6V House Battery Bank Capacity - SailNet Community

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Old 02-20-2009
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6V House Battery Bank Capacity

The instruction manual for my Link 20 battery monitors says that for setting the devices' battery capacity; "Two Trojan T-105 6 volt golf cart batteries in series would use the factory default 200 amp hours for capacity". Coincidentally this is exactly the battery bank setup that I have installed.

I had thought that these Trojans were actually rated at 225 amp hours each. However my main question is - when assessing the overall capacity of this bank of house batteries, do I have around 200 or around 400 amp hours available?
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2 batts x(6volts * 200amps) = 2400watts
2400 watts / 12volts = 200amps.

So you have approx. 200 amps when used at 12 volts.

Paul L
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Neil,

Yes, Paul's math is correct.

The T-105's are rated for 225AH each (20-hour rate) @ 6 volts.

If you connect them in series, you get 12V @ 225AH capacity.

If you connect them in parallel, you'd get 6V @ 450AH capacity.

In order to get 450AH capacity @ 12V, you'd need four batteries, connected in series/parallel.

Bill
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Neil, if you connecting them in series to create 12V system the capacity of the system will be around 225AH. There are many threads on this forum (and others) that explain the electrical boat design and calculations in details. In addition I suggest that you consult a book "The 12-Volt Bible for Boats".

Chris
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When you connect batteries in series, you add the voltage, but the amp-hours remain unchanged.

When you connect batteries in parallel, you add the amp-hours, but the voltage remains unchanged.
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Not Good News!

Well in that case it looks like I only have at most 60-70 amp hours of usable house battery storage. Unless I spend significant time and money adding another pair of Trojans I am going to have to economize on power consumption. This means putting more bags of ice in the fridge and a purge on power hungry lighting appliances. Out go the halogens and in come fluorescents and LEDs. However spending money on top end stuff like Alpenglow and Sensibulbs soon adds up as well.
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200 amp hours will give you closer to 100 amps of useable power. infact if the batteries are 225 i would set the link to use 100 amps as full, that way when you reach 100 amps used you are still at 60 % charge
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Extended Cruising

Yes 100 amp plus hours on a day trip starting with fully charged batteries. But as I understand it, on an extended cruise, assuming that it is not practical to engine charge above around 75% full and 50% is the lowest discharge point, then a 225 amp hour battery bank only stores a usable 56 amp hours.
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Neil, you are correct but I would use the 225AH figure and 50 % to 80 %, which is the point where smart regulators switch from bulk charge, giving you a daily useage available of 68AH. In order to see if this is sufficient capacity you willl have to do a detailed daily electrical consumption chart. You also need to know the alternator charge rate. Most standard alternators with standard regulators drop of dramatically after a few minutes requiring many hours to give back that 68AHs.
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The conventional figures actually use the 50% and 85% points, as noted in the Xantrex literature. This gives you closer to 79 amp-hours to use.
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