[Solarsail]

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.

 

[Solarsail]

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?

 

[Solarsail]

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"?

 

[Solarsail]

-- 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.

 

[Solarsail]

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.