17 cells vs 16

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17 cells vs 16

Postby mmayberry » Wed Feb 19, 2020 9:07 am

Gday all

I currently have a 48 vdc pack of Winston lifepo4 - 16 x 160 ah cells. Using the ev works cell top monitors and their BMS. All has been going well for 3 years now. I charge the pack to 56v (3.5v each cell) and float at 53 (absorb only 20 mins once it hits 56).

I've been thinking of whether it would have been better to run 17 cells - a bit of extra capacity but also to keep the upper voltages down - say charge to only say 3.41v each cell (58v) and float at 56 (3.29). Maybe absorb for longer, say 1 hour. Any suggestions/advice?
Thanks
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Re: 17 cells vs 16

Postby davef » Thu Feb 20, 2020 3:54 pm

I tried lowering from 3.55V to 3.4V using an Electrodaus SBMS60 solar controller and run into an issue with SOC calibration.

Contrary to your charging regime he recommends "bulk", ie up to C/5 no absorption and no float. I think the suggestion for non-LiFePO4 chargers is to set absorption and float to low values so that they don't kick-in.

Reduction in charge rate allows you to maybe cram a bit more but my guess is you are better going to 17 cells and to stop charging when they get to your target. He suggests to charge at the full rate with no cut-back to obtain maximum cycle-life, ie some issue with cycle-life versus time spent charging. He might provide referenceshttp://www.electrodacus.com. Probably worth having a look around his forum.
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Re: 17 cells vs 16

Postby australsolarier » Fri Feb 21, 2020 9:48 am

48V systems run with lifepo4 16 cells. you will look into all kinds of problems running it with 17 cells.
to properly charge a lifepo4 battery you should use end amps. the programmed absorption time does not mean all that much with lifepo4 batteries. (maybe as a max time limit)
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Re: 17 cells vs 16

Postby davef » Sat Feb 22, 2020 5:01 am

Glad I qualified my response with the word "guess".

Just so I learn something here what are the problems with going to 17 cells? Perhaps finding a BMS that would handle the extra cell?

Thanks,
Dave
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Re: 17 cells vs 16

Postby australsolarier » Sat Feb 22, 2020 6:35 am

davef,
yes a 17 cell BMS is more difficult and probably more expensive to find. for example batrium can be configured 17 cell, but is horrendously expensive, at least for a small experimental system.

it can also limit the choice of inverters, as all the voltages get pushed 3.3V higher. so V high cutoffs and low cut offs are different also and possibly not programmable anymore (same problem with the BMS)

also the solar charger might have the same limits. although might just slide in there. but certainly not his ev-works BMS, low and high cutoff's are set unprogrammable and i am not sure you could daisy chain a 17th cell.

rereading the original post, mmayberry is only 20 m in absorption mode so i don't think he is even fully charging the battery. his float voltage is a bit lower than most, his is 53V as compared to 53.6 commonly recommended. people have been going with those settings for a longish time.
what however seems to drastically shorten the lifetime of lifepo2's is heat. that's the reason i installed an aircon in the power room. if the battery's life is extended 10% only it has paid for the cost. besides electronics runs better and their life is also greatly prolonged.
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Re: 17 cells vs 16

Postby davef » Sat Feb 22, 2020 8:40 am

austrasolarier,

Thank you for the comments. I have a ventilation fan that comes on at 30C and an alarm on battery temperature ... can't afford air-con!

Cheers,
Dave
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Re: 17 cells vs 16

Postby mmayberry » Sat Feb 22, 2020 9:33 am

Pretty sure my FM80 has set points up to 80vdc and the Victron Multiplus works between 38-66 vdc.

But the BMS could be the issue. I assumed my BMS could have another celltop monitor added, as the cut off points are 2.5v lvd and 4v hvd, and the BMS just receives a signal if they go outside of this and opens relays, think this is the only communication between the cell top monitors and BMS, don't think they individually report to the BMS. The bullt-in shunt, once it hits 56v reports 100% but allows for higher voltage up to what I assume would be a cutoff of 16 x 4v = 64v. Might give the manufacturer a call.
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Re: 17 cells vs 16

Postby australsolarier » Sat Feb 22, 2020 11:06 am

my ev-power bms also has a total max and minimum cutoff. (47.2 and 60V respectively)
and i doubt you can daisy chain a 17th cell.
my ev-power bms has a hall effect sensor not a shunt.
and yes it reports 100% SOC when it hits a charging V of 56 even though the battery might only be 70% SOC in actual fact. to me that's absolutely unrelable. and it is not the shunt does absolutely not hit 56V. it would mean something like 10 million amps going through the shunt. it just takes the battery voltage. it doesn't count the amps like the midnite classic whisbang jr shunt does.
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Re: 17 cells vs 16

Postby dRdoS7 » Sun Aug 30, 2020 1:59 pm

Hi,

mmayberry wrote:Gday all

I currently have a 48 vdc pack of Winston lifepo4 - 16 x 160 ah cells. Using the ev works cell top monitors and their BMS. All has been going well for 3 years now. I charge the pack to 56v (3.5v each cell) and float at 53 (absorb only 20 mins once it hits 56).

I've been thinking of whether it would have been better to run 17 cells - a bit of extra capacity but also to keep the upper voltages down - say charge to only say 3.41v each cell (58v) and float at 56 (3.29). Maybe absorb for longer, say 1 hour. Any suggestions/advice?
Thanks


Did you try 17 cells?

I'm planning to buy 18 cells next week, with a suitable BMS, and wondering how you got on.

If 18 doesn't work out for me, I'll buy a second lot of either 14 or 16, and make it 2 banks of 16 or 17.

My chargers & inverter will cope just fine.

My NiFe experience has not been that good last 12 months, currently in discussions for return & refund, so rather than go to LA, I've decided to go for LFP.

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Re: 17 cells vs 16

Postby australsolarier » Fri Sep 11, 2020 8:17 am

drdos7,
don't waste your money and time, just go 16 cells. there is a reason all the systems are 16 cells.
and yes, the forum is dead, i do not get notification emails anymore
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