LiFePO4 High Capacity Cell Charging

phazaar · Aug 13, 2018

Okay, so having done a ton of research after being pointed in the right direction in my last thread, I think I'm making progress with my battery system.

In short, I've got 4 3.2v 200Ah LiFePO4 cells in series, giving me a 12v battery for off grid use, getting charged by 12V solar, a generator, or even a vehicle alternator in a pinch. I'm going to go BMS-less and just use fuses to ensure excessive current isn't drawn accidentally, and maybe a battery monitoring system to keep an eye on temperatures and cell voltages. I'll bottom balance the cells, then charge to near capacity, measure one full discharge as a bit of a 'battery gauge', then charge back to full before installation. From there I'll periodically balance the cells manually if they need it.

To do the bulk charges, and later to work off the generator, I've got a Sterling PCU1260, which has a LiFePO4 mode, delivering 60 amps.

My question is, what do I use to do individual cell charges if one is lower than the others? Lots of guides just say 'cut off the charge to each cell when it reaches X' and 'top off the low cells' etc, without any mention of what one would use to do that, or with a recommendation to use a 300mA charger in one case. Seems fine to use on a 2000mAh cell, but is there no higher current delivery method for these high capacity cells? Perhaps I'm just being silly, if so, please feel free to laugh (but also do correct me pleeeeease haha!)...

Voltron · Aug 13, 2018

Instead of topping of low cells, you could use a light or a load resistor to take down the voltage on the individual high ones.

phazaar · Aug 13, 2018

I more meant for initial charges than later balancing - they’re not currently balanced, but in order to maximise capacity I assume I should aim to get them all high rather than to undercharge (or bleed off) the higher ones?

Voltron · Aug 13, 2018

As soon as you bleed off the high ones you start bulk charging again. It's tedious, but that's the life of a human BMS. It varies with battery type, but after a few rounds of watching the voltage rebound when the load is removed, I've gotten good at bringing down four or five high ones at a time and having them come out at a target voltage within a few tenths. The closer it gets through, it keeps getting harder as there's more high ones.. So I'm usually topping off low ones at the same time.

BVH · Aug 13, 2018

Why not make up a balance harness for your 4S pack and use a small capacity RC balance charger to balance charge when you need to. That's what I do with my 9S/100 Ah LiFeP04 prismatic pack I use for similar uses as you do. If you have a DC power supply, your choice of low-cost chargers is quite wide. If you need a charger that works directly off house current then your choices are less. Many have 1 Amp or more of balance drain.

You could also use the small charger to charge each cell individually to the same termination voltage after you do a bulk charge with your other charger thereby accomplishing your balancing needs.

jonyjoe303 · Aug 13, 2018

I use these 4 amp usb load tester to bleed high cells, these work great if you not charging at too high amps. Even most balance chargers bleed at less then 1 amp. If your charging at 16 amps and the battery is very unbalanced, this will keep up with it, the charger will still be charging at 16 amps while bleeding the high cell. They are adjustable with the knobs, so you can bleed with less amps, cost about 9 dollars. Usually there is only 1 or 2 cells that will drift out of balance, so you don't need 4 of these.
View attachment 1

Also I found out that my lifepo4 always drifts out of balance near the high end, even using a balance charger every once in a while doesnt do anything. Eventually I bought a 4s active balancers (90 dollars). These are always balancing while charging and discharging they peak at 10 amps per cell, so you can fast charge at higher amps, they don't bleed the amps, they transfer from high to low cells. After 6 months of use, they perform outstanding, no more balancing problems, no more undercharge battery because the BMS shutoff charging too early. If you are going to go without a BMS, you need active balancers, that way the overall voltage will always be a balanced battery. If one cell is too high and 3 cells are too low, The overall voltage might be within limits, but that high cell will be over the limit. These work in the background , no settings, no on/off switch, you connect to the battery (I use a common 4s balance connecter) and it does its job. I leave connected to my 110ah lifepo4 24/7, its power use is insignificant on a large battery pack.

phazaar · Aug 14, 2018

BVH said:
Why not make up a balance harness for your 4S pack and use a small capacity RC balance charger to balance charge when you need to. That's what I do with my 9S/100 Ah LiFeP04 prismatic pack I use for similar uses as you do. If you have a DC power supply, your choice of low-cost chargers is quite wide. If you need a charger that works directly off house current then your choices are less. Many have 1 Amp or more of balance drain.

You could also use the small charger to charge each cell individually to the same termination voltage after you do a bulk charge with your other charger thereby accomplishing your balancing needs.

"Total lack of experience" might answer your 'why not'

Jokes aside, any chance of a link to suitable apparatus? The one thing I'm really struggling with in this process is the incredibly wide variety of products that all nominally do the same thing, whilst clearly having real differences along the way! If I can balance charge off the 12V input from the panels/split charge relay/60A 240->12V charger I have, that would be ideal. If that's too much, I suppose I don't need to balance charge too often, so could probably get by with less?

phazaar · Aug 14, 2018

jonyjoe303 said:
I use these 4 amp usb load tester to bleed high cells, these work great if you not charging at too high amps. Even most balance chargers bleed at less then 1 amp. If your charging at 16 amps and the battery is very unbalanced, this will keep up with it, the charger will still be charging at 16 amps while bleeding the high cell. They are adjustable with the knobs, so you can bleed with less amps, cost about 9 dollars. Usually there is only 1 or 2 cells that will drift out of balance, so you don't need 4 of these.
usb load tester.jpg

Also I found out that my lifepo4 always drifts out of balance near the high end, even using a balance charger every once in a while doesnt do anything. Eventually I bought a 4s active balancers (90 dollars). These are always balancing while charging and discharging they peak at 10 amps per cell, so you can fast charge at higher amps, they don't bleed the amps, they transfer from high to low cells. After 6 months of use, they perform outstanding, no more balancing problems, no more undercharge battery because the BMS shutoff charging too early. If you are going to go without a BMS, you need active balancers, that way the overall voltage will always be a balanced battery. If one cell is too high and 3 cells are too low, The overall voltage might be within limits, but that high cell will be over the limit. These work in the background , no settings, no on/off switch, you connect to the battery (I use a common 4s balance connecter) and it does its job. I leave connected to my 110ah lifepo4 24/7, its power use is insignificant on a large battery pack.
active balancers.jpg

Loving the load tester idea - that's certainly a good first step. And amazon-able too, albeit for a bit of a hike in price.

The active balancer sounds interesting - can you link me to one please? I'm just getting loads of BMSs when I google. Also when you say they peak at 10A per cell, does this mean this is the fastest you can charge all the time, or just whilst balancing? 10A per cell is only 2/3rds of what my bulk charger can provide.

Thanks for all the help so far guys!

phazaar · Aug 16, 2018

jonyjoe303 said:
Eventually I bought a 4s active balancers (90 dollars).

I found these https://www.aliexpress.com/item/3S-to-9S-lithium-battery-balancer-which-keep-voltage-difference-within-10mV-the-best-solution-to/1950876697.html which look identical to what you've got, and look just the ticket.

As a bit of a follow-up, what are the other pieces you're using? Searching things like 'multiple cell monitor' etc is just giving me HUGE amounts of stuff that doesn't appear very useful!

In addition, I've potentially got a line on 4 extra cells. If I was to put these in parallel for double capacity, how would balancing work? Would I set it up as two separate 4s batteries (and two balancers), would it be putting sets of 2 cells in parallel and creating one 4s battery, and if that, how would balancing between those paralleled cells work?

Really grateful for any prods in the right direction

LiFePO4 High Capacity Cell Charging

phazaar

1 mW

Voltron

1 MW

phazaar

1 mW

Voltron

1 MW

BVH

1 kW

jonyjoe303

100 W

phazaar

1 mW

phazaar

1 mW

phazaar

1 mW

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