Real Usable KWh standard?

[Moonshot]

Hi all, I'm on my way to building my own custom light EV... timeline TBD.

I'm at the early stages now and comparing KWh of different battery options. However I'm not sure if all stated capacities are apple to apples in comparison. The question comes down to whether the capacity refers to the difference between maximum safe voltage and minimum safe voltage, or the difference between maximum voltage down to zero volts, so that they can unrealistically inflate their marketing material.

Either one is "fine" as long as it is at least followed by all players involved. I just want to know what it is.

The two options I'm currently considering are the LiFePo4 cells from Chinese sources like these: Aliexpress Lifepo4 280Ah

... or on a totally different level, one battery module from a Tesla Model 3 LR (assuming it can be properly integrated atm). If the M3LR has 82KWh and one module is basically one quarter of that then can I expect about 20KWh of usable capacity?

If there is a way to know how to compare these apples to apples I'd like to hear it.

 

[Moonshot]

I should have elaborated on the LiFePo4 cells.

They are "280Ah" so that leads me to multiply that by their nominal voltage of 3.2V for a total of 896Wh per cell. I would need 30 of them for a 96V pack so that would be 26.880KWh of capacity. The question is then, is that a proper number to compare apples to apples to the Tesla M3LR battery module of about 20KWh roughly estimated.

 

[Chalo]

If you deal with fly-by-night Chinese hustlers, you have to accept that their claims and specs are whatever they believe will sell. There's basically nothing to hold them accountable for inaccurate claims. AliExpress contains some impressive bargains, but overall it's a wretched hive of scum and villainy. I'm below 50% so far in receiving what was described in an AliExpress listing. They'll send you something that's clearly not what they represented, and then when you complain, they'll tell you you're mistaken.

I would definitely either acquire sample cells and test them, or resign myself to the likelihood that they'll dramatically underperform their ratings.

 

[Moonshot]

but overall it's a wretched hive of scum and villainy.

LOL!

Yes I'm as cautious as I can be when I'm looking at spending a lot of money there. I look for sellers that are reputable and preferred by the groups that buy large quantities of that kind of product. So far I've received everything I've ordered on AliExpress to a satisfactory level. I've never ordered batteries there though.

I'm really just trying to establish whether capacity ratings are based on the difference between full charge and 0V, or full charge and minimum safe Voltage. Then is that enough of a standard to be comparing capacities of different battery options?

Sometimes I feel like I try to overly describe my question so that it seems like I'm looking for a complex answer. Now is probably one of those times.

 

[neptronix]

kw-hrs is calculated by:
amp-hours x nominal ( average ) voltage at X amount of draw ( C rate )

A common scam in rating is to use a C-rate like 0.02C ( IE way under the maximum load ) so that voltage drop is ultra low, therefore the nominal voltage is at it's highest across the discharge curve, and that's how we get an amazing 280whrs/kg out of a new 18650 cell, but when we load it, we get significantly less than that because the voltage sag is higher ( whether it's way higher, or a little higher, depends on the internal resistance of the cell; IE how powerful it is )

More simply, take 10% off most kw-hrs calculations from vendors because the industry is BSing a lot of us.

Even more simply, if you have a quarter of the amp hours of a given pack, then you have a fourth of the kilowatt hours, per the calculation. :wink:

 

[amberwolf]

I'm really just trying to establish whether capacity ratings are based on the difference between full charge and 0V, or full charge and minimum safe Voltage. Then is that enough of a standard to be comparing capacities of different battery options?

It *should* be, more or less, the latter, but as Neptronix noted, it also depends on the conditions under which the rating tests were performed (if they even tested it and aren't just using calculations based on perfect conditions rather than cell-manufacturer spec sheets, etc).

But for comparing, you can only really compare options from the same seller, and even then only if they made them themselves and are being equally honest or dishonest about all of their products. If they buy them from somewhere else, especially multiple somewhere elses (which you can never know), then you're dealing with *at least* two levels of someone else's decisions about how to represent the products.

The essential point is that unless you *know for sure from personal experience* that the specific seller and manufacturer in any particular case is honest and *using* the standard method, you should assume there *isn't* a standard.

The normal way to calculate battery capacity is as Neptronix stated, if you want to use that for reference in testing what you get.

But don't expect any particular seller to hold to it.

A good general rule to follow is, especially with batteries: The better the deal you think you are getting, the more likely you are being scammed.

If you're not sure what you're getting, ...test it when you get it, and if it doesn't meet the stated specifications in any way, contact your choice of ways to return/refund it (payment method, seller, site seller is hosted by, etc). No guarantee you'll get your money back or the product you expected, but worth a try.


And note with batteries (vs just cells) that it's not just the ratings they provide that are too-frequently dishonest. Too often, the cells in a battery are not from the same batch, not well-matched, not even matched at all.

This is something that applies also to purchasing a bunch of cells to make your own battery with--if you can communicate to the seller sufficiently well (not a guarantee regardless of languages spoken), get all cells from the same batch, and if they have the testing equipment (all manufacturers should, most sellers that aren't manufacturers don't), have them test the cells and send you only ones that are matched within as close a tolerance as they can get you, for all characteristics; your pack will work better and stay balanced better for a longer time that way.


They can even be random recycled garbage cells of random actual capacities. Build quality of interconnects, wiring, packaging...well, you can look around the forum at the various battery problem / troubleshooting / repair threads, and get some idea of the range of issues you may encounter.

None of that takes the BMS into account, and it's various settings, limitations, and designs, or manufacturing quality (or lack thereof).


I would like to be able to be less cynical about this stuff, but every new thread / post about a bad battery or a "brand new" kit, bike, system, etc., that includes a battery problem just makes the cynicism worse, after we see the root cause(s).

 

[john61ct]

Use Ah as your primary unit for energy capacity when talking about a given chemistry, voltage is a given

Wh and Kwh are derived from that, and only useful if comparing packs at different S-counts.