Good thesis on preserving the life of LiFePo4 lithium batteries

[john61ct]

The above discussions on developing hyper-accurate SoC metering that works without operator supervision, or even the ability to calibrate one SoC meter off another,

have little to do with day to day operations of normal "mere mortal" systems.

In other words theoretical more than actually needed for most owners.

[CatNewBee]

Originally Posted by rgleason

So the BMS should have cell temp? Or are you doing this manually every month? I assume not from your previous posts.

The one I use has it all, coulomb counter, shunt to measure current and adjust the measured cell voltage to the current load, active balancing, calculation of cell resistance, two external temp sensors to recognize thermal issues (one sensor between a pair of cells), also a temp sensor in the BMS and all set points configurable, 2 pairs of galvanic isolated outputs - 2 NO/NC relays, 2 opto-coupler, on request they can have individual set points by custom programming, 2 interfaces, one serial for programming or connecting the optional touch display and one CAN-BUS.

[rgleason]

There are strong opinions and differences in what type of system is appropriate for LiFePo4 Batteries:


No-BMS

Johnct: "...developing hyper-accurate SoC metering that works without operator supervision, or even the ability to calibrate one SoC meter off another, have little to do with day to day operations of normal "mere mortal" systems."

• Stop Charging 3.45vpc 13.80v
• Stop Discharging 2.975vpc 11.90v
• Difference 0.475vpc 1.9v

Full BMS

CatNewBee: "The one I use has it all, coulomb counter, shunt to measure current and adjust the measured cell voltage to the current load, active balancing, calculation of cell resistance, two external temp sensors to recognize thermal issues (one sensor between a pair of cells), also a temp sensor in the BMS and all set points configurable, 2 pairs of galvanic isolated outputs - 2 NO/NC relays, 2 opto-coupler, on request they can have individual set points by custom programming, 2 interfaces, one serial for programming or connecting the optional touch display and one CAN-BUS."

• Stop Charging 3.55vpc 14.2v
• Stop Discharging 2.90vpc 11.60v
• Difference 0.65vpc 2.6v

I wonder if the different approaches are related to the charge regimen selected, since the concern is what happens if you happen to get weaker cells that don't charge quickly enough, or discharge too rapidly?

CatNewBee cycles his batteries over a wider range, gaining usable ah, starting higher, at 0.3v below the Victron preset, but stopping discharge shorter than Johnct, by 0.3v, showing confidence in the BMS active balancing system.

Johnct has a simpler system which utilizes a range 73% less than CatNewBee in order to avoid problems with cell balance and to prolong cycling life, according to his judgement. His Stop Charge point is 0.4v lower than CatNewBee and his Stop Discharge point is 0.3v higher.

The critical questions that need to be answered are:

• Is the cost of the BMS system worthwhile?
• Could that added BMS cost be used to purchase additional battery capacity to be used in a more conservative way to avoid damaging weaker cells?
• Does the passive non-BMS approach work over the longer term? IE: Will weaker cells eventually fail? What routine is followed to check the condition of all cells and to reestablish balance? How long does that balance last? How difficult is this to do for a "mere mortal"?
• Does the passive non-BMS approach actually result in greater cycles and battery life?

I hope that some of the posts following will try to address these questions which are admittedly hard to quantify, so that we can refine our understanding of this new chemistry and resolve some of the differences in use, because I believe it confuses the market and potential DIYers. Perhaps there are two viable approaches to the the use of this chemistry.

There is no need to fight about these approaches, they are simply different and very useful experiments which we need to learn from. I hope we can have a good discussion about the pros and cons, by experienced users.

Thank you all for contributing!

[rgleason]

Also I wanted to clarify my understanding of the terminology used for various states of charge in connection with LiFePo4, along with a consolidation of the ranges in common use.

• Initial Charge ------------------ 4.00vpc (16.00v)
• Manuf. Maximum Voltage------ 3.65vpc (14.60v) cell destruction
• HVD High Voltage Disconnect-- 3.50vpc (14.00v) (varies by system)
• High Voltage Alarm (User set)--
• Stop Charging (Victron)---------3.625vpc (14.50v)
• Stop Charging (range used)---- 3.65-3.45vpc (14.50-13.80v)
• Float & Storage (Victron)------- 3.375vpc (13.50v) 95% SoC
• House Load Float (Victron)----- 3.35vpc (13.40v)
• Resting, Charged, normal state 3.20vpc (12.80v) Johnct
• Floating, Storage long term---- 3.20vpc (12.80v) Johnct
• Low Voltage Alarm (User set)--
• Stop Discharging----------------3.20-2.90vpc (11.00-12.80v)
• LVD Low Voltage Disconnect--- 2.90vpc (11.90v)
• Minimum Voltage---------------2.80vpc (11.20v) cell destruction

The exact figures are likely to change with battery manufacturer.

I believe Victron, Mainesail, Catnewbee, Johnct & Travelerw use Winston.

[CatNewBee]

Hi rgleason, I guess, time will tell. My battery survived the first season pretty well heavily used and abused.

I am pro BMS, and especially pro a high sophisticated one in MY use case. I have the largest available cells on board and they are also the most expensives (4 x 1000Ah).

Well, prices per Ah are almost linear, but I do not want to replace a cell for 1200€ due to bad luck, reluctance or man made faults.

So the price for the BMS is for this size of battery appropriate and justified. I would not spent the money for this BMS in a 100Ah battery, but then may spent 80..100€ for a much simpler passive pre-set system.

Anyway. If you look at the cell curve you will see the shoulders we all are talking about. Without permanent automatic monitoring chances are very high, that one cells runs off unnoticed after a while, while the other cells have a safe voltage.

At a "normal" SOC somewhere in the middle, all cells have a similar voltage, you won't notice the differences, even if you measure them manually. The problem is either near end of charging or end of discharging, where you may miss the imbalances and they built up over time. You may think, you have still 20% left, but one cell can already be below safe voltage while the other are ok, while charging you think you stop at 80% or whatever, but in fact one cell is above the 100% at this time. This small violations add up and destroy some capacity, so on next cycle this cell will drift even faster. It is irreversible damage. Can you be sure to watch the cells individually on every charge cycle and on every use?

If you really not like the BMS / balancing go for a much cheaper drop in battery pack without balancers and do not use it on the edge of full / empty it may be good for 1000 cycles so - still better than AGM.

We can discuss the set points, if they are appropriate or not.

I do believe sailors and campers are out there to enjoy the ride and not to monitor battery cells. Any device that can take this away from the user makes life easier and is worth it. I mean - you can manually measure current and voltage of your pack and save the money for the battery meter gauge - would you do that? How about your tank gauge? A dip stick will do the trick too. We do many things for comfort, safety and peace of mind - the BMS is definitely one of those Items when it comes to LFP.

John is very cautious about longevity of his LFP and also very passionate - I do believe him to watch his cells all day long and keep them safe and sound. But he is not an average sailor and he is not using the battery for heavy loads. So it may work for him.

In contrast - we enjoy an electric galley with all bells and whistles - and the Admiral rules there. I am simply reluctant to measure the cells all the time she is doing magic in the galley. I do prefer to do other things at this time. I am also not willing to mess around with charging sources all day long - I prefer sailing or diving or swimming or enjoying a sun-downer knowing my battery is just fine without my intervention.

[CatNewBee]

Originally Posted by rgleason

Also I wanted to clarify my understanding of the terminology used for various states of charge in connection with LiFePo4, along with a consolidation of the ranges in common use.

• Initial Charge ------------------ 4.00vpc (16.00v)
• Manuf. Maximum Voltage------ 3.65vpc (14.60v) cell destruction
• HVD High Voltage Disconnect-- 3.50vpc (14.00v) (varies by system)
• High Voltage Alarm (User set)--
• Stop Charging (Victron)---------3.625vpc (14.50v)
• Stop Charging (range used)---- 3.65-3.45vpc (14.50-13.80v)
• Float & Storage (Victron)------- 3.375vpc (13.50v) 95% SoC
• House Load Float (Victron)----- 3.35vpc (13.40v)
• Resting, Charged, normal state 3.20vpc (12.80v) Johnct
• Floating, Storage long term---- 3.20vpc (12.80v) Johnct
• Low Voltage Alarm (User set)--
• Stop Discharging----------------3.20-2.90vpc (11.00-12.80v)
• LVD Low Voltage Disconnect--- 2.90vpc (11.90v)
• Minimum Voltage---------------2.80vpc (11.20v) cell destruction

The exact figures are likely to change with battery manufacturer.

I believe Victron, Mainesail, Catnewbee, Johnct & Travelerw use Winston.

Well this values vary. There are BMS manufacturer with other settings.

Regarding Winston, 3.65V is the End of Charge recommendation, cells die above 4.0V - this is the maximum allowed voltage. But charging above 3.65V is not recommended and does not add up much to the capacity. Some pre-set balancing modules fire up the OVP signal at 3.75V to allow charger and slightly out of balance cells to finish charging at 3.65V and also balance out inequalities starting balancing at 3.55V.

Consensus between us in the forum is, we can stop charging earlier with losing some 5..15% and no other negative side effects just for the peace of mind and for a longer battery life, but then the balancing must start earlier to have a chance to keep the cells close to each other. If you not adjust the balancing start voltage you would accept larger differences and risk OVP occurrences, because the balancer will kick in very late.

Exampe: Stop charging at 14.2V (3.55V) , start balancing 3.55V, OVP 3.65V
If all cells are in balance, each cell will be at 90%SOC and 3.55V when the charger stops or switches to float, all good.

Imbalance of one cell at 100% SOC 3.65V, the other cells will have 3.51V at 80% SOC and the charger still charges (14.18V), but the imbalance leads to disconnect of the bank - thanks to the BMS. The balancer usually divert a very small amount of energy - around 1...1.5A for balancing, if your charger delivers 30A - this are only 3% of the current - so it takes a long time to balance out the cells ( it means 3 Cells at 80%SOC are charged at 30A and the one cell is charged at 29A because of the active balancer) . The shorter the time is, the more cycles it will take. At some point the cell simply cannot be balanced sufficiently and will always have a higher voltage than the others and will slowly degrade more than the other 3, because it is notoriously charged to 100%, while the others remain under 80%. Without a BMS this will happen even faster, because there is no balancing at all.

Empty cells - as shipped by Winston - are around 3.3V or 13.2V. Initializing them to something like 3.8...4.0V takes a lot of time. 12.8V is already quite low (almost empty), but still safe.

[rgleason]

I am not like Johnct (and I am uncertain how he uses his LiFePo4), and am in this category "...knowing my battery is just fine without my intervention". I am willing to pay for a BMS in a single box with wires and install it, but not more than necessary. For a 250-300ah LiFePo4 what features would be appropriate from the REC ABMS? Are there any features that could be dropped? Do you have a less expensive alternative in mind?

Features I think would need from your list:
......one encapsulated unit rather than modules attached to terminals.
..... cell voltage monitoring
..... current monitoring, SOC, SOH monitoring
..... cell temperature and BMS temperature monitoring
..... cell balancing
..... signals for HVD/OVP and LVD/LVP, 2 independent signals
......programmable - configurable set points.
......display of all relevant information
......reset and self recovery

So it looks like the REC ABMS would be the one for $500+! I was looking at the Orion Jr (not active balancing) which seemed less expensive or possibly the 123smart BMS (this has separate boards on the terminals!)

I would also need two or three BlueSeas 7610 relays I think. I would like to shorten the cable runs by eliminating the 6' runs up and back to the 3-way switch, and use control wires to the relays in the battery location. do you think that would work for:

• house charging bus
• house load bus
• start battery disconnect

Catnewbee - REC active ABMS

- programmable BMS that provides
..... static signals for OVP / LVP
..... aux signals that can be used at different set-points
..... cell voltage monitoring
..... current monitoring, SOC, SOH monitoring
..... cell temperature and BMS temperature monitoring
..... cell balancing
- Logic that translates signals to impulses
- bi-stable solenoids for high current switching
- logic that copes with legacy sources (BMV712)

I chose the REC product for this reasons:
- active balancing
- completely configurable set points
- on request 2 independent sets of signals for OVP /LVP
- self-resetting error conditions, self recovery from failures
- one single encapsulated unit, no cell modules
- takes current and cell resistance into account
- galvanically isolated interfaces
- display of all relevant information SOC, SOH, time remaining, pack voltage, current, Ah drawn, cell voltages, cell resistance, cell temp, BMS temp, max/min cell voltage indicating whitch cells will be balanced, cell cycles...

Catnewbee wrote:

Now to the BMS - the main meaning of this is, as stated before, to balance the cells, so no one runs off in contrast to the others to ensure all (legacy) charger and protection logic based on battery pack voltage thresholds works properly.

Second The BMS fires two independent signals (that can be joined for some purposes if you do not have separate charge / discharge buses - e.g when paralleling independent batteries with own BMS'es) - this are LVP (low voltage protection) and OVP (over voltage protection), some have different names for them, but all have the same meaning. If any one of the cells goes out of range, the according signal will be set. You then CAN and SHOULD disconnect either the chargers or the loads. This can be done grecefull by a signal to turn off the device or by brute force by disconnecting with a power solenoid. This is considered a last resort of protection - like a fuse that blows. This is not meant to control the charging sources / loads in regular operations, the settings of them must be lower than the OVP settings. IF you set OVP to 3.65V (14.6V Battery pack voltage on balanced cells), you should program your chargers to lets say 14.4V maximum, so OVP does not occur as long as the cells are almost in balance. I guess, you got the point.

[CatNewBee]

I know about a system, that is not configurable (factory pre-set for Winston cells) not using cell modules, but having 2 outputs for OVP and LVP and having 3.5A balancing current - what is quite good. It is not encapsulated, so you need to provide an enclosure, but it has a display built-in for the cell voltages.
It has a serial interface on the board, but it is not meant for tweaking by the customer.

Litrade BMS
It is around 200€, comparable cheap and can handle 4 to 8 cells (12V / 24V systems).

The Thresholds are for my taste a little too excessive:

Start Balancing: 3,4V
OVP: 3,85V
UVP: 2,7V delayed for high loads
Acceptable difference: 0.02V (no balancing for small imbalances)
Balancing Current 3.5A (up to 15W per cell), there is a temp sensor on the board and a contact for a fan to cool the board if necessary.

The 123BMS is almost double the price (379€) and uses daisy-chained cell modules.
123 Smart BMS User Manual, configurable by jumper settings / App.

Also the ECS system uses pre-set cell modules.

The REC BMS is modular too, the pre-configured BMS is not 500€. It is 205,00€ including one temp Sensor. You may need the cable set too for 25€ and let them pre-set your desired values for the thresholds and for your shunt, the second temp sensor is optional and adds 6.50€. Shipping (25€) and potentially taxes on top - you may end up with 270...300€

The touch display is sold separately - if you want it - for 130€ - not necessary, but nice
The PC- Software plus Adapter cable (USB/Serial) adds 55€ not necessary, but empowers you to tweak all settings.

The complete system with software and shipping and without the display and shunt was around 320€, with display 440€, but ask them for a current price list.

[Delfin]

Originally Posted by rgleason

There are strong opinions and differences in what type of system is appropriate for LiFePo4 Batteries:


No-BMS

Johnct: "...developing hyper-accurate SoC metering that works without operator supervision, or even the ability to calibrate one SoC meter off another, have little to do with day to day operations of normal "mere mortal" systems."

• Stop Charging 3.45vpc 13.80v
• Stop Discharging 2.975vpc 11.90v
• Difference 0.475vpc 1.9v

Full BMS

CatNewBee: "The one I use has it all, coulomb counter, shunt to measure current and adjust the measured cell voltage to the current load, active balancing, calculation of cell resistance, two external temp sensors to recognize thermal issues (one sensor between a pair of cells), also a temp sensor in the BMS and all set points configurable, 2 pairs of galvanic isolated outputs - 2 NO/NC relays, 2 opto-coupler, on request they can have individual set points by custom programming, 2 interfaces, one serial for programming or connecting the optional touch display and one CAN-BUS."

• Stop Charging 3.55vpc 14.2v
• Stop Discharging 2.90vpc 11.60v
• Difference 0.65vpc 2.6v

I wonder if the different approaches are related to the charge regimen selected, since the concern is what happens if you happen to get weaker cells that don't charge quickly enough, or discharge too rapidly?

CatNewBee cycles his batteries over a wider range, gaining usable ah, starting higher, at 0.3v below the Victron preset, but stopping discharge shorter than Johnct, by 0.3v, showing confidence in the BMS active balancing system.

Johnct has a simpler system which utilizes a range 73% less than CatNewBee in order to avoid problems with cell balance and to prolong cycling life, according to his judgement. His Stop Charge point is 0.4v lower than CatNewBee and his Stop Discharge point is 0.3v higher.

The critical questions that need to be answered are:

• Is the cost of the BMS system worthwhile?
• Could that added BMS cost be used to purchase additional battery capacity to be used in a more conservative way to avoid damaging weaker cells?
• Does the passive non-BMS approach work over the longer term? IE: Will weaker cells eventually fail? What routine is followed to check the condition of all cells and to reestablish balance? How long does that balance last? How difficult is this to do for a "mere mortal"?
• Does the passive non-BMS approach actually result in greater cycles and battery life?

I hope that some of the posts following will try to address these questions which are admittedly hard to quantify, so that we can refine our understanding of this new chemistry and resolve some of the differences in use, because I believe it confuses the market and potential DIYers. Perhaps there are two viable approaches to the the use of this chemistry.

There is no need to fight about these approaches, they are simply different and very useful experiments which we need to learn from. I hope we can have a good discussion about the pros and cons, by experienced users.

Thank you all for contributing!

I asked a similar question above regarding what seemed to be a divergence of opinion on whether balancing was necessary or desirable as a routine matter or not, but I don't think I got an answer from Nebster or CMS, to whom the question was directed. I don't pay much attention to people who have strong opinions but no actual experience with the technology since there are many non-poseurs (CMS, Nebster, Traveller, CatNewBe to name a few) to learn from.

In my mind, the question of yes or no on a BMS is a separate one from yes or no on periodic balancing. I see my BMS as a safety feature that I hope I never use, so I am of the no balance contingent, based on CMS's empirical testing but would like to learn from those who have reasons to take a different approach. But I can give you an example of the importance of the BMS as a safety feature that just happened to me that illustrates the point.

Delfin is out of the water having her bottom sand blasted and re-epoxied after 18 years of the same coatings and a whole lot of build up of non-ablative paint. She is inside a large shed, completely tented for the process and I had left her with the BMS off as well as the starter bank since no current was required for anything. The yard foreman wanted to check the inside of the hull before doing some spot welding, so turned on the BMS for ER lights. So far, so good. However, he also thought it would be good to hook her up to shore power. The Trace 4000 charger came on, with settings appropriate for an LA bank - 28.8 bulk, 2 hour absorption, 26.8 float. When I came in the next morning, the bank was at 27.6 volts. I don't think/hope any damage occurred, but without a BMS to shunt current from any individual cell reaching 29.2 volts, it certainly could have. Point is, s**t happens, so safety features like a BMS are highly desirable and worth the cost IMO.

[john61ct]

The higher charge setpoints by some come from simply accepting "vendors know best".

I do not know of any need to vary them by cell manufacturer, the Yttrium in Winstons apparently only affects the operating temps range, and even that has not been well test / verified.

I do not advocate for "no BMS", leave that decision to others.

Personally, using "a BMS" so far does not provide functionality I need, and most of their focus is on features I do not need.

BMS **functionality** is critical to longevity. Redundancy of key protections is important.

The degree of "set and forget" automation varies by use case, preferences of those **operating** the system - owner & installer's may be very different.

The more automation, the more complex and thus less fault tolerant.

[rgleason]

Catnewbee and Johnct, thank you, this is helpful to understanding two different approachs. Since I will be looking into BMS further, Catnewbee's observations are a good guide.

Johnct, these are intriging statements, that.could prove to be true:
"Personally, using "a BMS" so far does not provide functionality I need, and most of their focus is on features I do not need.

BMS **functionality** is critical to longevity. Redundancy of key protections is important.

The degree of "set and forget" automation varies by use case, preferences of those **operating** the system - owner & installer's may be very different.

The more automation, the more complex and thus less fault tolerant."

Can you detail the primary.BMD functions and features you are looking for? Also what features does your present BMS have and how do you achieve redundency?

[CatNewBee]

Originally Posted by Delfin

I asked a similar question above regarding what seemed to be a divergence of opinion on whether balancing was necessary or desirable as a routine matter or not, but I don't think I got an answer from Nebster or CMS, to whom the question was directed. I don't pay much attention to people who have strong opinions but no actual experience with the technology since there are many non-poseurs (CMS, Nebster, Traveller, CatNewBe to name a few) to learn from.

In my mind, the question of yes or no on a BMS is a separate one from yes or no on periodic balancing. I see my BMS as a safety feature that I hope I never use, so I am of the no balance contingent, based on CMS's empirical testing but would like to learn from those who have reasons to take a different approach. But I can give you an example of the importance of the BMS as a safety feature that just happened to me that illustrates the point.

Delfin is out of the water having her bottom sand blasted and re-epoxied after 18 years of the same coatings and a whole lot of build up of non-ablative paint. She is inside a large shed, completely tented for the process and I had left her with the BMS off as well as the starter bank since no current was required for anything. The yard foreman wanted to check the inside of the hull before doing some spot welding, so turned on the BMS for ER lights. So far, so good. However, he also thought it would be good to hook her up to shore power. The Trace 4000 charger came on, with settings appropriate for an LA bank - 28.8 bulk, 2 hour absorption, 26.8 float. When I came in the next morning, the bank was at 27.6 volts. I don't think/hope any damage occurred, but without a BMS to shunt current from any individual cell reaching 29.2 volts, it certainly could have. Point is, s**t happens, so safety features like a BMS are highly desirable and worth the cost IMO.

... even if I repeat myself, a BMS is primary a system, that balances the cells it is not just a Battery Protection System, it is a Management System.

When the balancing works, any battery protection based on battery voltage will work - like a battery protector or a charge controller.

Are the cell out of balance this devices will fail to protect the battery.

So that is the reason, a BMS does provide additionaly cell voltage based signals to cut off the current (either charge or discharge).

This is also the redundancy of protection - bacause the battery is primary protected by the regular electronics, like chargers or deep discharge protection devices. They of course must be set to the right set-points, because FLA deep discharge is almost 1V below the LFP setting.

Your target should be to never ever experience a OVP / UVP cut-off from the BMS, the regular logic should engage before that happens as long as the BMS is working properly and the cells are balanced.

It is also fake news that balancing always hapens. Normaly (top balancing) it hapens only while charging and it just reduces the charge current for an individual cell. Many better system even prevent balancing if the maximal cell difference is less than 20mV. There is no reason to neglect the balancing function of the BMS. It would ridicule the whole cocept of an optimal battery with equal cells and also compromise any logic based on battery voltage..

[john61ct]

At core, most essential BMS functionality are to prevent damage to the bank.

HVD
LVD
Temp protection

What is done at what "level" cell / pack / bank,

alarms vs cutoffs and

how much redundancy, is up to the user.

Likewise, my take on cell-level balancing functionality

initial vs ongoing
active vs monitoring + manual

you've read dozens of times by now, no need to start that up again.

Then you get into what I call "upper / user level" controls over daily usage, in a robust setup should prevent the above protective BMS functionality from ever getting triggered unless something fails.

Charge regulation, isolating / combining, protecting the alternators, Float the bank or not, loads buss separate from charging buss, LVDs on specific circuits or devices to prioritize Essentials at lower SoC.

Some may say all the above is "BMS functionality" but I just use that term for the bank-survival innermost-layer protections.

maybe overcurrent protection, but that's just CP afaic

[rgleason]

Johnct, not sure I've learned much more about the details of your BMS or how you use your system, but you have summarize/listed key elements/choices/characteristics of a BMS. Thanks for sharing that.

[rgleason]

Revised Bank Regimen

• Manuf. Maximum Voltage------ 3.65vpc (14.60v) cell destruction
• HVD High Voltage Disconnect-- 3.50vpc (14.00v) BMS
• High Voltage Alarm (User set)-- Balmar SG200 Alert
• Stop Charging ------------------ 3.65-3.45vpc (14.50-13.80v) Reg
• Rest, Charged, Float, Storeage-3.20vpc (12.80v) Johnct
• Stop Discharging-----------------3.20-2.90vpc (11.00-12.80v)
• Low Voltage Alarm (User set)---Balmar SG200 Alert
• LVD Low Voltage Disconnect--- 2.90vpc (11.90v) BMS
• Minimum Voltage---------------- 2.80vpc (11.20v) cell destruction

What controls?

• Stop Charging? Alternator Regulator, Shore Charger settings?
• Stop Discharging? User Action?

CatNewBee preferred values

• Start Balancing: 3,4V
• OVP: 3,85V
• UVP: 2,7V delayed for high loads
• Acceptable difference: 0.02V (no balancing for small imbalances)
• Balancing Current 3.5A (up to 15W per cell), there is a temp sensor on the board and a contact for a fan to cool the board if necessary.