LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[Moonos]

lifepo4 kickstarter - http://www.kickstarter.com/projects/...it/description

What do people think?

I don't currently have a boat but expect to get a 40 foot cat as a liveaboard in the next 2 years. I am an industrial electrician and refrigeration mechanic. i will most definitely be looking for something to enable 8 hours of air con off battery.

[toddedger]

Personally I think solar charge control and BMS in the same box is a bad idea for boats.

[NahanniV]

Originally Posted by toddedger

Personally I think solar charge control and BMS in the same box is a bad idea for boats.

Do you have a particular reason ?

[CGirvan]

Originally Posted by toddedger

Personally I think solar charge control and BMS in the same box is a bad idea for boats.

I'm not sure why this would be, I've often wondered why this hasn't been done before.

With LiPos need for no float charge it seems logical for the controller to bulk charge switch to absorb then when down to a set low amp flow switch off and reset the amp counter to 100%

If it's disconnecting the charge when full it would be taking the place of the BMS anyway.

Adding a low voltage disconnect would seem to cover all the possibilities.

[exMaggieDrum]

Is there a reasonable way to filter out high voltage ripple from solar controllers? It would be nice if it were built in to a controller but with LA no so important.

[toddedger]

The concept is to have charging sources self regulate and shut down at a certain voltage, and if they fail to do that the the BMS steps in as a safety factor. If you put it all in one box and it fails then what? I use separate solar controllers for each solar panel, if one panel gets shaded the others are still putting out full capacity. If one controller goes down it doesn't take the rest of the system down. I have a lot more faith in a controller made by a company with a track record of making controllers. I would not recommend using a newly designed controlle/BMS for a cabin in the woods let alone a boat sailing in the middle of nowhere, but that's just my personal opinion.

[OceanSeaSpray]

Merging BMS and charge management is what I have done. My BMS fully controls solar charging and it eliminated all of the crazy issues arising from trying to use so-called charge controllers that keep doing the opposite of what they should - like starting a new cycle every morning when the light comes up. I throw them out almost systematically now.

The only qualified piece of gear to make charging decisions in a LFP system is the BMS, because it should have access to current, cell voltages, SoC etc, unlike the wannabe charge controllers.
This doesn't mean that voltage regulation can't or shouldn't be implemented externally, which is not the same as charge control.

I don't see a conflict between protection and issuing charge control signals. The power is still switched/regulated outside the BMS and the protection function is independent.
In the case of solar, if the array is matched to the battery voltage, switching it in and out gives excellent performance and charge control capability.

It has been so successful that I am about to build the prototype for a second generation of the hardware and the key difference is a lot more and much smarter battery management capability, including and especially for charge/discharge control.

[exMaggieDrum]

Originally Posted by OceanSeaSpray

Merging BMS and charge management is what I have done. My BMS fully controls solar charging and it eliminated all of the crazy issues arising from trying to use so-called charge controllers that keep doing the opposite of what they should - like starting a new cycle every morning when the light comes up. I throw them out almost systematically now.

The only qualified piece of gear to make charging decisions in a LFP system is the BMS, because it should have access to current, cell voltages, SoC etc, unlike the wannabe charge controllers.
This doesn't mean that voltage regulation can't or shouldn't be implemented externally, which is not the same as charge control.

I don't see a conflict between protection and issuing charge control signals. The power is still switched/regulated outside the BMS and the protection function is independent.
In the case of solar, if the array is matched to the battery voltage, switching it in and out gives excellent performance and charge control capability.

It has been so successful that I am about to build the prototype for a second generation of the hardware and the key difference is a lot more and much smarter battery management capability, including and especially for charge/discharge control.

My plan, and hope, is to build out my total system as you state. Good points. Could not agree more about the separation of control and protection. Some have done away with the protection part and just rely on the control part. I don't feel that brave, as you do not. I have had too many problems with my own and customers various types of charge controllers to put full faith in them always behaving nice.

[alctel]

Wouldn't having the BMS and charge controllers seperate give you more redundancy?

My plan is to have the charge controllers all programmed to stop charging at 3.5/14 and then have the HVC in the BMS set at 3.6/14.4 in case something goes wrong (with no auto balance)

This also allows me to have separate charge controllers for the different solar positions on the boat

[kmacdonald]

There has been considerable discussion of the proper voltage to charge LiFePO4 batteries. The manufacturers say to charge at 3.65V per cell. That is probably fine as long as the cells are connected in parallel. That way none of the cells will exceed 3.65V. It would be nice to be able to easily charge them that way without having to rewire the battery. That would essentially balance the cells also. Any ideas?

[CharlieJ]

3.65VPC will unnecessarily stress the LFP cells and will lead to premature failure. A better plan is to charge them to about 0.9C and then stop charging. For most cell manufacturers, 0.9C corresponds to a charging voltage of from 3.35VPC to 3.45VPC.

I highly recommend MaineSail's excellent blog: LiFePO4 Batteries - Thoughts & Musings Photo Gallery by Compass Marine How To at pbase.com

[exMaggieDrum]

Originally Posted by kmacdonald

There has been considerable discussion of the proper voltage to charge LiFePO4 batteries. The manufacturers say to charge at 3.65V per cell. That is probably fine as long as the cells are connected in parallel. That way none of the cells will exceed 3.65V. It would be nice to be able to easily charge them that way without having to rewire the battery. That would essentially balance the cells also. Any ideas?

My understanding is that 365v is the maximum charge voltage, not the best. It seems the consensus on here is that the best routine "full" charge voltage is much less than this. I am top balancing my new cells to 3.65v but I hope to not push them that high in service, ever. See other sources as noted.

[OceanSeaSpray]

Originally Posted by CharlieJ

3.65VPC will unnecessarily stress the LFP cells and will lead to premature failure. A better plan is to charge them to about 0.9C and then stop charging. For most cell manufacturers, 0.9C corresponds to a charging voltage of from 3.35VPC to 3.45VPC.

I highly recommend MaineSail's excellent blog: LiFePO4 Batteries - Thoughts & Musings Photo Gallery by Compass Marine How To at pbase.com

It is a misconception to believe that SoC can be controlled using voltage. 3.4V will still charge a LFP cell to 100% given enough time and 3.3V will struggle to achieve 30% no matter what. The transition is so steep that it makes voltage unusable.



The data for the graph came from a published experiment by Powerstream, which I plotted in a different and much more interesting way some time ago while looking at the characteristics of LFP cells.

What would better control SoC is using the combination of end voltage and residual current, but since SLA charge controllers are completely unable to deal with it, all the LFP systems discussed here rely on a fragile balance between "settings" and a necessary consumption to try and work more or less acceptably most of the time.
This balance is easily upset if something changes, like leaving the boat for a time or going from summer to winter.

This is what quickly led me to moving away from using any kind of independent solar charge controller. I now have one control wire going from the BMS to a solid-state switch and no settings at all. Charging LFPs is in fact a part of battery management and this is ultimately inescapable.

[thomasow]

The concerns of voltage-only charge termination is not restricted to LFP deployments, FLA/SLA technology also presents several compromises when using voltage-only as a termination method, chiefly chronic undercharging. Traditionally charging sources would get around this via extending time-based charging cycles – relying on the battery to accept slight overcharging with minimal damage (perhaps increased water consumption in the case of FLA – SLA needs to be careful here). But of course LFP technology cannot do this.

Truth is, for most all battery technologies monitoring the acceptance amps in combination with voltage is truly one of the best way to reliable charge a battery w/o damaging it. It is why 3 years ago I developed the open source Alternator Regulator that includes current monitoring/regulation capabilities.

Which brings up another interesting topic: There is the question of ‘What to do about Float’ with FLP batteries. Many point to ‘Disable everything’, which is certainly in line with the common EV usage model of charge then discharge, but that usage pattern is not how house batteries are used. We do not often have the pure Charge and then Discharge states. Example: supporting a washer/dryer while underway via an inverter; is nice to let the alternator carry that load vs. depleting the ‘fully charged’ battery before we anchor for the evening.

Recently have been looking at active regulation of the battery current. Rather then a voltage set point for ‘float’ regulate instead to a current set point. Specifically 0A. So when the FLP battery reaches its fully charged state, we then manage the alternator to a 0A net current into/out of the battery – while still being able to support existing house loads. Of course voltage is still monitored, but it typically is well under the ‘recommended float voltage’. Early tests seem promising, and I am curious of others thoughts on this approach.

[marujo.sortudo]

Regulating to 0A is an interesting idea. I imagine it would work better if the battery was only 95% charged. Harder to monitor charge acceptance on a full battery, perhaps? I do a similar thing by floating to 13.3V with our LiFePO4 bank. Not quite the same, but what my regulator is capable of and we rarely do long runs anyway.