2005 NB, battery woes

  1. My model of MX-5 is: 2005 NB

I discovered the battery in my NB was completely flat this morning (at 10:30am). I took it inside and it measured 2.5 volts. I hadn’t used the car since Wednesday last week (most unusual for me) and it’s been unseasonably cold for the last few nights but even so, something’s amiss.

It’s a nearly new Panasonic battery, bought last November from the local Mazda main dealer. I replaced the Yuasa battery that was in the car because it was unvented and I was worried about fumes causing corrosion. Miraculously it was still reading 12.5v although it has been sitting (indoors) unloved and forgotten for the last 5 months. I put the Yuasa in the car and it fired right up so there’s nothing wrong with the starter circuit. (Quite impressed with that, nice one Yuasa!)

I’ve had the Panasonic battery on charge since 11:00 and at 4pm it was at 12.2v. Not ideal but I wanted to check the charge circuit so I put it back in the car and after a sluggish start it fired up and is being charged at a healthy 14.4v.

I’ve taken the battery back out and put it on charge again. I will disconnect it when I go to bed tonight, make a note of the voltage and then check it again tomorrow morning to see if it’s holding charge.

If it is, I’ll put it back in the car and put my multimeter in series with it on amps to see if there’s something draining it. (With the car turned off obviously…)

I checked to see if anything was inadvertently left switched on (lights, wipers etc,) but no.

Comments most welcome, thanks in advance.

If a battery has been down to 2.5V, its life expectancy will be much reduced. The plates may be warped or partly disintegrated. With everything turned off, try an ammeter between the battery negative terminal and its disconnected lead. If there is no current, or only a few mA, it was probably just a bad battery.

Good guide here: Testing for Parasitic Draw with a Multimeter | Test Meter Pro

An NB wouldn’t fall into the “modern car” category here and should be more easily diagnosable. It’s mire difficult when you have keyless entry and 101 things hanging off a CANBUS.

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I hate parasitic battery draws, I spent ages trying to track them down last year on my NB2 with some success but many blown ammeter fuses :roll_eyes: Don’t forget that if you leave even a door open when you’re testing it’s enough to give a false reading!

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Thanks, that article is very useful. That’s how I was going to test for battery drain anyway, but pulling fuses to isolate circuits hadn’t occurred to me. Obvious really! I have a schematic and I was an electronics test engineer a long time ago (in a galaxy far, far away…).

Thanks for that. Hadn’t thought about the doors!

Did the parasitic draw test just now - something is drawing 920mA. Oh dear.

It’s cold, wet, windy and generally horrible outside today, so the fuse pulling will have to wait for better weather. Bedtime reading tonight will be the schematics. Ho hum.

That’ll do it… time to start pulling fuses it’s usually the accessory fuse which is a good one to start with (look for “room” fuse in the manual)

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Thanks :+1:

Unless you’ve changed something else electrical, bet its a failing alternator (diode).

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Good call.

However, there are several possibilities.

Here is just one which I’ve seen twice on my cars in the past, Bosch and Delco alternators.
In each case the diode was fine, BUT it had not been potted properly and some silicone sealer was missing exposing one of the high-current diode chips. A quick squirt of isopropyl to remove the carbon brush dust contaminating (ie shorting) it, and some bath sealer to restore correct protection, and they then lasted the remaining life of those cars.

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Well, I’m scratching my head here.

I tried pulling all the fuses in the engine compartment box (except the main 100A one, for some reason that won’t budge) and no change on the meter. Consistently reading 920mA (Meter in series with battery on 10A dc setting.)

I had a look at the inside fuse box and quickly gave up. I’m 71, my close-up vision is not brilliant and the combination of access and seeing the fuses - and not dropping any into the nether regions - made me realise I’d reached my limits. Bit gutted with that because back in the old days I’d be all over a job like that. Better safe than sorry though. Ho hum, more of the joys of old age.

So, I called a local mechanic who has been looking after my family’s cars for many years, charged up the battery and he came and collected it on Wednesday. He brought the car back in the evening and said he couldn’t find anything wrong with it. Very little draw, tested with two different devices, battery also checked out fine, started up fine when he came to bring it back.

So, I checked the battery volts that evening - 12.6v

Checked it yesterday morning -12.57v.

Took it for a run, queued up for some Tesco Momentum, stop/start four times, went shopping stop/start, drove home, total run only 15 miles, all entirely uneventful, checked the battery - 12.6v.

Just checked it again this morning - 12.55v.


If it happens again I’ll have to bribe my son with a curry and get him to drive down (from Marlow) and get him to help me with more fuse pulling. He’s good with cars and can see a lot better than me.

In the meantime, the sun is out and I’m off for some top down motoring.

I have a MK2 NB1. I replaced the baterry with an OEM baterry and replaced the Alternator at the same time.
That was 3.5 years ago and with over winter garage layups from November through March. No trouble at all starting without any additional charging.
Ive fitted an combined Voltmeter,Thermometer and USB charger from Amazon that plugs into the Cigarette Lighter socket- extremely uselful.

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That’s three mentions of the alternator. My mechanic also said they are often responsible for parasitic drain. He also mentioned alarm systems and after-market sound systems as among the usual suspects.

The car has only done 33k miles but it is 17 years old. How does one test an alternator? It’s charging the battery OK, (at around 14.4volts).

The main fuse won’t come out by pulling because it’s bolted in from the side. If you can reach it try disconnecting the alternator multiplug and see if that changes the parasitic drain. Sometimes the exciter circuit leaks power through it’s electronics. It’s supposed to be insulated from the main engine live which it shares with the starter wiring, some don’t work 100% insulated. (I suspect mine is one of them!)


Thanks for that, will give it a go :+1:

Is it still 920ma? That will drain the battery to a point where it won’t start in less than a day! Are you sure you didn’t have the boot light on (shared with number plate lights on an NB) or a door open? Even the bonnet catch will draw a current on later NB cars as it has an alarm sensor on it. When I check my battery I flick the boot catch closed with a screwdriver so the car thinks it’s shut

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you don’t need to (and shouldn’t) remove the fuses to measure parasitic load, you can check the voltage drop across the fuse. (the blade fuses have holes in the back just for this), and there are tables relating to fuse value and millivolt drop across the fuse.

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Thanks for that - I’ve been doing the measurements with the boot open (bit difficult otherwise!). Never thought to flip the boot catch. Will do that.

Thanks - I watched a youtube video on that method last night.

Thanks guys, I appreciate the feedback :+1:

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