Lead Acid Battery notes

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All batteries are rated for 20h discharge. If your discharge current will discharge them faster - expected capacity can be significantly lower than written on battery

lead_acid_discharge_curve.jpg
For example at this image is 12Ah battery. 0.6A as "intended" is exactly capacity discharge rate(0.05C), but increasing current 4x times will lead to 4h runtime, instead of 5h.
lead_acid_discharge_curve2.gif

Don't expect battery to give you 100% of capacity and even don't try to be close to that.

dod_vs_cycles.jpg
Discharging battery too much shortening it's lifetime
Using from 75Ah battery approx 52Ah of charge will lead to ~300cycles lifetime, less than 1 year of service life with daily cycling. While using 22Ah(from 75Ah battery) will result in 1300+ hours of service life.
Do the math and don't be greedy "saving" a little on size of battery. Just adding 2.36 times more capacity will lead to ~4 times longer service time. Thats without calculation of labor required to change batteries and expected downtime


Charging rate for lead acid is SLOW and dynamic rate,slowing at the end, close to log(x)

tlv1228v1_-_12v_2.8ah_sealed_lead_acid_battery_with_f1_terminals_-_charging_characteristics.jpg
For example if system is designed for 40% DoD, you need 6 hours charging time for missing 60%. And 40% of them you will get in 2 hours.
NOTE: Undercharging battery(not reaching 100%, for example keeping battery in 40%DoD to 80%DoD range, without reaching ~100%) will lead to sulfation of electrodes and shorted service life.

Leaving battery in hot environment will shorten battery life

sealed-lead-acid-battery.gif
Each 8-10C after 20C "manufacturer expected temperature" shorten your battery life 50%.


Some example math for financially efficient setup (DRAFT)

For example you have datacenter with average current 10A (2.2kWh), and you need to provide 2hr backup runtime daily due unstable electricity, let's assume we are in country with supply issues
Current prices is taken for example for AGM Crown 110Ah = $235.00, which (very crude) means 2.1363$ per Ah.
Consumed current from single "theoretical battery" 12V is 183.33A. With required 2hr runtime it is 366.66Ah of capacity.
This gives us assumption if we want to reach not more than 30% DoD, our total capacity should be 1222.2Ah. As our discharging rate is 0.15C. Approximate loss on such discharge rate is 10%, so we need to add them in equation, resulting in 1344.42Ah required capacity.

Which is $2872, with expected service time in cooled room will be ~7 years, will result to $410.28/year or $34.19/month

But for example if owner is greedy, and he install bare minimum (70% DoD, UPS will cut off) outdoor, with summer temperatures that might heat small room to 45C. Let's assume with relatively cold winter it will be 30C-35C average temperature.
Required capacity of storage 476.658Ah with discharge rate 0.4C. Approx capacity loss on such discharge rate 30% (it will cut off much earlier), so we need to top capacity 619.6554Ah. Cost of setup $1323.76
Approximate service life of setup is 1.5 year, with temperature "issue" it is shortened to 0.75 year.
Approximate cost of running $1764/year or $147 per month

One more nail to coffin: At approx 50-70% of service life UPS might cut off earlier than 2 hour required runtime.

Thats good very approximate example, how twice cheaper improper setup will lead to 4-4.5 higher operational expenses.

P.S.

  • Sure i know that backup system never use 12V, and usually batteries are in series.
  • Do not put battery in same airspace as servers, if it is not sealed batteries. Acid vapors will quickly kill all PCBs in your aircooled equipment.