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

[tanglewood]

Originally Posted by CatNewBee

Hi there, there is a lot of misinformation and fear regarding LFP charge regimes.

First of all, do you use your battery or are you talking about storage?

When using it, there is no problem with float at all. you stop charging between 14.4...14.6V, the battery is then almost (97%)/ full (100%). Charging current depends only on the inner resistance / capabilities of the charger, because impedance of LFP is very low, and it takes all current you throw at it. I float mine at 13.5V, what means the battery discharges to 94...95% to the loads on board before it then stays around 95% without delivering more current, all loads run then on solar alone as long as the sun yelds more power than needed, the battery does not take any current on float.
There is no overcharging at all, cell voltage is then around 3.375V.

The battery is cycled during the evening and night for the fridges, freezer, lights, entertainment, a/c, watermaker, cooking, hot water for the shower, whatever.

Most of the logevity posts are based on fear and misinformation / lack of understanding of basic principles of electric circuits, voltage and current.

It is true, that LFP cells like high currents and being cycled, they are more robust than FLA, and it is true, that for storage when not in use for a long time, they are happyer when staying around 50% SOC (pack voltage 13.2V, cell voltage 3.3V). But this is not relevant when you use them daily.

The "don't float" mantra should really be "stop charging", i.e. stop sending current to the battery. That's accomplished by lowering the float voltage to a point where the charger/solar is powering loads, but not continuing to push current into the batteries.


Besides, how many chargers have a float that can be turned off? Not many, so you need to do something with it. Just turn down the voltage. 3.30 to 3.35Vpc works well.


And as CatNewBee says, come sundown, you will be cycling the batteries anyway so all is good.

[JmanC]

Originally Posted by Delfin

The simple solution that some use to address all of the above is to use a small starter bank as the destination source for on-going charge capacity, like solar. Use the Li bank to the DoD you want, re-charge, then take it off line and let charge sources provide on-going power needs via a lead acid starter bank. Sun goes down, bring the Li bank online. Rinse and repeat.

Thanks

I'm not exactly sure how this setup works, can you please give a bit more details.

[Delfin]

Originally Posted by JmanC

Thanks

I'm not exactly sure how this setup works, can you please give a bit more details.

A lead acid battery is isolated or added as the starter battery bank, separate from the LiPO house bank. They can be managed through a "1 - 2 - both" switch, or an automatic charging relay (I use one from Blue Seas). You then use the LiPO bank like a fuel tank; fill it, use it, re-fill it. If full, you direct charge sources from the LiPO bank to the starter bank, and run house loads off that when there is excess charging capacity available from solar, engine, genset, shore power, etc. So, the only time you charge the LiPO bank is when it requires it, and if you don't need its stored energy, you run off the lead acid battery.

The alternative is to do what Tanglewood suggests and I mentioned - set float voltage to something below the resting voltage of a charged LiPO battery. If everything works, then charge current is only going to the LiPO battery when demand exceeds float current supply, avoiding the scenario where the LiPO bank is full but still connected to a charge source, which just generates heat and shortens the life of the battery.

Which route you want to go is kind of influenced by your cruising style. I can recharge my LiPO bank in 90 minutes from the genset, so a once a day "fill the tank" charge cycle managed as I describe is no great burden. I don't have solar, but if I did, the same charging routine would work just fine.

[JmanC]

Originally Posted by Delfin

A lead acid battery is isolated or added as the starter battery bank, separate from the LiPO house bank. They can be managed through a "1 - 2 - both" switch, or an automatic charging relay (I use one from Blue Seas). You then use the LiPO bank like a fuel tank; fill it, use it, re-fill it. If full, you direct charge sources from the LiPO bank to the starter bank, and run house loads off that when there is excess charging capacity available from solar, engine, genset, shore power, etc. So, the only time you charge the LiPO bank is when it requires it, and if you don't need its stored energy, you run off the lead acid battery.

The alternative is to do what Tanglewood suggests and I mentioned - set float voltage to something below the resting voltage of a charged LiPO battery. If everything works, then charge current is only going to the LiPO battery when demand exceeds float current supply, avoiding the scenario where the LiPO bank is full but still connected to a charge source, which just generates heat and shortens the life of the battery.

Which route you want to go is kind of influenced by your cruising style. I can recharge my LiPO bank in 90 minutes from the genset, so a once a day "fill the tank" charge cycle managed as I describe is no great burden. I don't have solar, but if I did, the same charging routine would work just fine.

Thanks, good ideas.

[JmanC]

If you charge a Lifepo4 bank to say just 13V or 13.2V & float at this voltage. Will this only charge to a certain level & never get full?

Can it be a simple way of cycling up to just 80-85% SoC?

[CatNewBee]

Do you buy expensive LFP batteries to store them or to use them?

(There is no simple way to charge to / cycle to 80%, you can do it only with a battery monitor counting the Ah in and out, and it must be synchronized from time to time by charging to full, also if you stop that low, the BMS has no chance to make his job balancing the cells)

[john61ct]

Originally Posted by JmanC

If you charge a Lifepo4 bank to say just 13V or 13.2V & float at this voltage. Will this only charge to a certain level & never get full?



Can it be a simple way of cycling up to just 80-85% SoC?

Charging barely flows starting at 13.2-3V

Would take a very long time, but at 13.3V would eventually get to gull enough, in practice maybe 95% SoC.

And yes, from 12V (safe 0%) up to say 13V would be a decent point for safe long-term isolated storage.

But can do the same faster charging with high amps higher target V, just stop earlier.

With an Ah meter makes such easier, voltage alone is such a rough gauge in the middle ranges, varies with current so 1 hour rest needed when calibrating.

[nebster]

Originally Posted by JmanC

If you charge a Lifepo4 bank to say just 13V or 13.2V & float at this voltage. Will this only charge to a certain level & never get full?

Can it be a simple way of cycling up to just 80-85% SoC?

Yes, you can achieve a reliable partial SOC with just a voltage threshold.

However, the time you need to spend to get it set correctly for your system are significant, because the precise voltage needed is quite sensitive to your wiring, where your charger(s) take their readings, and so on. Even the temperature of the battery will shift the voltage slightly.

I use an absorption threshold voltage on my pack that reliably achieves ~92% of true SOC every time, which is the point at which I arbitrarily declare that my battery is "full." I also use a float voltage that reliably holds the pack at about 65% to 70% true SOC when on shore power for an extended period. I re-trigger absorption charging the night before we plan to leave the grid, so the pack is back to "full" for the next morning.

[witzgall]

The SG200 does not have to synchronize from time to time, once it has learned the batteries being monitored. For LiFePo4, our experience is that takes 1-3 cycles to happen. In fact, after this happens, you can disconnect the SG200, say when putting up for the winter, and then reconnect in the spring. The SoC will be reasonably accurate at this point, if allowed to rest (no loads or charging) for a bit when the reconnection happens.

Chris

Originally Posted by CatNewBee

Do you buy expensive LFP batteries to store them or to use them?

(There is no simple way to charge to / cycle to 80%, you can do it only with a battery monitor counting the Ah in and out, and it must be synchronized from time to time by charging to full, also if you stop that low, the BMS has no chance to make his job balancing the cells)

[nebster]

Originally Posted by witzgall

The SG200 does not have to synchronize from time to time, once it has learned the batteries being monitored. For LiFePo4, our experience is that takes 1-3 cycles to happen. In fact, after this happens, you can disconnect the SG200, say when putting up for the winter, and then reconnect in the spring. The SoC will be reasonably accurate at this point, if allowed to rest (no loads or charging) for a bit when the reconnection happens.

Chris

This sounds like a great product. However, it appears to be proprietary with its data network. Is there already, or are there any plans to provide, a way to export the calculated SOC?

[Delfin]

Originally Posted by witzgall

The SG200 does not have to synchronize from time to time, once it has learned the batteries being monitored. For LiFePo4, our experience is that takes 1-3 cycles to happen. In fact, after this happens, you can disconnect the SG200, say when putting up for the winter, and then reconnect in the spring. The SoC will be reasonably accurate at this point, if allowed to rest (no loads or charging) for a bit when the reconnection happens.

Chris

How many cycles before SoH is accurate?

[witzgall]

Originally Posted by Delfin

How many cycles before SoH is accurate?

It depends on the chemistry, and the condition of the batteries. LiFePo4 is the fastest to learn. In our labs, we have seen it between 1-3 for LiFePo4, and 2-6 for all of the others.

Chris

[witzgall]

I am not sure what you mean by "Export the calculated SoC?"

The forthcoming Bluetooth App will allow you to view it with a phone/tablet, utilizing the optional bluetooth gateway.

We would like to make the data available on a N2k network. One issue there is that the only display company we know of that shows/reports State of Health is Maretron. We are also looking at HTML5. I cannot give you a timeline.

Chris

Originally Posted by nebster

This sounds like a great product. However, it appears to be proprietary with its data network. Is there already, or are there any plans to provide, a way to export the calculated SOC?

[nebster]

Originally Posted by witzgall

We would like to make the data available on a N2k network. One issue there is that the only display company we know of that shows/reports State of Health is Maretron. We are also looking at HTML5. I cannot give you a timeline.

Perfect, something like N2k would work, although modbusTCP or mqtt over LAN would be great and more open as well. HTML might also allow us hackers to probe the server for the values as well.

I'm glad to hear it's in the pipeline, one way or another. Thanks!

[caveman22]

Originally Posted by Pelagic

Never heard of any problems from the engineers and on my own boat with Victron Isolation Transformer, Skyla Charger and Multiplus , never a problem

Anyone have experience with the Victron Lithium-Ion HE Batteries ? They are Lithium Nickel Manganese Cobalt Oxide (NMC) technology, energy density at 180kw/kg is outstanding, 24V, air cooled but with only IP22 rating.

Yes expensive but if weight matters (and it does to me) possibly worth it. Wondering if they are robust enough for a sailing cat. Certainly marketed as "Marine".

Possible advantages in assembling a full Victron system from Batteries, BMS, to Inverter aside from the hit to the wallet ?

Like to hear your thoughts and any comparable NMC batteries out there.