Q What could be causing my batteries to fail?

I purchased a new Pilote 650GJ in June 2021. During my initial trips, I noticed that, when I parked up after driving for several hours, the leisure battery was completely flat.

It transpired that, due to a wiring error, the charging system was not supplying charge to the leisure battery and, meanwhile, the fridge was drawing 15A from the leisure battery while the engine was running. As a consequence, the leisure battery failed and was replaced under warranty.

In July this year, I drove for three hours to Yorkshire. I parked up on site for two nights. When I tried to start the engine to drive off on the third day, it would not start.

Subsequent investigations showed a faulty cell in the starter battery and this was also replaced under warranty in August, having lasted just over a year and only 10,000 miles. I suggested to the dealer that there could be a common cause for both batteries to fail so early but it said the two cases were unrelated.

Last week, and two days after a two-hour journey, the vehicle would not start (no load terminal voltage measured at 7.8V). After a jump start, running the engine for half an hour then switching off, the voltage settled at 12.5V, but after 24 hours it had fallen to 11.9V. I monitored the voltage across the battery terminals during starting and it drops to 9.4V.

I've booked it into the dealer under warranty, but wondered if you could give me some suggestions as to the possible cause so that I'm not fobbed off with a new battery only to have the same problem in a few months' time post warranty.

The Fiat Ducato has a smart alternator and the B2B charger is a Redarc BCDC1225.

Eric Grant A When you are parked up, the residual load on your starter battery should not be more than about 30 milliamperes. One can guess of all sorts of things that can cause much greater discharge currents when everything is ostensibly off. Classic items are aftermarket infotainment systems, aftermarket alarms, aftermarket trackers and, yes, that B2B unit may also be the problem.

However, there is no point guessing. There is also no point in your dealer guessing and changing the first item that comes to mind.

If it were me, I would disconnect the negative main cable from the starter battery negative terminal and then connect an ammeter between the battery negative terminal and the lifted connection. This will you show the current flow when everything is supposed to be off.

If the current (amps) was significantly greater than 30mA then, progressively, I would be pulling fuses or disconnecting services (such as that B2B) to see where the amps are going. However, this task is not for a novice and you do require an electrical technician with a thinking head to get to the bottom of this.

This logic and method of testing applies equally if you had problems with the leisure battery discharging.

Clive Mott