T&L Publications

How long would a 10ah and a 19ah (two different batteries)but both 12volts lead acid batteries last when using around 70 watts.<p>Thanks
Myp71
Ryan

70W at 12V = 5.83A<p>A fully charged 10Ah battery will deliver this current for 10 divided by 5.83 = 1.7 hours.<p>A fully charged 19Ah battery will deliver this current for 19 divided by 5.83 = 3.2 hours.

David is correct on the discharge current.
However the lead-acid battery capacity is based on a 20 hour discharge rate. Which means the 20 hour rate for the 10AH battery would be .5A.
But since it is being discharged at 5.83Amps the the would be closer to 1.25 hours to a discharge voltage of 11 volts.
The 19AH battery normal discharge current for the 20 hour rate would be 19/20 or .95Amps. So at the discharge rate of 5.83Amps it would last very close to 2.5 hours to the discharge voltage of 11 volts.
These discharge figures were taken from a set of characteristic discharge curves for lead-acid batteries at a discharge temperature of 68 degrees F. Of course the discharge rate is assumed that the battery is fully charged before the discharge period.
Ned

For lead acid batteries, that amp-hour rating is usually cited over a 20 hour period. For example, 10 Ahrs/20hrs = 0.5A . If you drew 0.5A out of the battery, you would expect to get the full 10Ahrs out of it.
But, the capacity decreases when you put heavy loads on it, expecially for lead acid batteries.
Page 6 of this link to powersonic batteries has a nice graph of sealed lead acid battery discharge time -vs- discharge rate.
Quickly looking at the graph, it looks like 55 min for the 10Ahr and 2 hours for the 19Ahr.<p>Scott

Another consideration is the cycle life of lead acid batteries (or most Lead acid, deep cycle, VRLA rechargable batteries). When "using" 100 % of the capacity of the battery, the long term cycle life will be affected negatively. In tests here, we saw 50 % reduction in cycle life (useable number of times of discharge and charge) when using 100 % Depth of Discharge (DOD) in the unit under power. A rule of thumb we use for design is to limit the repetative discharge to 50%. 100% DOD is not a show stopper, just not something to repeat. Deep cycle batteries are better behaved when going to 100% but still experience a limited cycle life when repeatably discharging the battery past 75% compared to deep cycles that are limited to 50%.