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

[glenlist]

For charging LiFePO4 batteries, these are reasonably compatible with 
lead acid chargers with the following caveats

1) Lifepo4 have approx zero temperature coefficient, so any temperature 
compensation on the lead acid charger, if present needs to be disabled. 
(if is a good lead acid charger, it will have compensation and a 
temperature probe)

2) Equalise mode on the lead acid charger , if present must be disabled. 
Equalise mode  is designed to deliberately overcharge the battery to 
being all cells to max. This will dramically overcharge the LIFEPO4 
cells. no good.

3) Low battery cutoff, like on a lead acid, is essential. Set for 2.7V 
per cell. LIFEPO4 cells if discharged below 2.7V per cell say all the 
way to 2.5,  must have initial charge rate of less than C/50 ideally 
until they reach 2.7/cell.

LIFEPO4 batteries, due to their all-off -capacity performance, will 
provide approximately 2x the effective useable capacity of a lead acid 
cell, especially at something like 0.333C discharge (3 hours). Most lead 
acid batteries are specified for 10 or 20 hour discharge rates .   
overall you are a factor of 4 ahead of weight !.

Now- time has marched on, and many modern chargers either have 
microprocessor contrained LIFEPO4 profiles or chips that know what they 
are doing, so finding a suitable charger is fairly straightforward. A 
pro lead acid charger will be fine most likely, also- because on a pro 
charger you would have control over options like temp comp and equalise 
mode.

feel free to contact me direct

glen


On 27/05/2024 12:39 am, John Ackermann N8UR via time-nuts wrote:
> I need to replace the batteries in my 28 VDC power distribution system 
> in the clock room.  It's currently two series 12V AGM batteries with 
> an IOTA Engineering float charger.
>
> This system has two purposes: (a) primary power for some OCXOs and 
> other gear; and (b) failover power for some AC/DC gear like HP 
> standards. Under normal conditions, power draw is 2 or 3 amps. During 
> mains outage, it could reach 10 amps.  Now that I have a house 
> generator, long run-time isn't a big issue so 25 Ah or so of capacity 
> is plenty.
>
> I haven't explored LiFePo4 batteries until now, and I'm having trouble 
> finding a charger that can provide significant continuous load current 
> while maintaining a battery floated across the line. Does anyone have 
> recommendations for something like this?
>
> Thanks,
> John
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