On 3/8/19 5:38 AM, Bob kb8tq wrote:
Hi
If you are headed into a “cost is no object” sort of battery backup bank for
the lab / shelter / cave
there are some interesting options out there. The RV world is very slowly
waking up to the fact that
big heavy lead acid batteries may not be ideal. For a ghastly amount of money,
you can now get
12V 100AH “drop in” (no they are not drop in) Lithiums with fully integrated
BMS systems in them.
They *appear* to be safe and come with cute things like 10 year warranties . Be
aware that you
can buy a nice 5065 on eBay for what a couple of them will set you back …..
I've used the A123 7Ah "brick" LiFePO4 that replace the standard Lead
Acid - they weigh a LOT less, and cost a LOT more ($100 vs <$10) -
They're pretty nifty devices. As you note internal battery management,
built in (easily replaceable) 30A fuse, etc.
They weigh about 850g (vs 2.6 kg) and work well at low temperatures (-20C)
One peculiarity is that if you discharge them below minimum voltage,
they cutoff and put out nothing. You have to put a charging voltage on
them to turn them back on. This fouls up some "smart" battery chargers
which use the "discharged" battery voltage to tell them that there's a
battery connected. So we always had our smart chargers for day to day
use, but had a couple wall wart trickle chargers around to turn the
battery back on.
100 Ah would set you back well over a kilobuck. But, you would be able
to lift it without you going over the OSHA 55lb/25kg "one person lift"
limit.
In return for your massive cash outlay, you get a battery that does not outgas
or have the need
to be watered along with the plants. It has a much flatter discharge voltage
curve than a lead acid.
Their peak current vs voltage generally is better than a lead acid. The BMS may
impact this if it
is an issue in your setup. They also is a *lot* lighter / smaller for the same
amount of useful
capacity. The built in BMS *should* take care of all of the fiddly little
issues that might burn the
house down. Lots to like other than the absolutely giant hole in the wallet ….
Bob
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