I only do 10 miles to work and back, this Winter my battery went flat, as I've been using the heated seats, heated windows, full fan, lights/lamps on etc

I've now cutback on the electric usage and want to rev the engine a few times before I turn the engine off.

I know it safe to rev it in Neutral as the transmission is not connected, but is it safe to do this with the electric handbrake on? (Not Park using the dial, just elec handbrake).

 

Revving the engine in neutral will not accomplish much if you're attempting to charge the battery. Either drive the car more or get a battery tender.

 

+1 as otherwise you might have to sit revving for 20 mins!

 

Take a look at the ctek battery conditioners, they make conditioners for a lot of top end cars , I have been using one for nearly 4 years now.

 

Its just mechanics have recommended turn off all electrics and rev the engine a few times before switching it off, which gives the battery a little more juice to start the next time.

I know that is true of non-automatic petrol engines, but maybe not for an automatic diesel start-stop XJ.

Thanks for the heads up on the CTEK ones, does anyone recommended a mains powered jump starter (not charger) for a 3.0 diesel?

EDIT: researched this and mains powered ones won't have enough umph to start it up so the CTEK looks a good solution. THanks!

 

apachegogo, think of it this way, the amount of energy that your battery has stored in it is measured in a unit called amp-hours. As the number implies, it is the number of amps times the hours that it can do something. Most automotive batteries are rated for 80 amp hours (not sure of the specific rating for the XJ, but it is a trivial point). So, that means you can run a 1 amp load for 80 hours or an 80 amp load for 1 hour or a 20 amp load for 4 hours before the battery is considered depleted.

So, if you are only revving the engine for a few seconds, to get any measureable power back into the battery, you would need a very large current to flow. Unfortunately, the alternator of the car is not capable of generating these sorts of currents. So, like was mentioned, you would need to either idle the motor for a period of time or simply take a slightly longer way home so the motor is spinning longer and therefore has more time to add power to the battery.

Along these same lines, there is some truth to what the mechanic is telling you in that raising the engine RPMs does help recharge the battery for the mere fact that at idle, the alternator can only put out about 50% of its rated current (this is a function of how the alternator works). So, by getting the engine RPMs up over 2000 RPM, this will allow the alternator to reach its 100% maximum output should the battery require it. If you want to learn more about this, let me know. It is fairly easy to understand, but helps if you have some electrical knowledge too.

 

Charging a flat battery takes 3-4 hours of driving when using the car's alternator. Revving the engine for a few seconds or minutes will make no noticeable difference.

 

Thanks folks, that does help.

I think I will need to check my electrical usage in the winter and keep a jump starter on the standby because I don't really want to drive longer and waste diesel/money to keep it charged!

 

As previously mentioned, an overnight "kick" from a tender or charger could solve your problem, if the battery is in good shape. Once you figure out the simple connect points it shouldn't take more than a minute or two to put it on and or take it off.
Luck

 

Jump starting flat batteries is not good for them either. Get a battery tender (lots of discussion here) and keep the car plugged in when not in use.

 

Thanks folks

 

One more thing;

The explanation by "Thermo" using ampere/hours was a very useful one. Taking off on that very subject I would suggest using a small output battery charger vs. a battery tender.

Assuming that you are exhausting your battery by the end of the day due to the heavy accessory uses, you need more of an overnight recharge than what a tender can perform. I do not use CTEK but, use a different brand; as far as I know most of those tenders charge at the rate of 0.5 Amps. So, assuming 12 hours between your coming home, shutting the car off and having to start it in the next morning, that is only 6 Amp/hours into a battery that is rated 90 ampere/hours. As you can tell that is a very minor amount of charge.

Tenders are specifically made for keeping the batteries up on cars that sit in the garage for long period of times without charging. Like my '10XKR with only 6k miles on it:-). For your specific needs, I would use an automatic charger with up to 6 amps charging rate. That should give you plenty of charges for the next morning. They are available online or in any auto store. You may want to add a charging connector that is easy to connect to, instead of having to access the battery under the trunk floor every time.

Also, be sure that when you use any kind of charger, place the charger negative lead on a body/chassis bolt, instead of the battery negative post. Otherwise the little device on the battery negative post will not register the amount of charges for the ECU and you will keep getting "low battery" alarms.