Thread: Battery Voltage, State Of Charge.

On pure technical terms, with any kind of battery, it is not possible to accuratelly determine state od charge on the battery just by measuring voltage. Or how much "juice" battery has in it. State of charge term i have seen abused by many sales and maketing departments.

Battery with high impedance (type of battery fault) can show you even 13.5 for example. But once you start loading it even with few amps, this voltage can drop to half or even more.

Battery cutout by voltage works when your battery is in prime condition. As it gets older and less effective, state of charge, or more important, it's abillity to start the car will be lower and lower.

If using for aux battery only then ok. But not for starting.

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Some years ago I was the manager of a couple of large telecommunication facilities where battery banks of very large flooded cells were the norm. Staff monitored and recorded battery condition by both terminal voltage and specific gravity.

Over the years as equipment changed, those very reliable banks were removed and replaced with UPS systems that had sealed cells where only the terminal voltage could be monitored. Monitoring was a scheduled routine which my plant staff followed.

One day we had a power outage so the batteries were supposed to hold the UPS working until the diesel generating plant came on line - usually less than 30 seconds.

Well, the UPS batteries didn't hold up and a lot of equipment went off line with significant disruption to services.

When we investigated, we found the batteries had gone high impedance and whilst the terminal voltage looked good, under load the voltage dropped markedly. We then started checking other UPS banks by looking at ripple voltages on the batteries at other installations in the buildings and at other sites and found more faulty ones from the same supplier.

Originally Posted by ROAMER_AUS

As it gets older and less effective, state of charge, or more important, it's abillity to start the car will be lower and lower.

If using for aux battery only then ok. But not for starting.

Sent from my GT-I9305T using AULRO mobile app

The Traxide isolator works by keeping both batteries connected so that when running accessories, in my case the Engle, you are drawing off both starting and aux batteries.

Now when the batteries reach 12 volts the isolator kicks in and isolates the starting battery.

So I now have a starting battery with 12 volts. What you are saying above is that if my battery is getting a bit old it is not going to start my Disco?

my experience was that it will behave exactly as it would if the controller wasnt there...

i had a crook start battery in my 300tdi disco with an sc80 battery controller in it and it started the car fine @ 11.9 volts so i wouldn't be too worried about not getting a start @ 12v.

battery got me home and fridges etc stayed cold

replaced battery a month or so later and it hadn't caused be grief other than cranking just ever so more slowly.

storm in a tea cup...

Originally Posted by p38arover

Some years ago I was the manager of a couple of large telecommunication facilities where battery banks of very large flooded cells were the norm. Staff monitored and recorded battery condition by both terminal voltage and specific gravity.

Over the years as equipment changed, those very reliable banks were removed and replaced with UPS systems that had sealed cells where only the terminal voltage could be monitored. Monitoring was a scheduled routine which my plant staff followed.

One day we had a power outage so the batteries were supposed to hold the UPS working until the diesel generating plant came on line - usually less than 30 seconds.

Well, the UPS batteries didn't hold up and a lot of equipment went off line with significant disruption to services.

When we investigated, we found the batteries had gone high impedance and whilst the terminal voltage looked good, under load the voltage dropped markedly. We then started checking other UPS banks by looking at ripple voltages on the batteries at other installations in the buildings and at other sites and found more faulty ones from the same supplier.

Exactly what I was talking about. I was UPS technician for years and have seen many cases like that. There is no substitute for regular discharge test.

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Hi Dave, and while this is correct, reality is that without my isolator, even if you battery was showing it is fully charged, if it's stuffed, it's stuffed and will not start your motor.

One of the advantages of having one of my isolators is, if a cranking battery is on it's last legs, because of the way my isolators work, you are far more likely to still be able to start your motor.

But once the cranking battery is stuffed, with or without one of my isolators, you need to get the cranking replaced.

Originally Posted by d2dave

The Traxide isolator works by keeping both batteries connected so that when running accessories, in my case the Engle, you are drawing off both starting and aux batteries.

Now when the batteries reach 12 volts the isolator kicks in and isolates the starting battery.

So I now have a starting battery with 12 volts. What you are saying above is that if my battery is getting a bit old it is not going to start my Disco?

I was saying that in general, regardles of type of battery and use, including the car, you can not determine state of battery by purely measuring the voltage. Now specific situations and cases can vary.

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Originally Posted by drivesafe

Hi Dave, and while this is correct, reality is that without my isolator, even if you battery was showing it is fully charged, if it's stuffed, it's stuffed and will not start your motor.

One of the advantages of having one of my isolators is, if a cranking battery is on it's last legs, because of the way my isolators work, you are far more likely to still be able to start your motor.

But once the cranking battery is stuffed, with or without one of my isolators, you need to get the cranking replaced.

Tim, would an SC80 exacerbate an aging cranking batt. or would it be the opposite by linking both you might extend the life of it ?
My scenario is that my cranking batt. (supercharge gold MF80) which is 7 years old but apart from one occasion where it failed to crank my tdi over (it was -6.7c overnight) it has not let me down.