Is Li-ion now viable for off-grid?

[karrak]

- during charging stage bypass board would need to bypass at some point full charging current which could be in tens of amps in value. None managed to do so yet and for good reason: you need to come up with circuit which would be bypassing those tens of amps under say 3V voltage that for linear circuit results in 100-200W of heat per cell which needs to dissipate somewhere. Making switching circuit with high efficiency at that voltage/current combination could be a challenge as well. I saw reference made here about bypass board capable of mere 3A which is only 10% of the low charge current for that battery.

The following graph shows the balance of my friends LFP 4yo battery at the end of charge. This battery is only slightly out of balance (less than 1%). As you can see by the time the voltages have diverged by 0.100V the charge current is down to nearly zero. If this system had balancing boards that were set to balance at 3.50V it wouldn't be balancing until around the 60 minute mark when the charge current is nearly zero.

BalVolts&ChargeCurrent.jpg

- these boards are prone to electrical leaks discharging cells they're connected to and unbalancing battery as the leak current value is specific for each individual board. Preventing this would require more precise design (and probably use of ASIC to cut down on parts count)

I haven't seen any evidence to support your assertion. I only know of one case where a balancing board failed and dragged down a cell. This fault was detected by individual cell monitoring before any damage was done.

- I still don't think these batteries can be connected in parallel which prevents increasing total Ah. Connecting them in series limits you with 4 batteries system. I'm referring to BattleBorn batteries here.

The Battle Born manual says they can be connected in parallel, why don't you think the batteries can be connected in parallel?

- I don't see how this would help during discharge process where 1 weak cell inside one battery could be 'masked' by stronger cells in other batteries so monitoring entire series voltage won't prevent damage of that single cell. There's no overall BMS reading each individual cell voltage to set off alarms. Putting some switch in the current path would require pretty low resistance from the switch. Besides loads won't appreciate sudden disappearance of power without warning. Trying to gracefully decrease voltage by introducing some 'resistance' inside the battery to lower its output voltage presents similar challenges due to currents involved.

The BMS within the Battle Born battery will disconnect the battery from the outside world if any cell goes outside its safe operating voltage range. It will also disconnect the battery if you try to draw too much current, overdischarge it, or overcharge it. The low resistance switch can easily be made with suitable switching FETs.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[sensij]

There's another property of bottom balance I like- during discharge cells come to lowest SOC all in synch allowing graceful shutdown of the loads if so required with spare Ah to go. In top balanced system BMS is required to monitor each cell looking for the weakest to drive this process. Since there's no way to exceed Ah capacity of the weakest cell in the battery I don't even know why ppl bother with top balance: it provides no energy benefit and only complicates things.

PNjunction 's post here address this point... in his experience with a top balanced system, setting a LVD above the low voltage knee is sufficient to protect the weakest cell, and two levels of low voltage detection can allow for more graceful shutdown (warning and disconnect).

I haven't seen any counter-arguments presented to the points karrak makes for his reasons to prefer top balance in this post (that thread went off the rails).

I don't think anyone is saying top or bottom balance is *always* better, but consolidating the arguments made for and against each approach would probably be helpful.

[max2k]

There's another property of bottom balance I like- during discharge cells come to lowest SOC all in synch allowing graceful shutdown of the loads if so required with spare Ah to go. In top balanced system BMS is required to monitor each cell looking for the weakest to drive this process. Since there's no way to exceed Ah capacity of the weakest cell in the battery I don't even know why ppl bother with top balance: it provides no energy benefit and only complicates things.

Active balance seems allow squeezing those few additional Ah at the cost of much greater complexity and I'd just buy extra cells to avoid that for all but very niche applications where it matters.

[Sunking]

Another solution SK was suggesting here recently is bottom balance the cells and then monitor weakest cell during charge to switch to CV on time leaving rest undercharged. In general, a series of cells Ah capacity is limited by weakest cell anyway so you either have to catch this moment during discharge leaving stronger cells with some residual charge (top balance) or during charge leaving stronger cells undercharged (bottom balance). It seems to me the latter is safer, does not require bypass boards and more 'load friendly'.

You would be correct, but not for everyone. Just for consumers like Karrak who do not understand battery tech.

Balance Boards aka Vampire Boards are default in just about every BMS made. They are nothing but TROUBLE waiting to happen. Good for battery manufactures, not so good for consumers. Balance Boards or Shunt By-Pass are Passive methods by just switching a current shunt in parallel with a cell to By-Pass a set amount of Current when the cells reach 3.65 volts. They stay on until the Bleed the battery down to 3.55 to 3.6 volts. Sometimes they fail and just Bleed your battery to death. Active Balancing is rarely used, mainly in EV's, and commercial Lithium Ion Battery Banks like the Powerwall. In ACTIVE Balancing there is no Top Balancing or Bottom Balancing. It is Middle Balancing. The BMS takes energy from higher SOC cells and gives them to lower SOC cells. Commercial systems also do not allow the customer to ever fully charge or discharge the bank. Point here is anything you as a consumer can buy is going to BE VAMPIRE BOARDS either External to the battery like Vampire Boards and a Control Unit, or built into a battery block like a 12 volt starter battery.

First problem with BMS systems, especially those that can be used for Solar are not coordinated with the Charger and cannot communicate. Bypass boards have a fixed amount of current they can bypass. Usually very low on the order of 150 ma up to 1 amp. Anything over that amount still goes through the fully charged battery. To be effective and limit overcharge, the charger needs to communicate so that when the first cell reaches 100%, it needs to limit charge current to the Vampire Board capacity. So if the first Vampire Board triggers with 10 amps of charge current flowing is only going to bypass 150 ma to 1 amp leaving 9 amps or more flowing through a fully charged battery. Good for manufactures, not consumers. .

Second issue is it requires you to charge to 100% SOC which is the last thing you want to do as it drastically cuts cycle life. Again good for the manufactures, not the consumer.

So OK you Top Balance your battery. So what do you know? The only thing you know is every cell is 100% SOC and that is it. It does not tell you what the capacity is. As the cells discharge, the cell voltages will become unequal. The further you discharge, the more unequal the cell voltages, and you do not know anything about capacity until the first cells reaches 2.5 volts and that happens real quick once you reach 3 volts. At 2.5 volts, you have 0 AH capacity left in one cell and all others stil have some charge left in them to destroy the empty cell. Good for manufactures, not consumers.

Bottom Balance mimics what commercial systems do. Run in the Middle with a twist. With Bottom Balance you know what the SOC is at any point because we start with a Reference point of ZERO. What is ZERO? 2.3 to 2.5 volts = 0 AH Capacity. No we have all cells of equal SOC and known capacity. We know where we are at. Now I can forget about Cell Voltages and can focus on pack voltages. If I have 4S I just set LVD to 12 volts or 3 volts per cell which is comfortable well above 2.0 Volt DEATH. I charge to 13.6 to 13.8 volts and I see 90 AH went in. I am good to 12 volts, get nervous at 12.4 volts and recharge.

Unfortunately commercial producers cannot make a Bottom Balance Circuit as cheaply and easily as they can Vampire Boards to Bleed cells. Active Balancing is complex and expensive. So consumers like Karrak have no choice but to play the manufactures games by the manufacture rules. They Top Balance which means or = By-Pass Bleeder Vampire Boards. Nearly every IC made for Lithium cells BLEED them. Active requires expensive caps and chokes plus FET's. To run Bottom Balance, you need to know what you are doing. If you do know how, then your batteries are going to perform better, last longer, and eliminates Vampire Board damage and over discharge risk which makes manufactures and Karrak very UNHAPPY and consumers Happy Happy.

At the end of the day when it applies to solar, a BMS is not going to be used. Traditional charging methods do not apply to solar. Solar cannot perform a full 3-Stage Charge on Lead Acid batteries, and last thing you want to do with Lithium fully charge the batteries off and turn off the charger with half a day of sun left to be used instead of batteries. The solution is the exact same for either Lead Acid or Lithium.

You turn your Charge Controller into a simple CC/CV charger. For lithium you set Bulk/Absorb/Float to 13.6 to 13.8 volts. The batteries will saturate around 90% SOC voltage during the day, and float while the panels provide power for your gizmos until dark comes saving your battery for night. FLA do the exact same thing except the voltage is higher up around 14.4 to 15 volts and prey they get to 100% before sun down and save the batteries for night. No BMS required.

Pretty simple to grasp.

[max2k]

I think I went off a little in my statements above:
- in the perfectly top balanced battery (cells top balanced manually before being assembled to a battery) the passive bypass board will be subject to much lesser currents at the end of charge. OTOH regardless of its topology it has no choice but to dissipate that cell voltage x bypass current power in form of heat, there's no easy way to make them 'efficient'. The charging energy has to go somewhere so it either can go into cell or heat up the environment, there's no way around this. One could potentially come up with some way to use this energy but that would further complicate already complex system.

I'm not sure how well top balanced battery remains top balanced over time as all unbalances would end up either hitting weaker cells or those bypass boards during charge process.

Another solution SK was suggesting here recently is bottom balance the cells and then monitor weakest cell during charge to switch to CV on time leaving rest undercharged. In general, a series of cells Ah capacity is limited by weakest cell anyway so you either have to catch this moment during discharge leaving stronger cells with some residual charge (top balance) or during charge leaving stronger cells undercharged (bottom balance). It seems to me the latter is safer, does not require bypass boards and more 'load friendly'.

[max2k]

An LFP battery that is top balanced, and has automatic passive cell balancers (the dreaded vampire boards) acts in exactly the same manner as a flooded LA battery and doesn't need to know the state of the other cells to remain balanced.

The automatic passive cell balancers in the LFP battery do the same job as an LA battery outgassing. In an LFP battery the cell balancers will start bypassing current when the voltage gets to their operating voltage of around 3.55V/cell (14.2V for a 12V battery). The LA battery will start bypassing current by converting water in the cell to hydrogen and oxygen when the cell voltage gets to around ~2.45V/cell.

If you hold a 12V LFP battery with automatic passive cell balancing that balance at 3.55V/cell at a charge voltage of 14.2V any cells above 3.55V/cell will bypass charge so that any cells that are below 3.55V will continue to be charged until they reach 3.55V. When the battery is balanced all cells will be at 3.55 volts, the overall battery voltage will be 14.2V and the charge current going into the battery will be zero.

As with LA batteries in series it is important to first fully charge and balance the individual batteries before you put them in series.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

I understand what you're saying and I'd agree theoretically about application of bypass boards for top balancing cells connected in series but I think you're over- simplifying what is going on:

- during charging stage bypass board would need to bypass at some point full charging current which could be in tens of amps in value. None managed to do so yet and for good reason: you need to come up with circuit which would be bypassing those tens of amps under say 3V voltage that for linear circuit results in 100-200W of heat per cell which needs to dissipate somewhere. Making switching circuit with high efficiency at that voltage/current combination could be a challenge as well. I saw reference made here about bypass board capable of mere 3A which is only 10% of the low charge current for that battery.

- these boards are prone to electrical leaks discharging cells they're connected to and unbalancing battery as the leak current value is specific for each individual board. Preventing this would require more precise design (and probably use of ASIC to cut down on parts count)

- I still don't think these batteries can be connected in parallel which prevents increasing total Ah. Connecting them in series limits you with 4 batteries system. I'm referring to BattleBorn batteries here.

- I don't see how this would help during discharge process where 1 weak cell inside one battery could be 'masked' by stronger cells in other batteries so monitoring entire series voltage won't prevent damage of that single cell. There's no overall BMS reading each individual cell voltage to set off alarms. Putting some switch in the current path would require pretty low resistance from the switch. Besides loads won't appreciate sudden disappearance of power without warning. Trying to gracefully decrease voltage by introducing some 'resistance' inside the battery to lower its output voltage presents similar challenges due to currents involved.

I think LFP is the best battery technology out there for off grid RV/residential application today but those BattleBorn batteries just don't cut it- may be they can be used individually but then again lead acid fits that niche just fine. Making your own battery bank out of raw cells will definitely work, I have no doubts.

[jflorey2]

He thinks if the balancing is taken care of that 12V battery blocks should be able to be stacked in series indefinitely.

I'll let him answer as to what he thinks, but I didn't get that at all from his posts.

[adoublee]

He's not worried about that. He's worried about balance between the four 12V battery blocks.

He thinks if the balancing is taken care of that 12V battery blocks should be able to be stacked in series indefinitely. Not so.

[karrak]

correct- each 12V battery includes 4 individual cells. When such battery is connected in series with another one and they both get charged/discharged from the ends of the string cells can go out of balance between batteries as their internal electronics has no clue what is happening inside another one.

An LFP battery that is top balanced, and has automatic passive cell balancers (the dreaded vampire boards) acts in exactly the same manner as a flooded LA battery and doesn't need to know the state of the other cells to remain balanced.

The automatic passive cell balancers in the LFP battery do the same job as an LA battery outgassing. In an LFP battery the cell balancers will start bypassing current when the voltage gets to their operating voltage of around 3.55V/cell (14.2V for a 12V battery). The LA battery will start bypassing current by converting water in the cell to hydrogen and oxygen when the cell voltage gets to around ~2.45V/cell.

If you hold a 12V LFP battery with automatic passive cell balancing that balance at 3.55V/cell at a charge voltage of 14.2V any cells above 3.55V/cell will bypass charge so that any cells that are below 3.55V will continue to be charged until they reach 3.55V. When the battery is balanced all cells will be at 3.55 volts, the overall battery voltage will be 14.2V and the charge current going into the battery will be zero.

As with LA batteries in series it is important to first fully charge and balance the individual batteries before you put them in series.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[max2k]

He's not worried about that. He's worried about balance between the four 12V battery blocks.

correct- each 12V battery includes 4 individual cells. When such battery is connected in series with another one and they both get charged/discharged from the ends of the string cells can go out of balance between batteries as their internal electronics has no clue what is happening inside another one.

[jflorey2]

So are you understanding that a PV module's maximum voltage limit is only subject to the voltage of the module itself and not that of a series-connected string of modules?

He's not worried about that. He's worried about balance between the four 12V battery blocks.

[adoublee]

if i understand their idea correctly all their electronics is inside those batteries. As such:
- all it can 'see' is its own 12V so in the perfect world I could stack them up in series to infinity.
- it can't possibly 'see' cells in other batteries connected in series and can only 'guess' what is going on in the ones connected in parallel.

So are you understanding that a PV module's maximum voltage limit is only subject to the voltage of the module itself and not that of a series-connected string of modules?

[max2k]

Their electronics are now a higher voltage so they can support 4 in series for 48V systems (they are updating their datasheets). Virtually no limit to parallel numbers due to their cell balancing.

if i understand their idea correctly all their electronics is inside those batteries. As such:
- all it can 'see' is its own 12V so in the perfect world I could stack them up in series to infinity.
- it can't possibly 'see' cells in other batteries connected in series and can only 'guess' what is going on in the ones connected in parallel.

if they want to make them useful for bigger system they would need to come up with some 'data link' and overall BMS- similar layout as SolarEdge came up with for panels/inverters: each battery has certain small absolutely necessary circuit and then some data protocol going over to main BMS. If they can manage that over the same 2 DC wires it would be great and installers would be thrilled. And the cost of course- I can't justify paying x2 for questionable benefit.

[Amy@altE]

that 12V battery costs x2 over raw cells currently available from multiple suppliers. I see systems here in 10kWh range 24-48V which calls for 200Ah @48V or 400Ah @24V Not sure those BattleBorn batteries will tolerate 4S2P setup or alike..Interesting product but I'm having problem to come up with application- way too expensive to replace SLA lead acid and not clearly intended for off grid case.

Their electronics are now a higher voltage so they can support 4 in series for 48V systems (they are updating their datasheets). Virtually no limit to parallel numbers due to their cell balancing.

Due to the high discharge capability, you can do a smaller battery bank than lead acid, and the high cycle count gives you a longer life. So over the life of the system, it can be more cost effective than lead.

Weight also is a big plus of these, so RVs, boats, and tiny houses will benefit greatly.

[Amy@altE]

Have you seen a product that a "drop-in" battery like this can drop into to get heat proportional to the need to keep above no-charging temperature (25degF listed in this case). Something that heats from available battery/charge controller voltage?

They are actually testing some options right now, such as a warming blanket.