New car battery -how long should charge last ?

mro

How long should a charge last on a new car battery when the car has not been used ?

I bought new car battery in December, battery was fully charged mid January.
Battery was only powering alarm system light which is flashing,

Now month later I find the battery is flat, when car has not been used over that period.

I would have thought battery would last months in these circumstances.

debtdebt

The battery usually does last months if it's sat on the shelf and not connected up to anything. If it's connected to a car, there will still be things which will draw power from it like the alarm, clocks, etc. Usually, this isn't enough to drain a battery so much that it won't start a car though. Maybe it's a problem with the car and something which isn't shutting down properly and drawing power from the battery.

Strider590

The "alarm light" uses next to nothing, the cars ECU on standby would use a lot more.

The total quiescent drain on the battery should be in the region of 20 to 40 milliamps, the little LED is probably in the region of 3mA.

So for a 50Ah battery, at 40mA quiescent, it would be flat as pancake in 1250 hours or 52 days.

A car battery would probably fail to start a car at anything under 1/3rd charge, so 2/3rd of 52 is 35, that means after 35 days the car might not start at all.

Now i've assume 50aH capacity, as this is an average, yours may be lower or higher, but equally your quiescent drain might be lower too.

I think that probably answers your question to the greatest extent possible

mro

Strider590 wrote: »

The "alarm light" uses next to nothing, the cars ECU on standby would use a lot more.

The total quiescent drain on the battery should be in the region of 20 to 40 milliamps, the little LED is probably in the region of 3mA.

So for a 50Ah battery, at 40mA quiescent, it would be flat as pancake in 1250 hours or 52 days.

A car battery would probably fail to start a car at anything under 1/3rd charge, so 2/3rd of 52 is 35, that means after 35 days the car might not start at all.

Now i've assume 50aH capacity, as this is an average, yours may be lower or higher, but equally your quiescent drain might be lower too.

I think that probably answers your question to the greatest extent possible

The new battery was charged about a month ago, last time I started car.

Car was started for short period after it had been charged in doors, so that would have drained it as well.

I'm surprised fully charged battery might only last that long, ~35 days, just over a month.

Just assumed it would last number of months.

societys_child

Regular short journeys, heater, aircon, heated screens, headlights, satnav, radio, bluetooth, phone on charge can take their toll, not forgetting that low temperatures can just about halve the capacity of the battery.
At this time of year it doesn't hurt to charge it now and again.

C_Mababejive

You need one of these..
https://shop.theaa.com/store/home/car-battery-solar-charger

It isnt designed to or capable of fully charging a battery but it will preserve the battery chemistry and help to replace low level discharge

mro

societys_child wrote: »

That's supposing the battery was fully charged in the first place.
Regular short journeys, heater, aircon, heated screens, headlights, satnav, radio, bluetooth, phone on charge can take their toll, not forgetting that low temperatures can just about halve the capacity of the battery.
Sounds like op needs a charger, charge it now and again.

Battery was charged indoors beforehand, car was started for short period ~5min & is not being used.

societys_child

Cold weather really takes the life out of a battery. Running it for 5 mins won't replace what was used to start it.

System

mro wrote: »

I'm surprised fully charged battery might only last that long, ~35 days, just over a month.

Just assumed it would last number of months.

When you turn your car off its never fully off on a modern car. If you want it to last months disconnect one of the battery terminals.

Mr.Generous

also I remember reading somewhere that batteries never run at 100% capacity, they are usually performing below the stated capacity after just a few weeks. They work best when charged and used in similar proportions just like if you were driving the car. This is because as a battery discharges, not only does it diminish its internal store of energy, but its internal resistance also increases (as the electrolyte becomes less and less conductive), and its open-circuit cell voltage decreases (as the chemicals become more and more dilute). The most deceptive change that a discharging battery exhibits is increased resistance. IE if you just slowly discharge it without any recharging it becomes less efficient as it discharges. Some kind of trickle charge would be ideal, or a 4ah charger on a timer switch coming on 30 mins per day and see how it goes.

barnaclebill

I had a 1992 MR2 that I used to drive every day for two months then leave it in the garage for four months, come home from sea and it would start up no problem. I next had a new 1998 MR2, did the same thing and got a sat call from my wife after 6 weeks that the car alarm was going off, the battery was drained and the alarm thought the car was being stolen.
The old MR2 alarm had to be set which I did not do when it was stored, the new model was always active so drained the battery, after that I used to disconnect the battery while at sea.
The old MR2 still had the original battery fitted when I sold it and it never needed a bulb changed in those 7 years.