Charging a battery - what is correct?

[Prawo Jazdy]

Hello,

I have a flat battery in a GT86. I've jump started it a couple of times since I've owned it, but apparently the alarm draws quite a lot of current, so if I don't drive it for a few days the battery is too flat to start the engine.

I have been lent a basic mains charger/ battery maintainer to get the battery fully charged before I next drive it. But I can't seem to find a definitive answer for:

- Do I need to disconnect the battery in order to charge it? Some sites say yes, some no.

- Can I charge it through the OBD port? (I have an adapter for this)

Thanks if anyone can shed any light.

[Magicmushroom666]

I've always just charged with the battery connected to the car and never had any trouble.

Only ever connected direct to the battery terminals though, not relying on any of the cars wiring!

I've found though that once a battery has been sitting flat for some time it never is quite the same again and ends up needing replacement.

[GreenV8S]

Don't charge it through the OBD port - it isn't designed for power transfer. Ideally connect directly to the battery - unless you have a stop/start system with a battery monitor, in which case follow the manufacturer's instructions.

[McGee_22]

If the battery is already low disconnect it and charge it with a proper battery charger as trying to use a trickle charger to fully charge an already low battery may overload a trickle charger.

Once the battery is in good shape then you can use a trickle charger, and as others have said, you can use it while the battery is connected to the car but connect the trickle charger connections straight to the battery as car electronics could mess with the charging current and charging routine.

[Belle427]

id get the battery tested as it sounds knackered if an alarm discharges it in a few days.

[E-bmw]

I doubt any current manufacturer would fit an alarm that "apparently draws a lot of current".

I suspect your battery is actually stuffed.

Fully charge it and when it is charged turn on the lights for a few seconds without starting the engine and then measure the voltage at the battery.

12.6 or 12.7 is a perfectly healthy battery which is fully charged.

Every 0.1 volt under that is a loss of 15/20% charge so 12.0 volts is actually basically dead, and unlikely to recover.

[trickywoo]

Standard lead acid batteries hate being fully discharged. It’s likely scrap.

Get a new one and if it discharges quickly again you’ll need to find the drain or have it permanently connected to a trickle charger.

[Prawo Jazdy]

E-bmw said:

I doubt any current manufacturer would fit an alarm that "apparently draws a lot of current".

The cause isn’t entirely clear, but batteries going flat and being replaced under warranty well before their expected life because of some sort of parasitic loss seems to have been common for GT86/BRZs according to owners forums.

However, it could also be a six year old battery, and that’s not an unusual life for a battery I understand.

Anyway, regardless of the cause, thanks for the replies and suggestion for how to test the battery.