Overview of the entire system is here
For the battery setup I have started with getting familiar with the different
Lithium chemistries that are currently in use and found that most people
suggest LiFePo4 for boats where energy density is not that critical but safety
is. In comparison to other Lithium chemistries LiFePo4 seems pretty easy to work
with and the likelihood thy they transform the boat into a giant firework is
Further, Lithium batteries require a battery management system (BMS) to operate
them safely and the quality of that BMS is important for the longevity of the
system. The most basic ones monitor high and low voltage cut offs and can
switch relays (internal or external) to cut power to/from the battery in case
the battery starts to wander outside high or low voltage cut off. Better ones
also monitor the temperature of the cells as Lithium batteries must not be
charged under a certain temperature or if they get too hot. This is important
for the design of the circuits around the bank, I'll get to that later. Another
thing the BMS can look at is cell balance. Good BMSs can balance cells and show
a very detailed picture of cell condition so that it is easy to see if the
whole pack is save to operate. Even more important, if it is not safe to
operate what has to be done to get it back to a safe state. For boating
applications it is super important that all the interaction with the battery
pack is automated as there is simply no way to manually act on an alarm that
gets triggered and requires manual intervention. That alarm would go off when
the weather is tough, you are on watch by yourself and it is 3AM. No one wants
that so the BMS needs to be good.
The circuits for charging and discharging a Lithium pack need to be
separated for a simple reason. If we run into an overcharge situation and the
pack disconnects we need a way to drain the pack. If we only have one circuit
we can't do that. That is the main issue I have with “drop in Lithium
installations” now that I understand more about the topic. What this means is
that we need a charging circuit that has all the charging sources connected:
solar, wind generator, shore charger, generator and so on. That circuit is then
connected to the pack via a relay that can be switched by the BMS if we
run into a over charge situation. All the consumers then need to be connected
to a second circuit that can be cut off if we run into an undercharge situation.
With this setup we can make sure that we can operate the pack in a safe way and
have ways to automatically recover from failure modes. Additionally we can cut
off charging if the temperature falls too low (if the BMS supports it) as
discharging is still possible at low temperatures, not ideal but possible.
Building large Lithium packs from prismatic cells can give us the option of
building optimally balanced packs with BMSs controlling every cell in the pack.
Balancing is important as the cells should never diverge too much from each
other. Building a large bank from batteries like Battle Born or Ruxiu mean that
we have packs or 4-5 prismatic cells with a BMS per battery and the batteries may
diverge over time. Having big packs and only one or two means we can better
balance the packs over time. More control down to the cell level also means we
can replace dead cells if we have to, we can take a cell out and
charge/discharge it if it diverges too much and the BMS can't get it back, in
short, we have options.
In terms of prismatic cells we have a few players, Winston being the most
popular at the moment. Many of the big brand Lithium batteries are Winston
cells on the inside. Looking at different prismatic cell producers I found GBS.
Their chemistry is save to operate to lower temperatures than Winston cells and
I like the physical form factor and their case design better. It looks like it
would make it easy to build very clean looking packs with cable management for
the balance leads all ready to go.
Next I'll get in touch with GBS and Battrium ta talk about options, prices and