OT: Car battery volt drop

[Fredxx]

I once went through a couple of batteries until I sussed it was the starter motor!

[Fredxx]

Thanks, I knew there was a relationship but thought it was flatter than the graph in the link.

You can either leave it for a while or apply a load for a short time.

The best way of checking is to pulse a known current and measure internal resistance. Though that tells you the stored charge rather than state of charge.

[Paul]

But that doesn't tell you enough about the situation, to be celebrating a single reading of OC voltage.

It's comforting reading the voltage and seeing a "normal" value after it's settled. But that's not a diagnostic as such.

The Smart Charger has more means at its disposal of determining battery health.

Your car doesn't charge it chock-full. And if you didn't find it full, what would you conclude ? (Battery ? Alternator ? Voltage regulator ?)

The Smart Charger has more opportunities for making measurements, than your car does. The car doesn't have a desulphation cycle, it doesn't use pulse charging (where the relaxation can be measured after each pulse).

Paul

[Fredxx]

It does give an indication of the state of charge. That is better than nothing.

Most smart alternators only charge up to 90% in normal use.

The usual criterion is the car won't start.

It's normally called an equalisation charge, and is more suited to stationary use, and used to stir up the electrolyte to prevent stratification.

If you know the capacity of battery then charge current at constant voltage is a good way to determine a specified state of charge.

Pulse charging is generally used to increase battery life and decrease charging time.

[alan_m]

But these parameters are being monitored and if you connect a OBDII device to the diagnostic port you can see such items on a smart phone or tablet.

[williamwright]

Couldn't the manufacturers fit a separate small battery dedicated to supplying the quiescent items? One that would last maybe ten days. With a user option to decide whether it should steal power from the main battery when it became depleted?

Bill

[newshound]

I had that problem too with a Mk 4 Cortina in the 1980's. I used to do all my own maintenance at the time, eventually took it to an outfit with one of those Fruit-Machine sized "computerised" testers, IIRC it was taking 600 plus amps (presumably because of shorted windings).

[Roger Hayter]

Solved a similar problem recently in an old (but only 20 years) mini with a cheap Chinese clamp meter. >400A and turning over very slowly.

[Fredxx]

At 50mA that would be less than the self discharge rate of a lead acid battery.

I therefore don't see the point, it would also make a car even more complex than it they are already. And you've have to replace two batteries rather than the one. For some cars a battery change is already a dealer operation.

[Ian Jackson]

Last year, during the first lockdown, after my new full-of-eltronic-gizzmos Fiesta had been left undriven for four weeks, I had great trouble with its battery (which eventually found had fallen to

8V). Essentially, the car was dead. The doors would not unlock, and I had to consult then manual to find where the hidden door-handle keyhole was. When I got it open, the alarm went off, and wouldn't stop.

I only have 'simple' battery chargers, and even with a homemade beast that can deliver over 20A I couldn't get the voltage to rise. I guess that at least two - and maybe three - cells had gone reverse-charged. After many hours of charging, and a rather too-hot charger, I did get the voltage up about 9.5V, after which I was able to start the car - and the on-charge voltage rapidly rose to around 14V.

In contrast, I had absolutely no trouble with my wife's Citroen C1 which, being essentially a 2CV in a modern body, has none of the electronic fripperies of my Fiesta. The battery was around 12V, and it started instantly.

So I guess that the Fiesta's battery was discharging at a lot more than the C1's. So yes - it certainly would be a good design feature if a car's electronics were powered from a separate battery, so that the main battery could always be available to start the car.

[Dave Plowman (News]

I have an ACT tester. Expensive bit of kit. But calculates the actual battery capacity. It's never let me down. Although I can generally tell when my own battery is past it anyway.

[Dave Plowman (News]

One would hope that if you go to those lengths, you'd know what the readings mean. Unlike the average motorist. But such gauges disappearing started long before code readers.

[Dave Plowman (News]

Not too many would be keen on a car which still sort of starts, but has to be taken to a garage to have all the things that rely on a memory reset? Nor would it be a small battery. Up to 50 mA is a common quiescent drain. Work out the size of battery needed to supply that for any length of time.

If you know the car is not going to be used for some time, disconnect the battery. At least then you won't need to buy a new one when you eventually want to use it.

[Dave Plowman (News]

Not so. A lead acid in good condition has a very low self discharge rate.

[Fredxx]

More than a 1,000 hours?

[Fredxx]

I empathise with your issue, but this is down to design. I am currently using a microcontroller that takes sub microAmp when in sleep mode but still providing a date and time function. It has to when working off a lithium button battery.

I also know a acquaintance that had a similar issue to yours. It was a Peugeot, and the discharge rate was solved after performing a firmware update for his radio.

I'm more disheartened that your battery was allowed to go to 8V. Most

12V battery items I have ever come across tend to turn off at 11V to save the battery from being destroyed. I guess there is the reduced voltage during starting issue to contend with.

It's not a very good advert for Ford!

[Fredxx]

The last Transit I owned had two lead acid batteries of equal size. During cranking and running they were connected together. At other times one was isolated from any gizzmos and so zero drain.

I guess that is one solution to the problem.

[Dave Plowman (News]

Yes.

A pal has a place in Spain. In better days goes there for at least 2 months at a time, leaving a car here. With the battery left connected, totally flat on return. And toast. Disconnected, it will start the car after re-connecting.

[Dave Plowman (News]

So effectively a larger battery. That is one solution, but an expensive one. And some will still leave it long enough for them to go flat.

[Fredxx]

I thought the self-discharge rate was quite high but you're spot on, as per usual.

This gives a self-discharge rate of 40% per year, I did see another article that said 5% per month, but give or take they're both in the same ball-park.

Apart from a remote locking receiver being active I really don't see the need for a current draw from anything else.

I would propose it would be cheaper to fix the current drain issues than the cost of a second battery.