[time-nuts] Re: Backup DC power with current battery technology?

[Tom Van Baak]

John,

I hope you take the plunge on LiFePo4, especially if you plan to put 
some gear in a car for mobile lab use. Maybe talk to off-grid or RV 
users for advice.

I looked into various Li battery solutions and was scared by the prices, 
learning curve, and churn of brands to trust. The unusual weight, size, 
price ratios weren't attractive for a home time lab so I still use AGM 
batteries for my 12/24 V backups. Deep cycle marine grade when they are 
on sale.

I get a couple new SLA / AGM each year and rotate out the ones that are 
in poor shape (5 to 10 years). It's convenient having a standard battery 
size & chemistry to deploy in multiples of 1 or 2 or 4 for various 
voltage and redundancy configurations. Also this way there are no 
accidents with different chargers vs. different battery types. I make 
heavy use of Powerpole connectors for everything: batteries, clocks, 
extension cables, and chargers.

For the clocks, I gave up on full-time connected float/chargers. Now I 
only use diode-OR to combine a live 24+ VDC mains power supply and one 
or two standby 12+12 batteries, with the p.s. voltage set a bit higher 
than the battery would be under load.

When I know that a battery is low -- due to self-drain or after a power 
failure -- I simply swap Powerpole cables, replacing a now-tired AGM 
with a recently-charged AGM, and the clock never notices. The tired AGM 
can then be charged on its own, in full isolation. This way the weird 
"smart" charging profile that modern chargers employ never conflicts 
with the clocks and it also keeps charger noise completely away from 
instrument ground and power.

If this manual process were a time sink I'd use an automated round-robin 
DPDT relay system to rotate batteries in and out as needed. Again, I 
gave up on the "triple" method, where a mains AC/DC power supply, a 
standby battery, and a float charger were all somehow connected 
together. I'm all ears if someone has a good solution for that. I tried 
using double conversion UPS systems but they were expensive, noisy, and 
ate batteries.

/tvb