Quick battery question.

Afternoon all. I've just upgraded my alternator to a 100amp jobbie on my defender with a 300tdi.
On checking the voltage I'm getting nearly 15 volts at the back of the alternator but less than 13 at the battery.
Am I to assume my battery is on its way out?
Forgive me if it's a daft question, I'm new to vehicle electrics.

Cheers

Tez

Poor connections, particularly due to corrosion can cause a voltage drop in high current circuits.

Before you condemn the battery, check the condition of all the connections in that circuit. Both the battery and the alternator are connected to the main fuseboard in the engine compartment and there is a 150 Amp fuselink in there. Make sure that all these connections are clean and tight.

Two ways to check:

1.With engine running use your multi meter to measure voltage between alternator +ve terminal and battery + terminal, it should be low (0.5 volts ish), do the same from the alternator body (earth) to the battery -ve terminal. If you get a higher reading on one or the other that should indicate which one is dropping the voltage and where to look.

2. Try using jump lead between alternator body and battery -ve terminal (make sure you don't connect to +ve!!). Start engine and measure battery voltage, if it has increased then you may have poor earth straps, if no change then it will be the positive wiring you need to look at.

Thanks for the replies and advise. Il try them tonight. I've checked the voltage this morning after driving to work and the voltage looked a lot better. Perhaps my failing alternator was draining the battery.