I learned today by reading the technical manual on Power-Sonic's VRLA/SLA batteries that they recommended the following cut-off voltages for batteries under load:
Load to Volts per cell = 12V equivalent @ 68 degrees F
0.05C (20 hour rate) to 1.75V/cell = 10.50V (Ah available = 100% of rated capacity)
0.10C (9 hour rate) to 1.75V/cell = 10.50V (Ah available = 90% of rated capacity)
0.20C (4 hour rate) to 1.75V/cell = 10.50V (Ah available = 80% of rated capacity)
0.33C* (2 hour rate) to 1.67V/cell = 10.00V (Ah available = 72% of rated capacity)
0.50C (80 min rate) to 1.67V/cell = 10.00V (Ah available = 65% of rated capacity)
1.00C (33 min rate) to 1.50V/cell = 9.00V (Ah available = 55% of rated capacity)
2.00C (12 min rate) to 1.50V/cell = 9.00V (Ah available = 40% of rated capacity)
3.00C (7 min rate) to 1.37V/cell = 8.22V (Ah available = 36% of rated capacity)
*This rating was not in the data from Power-Sonic - I estimated it based on the other values because I needed a load value between .2C and .5C.
Of course other manufacturers will have somewhat different specifications, but I think these are pretty reasonable numbers and I intend to use them on future tests of VRLA/SLA batteries with my new battery analyzer in place of the default setting of 11.4V.
I also found the following interesting:
- These recommendations cover batteries ranging from 0.5AH to 100.0Ah.
- A 1C discharge rate reduces the Ah capacity rating to about 1/2 of the 20 hour rating (which is how most batteries are marketed).
- A 3C discharge rate reduces the Ah capacity rating to about 1/3 of the 20 hour rate.
- In the 400VA UPS units I have been modifying, 7Ah rated batteries were used. With a full load, this corresponds to a 3C discharge rate and should last about 7 minutes.
- Replacing a 7Ah battery in a 400VA UPS with a battery* rated at 40Ah gives 70 minutes of run-time at full load. A 10x increase in run-time in exchange for a 6x increase in rated capacity.
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