well BC4,put the link up, so anyone who wants to can learn more about it.

PNjunction

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Blackcherry04 wrote: »

It came from the same place as your Information, The Internet. Didn't you forget about mentioning temperature of the battery before attempting to charge ??

For charging, that would be about 0C / 32F. Discharge you can use in much lower temperatures. So they wouldn't be my first choice for sub-freezing temps, unless you control the environment, have them indoors, etc.

Here is the manual for the Winston batteries:

http://en.winston-battery.com/index.php/products/download-center

If using CALB cells, then the manuals that can be found on this page may be of interest:

http://store.evtv.me/proddetail.php?prod=ca100fi

The listings typically show 2000 cycles at 100% DOD. HOWEVER, if taken to that low of a level, and a full-current charge is applied, instead of a low-current until 3.2v is reached, then that cycle life is a LOT less! Usually accompanied by the swelling effect. So there's the catch. Instead, we have to be happy with 3000 cycles or so by only taking them down to 80% DOD, unless you have a way to control current at such low SOC's. Easier just to not go beyond 80% DOD in the first place and be able to apply full current to them.

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

zoneblue wrote: »

Well BC4,put the link up, so anyone who wants to can learn more about it.

If I had a link, I'd of posted it already. I thought it came from a boaters forum, but is was one where the photos don't show on the tread, just the link to the photo. I only found the photo because I was looking through the 1,000's of photos I have. I asked a question

How often do your FLA batteries come out looking like this after a .02 volt overcharge ??

Your answer to the question is ??

Cariboocoot

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

How often do your FLA batteries come out looking like this after a .02 volt overcharge ??

I can answer this one: never.
For one thing you wouldn't even measure 0.02 Volts difference.
And if you mean 0.2 Volts the answer is still never.
In fact it's also never if you mean 2.0 Volts.

Flooded cells are very tolerant of abuse. Any bulging is usually caused by positive plate oxidation due to excessively long Absorb times.

AGM's will not tolerate such over-Voltage, but they will vent not bulge. After that venting they are damaged and won't get any better.

GEL's can explode like bombs.

If you're not willing to admit to the disadvantages of a technology you're only fooling yourself. Perhaps expensively.

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

If I recall correctly, it was a boater that went to his boat in the winter and tried to charge LiFeP04 battery bank. That was why I asked about the temperature and it not being included in the charging discussion. It wouldn't be odd for someone to turn on a Inverter / Charger with a 125 amp output and to try to charge a battery bank below 32° F. I could easily see any of my customers doing it. A BMS might have caught it, probably not unless it had some kind of a temperature shutoff and / or current cutback.

Cariboocoot

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Blackcherry04 wrote: »

If I recall correctly, it was a boater that went to his boat in the winter and tried to charge LiFeP04 battery bank. That was why I asked about the temperature and it not being included in the charging discussion. It wouldn't be odd for someone to turn on a Inverter / Charger with a 125 amp output and to try to charge a battery bank below 32° F. I could easily see any of my customers doing it. A BMS might have caught it, probably not unless it had some kind of a temperature shutoff and / or current cutback.

Good point: some inverter-chargers start out at their maximum current then adjust downward. Lead-acid batteries don't care if they get a momentary hit of high Amps. Usually this is only a problem for small generators. What happens to a LiFePo under these circumstances? If it is this distortion pictured then that is bad; instant death.

The Outback, as BC04 implied, starts out at zero current and runs up. Better for any battery in fact.

zoneblue

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Every three years i pull a 7Ah AGM out of a friends APC UPS, that looks like that, it fact worse. Usually its so swollen its hard to get out. Those older APCs were known to float too high.

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

I have downloaded a few of the BMS manuals and they seem to use a scheme with a large contactor ( N/O ) that is engaged 24/7, 100% of the time to protect the bank if used with Solar. I'v tried to add up the tare losses without testing one on a bank and it's more than you'd think. Even if you tried using a latching contactor that wouldn't work because the BMS supplies current all the time to the cutoff and opens the contactor on fault. A N/C contactor would seem better and open it on a fault.

In Solar one would have to contemplate what would happen once a protection circuit is activated by the BMS as it requires a manual reset. If the Inverter and charge controller were both disconnected, then there would be no charging of the bank and no loads, but it would still have the BMS drawing power. If the LVD of the Inverter disconnected it would stay off until the voltage would rise to the programmed level with the charge controller connected ( will be fixed on some Inverters ). Depending on the loads on the Inverter this could trigger a runaway of on and off cycles as the voltage would rise and then be pulled back down, not good. If the Charge controller stayed connected to the bank, who knows what might happen, I guess it would depend on the fault. Things to think about on a system where someone is not there to monitor it 24/7.

There is another issue with any inverter on grid in pass through. Anytime the bank disconnected you'd lose the Inverter and any loads would be terminated. Not good if you have a refrigerator on it.

In a EV car it's controlled by the ignition key / switch, so they are only engaged when it's being used. Another issue is some of the BMS's need 12v input, that means that you have to have a axillary 12v battery to make them function with a 24v / 48v system. Yes, you could have a DC converter , but again, tare losses.

I am sure as markets mature there will be some BMS systems that will be tailored to different industries and their voltage requirements.

Mountain Don

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Blackcherry04, that is interesting info. In our case our off grid cabin is normally inaccessible by wheeled vehicle from mid December to sometime in mid March to the end of April, depending on snowfall and spring melting. We leave the FLA's sitting there all winter connected to the operating CC, since the first winter the system was up; 2009. Never had any issues. Every time we would snowshoe in the batteries were happily full and waiting for use.

The question of whether or not someone would trust leaving his FLA batteries alone for 4 mounths, or if they would rather have LifePO4's sitting there was previously asked by an ardent promoter. I had considered posting my personal preference back then, but didn't as I had no tech info other than personal experience with FLA. What you posted seems to reinforce my 'gut' feelings with the present technological state.

Cariboocoot

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Mountain Don; I leave my flooded cells over the Winter the same way: inverter disconnected, charge controller running. No problems. Our Cariboo Winters can be pretty harsh too; low sun, low temperatures.

The key to this is that they are charged to begin with; a charged battery won't freeze. The cold then helps by slowing the self-discharge rate. You could say they're in cryogenic storage.

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

I never thought much about a totally unattended system with LiFeP04. Since no float is necessary, although there is some self ( < 3% per mo ) discharge I guess you could rig up a VCS with a relay and a contactor that would connect the Charge Controller at some low voltage level and disconnect at a high point. Anything you add is a load, it would protect the bank, but one might be better off to disconnect everything and take the risk. Some of those Digital Panel meters will pull 3-4 watts, no clue what a VCS controller would pull.

karrak

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Blackcherry04 wrote: »

I have downloaded a few of the BMS manuals and they seem to use a scheme with a large contactor ( N/O ) that is engaged 24/7, 100% of the time to protect the bank if used with Solar. I'v tried to add up the tare losses without testing one on a bank and it's more than you'd think. Even if you tried using a latching contactor that wouldn't work because the BMS supplies current all the time to the cutoff and opens the contactor on fault. A N/C contactor would seem better and open it on a fault.

Could you provide the links to the manuals. I certainly wouldn't leave my LFP battery for any period of time with a BMS that would be capable of running the battery flat.

If I were to leave my LFP battery for a long period and there was no need to have loads like fridges etc turned on I would just disconnect the battery from everything for the period it is not used. With the low leakage of less than 3% per month and the fact that the battery is not being damaged by not being fully charged means LFP batteries can be left for several years like this.

In Solar one would have to contemplate what would happen once a protection circuit is activated by the BMS as it requires a manual reset. If the Inverter and charge controller were both disconnected, then there would be no charging of the bank and no loads, but it would still have the BMS drawing power. If the LVD of the Inverter disconnected it would stay off until the voltage would rise to the programmed level with the charge controller connected ( will be fixed on some Inverters ). Depending on the loads on the Inverter this could trigger a runaway of on and off cycles as the voltage would rise and then be pulled back down, not good. If the Charge controller stayed connected to the bank, who knows what might happen, I guess it would depend on the fault. Things to think about on a system where someone is not there to monitor it 24/7.

There is another issue with any inverter on grid in pass through. Anytime the bank disconnected you'd lose the Inverter and any loads would be terminated. Not good if you have a refrigerator on it.

Why should any fault require a manual reset? A LVD fault should automatically reset when the battery has some charge put into it. A HVD fault should reset when the high voltage fault disappears. I would think the same thing should apply to Lead Acid batteries to stop them be damaged. For off-grid use a HVD fault should either stop the charge controller charging the battery and/or disconnect the panels from the controller. For the LVD fault either shut down the inverter or disconnect the loads. Don't put a relay that disconnects the battery from the solar controller.

I am sure as markets mature there will be some BMS systems that will be tailored to different industries and their voltage requirements.

This is a problem that there may not be any plug and pray BMS systems that suit off-grid systems. There are however enough commercially available components to make up a BMS system that would be suitable for off-grid use.

Simon

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Here is one for you, I'll look up a couple more, can't you use google ??

https://store-0862a.mybigcommerce.com/template/files/HousePower%20BMS.pdf

karrak

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Mountain Don wrote: »

Blackcherry04, that is interesting info. In our case our off grid cabin is normally inaccessible by wheeled vehicle from mid December to sometime in mid March to the end of April, depending on snowfall and spring melting. We leave the FLA's sitting there all winter connected to the operating CC, since the first winter the system was up; 2009. Never had any issues. Every time we would snowshoe in the batteries were happily full and waiting for use.

What if you get a fault that stops your CC supplying charge to the battery from the solar panels? The CC is still drawing power from the battery and the battery is self discharging.

I would prefer to have the battery totally disconnected so that no external faults can damage it.

Simon

karrak

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Blackcherry04 wrote: »

Here is one for you, I'll look up a couple more, can't you use google ??

Thanks for the link, I know about this one, you might know of some that I don't. I am always interested in getting more information.

Simon

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

karrak wrote: »

Thanks for the link, I know about this one, you might know of some that I don't. I am always interested in getting more information.

Simon

Ok, you knew about that one, but you posted like you knew nothing about the manual reset's and the contactor scheme to disconnect the battery bank in you posts.

If BMS alerts are not acted upon and bank voltage continues to get worse, it will reach protection levels,
at which point BMS will drop the main contactor and isolate the bank from all circuits. To engage the
main contactor again, press Reset button. If bank voltage is still at protection levels, BMS will allow 60
seconds delay and will drop main contactor again. The delay is designed to give you opportunity to start
charging to get the bank voltage above protection levels.

You have 32 cells, how many balance boards do you have ?? How much power do they collectively draw from your bank ?? If you were disconnecting the battery what would you do about them and their long term power draw ??

karrak

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Blackcherry04 wrote: »

Ok, you knew about that one, but you posted like you knew nothing about the manual reset's and the contactor scheme to disconnect the battery bank in you posts.

I never got past looking at the basic specs for this one. I rejected it as it balances at a fixed 3.6 volts which is too high and none of the alarm/disconnect outputs are programmable.

You have 32 cells, how many balance boards do you have ?? How much power do they collectively draw from your bank ?? If you were disconnecting the battery what would you do about them and their long term power draw ??

I don't have any balance boards, if I did have I would have 8 and if I designed them or purchased off the shelf units I would make sure that their current consumption was low enough to not flatten the battery, would aim to have the current draw less than the battery leakage current. Both my battery monitoring system and the Cellog 8 I have as a backup can be easily unplugged from the battery if I needed to do so.

Simon

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

karrak wrote: »

I never got past looking at the basic specs for this one. I rejected it as it balances at a fixed 3.6 volts which is too high and none of the alarm/disconnect outputs are programmable.



I don't have any balance boards, if I did have I would have 8 and if I designed them or purchased off the shelf units I would make sure that their current consumption was low enough to not flatten the battery, would aim to have the current draw less than the battery leakage current. Both my battery monitoring system and the Cellog 8 I have as a backup can be easily unplugged from the battery if I needed to do so.

Simon

Seems like it's all about opinions in here. It's funny because 3.6v ( actually 3.55v ) is what Winston recommends for their Batteries, I guess you know more than they do.

Reed

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Karrak

The BMS on our Manzanita (CALB cells) absorbs to 3.6 V and floats at 3.4 V. Currently harvesting 4 to 5 kW-hrs from the flat mounted panels on our 5th wheel.

We spent 12 weeks traveling about Honduras and Guatemala this last winter and left our rig at son's place at 8000' in mountains of northern NM (just west of Las Vegas, NM) and it dropped to -25 F. Had neglected to drain black/grey water just before the real freeze started and the valves were frozen. The rig was left in the open so they battery bank was charged to float on those days where there was not a blanket of snow on the panels. The panels still got 200 or more W-hrs even on those days and the snow never lasted more than a week or two. This is not the same problem that one has in Canada or Alaska. I would think that if the panels were inclined at 45 degrees, then the snow would slough off.
Reed

karrak

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Blackcherry04 wrote: »

Seems like it's all about opinions in here. It's funny because 3.6v ( actually 3.55v ) is what Winston recommends for their Batteries, I guess you know more than they do.

This is part of the reply I received from Winston Batteries on this very subject

"3, You are absolutely correct that there is only a few percent SOC difference between charging the batteries into 3.65V and 3.425V at a charging current < 0.1C. Charging to only 3.425V can make a difference to their life span."

Simon

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

karrak wrote: »

This is part of the reply I received from Winston Batteries on this very subject

"3, You are absolutely correct that there is only a few percent SOC difference between charging the batteries into 3.65V and 3.425V at a charging current < 0.1C. Charging to only 3.425V can make a difference to their life span."

Simon

It's your business if your a 95% to 50% or 100% to 80%, no one cares. I see that same reply with different voltages, so do whatever makes you happy. What you don't seem to get is that 3.6V is a balancing voltage. That is not a voltage you have to use daily as a charging voltage.

karrak

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Reed wrote: »

The BMS on our Manzanita (CALB cells) absorbs to 3.6 V and floats at 3.4 V.

There has been quite allot of discussion on what voltage to charge LFP batteries up to at the low charge rates that we get from solar panels.

My take on this is that high voltage and high temperature will shorten the lifespan of LFP batteries so one should keep the charge voltage and the battery temperature as low as possible to maximise their life. There is only a few % difference in the amount of charge stored between taking the voltage from 3.45 to 3.6 volts so why not take the voltage to only 3.45 volts and maybe get more lifespan from the battery. For the same reason I have a float voltage of 3.3 volts which is close to the resting full voltage of the cells. Taking the cells to 3.6 volts can also aggravate any cell imbalance which is not a problem in your case as you have cell balancing hardware installed.

I don't know how much of a difference this will make but I can't see any downside to keeping the cells at a lower voltage except for loosing a few percent of storage.

Simon

Cariboocoot

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

karrak wrote: »

I don't know how much of a difference this will make but I can't see any downside to keeping the cells at a lower voltage except for loosing a few percent of storage.

Simon

The question would be how much this affects the operating range, i.e. actual usable power.

Lead-acid has the really irritating characteristic of not wanting to go below 50% SOC if you want to get long lifespan (and avoid ever increasing current per Watt as Voltage drops). So in essence only half the battery is 'available': 100 Amp hours = 50 Amp hours usable.

Now if the LiFePo is going to be 'kept' at, say, 90% SOC as a maximum (they don't suffer from the low SOC sulphation problem of lead-acid) just how low can they be taken and reliably recharged from without lifespan sacrifice? It would need to be 40% to make up the same range (40-90 = 50%). If they can go lower and still be brought back (it has been established there is less Voltage difference per SOC point than with lead-acid so the current increase per Watt is not an issue) then they have a capacity advantage.

At the other end of the spectrum is how low the Voltage will be at these lower SOC levels and how that will affect loads (especially non-inverter loads).

I think PNJunction's system avoiding the BMS is looking like a good idea as the monitoring seems to present more problems than it solves.

Blackcherry04

Re: Comparative specifications of LIFePO4 & Flooded Lead Acid batteries

Cariboocoot wrote: »

I think PNJunction's system avoiding the BMS is looking like a good idea as the monitoring seems to present more problems than it solves.

He can do that because he has a 20 amp hr 12v battery and a 40amp hr 12v battery that he posts about. If you have a 48V battery with 64 cells there no way to ever keep them balanced. His advise is to use a fluke meter and a single cell charger to keep them balanced, totally impossible to do with a Solar size bank. They probably do that over on the RC forums on a small battery pack.

Think of balancing like a EQ in FLA, your using it to balance cells, you do it with a over voltage, then you can do it with balancing boards as necessary. The board will start shunting the current as each cells voltage reaches the set point, while the others come up.