LiFePO4 bank power cycling policy Q

[vas]

hi,

ready to fit the battery bank onboard (waiting for ever for some tiny chips to arrive from all over the place!), should have connected and working by the w/e
One issue that needs some thought is charging/discharging policy
What I mean, when on board in the summer, I may need 150Ah (out of the 300 available) I may need a bit more but that's the ballpark figure.
Off season, consumption is minimal 5-10Ah max.

So on season, I can have the bank starting balancing at 3.45V per cell and stop at 3.5 with a tail current of 1.5-2A (passive balancing copes with that) they'll dip as far as necessary and next day solar with replenish - caters for the odd overcast day as well.

What do I do off season with sunny weather though? Down here that's the norm for April, May as well as most of Oct. FE, over the last week, with one fridge running, in the morning Trojans are at 95% and by 9:00 they are back at 100% - ambient temps are still low...
I can program the BMS to keep a similar policy by not turning on the solar if SoC is >50% and only when it reaches that, solar is enabled and brings it back to 80-85% whatever the 3.45-3.5V per cell is.
That would mean solar is on say once a week or so and bank is cycled once (if so) a week compared to 75-80% daily!

Makes sense in terms of improving the longevity of the cells?

any pointers on approaches followed by the experts?

cheers

V.

 

[roaringgirl]

I don't think LFP cares if you cycle it or not.

 

[vas]

that's not in line with what I've come across tbh.
For sure it doesn't like to be staying for ever fully charged. Easiest for me would be to have "winter" settings where I charge up to 3.4V only, or even less...
Also not quite clear (rather not clear at all!) if a 90%-80% cycle for 100days counts as no cycles to the battery or 100 or 50, or whatever. It's all rather vague.

 

[Jokani]

Are you on board during the off season?

 

[vas]

nope I'm not, I'm at home (500metres away)

 

[Zing]

The kindest thing for your batteries is to cycle them well within their capacity. Say at 80% down to 20% or less if feasible. If storing them, as effectively you are doing in the off season, then something like 30-40% is recommended. You could ‘float’ charge at that level with a low setting on your solar controller.

 

[Jokani]

Vas, I came to the conclusion that charging of any kind will always reduce longevity , so in the season when cruising, I'll have the LiFePO4's connected, but off season, I disconnect the LiFePO4 and rely on a smaller LA. That way I cam leave the LiFePO4 at something like 50% charge, which i believe is optimal. Might be worth you thinking about. something similar. Also has the benefit that if things go awry while you are away, only the cheap LA is at risk.

 

[vas]

thanks, well looks like that can be easily implemented as I'm keeping the halfdead T105s onboard.
In which case I can run the LiFePO4 to 50% in October, disconnect them via the contactor and only have their BMS running (to monitor SoC).
Once every couple of months I could do a 80% charge and then run down again to 50% with the watermaker, rinse-repeat

cheers
V.

 

[Jokani]

That is similar to what I do, I have 2 x Trojan T105 AGMs in parallel with the the LiFePO4, as they can both have the same charging regime. When I stop sailing I discharge the LiFePO4 to 50%, and disconnect them via the contactor. Only been up for one season, but seems to be working for me.

 

[vas]

That is similar to what I do, I have 2 x Trojan T105 AGMs in parallel with the the LiFePO4, as they can both have the same charging regime. When I stop sailing I discharge the LiFePO4 to 50%, and disconnect them via the contactor. Only been up for one season, but seems to be working for me.

so throughout the season you have the LiFePO4 profile charging in parallel both banks?
That's my plan also and on top avoid buying a B2B charger for the alternator / Sterling advanced regulator which only does 5% (if that!) of battery charging and just disconnect the LiFePO4 bank on long motoring sessions where alternator may overcharge them.

PS. T105 are not AGMs they are FLA surely?

 

[Jokani]

Yes

These are the T105 AGM variant I have:

 

[vas]

OK, at least they don't call them T105 but SAGM 06 whatever as that would be confusing . They are keeping the footprint of the T105 it seems.
Mind absorption V is 28.8 which is much higher than what LiFePO4 would be happy with which is a similar issue with the T105s I have.
I guess you're happy to undercharge them a bit and keep LiFePO4 happy.

V.

 

[Jokani]

28.8v is for a 24v system, I am 12v so 14.4v, which is not dissimilar to my LiFePO4

 

[vas]

hm, so you charge them to 14.4/4 = 3.6V regularly? you are on the brave side, I'll be stopping at 3.5V ?

 

[Poey50]

Late to this but for any length of time leaving the cells, summer or winter, the pack is left around 50-60% and the Victron Smart Solar is given my storage setting of 13.3 volts for both absorption and float.

For longevity the smaller the range used the better. If you went from 45-55% they could be listed in your will, so something in between, 20-85% for example, would be a good compromise.

 

[vas]

listed in my will, hm is that meant to be an attractive proposition?

so where would you place the 85% SoC in per cell V? where would you start balancing and where would you stop charging?
was planning for 3.45 balance, 3.5 stop

 

[Poey50]

listed in my will, hm is that meant to be an attractive proposition?

so where would you place the 85% SoC in per cell V? where would you start balancing and where would you stop charging?
was planning for 3.45 balance, 3.5 stop

Only after a long, happy and prosperous life, obvs.

For storage I just rely on the Victron Smart Solar programme to control everything. But in general balancing is 3.4v per cell. 3.45 may not give you a long enough balance period (depending on the balancing capacity of your BMS) but you can experiment with it. In my experience you have to let your system teach you how to tweak it. I always know what my cells and pack voltage are doing so in practice I tend to simply manually stop charging when it gets to the 85-90% level. It is good to have these things automated to avoid overcharge for example but also I have found it essential to be easily able to switch off charge systems manually. So with my alternator charging, the Balmar MC614 external regulator is set to charge to a pack voltage of around 90% but at the end of a trip I want to leave the pack at about 60% SOC so, rather than reprogram (madness on the MC614 with the magnetic reed switch) I simply flip a switch which safely stops alternator charging when it gets to the right level (assuming it was lower than that when the engine started - not always the case).