AGM

[ifmdalvey]

A slight hijack of the thread but i realise some members reading this have far greater knowledge than myself. I have 2 group 27 wet cell batteries, one of which is failing and cannot hold its charge, is there any reason why i cannot but an AGM battery as a replacement and run it alongside my serviceable wet cell. Can you mix battery types ?

 

[Maine Sail]

You ideally should not mix different type of batteries in the same bank as their charge acceptance rates can vary widely. You are also not best served to mix batteries of different ages in the same bank or you can shorten the life of the new battery.

So, if you have two group 27's in parallel as your house bank you would be best served to replace the pair of them at the same time, with the same technology as your starter battery, unless you have a charging system designed to accommodate charging banks of batteries of different types like AGM / Wet or Gel / Wet.

If you don't have the ability to recharge AGM's to 100% very often, like being on a mooring in Maine, you may find they suffer a rather short life for the additional expense when compared to regular old wet cells...

 

[ifmdalvey]

Thanks Maine Sail
Further to my last, my Pearson 31 only has 2 batteries both are 100AH marine batteries of the same type, should i therefore be buying one deep cycle and one starter battery or not mix types? i do run a 100 watt solar panel which works quite well when its not foggy.

 

[Maine Sail]

Thanks Maine Sail
Further to my last, my Pearson 31 only has 2 batteries both are 100AH marine batteries of the same type, should i therefore be buying one deep cycle and one starter battery or not mix types? i do run a 100 watt solar panel which works quite well when its not foggy.

For a 31 footer a single 100Ah battery is a very small bank leaving you with only about 30 usable Ah's if you want to maintain your batteries by never drawing them below 50% state of charge. If starting from the mooring on a Friday you would have about 50 usable Ah's before hitting a 50% state of charge but if off cruising your alternator will rarely get them back to much above 80% SOC or 30 usable Ah's..

You can use either a start or deep cycle to start your Yanmar as they require very little in terms of cranking amps. I use deep cycles on my own boat as they make for better back up batteries in the event something happened to my house bank. You might try and find a place for a separate starter battery so you can combine the two group 27's into one larger house bank then have a dedicated reserve battery.

The less you discharge a bank in terms of % of full the longer it will last. Thus, larger banks last longer than a smaller one when drawing the same Ah's off of them.

Removing 75 Ah's from a 100 Ah battery puts it at 25% state of charge. This will shorten its cycle life. By combining your two 27's for 200 Ah's and drawing the same 75 Ah's you would be drawing down to about 63% state of charge which is a good safe level of discharge for a longer cycle life.