Jump to content

USB Socket Current Drain


Recommended Posts

I heve recently fitted one of these USB sockets under the dash.  It is wired into a fused permanent supply. 

USB Socket

It has a small LED voltmeter display in the centre which is 'on' as long as power is supplied to the unit.  The spec states "The car charger will consume 30mA current at most when the vehicle has been switched off"

According to my calcs, (electricity isn't really my thing) that means it would take 100 days to drain my 72 AH battery which wouldn't be a problem as I have a battery cut off that I use when I lay the car up for long periods.  Is my assumption correct or should I be looking for a switched supply to wire it into?

Rgds Ian

 

Link to post
Share on other sites

it makes most sense to power if from a switched supply Ian but of course then you can't charge things with the ignition off, as the 3A doesn't have an aux position on the ignition switch.

Better than a flat battery though. It's 100 days to fully flat as your calculation shows - but maybe only 30 days to 'can't start the car'. It isn't good for a car battery to be left for any length of time at partial charge as it can result in plate sulphation. 

Link to post
Share on other sites
2 hours ago, RobH said:

it makes most sense to power if from a switched supply Ian but of course then you can't charge things with the ignition off, as the 3A doesn't have an aux position on the ignition switch.

Better than a flat battery though. It's 100 days to fully flat as your calculation shows - but maybe only 30 days to 'can't start the car'. It isn't good for a car battery to be left for any length of time at partial charge as it can result in plate sulphation. 

Could you not fit an ignition switch from a 4A/5/early 6 which have accessory position?

Link to post
Share on other sites

Your calculation ignores the internal resistance of the battery and the fact that the battery does not discharge linearly.

My guess is you would have a flat battery a lot sooner than 100 days.

I would definitely use a switched supply, or keep the battery on charge using a smart charger.

TT

Link to post
Share on other sites

Do you have any other devices connected - trackers and modern radios are quite pernicious as they too draw a small current even when switched off?

I would wire up most accessories that draw a current so they are off/disconnected when the ignition is off. With radio's it is a balance as they lose their tuning and bluetooth phone settings without a permanant live.

Link to post
Share on other sites
3 hours ago, Andy Moltu said:

Could you not fit an ignition switch from a 4A/5/early 6 which have accessory position?

Might be a problem(will be a problem) with your insurance leaving a key in the car while unattended for any length of time?. I am assuming the key has to be present in the ignition for the accessory position.

Link to post
Share on other sites
3 minutes ago, Andy Moltu said:

With radio's it is a balance as they lose their tuning and bluetooth phone settings without a permanant live.

Yes but the power to keep those live is generally much less than 30mA.  

[A bit of a digression but it really bugs me that most modern electronics devices no longer have a real on/off switch. The 'off' button just puts them into a standby mode.  That means that when you come to use them after some weeks or months, the batteries are often flat because of the small current drawn continuously in standby. I either resort to fitting a pukka miniature switch if there is room in the case, or pulling the battery but it really should not be necessary.

Link to post
Share on other sites

Many thks to all, the consensus appears to be that I should fit it too a switched supply.  Not a problem really, just an extra length of wire, I plumbed it to the permanent live because it was very convenient.

Rgds Ian

Link to post
Share on other sites

The Mk1, Mk2 and Mk3 MX5 have a "Dark Current" of 20 milli amps.

That is what the car uses to keep the electonics alive for instance the feed to keep the radio preset stations in the memory and other small housekeeping loads alive.

In most cases it will keep the battery alive for around 2 months and still be able to start the car.

If there is more coming out of the battery than 20 ma then that is an issue and the extra current use needs sorted by removing fuses one at a time to find the issue.

A smart battery charger used once a month while the car is not used will keep the battery topped up will stop any issues.

Therefore yes a "total dark current" of 20ma is OK.

Link to post
Share on other sites
On 7/30/2020 at 5:32 PM, RobH said:

[A bit of a digression but it really bugs me that most modern electronics devices no longer have a real on/off switch. The 'off' button just puts them into a standby mode.  That means that when you come to use them after some weeks or months, the batteries are often flat because of the small current drawn continuously in standby. I either resort to fitting a pukka miniature switch if there is room in the case, or pulling the battery but it really should not be necessary.

Further digression, but this is life with a 20 year old Boxster. Unless used within 8/9 days everything electronic shuts down. The battery is under the bonnet. The 'bonnet open' lever is unlocked by a microswitch. Result - one cannot open the bonnet to jump the battery. Genius design. There are various time-consuming manual responses. There have probably been more words written on Porsche forums about this than any other issue. About 15 year ago I wrote to Porsche suggesting that they incorporate some sort of solar panel technology in their designs. Got a very snotty response.

Miles

Link to post
Share on other sites

Another consideration: 

If you have a tracker in your car, this is often directly connected to the battery so will also add to the dark current.

I keep my TR connected to a smart charger when it is not being used, so that it will turn the engine over easily when needed. (It may take a while to start due to mechanical pump, but that's another story).

TT

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.