Mk 3 battery not holding charge

[redchris70]

Hello All,

New to the forum and looking for some advice about my partner's 2008 Mk 3.

She's had the car about 3 years now. Last summer she had a flat battery and after a quick check we realised it still had the original Mazda battery in. I just though it was a case of an old battery reaching the end of it's life and bought new replacement which was fitted and away we went. Until the next time the car was sat for a few days and sure enough, it failed to start again. I got my mate who's an electrician (not auto but really knows his stuff) to check for a current drain but everything checked out ok. I returned the battery to the shop and got a replacement. The car has been driven regularly until Christmas hols when it sat for a few days, and again the battery is totally flat.

I jumped it and took it to my local independent garage who again checked for a current drain and found nothing. And here we are...2 new batteries in 6 months that have failed to hold their charge...2 people checking for current drain and telling me everything checks out ok.

Cards on the table - I'm not in the least bit mechanically minded. A good friend of mine had issues with a BMW 5 series recently which turned out to be the voltage regulator in the alternator. He told me that everything checked out OK when the car was sat still but monitoring when the car was being driven showed the voltage at the battery spiking badly.

Do you think my symptoms might be being caused by a bad alternator/voltage regulator?

Is there a way of testing this and if so how is it done?

Does anyone have any other suggestions of what to check or do?

We can obviously buy another replacement battery but I am loathed to do so if something is causing them to fail.

Thanks in advance for any help you can offer. We're at our wits end!

[blackyb]

What you have said is sometimes the cause of a battery drain, it can also be caused by the alarm/immobiliser but the amount of battery drain you mention seems to point to the regulator/alternator issue.

Have you put a volt meter across the battery while the engines running to see what the alternator is kicking out and if its constant.

As for replacing or checking the regulator, I have no idea as I go to my local auto spark when things get too technical.

[redchris70]

Thanks for your reply. The voltage has been checked with the engine running but the car sat still and it checks out ok. My mates BMW did the same thing and he only noticed fluctuating voltage when monitoring whilst driving. I'm wondering if anyone knows if and how it is possible to run such a check on the MX5?

Also, I wonder if it is possible to replace with voltage regulator within the alternator as a quick google search shows new alternators coming in at £400-600

[blackyb]

I have a little plug in device that goes into the cig lighter socket that gives me a constant voltage reading (off the bay that is e)

Not sure about how to fit a new regulator into an alternator or indeed if it is possible, do you have a local repair shop that repairs alternators.

Most alternateors are sold as exchange unit deals.

Maybe not the answer you want but might be worth a look on Google for a local repair shop or youtube as someone will have posted something up if a DIY repair is an option.

[Mazda Mender]

Anything aftermarket been fitted?....there is defo a bad drain somewhere, but i have know alternators to seem fine on tick over but not charging when running/under load.
M-m

[safetymatch]

This is a very common problem for a mk3 MX5, there is nothing wrong with your car or your battery, basically it is called dark current drain.

When the car is parked up the battery is still being used as the ecu is still monitoring all the electronics, the clock and radio codes, the central locking and alarm functions are all still active even though they are only drawing a very small (dark current) charge.

You have to remember the battery is only 2/3rds the size of a standard car battery and will start to flatten out within 10 days if your lucky.

Solution....get yourself a conditioning charger and use it when the car is parked up, or use the car more often.

Search any MX5 forums and you will get the same answer.

[Mazda Mender]

Jump starting a gel battery will not charge it...as SM as stated a gel battery needs to be "reconditioned" with a trickle charger or smart charger...normal charging will kill it...with the swap you have done i am guessing you have swapped out for a wet cell battery ?..which is not great for the roadster any way...but bar that if fully charged correct it should last longer than 2 days stood still wet or gel types...
M-m