Thread: Batteries, CCA and Amps

Originally Posted by vnx205

I thought I would take advantage of the information here and carry out a little experiment to see what voltage was needed to start my 300 Tdi.

The battery is on its last legs and will need to be replaced very soon. If I don't use it for over a week, I am likely to have to top it up with the battery charger. Hardly surprising as the battery is the same one that was in the vehicle when I bought it four and a half years ago.

Anyway after not using the vehicle for five days, I measured the voltage today at 11.5 volts, which according to the table is only about 20% charge. I was a bit surprised that the voltage didn't change perceptibly when I turned on the parking lights. Is that what you would expect? Or don't the parking lights produce enough load to change the voltage?

There was just enough power to allow the glow plugs to operate normally and start the engine, but there wasn't much to spare. It started, but only just.

I realise that running a battery down to 20% is not advisable, but I had hoped that this little experiment would give me some useful information. I thought that if I ever fitted a battery monitor or some sort of volt meter, it would be helpful to know what voltage was needed to start the engine.

Was I wasting my time? Does the figure I came up with of 11.5 being well below what is safe tally with accepted ideas?

Hi vnx205, over the years I have tested many vehicles for how low the battery can be and the vehicle will still start.

Over 20 years ago, when I first started testing while developing the setting for my ( then ) new SC40 battery isolator, I found most vehicles would start from batteries with as low as 11.75 volts ( 30% SoC ) but some older ( thats older even back then ) diesel powered 4x4s, found it hard if not impossible to start with a battery voltage of 11.75v, but all vehicles tested at the time would start with a battery voltage of 12.0v ( 50% SoC ).

By about 2003, all the vehicles I tested had no problems starting off a battery with as low as 11.5v ( about 20% SoC ).

So your findings just confirm what I have come across.

The one thing you didn’t post up is what was the voltage of the battery about 10 hours after you last drove the vehicle and how long was that last drive.

Some clarification of what the voltage reading and State of Charge ( SoC ) of a battery means in relation to the storage capacity of the same battery.

The SoC of a battery relates to the amount of charge in the USEABLE capacity of a battery.

Put another way, if you have a new 100 Ah battery and the SoC voltage is 12.75v, the you have fully charged battery and you have 100 Ah available for use.

If the battery is getting on a bit and some of the battery’s surface is sulphated and only about 80% of the battery’s original capacity is available. If the battery is fully charged it will still have an SoC voltage of 12.75v, this is for the usable 80% of the battery.

The SoC only indicates the charged state of the useable portion of the battery, it does not give you any indication as to just how much of the battery’s original 100 Ah you actually have available.

Hi vnx205, thats close but to use your tank comparison.

It would be like having two tanks side by side at the same high from the ground. One new and one old.

The new one is full of water but the old one has accumulated a fair bit of sediment.

Both tanks will have the same pressure ( voltage ) when they are full but the older tank runs out of water sooner.

The more sediment, the less the older tank holds but it will still have the same pressure ( voltage ) when it’s full.