How low can a battery go...?

Chasing down battery drain issues this week, I went to start the car and the battery was as dead as a Dodo :frowning: at 5.9v and falling! Managed to stop it falling by turning off boot lights and recharged the battery. Trouble is that it was fine three days ago when I fitted the new car radio & gauges. I took the battery to my local garage who did a drop test on the battery and it came up fine (new in April)
So believing in cause & effect I unplugged the radio and gauges and did a parasitic drain test on the battery by connecting earth terminal and jumping across the live wire and live terminal with an ammeter. This showed 0.077 amps (77 milliamps) which is a normal amount to be expected from a car with an immobiliser or alarm in resting state. Then connected the radio & gauges and got the same 77 milliamp.
Hmm, took it for a good 30 minute drive to top up the battery with the alternator which was reading 14.4v when car was running. The resting state is about 12.8v with ignition off and I will check resting voltage again tonight and in the morning to see if it goes down below the 12.5v that I’d expect.

The thing I’m worried about is a battery that’s reached as low as 5.9v have I damaged it irrecoverably? If I have wouldn’t that have showed up on the battery test?

5.9 volts is way too low, I am sorry to say; there will be a price to pay for letting any battery, original Panasonic or otherwise drop to this level. I would say that battery voltage should always be kept above 12.3 volts. Recoverability is much more achievable on an AGM or gel battery rather than a lead acid.
What MK car?
Parastic current of 77ma is too high in my opinion, if MK1 - MK2.5 should be <30ma and perhaps <40ma for MK3.
There has to be a drain issue here and you seem to know what you are looking for with parasitic current theory and tests; what has changed?


What’s changed? erm quite a lot including new engine & ECU, changed alternator from external regulated to internal, bi-passed the factory immobiliser & too many parts to mention in the Rocketeer V6 Jaguar engine conversion. Reading on Pistonheads they mention a drain being acceptable between 50-80 milliamps on a modern car, hence that’s where I was getting the 77 milliamp being within range. But I still don’t think that a 40ah battery would get drained totally in just three days by a 0.077 amp parasitic drain? surely that would take at least 519 hours (or 21 days) according to my calculations?

One thing that did happen with the battery was that on the first test drive I didn’t realise that the alternator wasn’t charging the battery so after 30 minutes it went flat and broke down. I limped home with a jump pack connected and then charged it back up overnight. The issue was that the Rocketeer uses a mk1 internally regulated alternator and the mk2.5 had one that was regulated by the ECU. So I had to change the signal wiring in the car to match the mk1 wiring. I’m guessing that 30 minutes of running without charge might have toasted the battery, but why is the battery drop test done by my local garage coming up saying everything is ok?


I am impressed!..but your battery clearly isn’t.
This is very basic stuff which you clearly appreciate; a bit like saying why am I fat really with calorific input/output.
Input/output honesty. Battery should charge at 14.6 volts, parasitic current should be <30ma and, well that is it really. I would say that 77ma parasitic current is too much but not enough to cause issue with a healthy battery when car in reasonably regular use. Our Eunos was at 285ma parasitic drain for up to 2 years but new battery, knackered stereo replaced and me getting around to understanding the theory, totally cured it. Daily use meant the battery was recovering until it got to the ripe old age of 2 and started to complain and who can blame it?

Installing the Rocketeer kit was no doubt rocket science/engineering; this isn’t. Letting your battery drop to 5.9 volts and questioning why and what to do about it should be very embarrassing?

:rofl: :rofl: yes it is a bit embarrassing, I guess I am just sanity checking why the blooming thing still seems to have some life left in it ! I just don’t want to spend £80 odd and toast another one…


Quite understandable, been there, done it, scrapped the stereo, sold the battery, etc.
I would hazard a guess that the issue is charging but at 5.9 volts, nothing will work. These cars can just about function at a base level of 9 volts but the starter will need around 12 volts and some battery capacity to turn over sufficiently for a start.
What is your battery charging at when running? Regardless of MK1 alternator system(inbuilt regulator control) or MK2 system(ecu regulator control), the voltage when comfortably idling across the battery terminals should be a steady 14.6 volts or very close. .

It’s charging voltage is 14.4 or 14.5 volts now I have changed the wiring

Something is not right here?
Charging rate is more or less correct, parasitic discharge too much but not excessive, newish battery and I assume the car has not been left for weeks without being run?
Diode discharge would show up on the charge rate.
You have created a miracle, albeit an unfortunate and useless one:-) A system that can miraculously lose energy with no logical explanation?
How tediously unsatisfactory is that?

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The car had been left for three days, measured the battery again tonight (9 hours later) and it’s settled at 12.54v which seems good to me. I’ll continue to monitor it tomorrow morning.

Another thought on this, even though the battery is charging at a normal rate when running, is it possible that there is excessive discharge at the same time? Perhaps something on the switched circuit is sapping power. You could try your ammeter test with the ignition on to see what the discharge rate is. You could also rig something up to test this when the engine is running as well but considerable care required.

Decided not to take car to work as this morning’s voltage was 12.3v so something is not right…
Tested parasitic drain again and depending on which way I hook up ammeter I either get -0.077 or 0.124amps so looks like I was reading it wrong :roll_eyes:
Interestingly voltage is 12.3v when connected to car and when disconnected slightly higher. So there must be a drain somewhere. Time to start pulling fuses I think…

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12.3 is not great.
You’d think anything over 12 would be fine for a 12v battery but as I understand it, it doesn’t work like that and the difference between 12.3 and 12.6v is significant.
For more resilience, you’re probably best off buying an AGM battery, which have inherently better Cold Cranking Amp (cca) figures, so have more oomph and are more tolerant of abuse. They also don’t leak.
Yuasa have a great reputation and warranty and are actually made in the UK

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I see that you have changed the ECU. Is it possible that this is the cause of your problems with something associated with the ECU is no longer connected in your application but the ECU still thinks ii is? What ECU are you using?

Got some time tonight to pull fuses & test with ammeter, Good news is that I temporarily undid the imobiliser alterations I had to do for the new ECU and the drain remained the same which is good news as it’s not that! Also removed all the engine bay fuses including the main 100 amp one - no difference. And I removed the 30amp fuse on the Rocketeer harness that takes constant power from the alternator main +ve terminal to the new ECU - still no difference. So that means it’s probably not the new ECU. Also disconnected the recently fitted stereo - no change there. Next to check is the funky wiring I did to change the car from external to internal regulated alternator. Then pulling all the ancillary fuses in the cabin is the only thing left! I bet it’s something daft like the factory Alarm (there’s an extra horn under the bonnet so I assume it has one?) or the bonnet opening sensor that is having a hissy fit because the ECU’s gone…

Just a random thought, but having removed so many fuses and disconnected things, it might now be an idea to listen very carefully to parts of the car when making and breaking the connection to the battery.

Quite a few 12V relays have 150 Ohm coils which take a similar current.

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Good point, you can definitely hear a relay for the rear number plate lights in the boot when the battery is connected, but nothing when the ammeter is connected (plus I disabled the switch) Can’t help thinking the alarm fuse might be a good one to check if I can find it.

Are you sure the relay you are hearing is for number plate lights?
I’m no expert but would such a low current draw pair of bulbs have their own relay??

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rather than pulling fuses, it is better to check the voltage drop (in millivolts) across the fuses while in circuit.
this can give the current drawn, without physically isolating the circuits.
there are tables of the drop voltage vs current for each of the fuse types and ratings.
and that’s why modern fuses have a small hole behind each of their connections.

testing like this avoids any extra ‘wake up’ current and many other oddities


That’s a good point that I wondered myself! There is some talk of the electric aerial using power when it gets old even when it’s not on. That’s also the same area the noise is coming from…

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thanks, I’ll check a few other fuses with the same method I’m using for now just for consistency, but it’s good to know. Apparently a prime candidate is the “Room” fuse…