Car battery left standing flat

Joe_Horner

We often see posts on here advising people that, because their car battery has been allowed to go flat, it'll be damaged and should be replaced. To balance that up a little, I thought I'd mention the battery on our 5 series diesel.

It was fitted in 2008 (not by us - genuine BMW battery all properly date stamped by the dealer) so is already 8 years old. Having got the Mondeo for my partner a couple of months ago the BM's been sitting and the battery went flat. I jumped it 4 weeks ago and used it for a couple of days commuting (14 miles each way so not enough to fully charge it) then left it again. It got to the point it wouldn't even try to work the central locking.

Last week I got it off the car and put it on a slow (about 3 amp) charge - that was always the first step in old-school battery maintenance before we were told we should just throw them away! It took the best part of 40 hours to fully charge, which matches up quite well with its 95Ah capacity and, at the end it, was showing a healthy voltage and even it's own condition indicator (built in hydrometer) was showing "good".

After resting for 4 days it's now showing a specific gravity of 1.270 +/- 3 across all cells, a resting voltage of 12.6v which it's held since settling off charge, and is holding over 11v while cranking a 2.5l diesel. That's pretty good performance for a new battery, never mind an 8 year old one that's been mistreated!

So, in the interests of money saving (in this case around £120 for a decent replacement), yes it is worth charging before condemning a flat battery :beer:

Strider590

I've got a type 015 in my garage that's now 7 years old, was replaced 3 years ago and kept in my garage, it'll hold a 12.7v charge for in excess of 6 months without topping up, it won't however EVER reach the maximum charging voltage of 14.4v (it'll just sit there gasing and overheating the charger) and when using it to power my exhaust gas tester last year it died in about 3 minutes flat.
I'm basically using it now for some "fun" solar projects.

Conversely, I have a 7.2ah 12v SLA battery, which has one damaged cell, only holds a charge of around 10.5v, but practically still has damn near to 7.2ah capacity.

Leaving them sitting flat is not the only thing that kills them, it's a combination of this AND the number of discharge cycles AND the alternator float charge voltage (which is why some Fords eat car batteries).

Joe_Horner

In the case of yours it's also unlikely to hold over 1.9v per cell under a 100 amp + discharge, which this one does. It then recovers virtually immediately back to 2.1v per cell.

Between the specific gravities (values and evenness across the cells), the time taken to charge (with charging current, a rough measure of capacity if it reaches full charge) and the cranking ability, this one is good to go without any doubts.

Agreed there are plenty of other things that will kill batteries, my point was simply that the advice often given to throw it away because it's been run flat and left for a few days is, to be polite, pessimistic regarding its chances of recovery if treated with a little care. It's also not "my suggestion" but old, established, practice from well before makers saw fit to promote the "throw it away" mentality

Incidentally, the gassing on yours is most likely due to your charger trying to float it at too high a current - the last few percent of charge should be at less than 1 amp or so for most normal starter batteries, which doesn't supply enough energy to cause gassing.

londonTiger

voltage isn't the be all and end all. Amps is just as important. Can it crank during the cold and deliver the 40 amps or whatever a car needs?

Modern cars are quite sensitive to poor current and can throw up all sorts of error messages if te car doesnt get the correct voltage/current it needs.

it's worth changing a battery after 5/6 years, at this stage you're just taking a chance, it will fail at some point, maybe in weeks, months perhaps even a year or two. But it's going to fail to start when you need to use the car to go somewhere and you'd wish you paid for a new battery then.

londonTiger

Strider590 wrote: »

I'm basically using it now for some "fun" solar projects.

if you have a 100w panel and use very light use from the rig like shed lighting or PIR floodlight for a very infrequently used patio in the evenings it might just suffice. But for longer durations you really need a decent deep cycle battery.

Joe_Horner

londonTiger wrote: »

voltage isn't the be all and end all. Amps is just as important. Can it crank during the cold and deliver the 40 amps or whatever a car needs?

Absolutely agree that voltage isn't everything.

But note the 11v while cranking - the initial cranking load on that 6 pot is around 300 amps so yes it can deliver your 40 amps without batting an eyelid . As for cold weather, don't have a fridge big enough to put it in but it's been below 6 degrees the last couple of evenings here while I was testing it so, yes, pretty sure it'd work just fine if I did have one. It might drop another half volt or so, but it'd cope.

Don't agree that you're "on borrowed time" at any specific age, no matter how much the battery sellers would like to think so.

General maintenance and cycles matter far more than time. A battery that's been rarely run flat, kept clean and topped up (yes, you can top up a "maintenance free") and used on a car that fires up quickly, with an alternator that doesn't over charge, can easily last a decade or more.

One that's being constantly over charged, low on electrolyte, regularly cranking for 20 seconds while the glow plugs fail to glow, or has a parasitic drain that takes it flat every week and is never fully recharged might last a year or two if you're lucky.

forgotmyname

All batteries are on borrowed time. Its just time before it fails.

The worst damage is leaving them for extended periods in a discharged state. Especially in freezing temperatures where a battery can actually freeze if discharged.

Run flat and recharged fairly quickly wont be an issue, a good and proper recharge may actually extend its life.

s_b

most modern batteries dont allow you to check each cell without damaging the casing
your battery is obviously old school where they put the lets see if its good hole of doom innit (the green light) but in fact delve deeper and this cover still pulls off to reveal the cells:D
i hate having to sell batteries to the tinkers for 5 slavs

bigjl

Alternators are not designed to fully recharge a flat battery.

Strider590

Joe_Horner wrote: »

Incidentally, the gassing on yours is most likely due to your charger trying to float it at too high a current - the last few percent of charge should be at less than 1 amp or so for most normal starter batteries, which doesn't supply enough energy to cause gassing.

The gassing is due to the fact the battery is no longer capable of being charged to 14.4v. Any decent charger will work by holding it's output voltage at 14.4v (or 14.8 for AGM) this will lead to the charger applying maximum current until it reaches that voltage..... Simple ohms law applies here.

I tried to charge it on my bench power supply, with the current limiter set to 0.2A, this didn't gas out, but after a week it still hadn't taken a charge.

It's interesting how car batteries can fail in many different ways and it's scary how people mess with them in an attempt to save the pennies.

Iceweasel

s_b wrote: »

i hate having to sell batteries to the tinkers for 5 slavs

I can't quite put my finger on it, but there is something a bit 'un-PC' about this statement. :think: