DIY Electric Car Forums banner

1 - 15 of 15 Posts

·
Registered
Joined
·
40 Posts
Discussion Starter #1 (Edited)
The LFP pack on the EV I acquired is dead and I want to get some power in the pack just to make sure everything works before I start dropping any more money into this pit.

As a quick-and-dirty solution just to get things checked out I want to use a dumb FLA charger to pump some juice into the cells, enough to get the donor EV rolling under its own power to verify I'm good to go on the transplant project. Or not, as the case may be. This will be a one-shot deal. I know the FLA charger is not the proper way to deal with my LFP pack in the long term.

The battery bricks are five 12.5Ah cells paralleled into a 3.2v 62.5Ah brick. I intend to series 4 bricks at a time for charging till I get through the 40 brick pack. (I assume that a 4s10p arrangement is a bad plan because of the balancing issues with LFP.)

Any suggestions on the safest way to do this (safe for the life of the cells and safe for my shop) that is likely to be successful for this one-off charge?

As a side note - I'm wanting to do this because I believe it will take me into next year to finish charging all fourty 62.5Ah bricks using something like the reasonably priced PowerStream 2A single cell charger. Am I mistaken about this?

Harmon
 

·
Registered
Joined
·
740 Posts
Higher the amps and voltage, the more careful you should be, disconnect soon as it hits your target, IMO 13.8v.

No need to hold Absorb, better not to at all above that point.

And only fill when you plan to use them, healthier to keep at low SoC if not being cycled
 

·
Registered
Joined
·
252 Posts
LFPs are the safest lithium ion batteries. However, when you say dead - does that mean below 1.0V per brick? If so, most likely the brick is useless, and will not hold a charge for too long. If it were a non-LFP cell such as a regular NMC cell, I would not try to charge it, as it can result in safety issues such as runaway behaviour. For LFP cells, I am not sure what the rules are though. So what is the voltage and how long has the brick been in that state?

If the bricks are above 2.0V, then string 4 in series together and use your FLA charger. Make sure you do not apply more than 4x3.5 = 14 volts for the series with this charger. Since you don't have a balancer (and they are pretty easy to install along with an LFP charger), you will have to continuously monitor the 4 cells to make sure none exceeds 3.5 or 3.6V. Also measure the current. The 5p bricks should be good for at least 30A, but since the state of health is not clear, I would not exceed 10A or 2A per cell for the time being. If the FLA insists on pushing more current, build a 0.2 to 0.5 ohm resistor by winding thin wire into a coil and putting it in series with the brick, to cut the current down to below 10A. Make sure the voltage rises as you charge, and the cells remain cool. Measure the time it takes to reach 3.6V and see if that is commensurate to 62.5Ah.

With a 2A LFP charger, it would take a long time to charge 40 bricks. But you can always get a 4s 14.4V 10A LFP charger from eBay. And for your test you only need to charge them 50%.

So if your plan is to test the EV, then just use a LA 12V battery to test. If you want to test the LFP bricks, then check their voltages. Below a dead threshold, it would mean the LFP is bunk. Above a minimum, it would mean the battery may be usable, but you have to monitor it for a while, see how it charges, what the temperature is, how it discharges. Between the threshold and the minimum, I am not sure what to expect.
 

·
Registered
Joined
·
252 Posts
Yes, LFP once dead flat don't come back, scrap.
John, what is the maximum voltage you would recommend for a 48V lithium ion battery pack? Assume it powers traction / propulsion 48V motor controllers, and electronics and mechanical devices (fridge?) that are rated at 48 V.

If FLA is rated 11.2 - 15V, then four in series would be 60V max. Is this a problem for 48V devices?

13s lithium is 13x4.2 = 54.6V
14s lithium is 58.8V

Is 14s lithium acceptable for a 48V standard, or must I stick to 13s?
 

·
Registered
Joined
·
740 Posts
Lithium-ion is a very general category, includes dozens of different specific chemistries, each with its own specs.

I only deal with LiFePO4 myself, and looks to me like that's not what you're talking about, so I'm not your man.
 

·
Registered
Joined
·
252 Posts
Please assume the lithium ion battery in question is max 4.2V. So is it acceptable to have a max voltage of 14 x 4.2V = 58.8V?

Let me rephrase - when four FLA are tied in series, what is the maximum voltage at 100% charge. Does it exceed the maximum voltage for a "48V standard"?

Is 60V max acceptable in a "48V standard"?
 

·
Registered
Joined
·
740 Posts
If FLA is rated 11.2 - 15V, then four in series would be 60V max. Is this a problem for 48V devices?
The *charging* profile varies by batt vendor and product line.

A given *load* device will have its own specs.

I take all loads offline when equalizing an FLA bank, that's the only time they'll see high voltages, depends on temp but some mfg spec 15.5+V.

Extrapolated to 4s yes that is 62+V, and yes many electronics would be too sensitive to stand that.

I think generally 14.75V, or 59V would be a reasonable upper limit.


Since I charge LFP at 3.45V max, 4s is 13.8V max, at 16s 55.2V max, so well within safe margin. I might get up to 14V or so for maintenance routines, also safe at 56V.
 

·
Registered
Joined
·
740 Posts
Is 60V max acceptable in a "48V standard"?
Pushing the above by only 1V, but likely OK.

Really need to check the specs of the device(s) you're concerned about.

If this is indeed LFP, why the heck are you pushing charge volts so high? Sacrificing IMO many hundreds of life cycles off the back end, for very little gain in AH capacity. Of course the batt maker will be happy about your doing that. . .
 

·
Registered
Joined
·
252 Posts
:) -- no this is not LFP. It is NMC (4.2V max, but I will charge to 4.15).

Another way to put the question - the 12V standard demands that all devices be able to operate from 11V to 15V. If they cannot operate at 15V, then they cannot call themselves a 12V (automotive) device, and if it break at 15V, then the purchase must be refunded.

If a motor controller maker claims to have a 12V controller, then it must handle 15V, otherwise it is false advertisement.

My question is - for 48V, is there also such a convention or standard? Since that is achieve by 4s FLA, and an FLA can be 15V, then if someone advertises a 48V motor and controller, it better work at 60V, without blowing up.
 

·
Registered
Joined
·
740 Posts
the 12V standard demands that all devices be able to operate from 11V to 15V. If they cannot operate at 15V, then they cannot call themselves a 12V (automotive) device, and if it break at 15V, then the purchase must be refunded.

If a motor controller maker claims to have a 12V controller, then it must handle 15V, otherwise it is false advertisement.
What international government or industry body created and enforces this standard?

Sounds like fantasy spun from whole cloth to me.

Links?

Perhaps some sort of informal de-facto understanding.

I doubt respected by the factories in third world countries making most such devices these days.

But IMO just multiply by 4 to generalize, but check the mfg specs for any important or expensive device you plan to buy, the tolerance range is often spelled out in the tech docs.
 

·
Registered
Joined
·
40 Posts
Discussion Starter #12
john, solarsail

Thanks for the feedback. That's the kind of info I needed.

Of the 40 packs 36 of them were at 3.2v, three of them were at 3.0v and one was at 1.8v. None of the low ones were adjacent to another low one. I'm thinking the 1.8v guy might be gone.

I'll get a single cell LFP charger for the four low ones and do the other 36 with the FLA charger to save some time.

Harmon
 

·
Registered
Joined
·
1,551 Posts
Just make sure that whatever you do the charger maxes out on voltage before the cells do or you have a voltage sensing shut off device

Both Pete in sacto and I have experienced OOPs events where cells got fried on a momentary lack of attention. Some people experienced cell ignition.

My $0.02 YMMV
 

·
Registered
Joined
·
1,773 Posts
:) -- no this is not LFP. It is NMC (4.2V max, but I will charge to 4.15).

Another way to put the question - the 12V standard demands that all devices be able to operate from 11V to 15V. If they cannot operate at 15V, then they cannot call themselves a 12V (automotive) device, and if it break at 15V, then the purchase must be refunded.

If a motor controller maker claims to have a 12V controller, then it must handle 15V, otherwise it is false advertisement.

My question is - for 48V, is there also such a convention or standard? Since that is achieve by 4s FLA, and an FLA can be 15V, then if someone advertises a 48V motor and controller, it better work at 60V, without blowing up.
I can help you out here, I build racing golf carts and they are 48 volts. Some Pb, many 16S LFP batteries.

LFP batteries are directly compatible with 12, 24, 36, 48, 60... volt Pb batteries. They can be used as drop in replacements. All of them use the exact same charger be it Pb or LFP. So you are kind of chasing your tail and spinning wheels. Nothing special is required. Just a slight voltage change and frame of mind. Super easy.

A 12 volt Pb battery operating range is 10.5 to 16 volts, and 4S LFP is 10 to 14.4 volts. A match made in heaven. The only thing you have to change in a Pb battery charger is the charge voltage. Today just about all Pb traction batteries use 14.2 to 14.6 volts charging. Any decent 48 volt battery charger can be set from 52.0 volts up to 64 volts and anywhere in between. For 16S LFP you set the charger to 54.0 to 55.2 volts and you are done with it. If you use a BMS and Top Balance occasionally raise the voltage to 57.6 volts so that the Vampire Boards turn on and Top Balance the cells. There is no reason to Top the cells every charge. Last thing you want is a fully charged LFP battery. So lower the voltage down a bit to 54.0 to 55.2 volts so the BMS never turns on and you gain double the cycle life. All you give up is a couple miles of range and gain twice the cycle life. . Or do what the battery manufactures want you to do. Charge to 100% every time so they can sell you new batteries sooner rather than later. It is good for them, not you.

Easy Peazy no thought or knowledge required.

I design a lot of off-grid solar and racing golf carts professionally using LFP and Pb. Makes no difference if it is LFP or PB, it s dang battery that gets the exact same charging voltage. You set the charger voltage to 54 volts, LVD to 48 volts, then battery life is good and worry free.
 

·
Registered
Joined
·
252 Posts
Thanks Sunking - very informative.

Out of curiosity, as weight and volume are important in racing (what is peak amperes?), and cost is important in solar (what size storage?), have you considered using NMC lithium ion cells (3.0V to 4.2V), such as used Leaf or Tesla packs, or new 18650 cells such as Panasonic 3.4Ah?

Comparing new Panasonic NCR18650B 3.4Ah to new LFP prismatic 100Ah, this is what I get:

Unit weight - 3.8 kg/kWh vs 8.9 kg/kWh
Unit volume - 2.2 L/kWh vs 6.0 L/kWh
Unit price - 272 $/kWh vs 375 $/kWh

Used Leaf and Tesla cells are less expensive than Panasonic.

Of course you would need a new charger and balancer. Add 20% cost to packaging 18650 cells.
 
1 - 15 of 15 Posts
Top