I’m looking for technical help or recommendations on: Battery running flat
This is the second time this has happened to me; I parked up the MX5 RF (ND) in the garage over Xmas break and when I went to start it, nothing/zip! Lots of warning lights etc but no response from the engine at all. The previous time this happened, the engine nearly turned once then died. On both occasions I was able to recharge the battery and the car sprang back into life on the 1st attempt. Clearly, there is a drain on the battery somewhere which is not obvious. Any tips or should I remove the battery next time? Your help would be appreciated.
Thanks!
This assumes the battery was fully charged to begin with.
Unfortunately, if you only do short journeys, and infrequently, and with HRW, and headlights, wipers, blower, seat heaters, and all the rest of the accessories, it will never be fully charged. At least half an hour to an hour of driving is needed to refill it if it was really low.
Sitting doing nothing it will have a dark current running all the security and door lock systems, somewhere between 30 and 40 mA. It doesn’t sound much when it takes a couple of hundred Amps to start the car.
However, that possible 40mA is there 24/7 whereas the 180Amp starting pulse might only be only a second or two. So during a day doing nothing the battery will lose about ten times more charge (~1Ah) than if it had started the engine (~0.1Ah).
Since you keep it in a garage, I suggest you use a Smart charger to keep it happy. There are several threads here on the topic.
Both my 2007 NC and my 2019 ND would barely last 3 weeks lack of use before the batteries went flat. My Toyota GT86 was similar but at least Toyota fitted a larger capacity battery FOC under warranty.
There is a TSB about it for the RF which has to do with the cruise control circuit remaining on and draining the battery. Worth getting it checked at the dealers.
All modern cars have a small ‘parasitic’ load (usually a few tens of milliamps) on the battery when parked. The key items powered up are alarms, radio (to maintain settings), central locking sensors, slight drain through alternator electronics, ECU etc. My Audi instruction book actually notes that the car manages these things when it’s parked to minimise battery discharge. It’s very common to see owners finding batteries are discharged after a few weeks. The topic crops up regularly on here. Car batteries are now generally smaller, to save weight and that combined with limited use leads to the problem. My NC stays on a CTEK maintainer to avoid the problem.
JS
Yes, as it turns the alarm on after a short delay; which is why I always turn the alarm off on mine as I also suffered a flat battery after the car was not used for 10 days during this lock down palaver. I also connect a CTEK to it one day a week to keep the battery topped up “just in case”.
I wondered about that for mine (2019 RF) being a relatively new car/battery it went flat after just 3-4 weeks in the garage, I assumed the boot light turned itself off, but maybe not? Ive not sat in there and checked!
Thanks everyone for your comments and suggestions. My RF is 3 and a bit years old and I did get a service recall to update the software regarding the cruise control battery discharge problem. I assume the Mazda garage obliged when I took it in. I also checked the boot light and it was off so this is not the issue either.
It seems like the best option is to get into the habit of switching off the alarm sensor and or buying a Ctek battery maintainer! A battery switch could also work depending on how it is fitted but I’m sure if a total switch off would mess up the electrics on locks etc.
