My MINI has been sitting in the garage for more than 3 weeks and I was getting worried about the battery. It has been snowing for several weeks now and it only has all season tires so I'm kind of scared to drive my MINI until I put on snow tires.

I saw several posts recommending the CTEK battery charger so I got one from a local shop. I paid $80 CAD. I looked in the manual and it says I can just leave it plugged in and it will not over charge the battery. Is that true? Should I unplug it once the battery is fully charged just to be on the safe side??

 

A good battery in a state of full or near-full charge, connected to a low amp, steady voltage/low amp maintenance charger can remain connected almost indefinitely.

If your charger is functioning correctly and is feeding a low amp, steady voltage trickle into your battery at a level between 12.7 and 13.25 volts, as long as the amps going in are very low (1.5 amps or less), you should be fine.

Standard lead-acid wet cell batteries are considered to be in a state of full charge at a minimum of around 2.12 volts per cell, or 12.7 total volts in a standard 6-cell 12 volt battery.

Most battery maintenance-level chargers hold at about 2.2 volts per cell, or 13.25 volts in a standard 12 volt battery and provide constant voltage at an amp or less into a fully-charged battery.

There are a number of high-quality battery maintenance chargers on the market, most of which are low amp chargers, designed to maintain a designated state of charge rather than having the power necessary to properly bring a fully-discharged battery up to charge. These chargers, if functioning properly, usually won't damage a healthy battery but they are not designed to bring a fully discharged battery back up to a state of full charge.

Chargers that can provide a maximum 4 or 5 amps, can taper the charge in response to the battery's ability to absorb a charge, and also have a hold or maintenance mode, can recharge a discharged battery eventually and can also maintain a full state of charge almost indefinitely without damaging a healthy battery. The CTEK is such a device.

Parasitic loads that exist on modern cars can drain a battery quickly. The cycles from 50% depth of discharge (2.01 volts per cell/12.06 volts in a standard 12 volt battery) back up to full charge are hard on thin battery plates and can intensify calcification leading to premature battery failure.

A 12 volt battery that is at 10.5 volts is considered to be 100% discharged, fully dead, and, while a strong 3 phase charge can often bring an otherwise healthy battery in this condition back from the dead, internal damage to the battery will not be fully reversible. And at least 8 amps of charging capacity is best to bring a fully-discharged standard sized car battery back properly.

The amount of lead in a battery, plate thickness, paste chemistry, and the way plates are tied together all impact how a battery receives and holds its charge.

Automotive batteries are designed to provide lots of cranking amps for a very short discharge period (a few seconds). This type of battery takes a multi-stage charge well, where a higher level of amps are added at the start of the charge cycle and then the amps taper as the voltage rises, culminating in a steady-voltage, very low amp (1.5 amps or less) maintenance "holding" charge.

Also, the colder the ambient temperature, the more difficult it is for a battery to receive and hold a charge.

Common practice is to NOT start the car with a low amp charger connected as the sudden high amp pull can be devastating to a maintenance charger's electronics.

Long way of saying, a battery will live longer and be generally happier and better able to accept a full charge from the car's charging system if it is maintained at full charge. Severe discharge cycles to below 50% depth of discharge kill batteries. Car batteries are designed to start cars daily, and be recharged by the car's internal system daily.

Cycles of discharge and recharge that are many days in duration between discharge and recharge stress modern car batteries and shorten their lives.

 

It's a pretty sophisticated charger, leave it plugged in.

 

I wouldn't worry too much about a dead battery after 3 weeks. Also our cars have AGM battery which is much better than the regular lead acid. If worried, I'd just get a BatteryTender Jr (bought mine $25 on sale) used for maintaining a charge, but it can also charge given enough time.

Your charger looks like a really nice charger.

 

Where exactly is the best place to attach the CTEK?

 

To the battery