Run Time
A sub-page of http://bicyclelighting.com
It can be quite complex to calculate the actual run time of a system because the actual battery rating cannot be directly used. Sealed lead-acid batteries are typically rated for 20 hour discharge rates (5% per hour, C=0.05), but their ratings decrease significantly at high discharge rates. For example, a 2.3AH, 12V sealed lead-acid battery has a 28WH rating only when the discharge current is 0.115 amps. With a 26 watt lighting system the discharge current is 2.2A, and the battery rating is about 1.3AH, only about 57% of its rated capacity. "C" refers to the discharge rate in terms of the capacity; a 7AH battery discharged at 0.35A/H would have a C rating of 0.05 while the same battery discharged at a rate of 1.4A/H would have a C rating of 0.5. There is a chart with this information at http://www.flex.com/~kalepa/techman.pdf. On my system, which has a 12V, 5AH battery driving two 14W headlamps and a 2W tail light (30W), I get about 1.7 hours of run time. So a 60WH battery runs 30W of lights for 1.7 hours, which is a 0.85 derating factor (much better than the chart below indicates). However I have not actually measured the current from the battery, I'm using the ratings from the lamps.
NiMH batteries do not suffer from the nearly the same problem in terms of discharge current lowering the capacity, in fact the current drain needs to be extremely high before the capacity goes down (at at low drain rates it actually goes up).
|
Battery
Type |
Volts |
AH
(Amp-Hours) |
Watt-Hours |
Watts
(Front + Rear) |
Discharge
Current (A) |
"C" |
Derating |
Run-Time
(Hours) |
|
Sealed Lead Acid |
12 |
7 |
84 |
15 |
1.3 |
0.2 |
0.80 |
4.48 |
|
Sealed Lead Acid |
12 |
5 |
60 |
15 |
1.3 |
0.3 |
0.75 |
3.00 |
|
Sealed Lead Acid |
12 |
3 |
36 |
15 |
1.3 |
0.4 |
0.70 |
1.68 |
|
Sealed Lead Acid |
12 |
2.3 |
28 |
15 |
1.3 |
0.5 |
0.65 |
1.20 |
|
Sealed Lead Acid |
12 |
7 |
84 |
26 |
2.2 |
0.3 |
0.71 |
2.31 |
|
Sealed Lead Acid |
12 |
5 |
60 |
26 |
2.2 |
0.4 |
0.70 |
1.62 |
|
Sealed Lead Acid |
12 |
3 |
36 |
26 |
2.2 |
0.7 |
0.60 |
0.83 |
|
Sealed Lead Acid |
12 |
2.3 |
28 |
26 |
2.2 |
0.9 |
0.57 |
0.60 |
|
NiMH |
12 |
2 |
24 |
15 |
1.3 |
0.6 |
1 |
1.60 |
|
NiMH |
12 |
2 |
24 |
26 |
2.2 |
1.1 |
1 |
0.92 |
|
NiMH |
12 |
2 |
24 |
36 |
3.0 |
1.0 |
1 |
0.67 |
|
NiMH |
12 |
7.5 |
90 |
15 |
1.3 |
0.2 |
1.1 |
6.60 |
|
NiMH |
12 |
7.5 |
90 |
26 |
2.2 |
0.3 |
1.1 |
3.81 |
|
NiMH |
12 |
7.5 |
90 |
36 |
3 |
0.4 |
1.0 |
3.00 |