Just bought 3.2v 38.4ah Lifepo4 38400mah Cell packs (24x18650)


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ken

New member
Joined
Nov 1, 2016
Messages
4
Hi just made a purchase off these packs, from

http://www.ebay.co.uk/itm/3-2v-38-4ah-Lifepo4-38400mah-Cell-packs-24x18650/142489634336?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I have bought 12 off these packs "one pack is 3.2 volts"and would like to know what charging and even discharging would be good to keep these in peek performance, I have a epever Tracer MPPT at 40A thinking that would be good to manage the solar charging side off things, at the moment Have 200 Watt panelsmounted on wall, facing sunrise and mostly midday facing.

and tips, hints and advice most welcome for this Britt appreciated
 

daromer

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Oct 8, 2016
Messages
5,662
Charge them up to 3.5V for long-life. 3.65V for maximum. Thats how I have my system setup. Low point is set to 2.8V Though dont really run them below 3.2V.

LiFePo4 have a very very flat curve! So its impossible to meassure SOC based on voltage unless they are at top or bottom. So BMS is HIGHLY needed with shunt if you want to keep track of them. Because you get no indication kind of when they are empty.

I run LiFePo4 myself and have above 80kWh of them.



image_mumuvc.jpg
 

DarkRaven

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Joined
Sep 2, 2017
Messages
1,223
Your Tracer is probably not made to charge lithium cells though so you have to do something about that. 4S LiFePo4 is, by design, close to 6S/12V Lead Acid in terms of voltage, but still require CCCV charging with proper cutoff and won't take float charging.

Characteristics of LiFePo4 cells may vary, like with any other chemistry, but yes, as daromer said, they typically have a very flat discharge curve. 3.2V is their usual, nominal voltage and they have like 90% or so of their capacity in the range of 3.2/3.3V. Above that and below that there is almost no noteworthy capacity. I would charge them to 3.4~3.5V and discharge them to 3.0~3.1V. If you discharge to 3.2V you might waste a lot of their capacity because that might still be in the range of their sweetspot.

As for cycle life, LiFePo4 usually lasts many cycles in the range on several thousand. Especially in you run them between 3.4 and 3.1V you can expect them to last a very long time.

You could look up the exact values given by the manufacturer in the datasheet, but I couldn't find one for these cells. They are made by PHET to be used in bigger packs so there is probably no official datasheet on the single cell. These might be power cells, able to supply high currents, since LiFePo4 is usually very robust in terms of discharge currents. And charge currents as well for that matter. Maybe it is worth making you own datasheet by observing their charge and discharge behaviour under several circumstances.

What are you doing with 12 of these packs though? 36V system? Not impossible, but maybe a bit unusual, unless you build a ebike battery with them :)
Unlike 3.6V and 3.7V LiIon (LiMn, LiCo, ...) and LiPo cells, where 3S is usually a bit low and 4S is usually a bit high for a 12V system, you can do a 4S configuration with LiFePo4 cells for a 12V system and it scales to 8S/24V, 12S/36V, 16S/48V...
 

karrak

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Joined
Sep 4, 2017
Messages
14
I agree with the recommended maximum cell charge voltage of 3.45-3.5V and minimum discharge voltage of 2.8v. The charge/discharge curves for LFP cells is very flat and also dependent on charge current.


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My battery is top balanced and I manually rebalance it. As it has got older the need for rebalancing has become less. I have not had to do a rebalance for over twelve months.

It was originally balanced by the supplier by placing all the cells in parallel and charging them up to 3.6V. They didn't do this properly and my original BMS (a Cellog 8) saved the battery from damage. IMO It is most important to do the initial commissioning balance properly!

I have programmed my solar charge controller to charge to 27.6V (3.45V/cell) and switch to a float voltage of 26.8V (3.35V/cell) when the charge current reduces to C/50. I would only recommend floating an LFP battery if being charged from solar and if the battery it is being used on a continuous basis.

Most of the time my battery would be above 50%SOC, the average over the past year is ~77%SOC and the lowest SOC was ~8%

Simon
 
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