# CAR BATTERY KEEPS GOING FLAT

#### birkee

There are so many problems mentioned in these forums, with regards to battery problems, that, as a retired battery laboratory testing technician, I thought I should give some guidance on car batteries.

A 12 Volt battery on a car, is 12 Volts ad infinitum (Unless completely shot!). This will be 12 Volts, until such time as a load is applied (turn on the lights etc) What happens when a load is applied, will indicate whether it is fully charged or flat. If it falls to less than 10.5 volts on load, you are in trouble. If it's less 10.5 volts OFF load, it's scrap. (About 2% recovery rate, but at much reduced AH capacity)

A good battery, is charged at 14.7 volts, and this will fall to around 13 volts when the charger is removed.

If the battery is allowed to discharge to less than 10.5 volts, it will decay rapidly, and needs charging IMMEDIATELY. If left at 10.5 volts or less for any time, you will probably NEVER recover the battery, even if recharged.
It MAY come back to 12 volts, but half the plates will probably be shorted out by paste falling off the plates, and shorting themselves out in the bottom of the battery. SOME plates may not get shorted out, which will still give the battery 12 volts when charged, but the Ampere hour capacity of the battery will be massively reduced. 8 positive plates, and 7 negative plates per cell, may be reduced to ONE of each still working. It will still give 12 volts after recharge, but the Ah capacity of the battery, could have gone from 40Ah to 5Ah for instance. May not even start the car!

Never judge a car battery by its voltage OFF LOAD!

If a battery has "off load" voltage of less than 10.5 volts, then chances are, that one cell already has an internal short (positive to negative plate) and charging it will be like putting air into a punctured tyre.

Whats the story with the dry cell batteries in the electric/hybrids cars?

If "off load" battery voltage is less than 10.5 volts, then chances are, that the battery already has one cell that has a short (positive to negative plate) and charging it, will be like putting air into a punctured tyre.

(i.e. NiCd / Selenium / Lithium, etc etc etc.)

Dry?
Most likely these are lead / acid batteries with gel, instead of liquid electrolyte. Common on smaller vehicles such as motorcycles, invalid chairs etc.
Very similar in some respects to wet batteries, but have a differrent requirement in other respects. One of them I seem to recall, was a maximum charging voltage of 14.4 volts instead of 14.7 volts. They are also sensitive to maximum current used whilst charging, as they can't dissipate the charging generated heat as effectively.

Birkee..... a common 12 volt lead/acid type battery is made up of six cells. Each cell is slightly above 2 volts, therefore giving the battery a potential voltage of around 12.6 volts (in the real world). However, as this voltage is variable, the manufacturers just call them '12 volt' batteries. A well charged battery, capable of starting the car, should be 12.2 volts or above. The battery's capacity to start the engine is rated by it's CCA reading (Cold Cranking Amps), and NOT by it's AH (Ampere Hour) rating as you suggest. This is only an indication of how long a battery would take to go flat if you left a load switched on. The typical charging rate of a car's alternator is 13.7 to 14.3 volts, depending on how much is already in the battery. Some Japanese alternators will get up to almost 15 volts, but after that, you're overcharging the battery. Lead acid batteries should never be fitted inside the passenger compartment of a car, just in case it turns over in an accident. Most of these types of car use Gel batteries. Cars with stop/start technology now use AGM (Advanced Glass Fibre Mat) type batteries, to cope with the demands placed on the battery by this system.

Sorry Jetex,
the lead acid battery is 12 volts under neutral conditions, and fully charged is virtually always 12.7 volts near as dammit when first removed from charge. Means nothing, if you can't get the current out because the plates are shot.
Volts will flow through a conductive hair, but current needs good thick conductors. That's how fuses work. Too much current, and bang.

The Cold Cranking Amps rating is irrelevant to the function of the battery.
This is determined by the vehicle manufacturer at the design stage. The larger the engine, the more CCA are required because of the larger loads. A small engine may have a battery with as little as a 30AH, and a large engine maybe as much 100Ah. Stick a fully charged 30AH battery in a Range Rover, and it may well not start the vehicle because it can't deliver the CCA load. e.g. 5 positive plates to deliver the current from each cell in a 30Ah battery, or 10 positive plates or more in a larger battery. (Ampere/hour spec) ......the CCA is a consequence, not a design spec,.... that's accomodated later, with thicker bus bars between cells so they don't burn out with higher currents.

A starter motor is initially a short circuit, UNTIL it starts turning, and could take as much as 2000 amps or so,.... then the current drawn falls to that required to turn over the engine, and the CCA comes into play.
Stick a big battery into a small car, no problem, try to start a large car with a small battery.....maybe problems.
Using a small battery to jump start a large car, and most of the output fom a small battery will go into supporting the cars fitted battery, not starting the car.

MOST charging systems run at about 14.7V, and have done since dynamo's. The reason the charging current changes, is that if you put in 14.7 volts, and the battery is AT 14.7V, then there is no voltage difference for ANY charging current to flow. (Voltage = Pressure...... Amps = volume. no pressure, no amps flow... Ohms Law)

If the battery is flat, and at about 10.5V, then there is 4.2V voltage difference to drive the current, and the battery takes all the charging current available from the source, which is why the ammount of current needs restricting, to prevent overheating of the battery.
(Probably restricted by a charger with a max output of 5 - 8 amps usually.)

The high current from the alternator after an engine start, last a short time, which the battery can accommodate, as the voltage soars quickly.

Having worked for the largest battery manufacturer in the U.K, and done competitor product testing, to evaluate THEIR products against ours, I can say they are all identical in function. The differences are in product quality (paste on plates quality) Quality control proceedures (rejecting plates below a certain standard when building) Control of Sulphric Acid gravity, etc etc etc.

Repeat: BATTERY VOLTAGE is no indication of battery condition.

PS JETEX.
The reason you see different voltages on different vehicle alternators, is because of the state of charge of the battery. If the battery is taking lots of current, it will be low, but will get up to 14.7V as the battery charges. (Less vehicle running loads)
Take a car with 13.8V alternator output on a long drive, and measure it again. If the charging is still 13.8v, then the vehicle running loads are 14.7v minus 13.8 volts, which maths will tell you, how much current is being used by the car, instead of charging the battery.

At 14.7 volts you CANNOT overcharge a Japanese battery.
(You think they would design that potential into their cars?)

Fibre glass mat batteries: Sulphuric acid soaked mat, that prevents spillage in an accident. Same batteries otherwise.

Maintainence free batteries: More Calcium used in plate manufacture, so less gassing to evaporate water in the acid. Result is slightly inferior current delivery under high load conditions. Doesn't matter until you have battery problems.

Just wondered on these things I have seen on ebay recently.
http://tinyurl.com/battrestore

whilst I am sceptical of these things, Im intrigued as to their claims, can you shed any light.

Also, do you have any advice on what to look for in a leisure battery? Im looking for a new one at the mo...

A helpful and informative post Birkee. Thank you.

Your response to TTE will also be interesting. I cannot imagine the guide being much use, because surely what ever the content, some company within the commercial market would already be exploiting the procedure?

Lou.

An interesting and informative post Birkee, thanks for that. Did you ever do any work with battery de-sulphaters ?. I found these not to be very good a restoring a failed battery, and therefore not worth the investment.

Sulphation is paste falling from the plates, and eventually building up in the bottom of the battery, and shorting out positive and negative plates.
What patented additive can get rid of paste shorting out plates, without removing it from plates also?

Where a battery has say, 6 pos and 5 neg plates per cell, you may get four pos plates shorted to 4 neg plates. You still get 12 volts, but the Ampere hours will be 2/6ths of the original cell capacity.
This is where most people are struggling with failing batteries....If you have shorted plates in cells, then the battery goes flat much quicker than expected, because it's Ah capacity will be reduced.

If the battery is less than 10v, then you have probably already lost one cell entirely. Repeat: Charging it, is like putting air into a punctured tyre. The battery self discharges and will go flat, even if left off the vehicle.

An interesting note, picked up years ago, is that for every engine start you do, you needed to drive at least 20 miles, to put back the Ah removed by that ONE engine start. I expect most of you have noticed, that when doing a bunch of short distance journeys in succession, the cranking speed of the engine get slower and slower, but as long as the engine starts, we ignore it.

I'm holding up a six year old battery at the moment, due to lots of short distance driving. (Disabled) Half a dozen short journeys, and I have to think about sticking a charger on it, whereas, when new, it could do dozens of starts. The Ah capacity is shot now, and I'm just letting the winter go by before replacing it.

Hope this saves you guys from wasting too much time with duff batteries, but we all need to try and save a few quid, and replacement is a last resort for ALL of us.

timtheenchanter

Restored batteries.
Empty the batteries, and wash out all the displaced paste from the bottom of the cells which cause the problems.
Refill with new Sulphuric acid electrolyte at 1.270 specific gravity, and recharge at 14.7 volts....... 12 volt battery as good as NEW?

Paste area on plates = current availability, Paste volume is Ah, so where have all the Ah's and current delivery gone when the paste has fallen off the plates?
Suspect a 40Ah battery will be more like 20Ah battery after this treatment, and will soon fail again.

Leisure batteries? Buy a reputable brand, and not an oriental looky-likey.
Tubular plates are better for deep cycling without recharges than flat plate batteries, but you would have to search for these, as they are mostly for commercial usage and expensive. (Used on things like fork lift trucks etc) Better than a car battery though, which doesn't like deep discharges.
If you use a flat plate (car battery) DON'T discharge it too low. NEVER less than 10.5 volts 'on load'. The voltage will lift back to 12 volts when the load is turned off, so the 'off load' voltage means nothing.
Get a cheap voltmeter (not multimeter) from Maplins, and attach it to the circuit, with red warning paint at the 10.5 volts level. (It uses no current to speak of, so is barely a 'load')

Just wondered on these things I have seen on ebay recently.
http://tinyurl.com/battrestore

whilst I am sceptical of these things, Im intrigued as to their claims, can you shed any light.

Also, do you have any advice on what to look for in a leisure battery? Im looking for a new one at the mo...

Don't waste yr money on ebay, Birkee has provided the same advice for nothing! Correctly done it will at least stop the self-discharge so the battery will hold what is left of its charge from one day to the next.

Re leisure batteries there is as he says no substitute for the correct plate construction. We used to have a boat with two batteries, one for starting and one for "domestic" loads such as lighting and electronics. Specialist boat batteries cost a fortune but Lucas's techical dept were very helpful and recommended their matching automotive and leisure batteries with the same (calcium) chemistry, you need to have this or they won't achieve the same state of charge when connected to the same alternator. AFAIR they were type 644 and 645 and the price was much more reasonable, they lasted about 3 or 4 years in pretty arduous service. We also fitted a Lucas caravan charging relay which saved having to remember to charge the domestic battery when the engine was running and disconnect it afterwards, much better than diodes or the usual changeover switch arrangement.

HTH

CHRISJC

Thanks for your input chrisjc, and a good (or used to be) recommendation for a leisure battery.
I hesitated in recommending a make, as that would be blamed on me, if problems ocurred later, but, one observation.
Lucas Batteries no longer exist, despite the badge, as Lucas Batteries were taken over by the Japanese company Yuasa. They used to supply people like Halford's / Rover etc, but not sure of their involvment now. They closed the B'ham factory, and moved to S. Wales.

Type 643 and 644 batteries. (around 90 - 95Ah, manuf dependant)
If memory serves, these are the same battery, but the pos & neg battery terminals are reversed relative to the box, to enable fitting to different vehicles etc. i.e. with the terminals nearest to you, you can have the positive terminal on the left or right according to requirements.

Type 643 and 644 batteries. (around 90 - 95Ah, manuf dependant)
If memory serves, these are the same battery, but the pos & neg battery terminals are reversed relative to the box.

It's a long while ago now, maybe the leisure battery was 622 or something, but you could get leisure and automotive batteries in the same case size and both were available with the terminals either way round. For some reason the batteries in our boat were opposite handed, maybe to discourage fitting them in the wrong position, though that would have resulted in both being reverse polarity as well and the damage would have been considerable! As I recall the reputable/expensive brands sold in chandleries for marine use were Tudor (what happened to them?) and Varta, there was a Swedish one too but I think just Varta rebranded.

I knew that Lucas had lost their independence but didn't know who had bought them. What became of their parts operation? We used to buy Lucas filters as well from the local trade counter, a fraction of the Volvo marine spares prices. (The engine was a 2 cyl diesel but IIRC the oil filter was the same as a Renault 12)

I was only talking about the 643 & 644 leisure battery recommended above, as regards polarity chrisjc.
Most batteries are available with both polarities as an option, but some have very little demand for them, and may be harder to get.
Of course the cases of different batteries can be the same size, it all depends how many plates and what type you put in the cells (what the spec is.) Why pay for the manufacture of 4 different box sizes. on 4 different moulds (also to be paid for) when ONE box will accommodate 4 specifications. Usually more common on the larger (leisure) batteries though, as the smaller ones have to fit in tight locations in cars.

Lucas sold out everything to concentrate as aerospace suppliers. Each type of subdivision was sold out to all sorts of companies, for them to try and cash in on the Lucas name. Who knows who's using it now?
Lucas was sold out to the Americans after this though, so not even aerospace survives now.

You may recognise brand names, but who makes them now?
They all keep losing out to the Far East in the end. Nobody can compete with their labour costs.

Tudor batteries were Mickey Mouse at automotive level, but Varta was O.K and are still available, then there's Exide or Bosch, usually pretty good, Trying to remember back, I think Delco was a good battery too, but try to find that name on a battery now!

Replies
2
Views
1K
Replies
6
Views
884
Replies
3
Views
855
Replies
4
Views
1K
Replies
1
Views
619