Smart Solar settings for lifepo battery

[dreadlocks]

boondocking loads and solar are so variable its nice to have a reasonably full tank when the sun stops cooperating.. sometimes that might be a bit earlier in the day than you'd want just due to the site's location..

but yeah if I were running fixed loads full time, even more room to make things conservative to get these things to last a lifetime.. great thing about having programable charge sources like Victron.

 

[jonyjoe101]

One thing with lithium is you need to verify the solar controller voltage is the same as what the voltage on the battery terminals are. If you don't plan to fully charge your battery its not a big deal, but if your like me and want maximum performance out of your battery, you need an accurate voltage reading. If you have voltage drop the lithium setting is not going to work.

At 14.4 volts your not going to get a fully charge battery, maybe 95 percent, less if you got voltage drop. But for drop-in lithium, 14.4 volts will prevent the battery bms from activating and causing voltage surges from controller. If the battery has a balance problem even 14.4 volts might be too high and cause the bms to activate.

14.2 volts would be too low for lifepo4, when the bulk/absorb is set too low, the controller reduces the amps to the battery. The closer the battery voltage to the bulk voltage, the lower the charge amps, the higher the bulk setting the more amps (fast charging) you will get. With solar and limited available light you want to fast charge your battery every time.

I have had my 220ah lifepo4 close to 3 years, connected to my 240 watt solar panel 24/7, I keep it fully charge to 100 percent every day. It not going to hurt the battery. Undercharging the lifepo4 won't hurt it either, but if you don't have a coulombmeter to track the amps, eventually the battery will be at 50 percent or zero percent when you need it most.

 

[dreadlocks]

If you have a BMV networked w/SmartSolar it has remote voltage sense capabilities.. and either way your Solar controller should always be as close to the battery as you can reasonably get, so voltage drop should be negligible unless you grossly underwired it.

I dunno what your talking about, its nearly a constant current until its basically full, bulk/absorb of 14.2 dont slow nothing down.. here's a 30A solid charge through a SmartSolar at 14.2



Whoo, 3 whole whopping years.. I'm going off the advise of research papers published showing 20 year lifespans for LFP in fleet vehicles if correctly coddled throughout their lifes.. published by companies like Tesla heavily invested in battery pack longevity.. a few measly amp hours not taking it up to 14.6v is no sweat off my balls.. If your hurting so hard that 5% breaks you then you, then you need more capacity..

 

[shade]

A great article on protection & management: http://nordkyndesign.com/protection-and-management-of-marine-lithium-battery-banks/

Do Not Confuse Protection and Control

A protection system with automated disconnection is just that: a last line of defence that should never be activated. Using the disconnection device(s) to terminate charging (a suggestion often formulated by DIY implementers) is out of the question. It breaches the system design boundaries, where the battery protection layer’s role is to mitigate any failure in the charge control system. More specifically, one role of the BMS is defending against a failure in maximum charging voltage regulation.

That's one in a series of articles. It's worth noting that most (all?) drop-in LFP batteries lack both protection & control systems, leaving it up to the user to add those functions, or roll the dice.

 

[luthj]

If you are using return current, its possible to adjust the absorb setpoint to compensate for wiring losses. For most vehicle type installs this is a kludge in my view. Just use appropriate wire to keep the voltage drop down. I do agree you should verify there are no excessive losses in the system, especially for DIY installs with less than professional gear and materials.


LFP will charge to the high 90s SOC on 13.6V (3.4Vpc). They will charge to 100% on 13.8V (3.45Vpc). It takes longer, and a lower return current termination setpoint is desired. Typically the reduced charge rate for the last 10% is undesirable, so voltages over 14V are used. When combined with a calibrated timer or return current termination, there is little impact on batter life from using ~14.2V. For those who want to eke out the maximum life, charging to the lower voltage is ideal.

Some BMS will not balance until the cells hit 3.6Vpc. That's just bad design. Most will start balancing any time there is a charging source, and the cells are more than ~10-25mV apart.

Keeping lithium at 100% all the time dramatically reduces its lifespan. A recent test charged some LFP cells to 100% SOC (zero return current at 14.2V). Then disconnect them, and let them sit for a whole year. IIRC capacity loss was nearly 10% in that year. This was at average ambient around 75F too!

The best approach for a daily cycled pack, is to only charge what you need. Since that's not a good solution for most, its best to charge to about 90-98% SOC via absorb voltage, and terminate with return current, a timer is fine too, just make sure to verify it once a year for accuracy. Then drop to a low float. If you look at the SOC vs open circuit votlage curve, pick a voltage around 50-75% SOC. This is typically 13.2-13.5V. This lets the charger carry most of the float loads, and the battery will be pulled down a few percent by the end of the day. This reduces stress on the electrolyte and plates, and still allows the occasional balance if needed. A well built lithium pack will not need more than 5-15 minutes at absorb to balance. Even an well used pack will not need more than an hour.

Lead also experiences capacity loss at 100% fully charged, its just that in lead sulfation at partial state of charge is much more significant. For lead batteries which spend a lot of time at float charge (backup supplies etc), the float voltage is lowered to reduce the degredation.

 

[luthj]

Here is an excellent paper about storing batteries at various temps and SOC levels. Note how elevated temperatures and SOC levels degrade the battery.

http://jes.ecsdl.org/content/163/9/A1872.full.pdf

 

[luthj]

I think its important to note there are plateaus in the capacity loss curve. Between 70 and 40% SOC there is not significant change, so anywhere in this range is fine. For optimal conditions, storing at 10-15% SOC is acceptable. Interestingly storing at 0% SOC didn't have a negative impact, so as long as the pack (and BMS!) is totally disconnected, storing at 5-10% SOC has some benefit. If this isn't possible, then storing in the 30-40% range makes sense.