Thread: Problem Charging Aux Batteries with Smart Charger

12.6 is fully charged (6x2.1).
Which battery was the C -Tek charging - the starting or second battery?
If batteries of different sizes are connected they may equalize at the capacity of the smallest battery, so the larger one may not be fully charged.

The alternator will provide far more power than a 5A CTEK. There is no comparison! The only reason for charging with the CTEK is if you are doing very short drives and the battery is not being fully charged in the time that you are driving. With 5A it takes 1 hour to provide 5AH (ignoring the battery charging inefficiency) so you can see how many hours it will take to charge a high AH set of batteries. The alternator will be far quicker to charge them when the engine is running.

Originally Posted by BobD

The alternator will provide far more power than a 5A CTEK. There is no comparison! The only reason for charging with the CTEK is if you are doing very short drives and the battery is not being fully charged in the time that you are driving. With 5A it takes 1 hour to provide 1AH (ignoring the battery charging inefficiency) so you can see how many hours it will take to charge a high AH set of batteries.

Umm, can you explain the charging rate here. To my simplistic uneducated mind and ignoring all charging and battery inefficiencies, I would have thought that if you needed to charge a battery with 50Ah with a 5A charger, then it would take approximately 50/5 = 10 hours. The way I am reading your calculation, it would take 50 x 1 = 50 hours. What am I missing?

Hi laughto and I think Bob might be relating to the fact the batteries were already at 95% SoC and as such, they would not draw more than 1 amp from any charger.


The charger would have gone into FLOAT mode very shortly after it was connected to the near fully charged batteries.


As such, it would drop the applied voltage down to something like 13.5v.


Whereas, as the alternator charges at a constant higher voltage all the time, the batteries would be maintained at a higher state of charger anyway.


Hi GD-4, at 12.65v in the morning, is about as high a state of charge you are going to get and is very good.


I can not make any suggestion for why the lower settled voltage when charging with the battery charger.

This chart will give you some idea of the actual state of charge of your batteries.


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Originally Posted by laughto

Umm, can you explain the charging rate here. To my simplistic uneducated mind and ignoring all charging and battery inefficiencies, I would have thought that if you needed to charge a battery with 50Ah with a 5A charger, then it would take approximately 50/5 = 10 hours. The way I am reading your calculation, it would take 50 x 1 = 50 hours. What am I missing?

Sorry mate! That was a typo. I got distracted half way through and meant to write 5AH in an hour. You are correct. 10 hours for 50AH when the battery is low. I have corrected the post!

What size is the alternator in these vehicles - 80Ah?
If it is then the alternator is (thinks carefully to avoid usual dodgy maths abilities) 16 times more powerful than a 5Ah battery charger. So it is certainly a lot more powerful.
Returning to my previous question, was the C-Tek connected to the starting or secondary battery, as that could affect the result.
Also, which battery are the readings you are quoting taken from - the starting or second battery?
Your two batteries are significantly different in size.

Add 100amps. They're 180A.

Battery size will have minimal impact on charging...

Don't forget to factor in that regardless of how much current an alternator produces it only provides head room *above* vehicle consumption.. so while the Alternator has a nameplate 16x that of the charger - it may only have 2x the charger output to give - I've seen cases where it's about parity!!!