Current Drain in Perspective

[MSS]

With many posts on current drain and charging times etc. I thought it would be useful to put things into perspective. So here are some calculations for members' perusal and comment. I have done the calculations to show the energy held in the battery or consumed from the battery as this (or the equivalent charge) is the most useful measure.

Key points to note:

1. A 72Ah battery will probably have a residual capacity of around 50Ah after a few years of use, storing a maximum of 2200kJ.

2. Starting does not consume as much energy as many people believe - around 7.5kJ when used with glow plugs.

3. The main factor to consider with a car that stands unused is the quiescent current drain of the electronics. Even for the ideal scenario of a 25mA to 30mA drain, it would require the car to be driven for up to an hour to bring the battery up to full charge after one week of standing. This is because despite the rating of the alternator, a battery can only accept charge at a maximum rate of around 1/10 of its capacity.

4. A 4A maintenance charger will take about 1.5 hours to replenish one week's worth of energy usage with the car standing unused. A 0.8A maintenance charger will require a few hours.

All the calculations assume a nominal 12V battery voltage and the numbers have been rounded. There are a lot of variables involved but this should provide a useful ballpark.

[MSS]

No comments, alternative views, additional information etc. from anyone?

Where is COLVERT. Surely there must be a spelling mistake in the post somewhere?

[COLVERT]

Originally Posted by MSS

No comments, alternative views, additional information etc. from anyone?

Where is COLVERT. Surely there must be a spelling mistake in the post somewhere?

There is.---Lol.

Fourth item down in your chart.


Mind you, I agree with the rest of your post except for one item.--My diesel starter motor takes 150 amps according to the tech spec.

[HarryM1BYT]

A flat battery takes so long to charge by driving, via the alternator, because once a battery is on charge - its voltage rises rapidly to oppose the flow of current into it. The only way to speed up the flow of current, is to increase the voltage.



Checking the discharge rate when parked, is very easy on the 75 - there is a bus-bar link in the under bonnet fuse panel, which carries all of the loads except the stater motor. Mine CDTi draws 20mA when parked and locked.

My 75 CDTi is used very irregularly, most of it's time is spent in my garage, so looking after my batteries have always been an issue.


I originally would aim for putting the battery on a Smart charge, every week or two - that sort of worked, but relied upon me to remember to put it on charge and take it off regularly - and I hate having to remember to do things.

It is not good for a battery to be left to discharge, nor is it good to have a battery on a permanent maintenance charge.

A better scheme I devised, was to have a dumb charger, powered from a 24 hour plug in timer, set to power the dumb charger for 20 minutes per day.

I have since replaced the plug in timer with a Smart plug, controlled by Alexa, to come on for 20 minutes per day.

Another thing I have done, is to make it easier to connect the charger...

I originally tried fitting a lighter plug onto the charger and plugging that in the lighter socket. It sort of worked, but not very well - the resistance of the wiring loom between socket and battery, seriously limited the ability to charge, so I would end up lifting the bonnet and attaching directly across the battery to get an effective charge.

Instead, I added a second socket, hidden behind the glovebox lid and just to the right, connected to the heavy duty cable for the towing socket.

The charger is fixed to a beam in the garage, with a long lead and ciggy plug hanging down, ready to plug into the car, next to the drivers door.

As ciggy lighter sockets are not absolutely reliable to connect to, I added a telltail....


Just an LED powered via a flasher circuit, across the charging lead where it dangles from the ceiling. It gives a brief flash, every few seconds when it sees 12v power from the battery and only draws around 0.5mA.

The flashing LED enables me confirm my ciggy lighter plug has a good connection before I get out of the car and provides me with a means to check at a glance that I remember to plug it in, each time I pop in the garage.

[MSS]

Originally Posted by COLVERT

There is.---Lol.

Fourth item down in your chart.


Mind you, I agree with the rest of your post except for one item.--My diesel starter motor takes 150 amps according to the tech spec.

My CDT starter is also rated at 150A. But some 3.0L V6 diesel starters are rated at 250A so I used this figure. This gave the worst case scenario and still showed that starting a car does not consume much energy in relative terms.


On most of our cars I have a fused connector coming out around the grill area. These are either a CTEK Comfort Connectors or a 2-pin SAE connector. I just plug the maintenance chargers into these as necessary.

[COLVERT]

Originally Posted by MSS

My CDT starter is also rated at 150A. But some 3.0L V6 diesel starters are rated at 250A so I used this figure. This gave the worst case scenario and still showed that starting a car does not consume much energy in relative terms.


On most of our cars I have a fused connector coming out around the grill area. These are either a CTEK Comfort Connectors or a 2-pin SAE connector. I just plug the maintenance chargers into these as necessary.

That spelling mistake though------


Bit your bum a bit did it.--so you-------

[MSS]

Originally Posted by COLVERT

That spelling mistake though------


Bit your bum a bit did it.--so you-------

I'm surprised only one of the mistakes was found. Losing that special touch, perhaps we are?

[COLVERT]

Originally Posted by MSS

I'm surprised only one of the mistakes was found. Losing that special touch, perhaps we are?

Definitely not losing it.---Can't be as so many folk tell me I'm touched.

[COLVERT]

Originally Posted by MSS

I'm surprised only one of the mistakes was found. Losing that special touch, perhaps we are?

OK---OK---A bit more info.---Lol.

Glow plugs.-The light goes out but the glow plug carries on working until the ECU tells it that the engine has no more need of it.--
Like the automatic enrichment of the fuel gets less as the engine warms up.

[COLVERT]

Originally Posted by HarryM1BYT

1/--A flat battery takes so long to charge by driving, via the alternator, because once a battery is on charge - its voltage rises rapidly to oppose the flow of current into it. The only way to speed up the flow of current, is to increase the voltage.



2/--Checking the discharge rate when parked, is very easy on the 75 - there is a bus-bar link in the under bonnet fuse panel, which carries all of the loads except the starter motor. Mine CDTi draws 20mA when parked and locked.
y 75 CDTi is used very irregularly, most of it's time is spent in my garage, so looking after my batteries have always been an issue.


3/--I originally would aim for putting the battery on a Smart charge, every week or two - that sort of worked, but relied upon me to remember to put it on charge and take it off regularly - and I hate having to remember to do things.

4/--It is not good for a battery to be left to discharge, nor is it good to have a battery on a permanent maintenance charge.

5/--A better scheme I devised, was to have a dumb charger, powered from a 24 hour plug in timer, set to power the dumb charger for 20 minutes per day.

6/--I have since replaced the plug in timer with a Smart plug, controlled by Alexa, to come on for 20 minutes per day.

Another thing I have done, is to make it easier to connect the charger...

7/--I originally tried fitting a lighter plug onto the charger and plugging that in the lighter socket. It sort of worked, but not very well - the resistance of the wiring loom between socket and battery, seriously limited the ability to charge, so I would end up lifting the bonnet and attaching directly across the battery to get an effective charge.

8/--Instead, I added a second socket, hidden behind the glove box lid and just to the right, connected to the heavy duty cable for the towing socket.

The charger is fixed to a beam in the garage, with a long lead and ciggy plug hanging down, ready to plug into the car, next to the drivers door.

As ciggy lighter sockets are not absolutely reliable to connect to, I added a tell tail....


Just an LED powered via a flasher circuit, across the charging lead where it dangles from the ceiling. It gives a brief flash, every few seconds when it sees 12v power from the battery and only draws around 0.5mA.

The flashing LED enables me confirm my ciggy lighter plug has a good connection before I get out of the car and provides me with a means to check at a glance that I remember to plug it in, each time I pop in the garage.

Bon jour Harry.

Item 1.--The charging rate on the car and the rate at which a charger puts current into a battery is about the same.---By increasing the voltage you also generate heat which can. if in excess. boil the water out of the electrolite.-

Item 3-- I use a solar charger which puts in around 1.2 watts to compensate for the normal internal losses of a lead acid car battery.

Item 7--The lead to the cigar lighter is heavy duty and thus will have minimum voltage loss. It can carry enough current to make the lighter Glow.



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