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MG Midget and Sprite Technical - Battery condition measurement

12V 17Ah M/cycle battery. Not been used for a while. Measures 4.8v across the posts.
Does this just indicate a flat battery or does this mean that some of the cells have packed up?
Its currently on charge, (pun) but am I wasting my time with this?

Guy
Guy Weller

Many batteries can be recovered from that position Guy although there are no guarentees. I find that charging for a while then discharging with a big load then repeating a number of times breaks down the internal resistance and brings the battery back to life.

However do take care. Many years ago when I was young free and single, without a care in the world and never doing mental risk assessing I almost lost my sight with this technique.

My method of discharging was to simply connect the 2 battery posts together with an iron bar. Did it on numerous occassions until one day the spark as the bar made contact with the post ignited the hydrogen gas above the battery. It exploded with such a huge bang that temporarily deafened me whilst the acid that blew everywhere hit me square in the face. Fortunately I found my way to the outside tap and was relieved that after washing out I opened my eyes and saw day light.

Must say I had a MGB in the garage that paintwork wise was never the same again.
Bob (Robert) Midget Turbo

On the same note, be careful when using a big battery charger, then connecting to the car & repeatedly trying to start it whilst under charger boost, i had a battery explode on me too, not nice.
Brad (Sprite IV 1380)

Thanks Bob, I will take care!
I have seen the results of an exploding battery before - not nice!

For clarification: does the low voltage necessarily indicate damaged cells, or is a low voltage reading normal just for a heavily discharged battery?
Guy Weller

if it goes flat it gets low voltage Guy, doesnt mean the cells have gone, you have to test properly

if it won't recharge after a couple of charge discharge cycles as Bob advises, chuck it

discharge by draining with a 21W 12v bulb across the terminals. Do it a few times and check the resting state after charging, each time.
Bill

Information I have read indicated that lead acid batteries allowed to discharge below about 25% capacity suffered from sulphation and therefore damaged if left that way for to very long. IIRC the damage is permanent but can be somewhat recovered in some cases. Plenty of info on the web but I can't find the links I had bookmarked at the moment. I've killed otherwise good batteries by being lazy and not putting them on a float charge every few months or so. LA batteries have a low self discharge rate compared to other common batteries such as NiCd or NiMh but do still require topping up to maintain condition.
David Billington

Lot of variation there, however. I have had "Mopar" brand batteries here (a Chrylser label on goodness knows whose product) which very consistently would lose a cell any time they discharged completely (and it did not take much to discharge them completely no matter how new they were). I could charge them up, they would indicate 13 volts. Try starting the car and the engine would get maybe one revolution and stop. Measure battery voltage again and it would be down to 10 volts.

OTOH, I have had Interstate batteries that have been run into the ground completely dead repeatedly and kept right on going as soon as they were charged up again. That poor battery lasted 8 years for me. A friend of mine used one in his late-model stock car with no alternator. Every week it would get pretty much used up all summer long. His batteries generally lasted two or three seasons, but it was the vibration of racing stiff suspensions on dirt tracks that killed them.
David "all charged up" Lieb
David Lieb

After seeing a battery explode, I'd not like to see that ever again! My ears were ringing for ages!
Clive Reddin

David B has it about right. As I understand it a battery needs to see action at least every 10-11 weeks or sulphation progressively kills the cells. They can still take a charge but capacity will be reduced. So plan to use the car over the winter or top up as necessary.
f pollock

Update,
Left it on charge overnight and it now measures 13.6v. So I just need to see if that will hold its charge for a reasonable length of time.

I said it was a M/cycle battery. It is in one of those Jump-start battery pack devices. A "cheapo" version bought from Aldi last year, so I was not expecting much. Main problem is that the cars have all been too reliable and I haven't used this for months. One should give them a boost charge from time to time but in the general excitement of life I just forgot.

Guy
Guy Weller

You could test it now it is charged you know...

Bill

?
Guy Weller

That makes recovering it very easy Guy. Simply use it to start a car or 2, might not the first couple of times depending how much internal resistance has built up, but with a bit of hard use it will recover its full capacity.

The last battery I brought back from the dead was on my Metro turbo. It had been in my friends garage for 4 years with the boot light on. Naturally the battery was nowhere. I began the process of forcing in charge and then discharging for about a week or so and eventually it recovered perfectly. Just needed to break down the sulphate.
Bob (Robert) Midget Turbo

wot Bob says

Disconnect your own battery in the car (Preferably with a non-ecu car) put the leads somewhere safe

and plug your dooberry in and start the car and run it for a while to add to the charge inside it


another question about the battery

is it lead acid or one of the more modern types calcium or such?

Ought to be on it somewhere if it is different...

They makers of these auxiliary devices expect them to lie around underutilised, so theinternal battery should be a bit robust.
Bill

can the fancy calcium ones be recovered?
had the same problem in the 928, the inside light was left on and the battery absolutely dead.
Tarquin

This thread was discussed between 29/11/2008 and 30/11/2008

MG Midget and Sprite Technical index

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