Hello, not sure its the right place to ask, but I have a 75 AH deep cycle battery(AGM) and I have a battery charger with 2 modes, 5.5A for car, 2A for motorcycle, as far as I remember, lower charging rate is usually better for longer battery life and better charging, my questions are if what I remember is even true, and if so, does long charging affects the battery? For example, maybe lower charging rate might be better, but if it takes very long to charge it makes it worse? Long story short, which mode should I use, the 5.5A or the 2A? Thanks

 

Higher charge current (higher voltage) causes battery to heat up and this evaporates water inside electrolyte (aka battery acid). It can also vent more hydrogen.
There are other factors, but as far as I know this is the main reason behind higher charging rate can shorten battery life.
Long charging shouldn't affect the battery. However, if you use a charger without charging control, it will continue to try to push 2A on a fully charged battery and that can also reduce life of the battery.
The charging current is determined by the voltage and if you charge at high current with devices connected to the battery, damage may occur depending on how sensitive your devices is to the voltage.

Lead acid batteries are very easy to charge and very forgiving as well.
Usually, you won't have any problem charging the battery 5A.

 

Thanks for the reply, the charger has a charging control, so it will stop charging once its done, in case the charging speed is not a factor for me, will charging at 2A be better for the long run? Or should I just charge at 5A?

 

2A will be slightly better. Especially for a small batteries.
Chargers with charge control will switch to maintenance mode once the battery is fully charged. It basically switches to same or slightly higher voltage of a fully charged battery and keep the voltage level there.

 

The only issue with charging at the lower amperage is time. If you have plenty of time for charging it's no issue. If you want to put 50 amps back into the battery in 10 or 12 hours you will need the higher charge rate.

 

Advice given on a boat forum I visit is that deep cycle AGM batteries "like" higher charge rates. I'm not sure how to verify this as manufacturers might recommend lower than ideal rates because it lessens other hazards (aka liabilities).

 

I'll be using the battery around once a month, sometimes more, sometimes less, so charging time is not really a problem..

 

I see, I wonder if there's a way to verify that.

 

A reputable battery manufacturer usually has a datasheet with charging/discharging rate graph. It might suggest the best charging rate and voltage.