Batteries
Hi,
All three battery types discussed here have their pros and cons. Lead
acid are probably the most robust and some versions use gell instead
of liquid electrolyte, which makes them even safer in terms of electrolyte
spills. Major disadvantage for a/c use is the poor energy density to weight
ratio. Aircraft style nicads are also quite robust, have long life and are
lighter than lead acid for equivalent capacity. Both the above will withstand
moderate rates of overcharge with no damage at all, other than electrolyte
loss. Lithium Iron are well proven, have been in use for years and are quite
safe, despite the occasional scare story re portable computers. However,
they have far more critical requirements in terms of charge / discharge
currents, voltages and temperatures. Energy density / weight ratio is very
high for these batteries.
I don't know what the dc rail is in the a/c, but if at 270v or so, as
someone else mentioned, it becomes very difficult to ensure that individual
cells within the pack match in terms of charge / discharge characteristics.
If there is mismatch, either in capacity or self discharge, some cells
discharge faster and will take differing times to recharge. Over time,
some cells become overcharged, while others never reach full capacity. This
is exacerbated by high charge current and discharge loads, in relation to
battery capacity.
The best way to solve this is to have separate monitor and charge circuits
for each individual cell, but his becomes expensive for large groups of cells.
Users like telcos do monitor in this way to maximise battery life, but
typically in groups of 3 or 6 cells. On a 28v system, there are typically 14
cells in series, whereas a 270v system needs 135. This just makes the problem
of mismatch monitoring more difficult, especially for Lithium Iron.
Yuasa are a world class manufacturer and my guess is that there is a problem
with the charging / monitoring electronics, or cooling. That or the batteries
are being driven too close to their design limit to save weight...