Our Grand voyager regularly will not start. we are told it is not the battery. The electrics seem to be able to turn themselves on at 22 C, even if we turn them off. we then turn the key and it just clicks. we then have to jump from another car. This happens maybe 50% of the time and 50% it starts fine. Its as though something just drains the battery from the electrics. Mechanics have failed to find the answer. I wonder if it is the battery misdiagnosed, the starter motor or what? Thanks for any suggestions as to what to do

This is a basic case of your battery and charging system needs to be PROPERLY tested.

First, the battery: If the machanics that you went to checked the battery with an old-school carbon-pile battery tester, this is not good enough for today's batteries used in today's high-tech cars. The battery should be tested with a tester that checks the conductivity of the battery. This is the only sure way to detect a sulfated or partially shorted cell. I have seen many batteries that pass the old carbon-pile test and fail miserably when conductivity is checked.

Battery Cables: The battery cables should be tested for voltage drop. This is the ONLY way that you can be sure the cables are in good condition and are connected properly. The POSITIVE cables should be checked for voltage drop between the battery and the starter as well as between the battery and the main fuse block under the hood. The NEGATIVE cables should be checked for voltage drop beween the battery and the engine block and from the battery to the body. The maximum allowable voltage drop with the cables loaded (headlamps on, blower motor on high) is 300 milivolts (0.3 volts) for the POSITIVE cables and 200 milivolts (0.2 volts) on the NEGATIVE cables. (most will have MUCH less voltage drop than this.)

Alternator: Again, old shool is not good enough. Alternators are regularly tested only by checking charging system voltage at idle. It should be between 13.7 and 14.3 volts. However, this is not the complete picture. A charging system tester should be used that can apply a measured load to the battery while checking the charging system voltage at an engine speed of 2000 RPM. This can detect things like weak alternator diodes or weak/loose connections in the alternator circuit. AC ripple should also be checked to make sure that the diodes in the alternator are not allowing an excessive amount of AC current to get to your battery.

If any problems are found in any of the tests above, these must be fixed before any further diagnosis can be performed.

Then there is the Ignition-Off Draw test. To put it simply, this test measures how muchcurrent is being drained from your battery with the ignition turned OFF, using either an inductive or in-line ammeter. Some draw is perfectly normal due to computer memory, radio station settings memory, etc.. The absolute maximum allowable draw with the ignition OFF is 300 miliamps (0.3 Amps) Most vehicles that I have tested are normally below 100 milliamps.

Although there could be a bunch more time needed to track down the actual source of a draw on the battery, the entire charging system and ignition off-draw tests listed above can be performed by a mechanic that is worth his salt in 1/2 hour (assuming we are talking about a properly equipped shop). So don't let anyone gouge you for excessive diagnostic time just for perfoming these tests and reporting back to you. Or to put it another way, within 1/2 hour they should be able to tell you which of these tests your vehicle failed and if more diagnostic time is needed, they should be able to give you a REAL GOOD reason WHY.