flat battery after standing overnight

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mikesu
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flat battery after standing overnight

Post by mikesu » Wed Mar 21, 2018 4:36 pm

My '71 Se5a has recently begun flattening the battery overnight. Clock isn't working - as normal, so I tried removing the underbonnet light's supply from the switch as suggested on here - no change, still flattened overnight. Next checked all the fuses with everything off - no volts on any of them that shouldn't have volts, so not the light switch shorting. Haven't checked the unfused circuits from the ignition switch yet, has anyone any ideas what else could allow the battery to drain away slowly?

Mike Sullivan


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1971 SE5a (6yrs)

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flat battery after standing overnight

Post by GTC Ian » Wed Mar 21, 2018 4:40 pm

Could be a duff battery. Maybe a cell has gone. First thing to check. Had the same thing on the SS1. New battery solved it


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flat battery after standing overnight

Post by philhoward » Wed Mar 21, 2018 4:41 pm

Another option is it draining through the alternator -being an SE5, does it still have the original 11AC and external controller or has it been converted to an ACR unit?

Have you tried disconnecting the battery overnight?


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flat battery after standing overnight

Post by Roger Pennington » Wed Mar 21, 2018 7:46 pm

I'm no electrical guru, so I may be wrong, but what I would try is I would disconnect the main battery lead, with everything switched off, then connect an ammeter between the lead and the battery. If there is no drain, that should show zero amps, if that's the case then it's probably the battery dying, as Ian says. If there is a drain, then you know it's something in the car - the next problem is to find it! That may mean disconnecting things until something removes the drain.


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flat battery after standing overnight

Post by DARK STAR » Wed Mar 21, 2018 8:01 pm

If the battery is ok, first thought is the alternator, or rather the diode which is integrated on later alternators but as Phil say was originally separate on Coupés and 5's.


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flat battery after standing overnight

Post by Oldconn » Thu Mar 22, 2018 9:11 am

My SE6A has the same problem. I eventually found it was the delay timer on the interior lights which was faulty. This may not apply to an SE5A, of course.


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flat battery after standing overnight

Post by mikesu » Fri Mar 23, 2018 11:30 am

Thanks, guys for your responses.
Re-reading my post, I see I forgot to mention that I tried a spare secondhand battery and then a new one - both flattened overnight. I'm reduced to removing the earth wire from the battery each night so the car is usable in the mornings!(!!)
At our Noggin this week, best advice was use a tail lamp with wires attached to check for drain as easier to see. Having removed each fuse in turn this morning and checked for a drain - still nothing. I think I'm just left with the two feeds from the ignition switch which are unfused ( Tacho and fuel level ). Will have to wait now until I can get under the dash to check those. (BTW no glimmer from the IGN light in the speedo either)

What would we do without Mr Painter's wiring diagram?

Mike S.


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flat battery after standing overnight

Post by scimjim » Fri Mar 23, 2018 11:32 am

If the ign light isn’t illuminating, that’s almost certainly the culprit. Disconnect the alternator overnight.


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flat battery after standing overnight

Post by mikesu » Fri Mar 23, 2018 11:36 am

Thanks Jim, Ign Light works OK when starting car, but what I meant was that there is sign of a drain via the Ignition switch and the lamp to the Alternator and ground.


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flat battery after standing overnight

Post by philhoward » Fri Mar 23, 2018 12:20 pm

That’s pointing to the alternator regulator then.


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flat battery after standing overnight

Post by mikesu » Sun May 27, 2018 3:59 pm

Hi, Guys, apologies for the long silence, but real life gets in the way. After my last post and continuing with the removal of the earth from the battery every night, I got on with an unrelated job (I think) of changing the OEM fusebox for some new blade fuses and holders. (from 12vplanet.co.uk - no connection, but good stuff and good service) All the necessary disturbance to achieve this, seems to have solved the battery draining problem, as well as ensuring that the headlights now work every time and not just when the wiring wanted to permit it!

many thanks for the support,

Mike S.


1970 MG B roadster (28yrs)
1971 SE5a (6yrs)

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