Car battery, unused -how long should charge last ?

mro

Should an unused charged car battery which is out of car remain charged for long period, months or even a year ?

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


I know when I have bought new car battery, date stamp shows they were made/charged long time before.

I charged relatively new, maybe 2 year old spare car battery recently, it was sitting in car disconnected & unused.
(The battery when charged turns & starts engine ok).

Month later it was pretty flat, not turning engine well, slow turning for short while then grinds to halt.


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

wgl2014

If it has been left discharged for any great length of time it may well be goosed now. Generally if s battery isn't going to be used for a while it's best to store it fully charged.

reeac

My experience with lead acid batteries in a boat which is used only between May and September is that the batteries need a top up charge every 2-3 months during the idle Winter period. My last pair of batteries lasted 10 years under this regime.

mro

wgl2014 wrote: »

If it has been left discharged for any great length of time it may well be goosed now. Generally if s battery isn't going to be used for a while it's best to store it fully charged.

No, it has been charged most of time.

mro

reeac wrote: »

My experience with lead acid batteries in a boat which is used only between May and September is that the batteries need a top up charge every 2-3 months during the idle Winter period. My last pair of batteries lasted 10 years under this regime.

So if battery is charged, sitting disconnected & unused, would you expect it to be charged, have reasonable charge in 2, 3, 4, 6 months or more ?

brianposter

I seem to remember that my car stored in the house for just over 6 months with battery disconnected would usually start. Depended a lot on whether petrol had to be pumped from the tank or not.

Gloomendoom

According to the source linked below, lead acid batteries self discharge at about 5% a month.

http://batteryuniversity.com/learn/article/elevating_self_discharge

mro

Gloomendoom wrote: »

According to the source linked below, lead acid batteries self discharge at about 5% a month.

http://batteryuniversity.com/learn/article/elevating_self_discharge

So good battery should have charge for good few months up to a year maybe.

lead acid has one of the lowest self-discharge rates and loses only about 5 percent per month. With usage and age, however, the flooded lead acid builds up sludge in the sediment trap, which causes a soft short when this semi-conductive substance reaches the plates.


Figure 6 illustrates the self-discharge of a lead acid battery at different ambient temperatures At a room temperature of 20°C (68°F), the self-discharge is roughly 3% per month and the battery can theoretically be stored of 12 months without recharge. With a warm temperature of 30°C (86°F), the self-discharge increases and a recharge will be needed after 6 months. Letting the battery drop below 60 percent SoC for some time causes sulfation.


Figure 6: Self-discharge of lead acid as a function of temperature.
Lead acid should never drop below 60% SoC. Charge more often when warm.

Farzackerly

AGM batteries have a much better chance of surviving uncharged for longer periods than conventional LA batteries. Still not a particularly good idea to forget about them though, and you should give them a refresher charge every three months if possible.
My personal observation is this - if a flooded LA battery is in use, its resiliance to casual mistreatment falls markedly with time, and it becomes progressively less tolerant of being forgotten about. This is compounded by the micro-drain of onboard electronics, but this is not a new problem; something as simple as a dashboard electric clock would drain a battery stone flat in a few weeks, back in the 70s.
If a LA battery of car-starting type is allowed to go entirely flat, its life diminishes. You can get away with that a few times, but as the battery gets older it's less likely to be tolerated and it can fail suddenly with the onset of colder weather and the greater demand of starting a cold engine from a cold battery.

Svein_Forkbeard

mro wrote: »

it was sitting in car disconnected & unused.

Can I ask why?

AdrianC

mro wrote: »

Should an unused charged car battery which is out of car remain charged for long period, months or even a year ?

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

Not a problem, ime.

I charged relatively new, maybe 2 year old spare car battery recently, it was sitting in car disconnected & unused.

Svein_Forkbeard wrote: »

Can I ask why?

I'm going to take a guess that the car was sat there unused...

Month later it was pretty flat, not turning engine well, slow turning for short while then grinds to halt.

Was that "months later" IN the car? If so, then that's probably your answer. Very few cars that are less than several decades old have no quiescent draw - alarms, clocks, radio memories, ECU memories, remote locking are all consuming power all the time.


I have an old Landy that, quite literally, has zero quiescent draw. Nothing whatsoever using power while it's turned off. No radio, no clock - let alone alarm/remote locking/ECU. It'll sit for a year with the battery (of unknown age and origin, definitely been flattened before) connected, and start first turn. SWMBO's 2cv was in the exact same boat until we fitted a modern radio. Now it needs the battery disconnecting if it's sat for more than a month or two.


One thing lead-acid batteries really don't like is being flat. Properly flatten them, and they very often don't recover properly, especially if they've been left flat.