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Jim,

Some quick thoughts about designing in lead-acid batteries in your system. The first is that the battery storage capacity will degrade over time so there is a balance between life of the battery and the number of batteries. This is complicated by the fact that the life of the battery is directly related to the number of charge/discharge cycles and how fast the battery is charged. End result is you will need more batteries than is readily apparent from the simple numbers.

If it was me I would be looking at a combination of batteries with ultracapacitors used to reduce the number of cycles and charge rate on the batteries.
 

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Hi Jim,

The primary use I see for ultracaps in an application like this is between the gas generator and batteries. The physics that work against the system is the gas generator is inefficient at low power levels and batteries should not be charged quickly. End result is a lot of power is wasted while charging the batteries. With an ultracap in between, the generator can run a short period of time and quickly charge the ultracap. The generator can then shutoff and batteries recharged by a trickle current from the ultracap. Check out:

http://www.maxwell.com/products/ultracapacitors/applications/application.aspx?sid=QUICK-CHARGE

Ultracaps don't have anywhere near the energy density of a lead acid battery so they are not much use as a prime energy source. The best example I have seen for their use is a scenario for battery operated carts with charging stations along the route such as a golf course. Pull up to the tee and plug in the cart. A quick burst of current charges the ultracap and then as the cart is driven off the battery is slowly charged.

The best way to estimate the size of ultracap is to profile the energy usage over time. Once this is done then the combination of ultracap and battery usage will determine the cycle rate of the generator. If the rate is greater than an hour then the system should be very efficient compared to battery alone and the battery life should be extended considerably.
 
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