[PSUBS-MAILIST] Lithium Batteries

[Antoine Delafargue via Personal_Submersibles]

also,
another cost issue with lithium ion batteries (incl LiFePO4) for our
applications is the Battery Management System (bms).
You cannot just add batteries in series and parallel like with lead acid,
because you need to control the voltage of each cell very accurately to
avoid over charging or one faulty cell debalancing/shorting the others, and
to avoid too brutal cells charging after prolonged rest, all of which could
lead to fires. The bms is also needed to manage cell balancing at the end
of the charges to bring up cells lagging in voltage up to full charge
whilst preventing the faster charging cells to exceed safe maximum voltage.
This requires extra cabling to purposely bleed off the faster charging
cells.

So you have several options:
-go with a fat battery with a built in bms. but that limits the amount of
kWh stored (in which case you may be better off with lead acid as weight is
not an issue in our subs), and constrains flexibility on dimensions.
-use certain models which have built in bms allowing for a certain pairing
in series/parallel. but those may be quite expensive as these are fancy
features and they duplicate electronics in each battery.
-make a dedicated pack and bms. Having a professional do a tailored bms can
double the cost of your battery. There are probably open source bms
architectures available out there (from the solar home systems,
survivalist, mountain biking etc. communities).

regards,
Antoine



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On Fri, Apr 10, 2020 at 3:10 AM River Dolfi via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Alan,
> I've seen the price comparisons that you're referencing. They're based on
> total cost of the lithium bank distributed over a "lifetime" of 3000 full
> discharges or so. After 3000 cycles, the batteries are at 80% of the
> original capacity. That is what they constitute as "failure".
> The problem is, most psubs, and most professional subs for that matter
> don't come close to 3000 total battery cycles. That makes it hard for most
> people to justify the initial purchase price.
>
> -River J. Dolfi
>
> 412-997-2526
> rdolfi7 at gmail.com
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