Top Balance Questions

[Ampster]

The data are sparse, but it's very clear that carrying higher temperatures AND holding high SOC will dramatically accelerate capacity fade in LFP.
.........
LFP is becoming cheap enough now that other factors start to drive the economic rationale for various packs. Not unlike the situation with PV.

Thanks @nebster. To your last point about lower cost, oversizing a pack could be a strategy to improve life. Concurrently someone started a thread about an industrial use where the goal was to maintain the pack at around 50 - 75% SOC for long life.

 

[Danny]

I am seeing this as a recurring theme, that folks that have a continual load have a higher charge and higher float level. @Ped is a full-time RVer i believe and he has an inverter and mini-fridge going 24/7 (i was paying attention to what you were saying!)

My RV sit in the driveway more than i'd like. If i were to float at 13.6, it would be micro-cycling all day long. My cells settle to 3.35v very quickly so my system would start a charge cycle from 13.6v to my max charge voltage several times an hour, all day long, everyday. I have my float value at 13.2v (3.3v) and it may be weeks between charge cycles (2 amp tender to engine battery and small misc drains). I might bump this up higher if i need to maximize battery capacity while traveling for some reason, but i've oversized my battery by design to avoid this scenario.

I have a charger box that that has a switch for home and away that I drive the bms input of my charger with. When I'm home or expect to be home soon it shuts the charger down at say 13.3 and won't turn it on until 13. When in away setting I charge to 13.8. On the charger, I charge and float to 13.8. This seems to work for my use pattern.

 

[Danny]

And this continually boggles my mind. The official stance of this website?

12V LiFePO4 Battery w/ BMS:

• Absorption: 14.5V
• Float: 13.6V
• Inverter Cut-off: 10.7V-12V (depending on size of load and voltage drop etc)

Design Your Own LiFePO4 Solar Power System

This is a complete system solution complete with: BMS Protection Features (over/under voltage/over-current/balancing) Low Temperature Cut-off Programmable Charge/Discharge Bandwidth Settings Matched...

www.mobile-solarpower.com

I agree. Lets say you don't do damage at pack 14.5 (3.625 vpc).
1. Manufacturer specs are 3.65 for my cheap cells. I think I am taking unreasonable risk if they go higher or lower than the manufacturer spec. Lets say they risks are personal safety and Cell damage. Personal risk in over charging can involve fire. We should not in any case ignore the manufacture specs.
2. It is more likely cells go out of balance above 13.8 pack voltage. Then we need to hassle with balancing that is not trustworthy and risky if you have a cheap BMS. BMS balancing should be and can be avoided by charging to lower 13.8 pack voltage in most cases.
3. Cell balance issues are unpredictable and sometimes arise from poor battery connections.
4. It is not unusual for cells below 13.8 to be off 10mv this leaves very little room for error then charging to 14.5.
5. There is very little energy going into the pack between 13.8 and 14.5. We are giving up almost nothing charging to 14.6 and risking a cell going over voltage.
6. Our BMS systems should be our last line of defense and we should not put them in the line of fire every charge cycle.

Maybe I am wrong on some of the points above? If I am correct, I see only risk recommending to beginners they charge to 14.5. Just I can charge to 14.5 does not mean I should charge to 14.5 because of the risks.

Santa Cruz Island Boat Fire on I Dive Boat in a place where I hang out. Do we really want to risk overcharging our cells for a few amp hours?

 

[Sojourner1]

You have your battery chemistry confused between fires and venting.

 

[Danny]

You have you battery chemistry confused between fires and venting.

right you are. I guess I couldn’t find a picture of my cheap Chinese $60 BMS catching fire. Ha.