hi I have 6 4v 200 amp hr batteries connected 3 3 in series and than in parallel
to keep them 12v they are deep cycle flooded lead acid.
1 how is the best way to charge them?
2. when fully charged what voltage should they hold 13.5v?
thanks.

 

Go read this.

 

Do a web search for the Manufacturer's data sheet for your specific make and model of battery. It will have the recommended "charge" voltage and current limit. It will also have a "float" voltage specific to the actual chemistry in your batteries. These numbers change a bit depending on how much Calcium the battery maker put in the plates.

Also take note of how these voltages need to be adjusted for ambient temperature. Sophisticated chargers have a thermistor that is bolted to the battery terminal so that the charger automatically adjusts the charge and float voltages as a function of temperature.

 

The ideal is a smart three step charger , bulk, float & maintenance charge. Some chargers also have an additional equalising charge feature, on a timer.

Deep cycle flooded cells are extremely robust and can take a hefty bulk charge. The general rule is 1/10th of the A/h rating. In your case that would be 40A. 14.4V is the ideal cut off voltage for bulk charge. You will notice that the current drops off significantly as the batteries approach 85-90% charge. The next step is float charge when the voltage is reduced to 13.8 V until the batteries are fully (> 90% charged.) If your charger has a maintenance mode then the charger will automatically keep your batteries topped up over a prolonged period to counter natural discharge ( which increases dramatically with temperature).

Deep cycle batteries do not like just sitting there and should be discharged to at least 50% of their capacity and fully recharged at least every two months. They should also never be left discharged (<11.5V) for any period more than a few days due to sulphation, where the lead sulphate irreversibly crystallises and reduces the battery capacity.

There is one more important item which is very important in deep cycle batteries. Over a period of time i.e.10-15 charge/discharge cycles, the voltage of the cells will begin to vary due to the small chemical differences of the cells and their internal temperatures while charging. This results in some cells never being fully charged and others being overcharged, as the same current passes through all the "in series " cells. You will need to increase the final voltage, after the bulk charge is complete, to about 15.5-16V and charge until all the cells start to gas freely. This ensures that every cell is charged to it's maximum capacity. This step is called an "equalisation charge". You may need to replenish the lost water in the cells after each equalisation charge and it is quite normal that some cells will need more water than others.

As mentioned by the other posters the temperature is important and the above figures relate to 25Deg C . If you are in a hot climate or confined space, with little or no air circulation, you may need to lower the charge voltage . If in a confined space don't pack the batteries close together, as a little natural convection is a great help.

Lead acid batteries will have a surface charge of about 13.5 Volts for a short while after charging. This will reduce after a few hours to 12.8 Volts, which is correct for a fully charged 6 cell (12V) battery in good condition.

I hope this information is of help to you.

 

thanks cork_ie did read the battery university thing but your explanation answered in plain language all my questions thank you.

 

You're very welcome. I'm delighted that my reply was of use to you.