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7s BMS how do you manage the parallel?
#11
(04-08-2019, 10:20 PM)friedpenguin Wrote: Plan was to use the cheap 'smart' BMS and monitor them. By keeping the packs smaller I could add them to my system one at a time each with their own BMS over time. As it stands I have seven 24v LifePo4 batteries each with their own BMS. I'll never run a load greater than what half the batteries can push at once just so there's always reserve to keep the critical stuff running 24/7. As it is I have a blocking diode on the battery connected to the Pi so if the voltage on the main bank caused a LVD the Pi would keep running (which has only happened once so far). By having smaller packs I'd be looking to minimize downtime by just pulling a small pack if it was out of spec with the rest of the system. The BMS would be there to prevent overcharge and offer LVD for any pack that drained faster than the rest. All the packs would then equalize each other. Even the battery with the blocking diode will get equalized to any higher voltage than it presently was.

small/replaceable packs == 'cartridge' style.  scales well.  good method if you need to get battery online 'fast' without waiting for full load of tested cells and/or if you wish/need to reassign cells to another task etc.

however... I say better to build each cartridge well, than to think/plan to replace them.  afterall, you're busy collecting more cells to expand/complete, not just maintain, right??!?

self-discharging cells are probably your biggest enemy.   as I read it you are not stressing your strings/battery. so therefore my advice would be ... to concentrate on eliminating (removing) ANY and ALL cells which don't 'sing well' with the others.  removing one single bad cell can help hundreds of other cells sing and dance in harmony ... truely.  and harmony is KEY ... whatever the battery is ...

also, ditto what Crimp Daddy says (above).  : )
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all systems nominal... rock on.
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#12
(04-08-2019, 10:20 PM)friedpenguin Wrote: Plan was to use the cheap 'smart' BMS and monitor them. By keeping the packs smaller I could add them to my system one at a time each with their own BMS over time. As it stands I have seven 24v LifePo4 batteries each with their own BMS. I'll never run a load greater than what half the batteries can push at once just so there's always reserve to keep the critical stuff running 24/7. As it is I have a blocking diode on the battery connected to the Pi so if the voltage on the main bank caused a LVD the Pi would keep running (which has only happened once so far). By having smaller packs I'd be looking to minimize downtime by just pulling a small pack if it was out of spec with the rest of the system. The BMS would be there to prevent overcharge and offer LVD for any pack that drained faster than the rest. All the packs would then equalize each other. Even the battery with the blocking diode will get equalized to any higher voltage than it presently was.

Hi Penguin,

I think it sounds like a good plan. I am thinking along similar lines. I've been very slowly moving into the electronics and battery area and I like the idea of 'hot-swappable' packs. I think any system has it's pro's a cons ... I don't think the 'standard' 80+p packs that are then series'ed into 7s or 14s is a bad idea by any means but if you have issues, and you need to remove a pack to resolve an issue, unless you happen to keep a few 80+p packs on hand, not being used, then your system, essentially, has to go done while you fix the problem. Moving forward, I hope to be largely off-grid and so having my battery system completely off-line doesn't seem to work well for me. Combined with my slow progress, means by building smaller packs (7s18p, only 126 cells) means I can use the system as I build system.

I am keen to follow your progress and see how you go. I currently am working with the idea of a 7s18p design, which, depending upon average cell capacity is, more or less, 1kwh per pack. I think, for me, any smaller is, as others have suggested, maybe more work than it's worth ...but time will tell I guess. I'm not sure I'll monitor every 7s18p pack but I will definitely have balance leads on each pack to allow it to happen if I move that way.
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#13
Looks like there are 2 threads in parallel about the same issue.

https://secondlifestorage.com/t-Single-s...9#pid48789
DCkiwi likes this post
1 kWp in Test
4 kWh battery target - plus Mobile Home battery
Ultra low cost
Electronics ? No clue. Am machinery engineer.
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