Illini,
There isn't much information to go on here, but there's no reason why the external lead acid batteries in series couldn't crank your engine. If your aircraft battery is capable of performing a battery start (it is), then the external batteries should be able to do the same thing.
The car batteries are typically rated at up to 1,000 cold cranking amps, and often are higher. How you were able to get 25 volts from the two batteries is a bit of a mystery, as your voltage should have read 24 or lower...but I don't think that plays a part in your problem.
The most likely cause of the failure to excite the starter, was an incorrect hook-up for the batteries. Either they were connected in series in reverse, or not grounded properly when attached to the airplane, or attached in reverse.
An alternate possibility is that the ground power receptacle was used to attach the external batteries. In many airplanes, a minimum value must be available from the aircraft battery in order to close the contacts for the ground power relay, in order to admit external power. If the aircraft battery is dead, no power exists to close the external relay. It doesn't matter what you have for voltage in such a case, because it's not going to put power on the airplane.
In such a situation, your only option is to hook the external power directly to the battery. In most cases this is inadvisable; you're far better off removing the batteries and charging them. However, in a pinch, you can stretch jumper cables directly to the terminal attachments on your aircraft battery, and crank the aircraft directly.
If your aircraft battery had an internal short, this could have provided an additional drain which wouldn't allow enough amperage to reach the starter solenoid.
A problem with charging any kind of battery in the airplane is the potential for sparks. Batteries reliease hydrogen, which is flammable, and in a confined space such as the battery compartment, explosive. Also, the initial rush of voltage to a drained battery can cause a voltage spike which can damage equipment hotwired to the aricraft bus, and even current limiters protecting other attached busses or elements.
Charging should never be done in the airplane, if at all possible. the only exception is passive charging via a GPU or other external power (which can be an APU or GPU, by the way), when external power is attached at system voltage, allowing the aircraft batteries to draw a normal charge in a similiar manner as if receiving generator voltage.
With respect to charging batteries, the lead acid and nicad can be charged together; the special requirement for nicad involves deep cycle charging, which is only done periodically. That's typically done by cell, with the battery opened in order to accomplish the full cycle. Nothing you need to worry about in the field, or as a pilot. A far greater danger of charging with a NiCad is the risk of thermal runaway. Once it starts, the battery will keep heating, and will either catch fire, or melt through the bottom of the aircraft. It happens, which is one good reason among many to switch back to lead acid. A wild or unregulated charging source is more likely to induce a thermal runaway (which can also occur naturally as a result of an internal short).
Your mechanic should know everything necessary for charging batteries. When charging, the filler caps should be loosened to allow the battery to more properly vent. Charge in a well ventilated area away from sparks or sources of ignition. Charge away from NiCad batteries, and if you test electrolyte (you should, or the mechanic should), be sure to use a dedicated instrument which is not used for testing NiCad. Don't look into the cells while charging, and wear eye protection when handling the battery, the caps, or the fluid.
If you'll be running external power while the battery is out (you can do this in some aircraft, and cannot in others), be sure to insulate both the battery terminals (particularly the positive, in the case of an aircraft with negative ground; (virtually all aircraft DC systems are negative ground). Consult your DoM or maintenance department for special precautions about putting power on the aircraft, especially with the battery removed. In some cases, circuit breakers must be pulled, etc.
Despite popular wisdom about cleaning batteries using baking soda, don't do it if you love your battery. It's acceptable to do so in the battery compartment if you're inclined, but wash it thoroughly and rise thoroughly; don't leave electrolyte, or baking soda in the compartment. Dry it completely before putting the battery back.
An obvious caution (but worthwhile, anyway); use caution removing and replacing the battery. aircraft batteries are often hard to reach, require manevering out of small spaces in a less than ideal posture, and involve lifting above the waist. All of these things can contribute to a quick back injury or a pulled muscle.
Also note the arrangement of the battery cables. Note the use of safety wire, if appropriate, on fasteners on the battery case. Ensure that when the battery is replaced, the positive terminal or cable makes no contact with any metal surface other than the battery, and that the battery is imobilized. Don't risk a fire. (I saw a battery melt in a Cessna 207 once in this manner).
Don't be too surprised about the automotive batteries. I've used them many times myself, starting turbine equipment, and even large radial engines. When I was operating a LR35, one airport we frequently used had a rack of lead acid car batteries for the external power source, on a small welding cart. It worked just fine.
One final possibility with your troubles may have been the state of charge for the aux batteries themselves. They just may not have been up to the task. It's always a consideration. Remember that you can still get a voltage indication, but not have any significant amperage behind the battery to drive the starter. Have those batteries checked, as well as your aircraft battery. Look for popped circuit breakers, blown current limiters, etc. Check all the possibilities.
Good luck!!
