Mixing 12 volt engine & thruster with 6 volt house

[scross]

Our Journey currently has six 12 volt lead acid batteries at 90 AH each. One for engine, one battery for thruster+windlass and four house batteries.
I replaced the engine and thruster batteries with new 12 volt 90 AH lead acid batteries last spring and am looking at replacing the four house batteries this spring. I am not interested in going to AGM type batteries.
For the battery charging & ACR experts out there I have a question about going to 6 volt house batteries.
Looking at four 6 volt 235 AH deep cycle instead of four 12 volt 90 AH batteries. The pluses for doing this are going from a total of 360 AH to 470 AH - a 30% boost. The negatives are slightly higher cost for the 6 volt batteries (an extra $120 in total) and an additional 60 pounds of weight. Space is not an issue.
I am not sure if having two pair of 6 volt batteries on the house circuit would cause any charging issue because of the differences vin AH capacities between those and my smaller AH engine and thruster batteries. The only time the house and engine batteries are connected is by the Emergency rotary switch in the cave.
Thoughts?

 

[BradOwens]

Are you going to connect the 6 volt batteries in two banks with two batteries in each bank connected in series to achieve 12 volts? If not, I fear some if not most electrical equipment won't work properly, including the inverter.

 

[scross]

Brad,
Yes. Sorry I wasn’t clear on that. House would be at 12 volts by having two pair of two 6 volt batteries in series to get to 12 volts and then those pair of series batteries connected in parallel to get to 12 volts at 470 AH.

 

[Submariner]

Lead acid batteries generally should not be used below 50% DOD (depth of discharge). Taking into account the absorption phase that starts around 15-20% DOD, means there's really only about 35% usable capacity with lead acid batteries.

Reading what you wrote, you should have 126 usable Ah today (35% of 360Ah). The upgrade you're looking at would get you to 164Ah usable. From a charging perspective, as long as it's two pairs of 12 volts batteries (2 6volt batteries connected in series), then connected in parallel, I would think it'd be fine from a charging perspective as long as all 4 batteries in your house bank are the same voltage and same Ah ratings.

I know you said you're not interested in AGM batteries. I just upgraded my house bank to a pair of Firefly Oasis G31 (AGM) batteries. Firefly AGM's are not normal AGM's. Two 12v G31 Firefly batteries (110Ah each) gives me 143Ah usable. (65% of 220Ah). Firefly batteries can be ran to 100% DOD (though, the manufacture recommends 80% DOD to get upwards of 1,300 cycles out of them).

Firefly Oasis batteries are offered in 12v G31 or they also have a L15 battery thats 2v@900Ah or 4v@450Ah configurations.
They're not cheap though. Over their lifetime they should be less expensive than replacing conventional lead acid batteries, while offering greater usable capacity. If I had the space (I don't), I'd run 3 of the L15 Firefly batteries for 450Ah (292Ah usable @ 65%).

 

[Red Raven]

Hi Al,

The difference in AH should not make a difference in charging systems or settings. As long as all the batteries are the same type (flooded in your case) you should be good. Note, you already have a significant difference in AH from your start battery (90AH) vs. your house bank (360AH) without an issue. The change to 470AH is actually a smaller difference than what you currently have between banks. The 6 V batteries do have to be the same AH. The charging systems will not “know” you have four 6v batteries in series/parallel vs. four 12v batteries all in parallel. Just looks like a big capacity 12V battery!

Curt

 

[scross]

Martin,
Thanks for the reply. I’m resisting AGMs and Lithium because of the much higher cost. I’m getting 4+ years of life from my flooded batteries and can’t see spending 50% or more for an extra year or so of life from the AGMs. Yes, the flooded due take more maintenance but my batteries are easy to access so that’s not a problem. I only cycle my batteries about 40 times a season so I’m cycling about 160-200 times over the life of the batteries.
My solar system is almost keeping up with my usage for extended stays on the hook so I’m just considering a little bit extra AH in case of multiple consecutive rainy/heavy overcast days.

 

[Submariner]

Martin,
Thanks for the reply. I’m resisting AGMs and Lithium because of the much higher cost. I’m getting 4+ years of life from my flooded batteries and can’t see spending 50% or more for an extra year or so of life from the AGMs. Yes, the flooded due take more maintenance but my batteries are easy to access so that’s not a problem. I only cycle my batteries about 40 times a season so I’m cycling about 160-200 times over the life of the batteries.
My solar system is almost keeping up with my usage for extended stays on the hook so I’m just considering a little bit extra AH in case of multiple consecutive rainy/heavy overcast days.

Totally understand. What changed my mind was the manufacture of Firefly stated life expectancy of the batteries to be 5-10 years in a marine application.

"These batteries are marine grade, sealed and maintenance free. Manufacturer estimates a 5 to 10 year life cycle. With a daily discharge of 50% the manufacturer projects 3600 cycles which give you just shy of 10 years of operational life."

We use our boat a lot, but to get that kind of usage is still well out of our reach.
I resisted LiON due to cost and the other electrical components that may need upgrading, such as battery charger, solar charger, etc...

 

[scross]

Curt,
Thanks for your comments. I’m going with the 6 volts!
Al

 

[knotflying]

I guess my initial question would be; Are you having and issue with the current amperage you have. As they say, "If it works, don't fix it."

 

[BB marine]

Looking at four 6 volt 235 AH deep cycle instead of four 12 volt 90 AH batteries. The pluses for doing this are going from a total of 360 AH to 470 AH - a 30% boost. The negatives are slightly higher cost for the 6 volt batteries (an extra $120 in total) and an additional 60 pounds of weight. Space is not an issue.

Just a thought that will accomplish your AH needs reduce 60 pounds ( I'm always reluctant to add weight to theses fine boats because they are weight sensitive. Simply install a battery switch between the thruster battery and the house bank. If you are at anchor the thruster battery is doing nothing and is not really needed when you pull the anchor so if it is 50% discharged it will be charged when you start the engine and head home. Using the thruster battery in parallel with the existing house bank will increase your capacity to 450 AH or 225 AH of usable compared to 470AH or 235AH ( (4) 6 volt batteries) there is not much difference in capacity. There is another advantage of this. If you find that the thruster battery is compromised or low in charge because of high wind and extended thruster use you can boost your available thruster battery capacity by turning the battery switch on combining banks.

I installed a four way battery switch used for my inverter battery switch On/Off/1 is house bank/ 2 is thruster bank/ both is house and thruster banks combined. When I turn the switch to both my house bank now has an additional battery. If I am not using the inverter I just keep it turned off. I am using the inverter battery switch for multiple purposes.

 

[scross]

Thanks all for the great suggestions and insights.
I ended up replacing the four flooded 12 volt house batteries with the same type. I found 12 volt replacements that gave me almost the same total Amp/Hours that the 6 volt setup would have provided.
I was looking at getting 470 Amp/Hours out of the 6 volt setup. The Crown 27HDC115 flooded batteries will give me 460 Amp/Hours and are quite a bit lighter than the 6 volt alternative. Crown has a good reputation- similar to Trojan.
In six years I’ll follow up to let everyone know how they worked out!