I have a Ford Focus 1.8 TDCI which has been a bad starter. It was fitted with a 60 AH battery and the motor always cranked relatively slowly. Most times it would eventually start after many attempts, but not always. Sometimes it would just not start, and inevitably the battery would go flat. The standing voltage measured 12.7v and the alternator is delivering 14.7v. I fitted a new starter motor and new  glow plugs which made no difference. Anyway I have now bought a new battery rated at 70 AH and it has transformed starting. Engine cranks much faster and it starts almost straight away even in cold temperatures. So, can anyone offer an opinion on the battery? Was the old battery US, or simply not man enough for the job? I know nothing about batteries and would be interested to hear from anyone that does. Seems to me that although the old battery seemed to check out OK it was probably on its way out.

4 hours ago, JohntH said:

I have a Ford Focus 1.8 TDCI which has been a bad starter. It was fitted with a 60 AH battery and the motor always cranked relatively slowly. Most times it would eventually start after many attempts, but not always. Sometimes it would just not start, and inevitably the battery would go flat. The standing voltage measured 12.7v and the alternator is delivering 14.7v. I fitted a new starter motor and new  glow plugs which made no difference. Anyway I have now bought a new battery rated at 70 AH and it has transformed starting. Engine cranks much faster and it starts almost straight away even in cold temperatures. So, can anyone offer an opinion on the battery? Was the old battery US, or simply not man enough for the job? I know nothing about batteries and would be interested to hear from anyone that does. Seems to me that although the old battery seemed to check out OK it was probably on its way out.

Doing research on car batteries soon becomes apparent that it’s a complex subject.On starting you should be looking at the cold crank amps figure and not the amp hours figure.On mine it’s 700 cca👍

Most likely wrong battery i would imagine the old one was either an 075 type 60ah and about 500-550cca 1.8 diesel should be a type 100 battery 70ah 620-670cca

More than likely a second hand dealer at some point in its history put on the wrong battery as he was being cheap just to shift it from his forecourt as the battery died while it was sat there.

cca basically mean peak out put so as much power as the battery can deliver in one hit for example 70ah battery with a constant 1 amp drain 70 hours life but at very low force for want a a better term. but when you need it to dump power in one go ie starting 620cca is the force behind more force behind the power quicker things move but for a lot less usable time.

cca is more important on diesel then petrol. diesel starter motor needs more current to overcome the higher compression ratio of diesel engine compared to petrol. then couple that demand on the power with the glowplugs simultaneously using loads of power and you will see that a diesel needs tons more electrical power in one hit to start the engine compared to petrol.