ABYC and Batteries

[mitiempo]

Fuse size has to do with wire gauge and load - not length of run.

For example 2/0 wire (ampacity of 281 amps in engine spaces) and a load of say 175 amps you fuse between the two. Maybe 225 amp fuse - whether the run is 2' or 30'.

[Lake-Effect]

Originally Posted by Ex-Calif

You double the positive run when calculating fuse size.

Because OP was asking if he should calculate based on round trip (24 feet) or 36 feet because he has an extra wire run. My question was whether the extra run was a positive cable because it was not clear from his post.

Small correction: you double the positive run when calculating WIRE size, to be within the desired V-drop (eg 3% or 10%) of the wiring run.

The point of fusing at the battery is to protect the wiring, in case of a short circuit resulting in current that would

• exceed the wire's ability to handle safely (fires, melting etc) and/or
• cause the battery to overheat or explode

... so it doesn't really matter whether the positive battery cable is 3 ft or 30 ft. The fuse current rating should be higher than the largest expected draw, but the same or lower than the wire's safe maximum current. (which is what mitiempo already said above)


For a long run between two batteries on the same bank, you do need a fuse near each battery on the positive cable, because there are two sources of potentially dangerous currents for that cable. You don't need a fuse on the long negative run, because all of the metal bits it could short against are also at ground potential or are isolated, and also because all the sources of positive potential are already themselves fused.


All things considered, it's not a great idea to to have the batteries of the same bank separated by a large distance. If more bank capacity is desired, either locate the extra battery much closer to the bank, or put in a larger battery. If you only have that remote location available for more batteries, consider making that a new (house) bank, that you can charge and draw from separately.

[mitiempo]

Originally Posted by Lake-Effect

For a long run between two batteries on the same bank, you do need a fuse near each battery on the positive cable, because there are two sources of potentially dangerous currents for that cable. You don't need a fuse on the long negative run, because all of the metal bits it could short against are also at ground potential or are isolated, and also because all the sources of positive potential are already themselves fused.


All things considered, it's not a great idea to to have the batteries of the same bank separated by a large distance. If more bank capacity is desired, either locate the extra battery much closer to the bank, or put in a larger battery. If you only have that remote location available for more batteries, consider making that a new (house) bank, that you can charge and draw from separately.

I agree. Part of the problem with widely separated batteries in the same bank is charging. Even 3% drop will mean that 14.4 at the close battery will mean less than 14 volts at the far battery. It's time to a full charge will increase dramatically because of this. In reality it is unlikely it will seldom or ever be fully charged away from the dock and will die an early death.

I always try to wire for zero voltage drop in charging circuits and at that distance it is impossible to do.

[Maine Sail]

Originally Posted by mitiempo

I agree. Part of the problem with widely separated batteries in the same bank is charging. Even 3% drop will mean that 14.4 at the close battery will mean less than 14 volts at the far battery. It's time to a full charge will increase dramatically because of this. In reality it is unlikely it will seldom or ever be fully charged away from the dock and will die an early death.

I always try to wire for zero voltage drop in charging circuits and at that distance it is impossible to do.

With proper wiring this is not a big issue but it means the bank and the charge sources need to be wired correctly, eg: not to one end of the bank but across it..

[smac999]

as the batteries get closer to charge the amps will drop and the voltage drop will reduce to near 0. so both batteries should hit full charge voltage.

[mitiempo]

Originally Posted by smac999

as the batteries get closer to charge the amps will drop and the voltage drop will reduce to near 0. so both batteries should hit full charge voltage.

Yes, eventually but it will take longer. Voltage is pressure.

[Sailing Drummer]

Well, I've learned (and in some cases re-learned) a lot reading your replies. And thank all of you for the information.

I was on the boat yesterday and I checked every possible place to either get the second battery closer to the first, move the first closer to where the second will be, or find a completely new area for both. And I've come up with nothing.

It's becoming more logical to not add another 4D and get rid of the 4D I currently have and come up with a better solution. And I'm pretty sure that solution is going to be 4 X 6volt batteries in serial/parallel that would fit in the space where the current 4D sits.

The cost of 4 new 6v is around the same as a new 4D plus all the wire I would need. And it would be much better from a voltage drop standpoint and I'd still get about the same amp capacity. Also, I may be able to sell the 4D to help recoup some of the expense.

So once again, thanks for the input and knowledge.

Larry

[mitiempo]

Good plan.

[hlev00]

If you are suggesting that your 4 6V batts, fitting into the roughly the same space as a 4D will give you the same as 2 4Ds, be sure you are accounting for the 6V amp-hours vs 12V amp-hours. Just sounds not quite right. The same rough size and weight should be about the same capacity.


Sent from my iPhone using Cruisers Sailing Forum

[ontherocks83]

Originally Posted by hlev00

If you are suggesting that your 4 6V batts, fitting into the roughly the same space as a 4D will give you the same as 2 4Ds, be sure you are accounting for the 6V amp-hours vs 12V amp-hours. Just sounds not quite right. The same rough size and weight should be about the same capacity.


Sent from my iPhone using Cruisers Sailing Forum

Actually depending on which batteries he is looking at it is absolutely correct. There are trojan 6v batteries that put in series parallel will put out 520ah where 2 4Ds are in the 360 to 400ah range.

[Sailing Drummer]

hlev00 - My battery box is the length of a 4D but wider. The fit will be close so I'm not sure if that battery box will work. I may have to go with a different box. As for the capacity - I will have 2 sets of 6v batteries consisting of 2 batteries in each set and they (each set) will be connected in serial. That will give me 12v at the same capacity as each battery (about 200-250 amps depending on which batteries I go with). So in theory each set will be like a 12v battery at about 200-250 amps. Then I will connect the 2 sets together in parallel. That will keep it at 12v but double the capacity to 4-500 amps.

Correct me if I'm wrong. But I think I've got it right.

Larry

[ontherocks83]

Originally Posted by Sailing Drummer

hlev00 - My battery box is the length of a 4D but wider. The fit will be close so I'm not sure if that battery box will work. I may have to go with a different box. As for the capacity - I will have 2 sets of 6v batteries consisting of 2 batteries in each set and they (each set) will be connected in serial. That will give me 12v at the same capacity as each battery (about 200-250 amps depending on which batteries I go with). So in theory each set will be like a 12v battery at about 200-250 amps. Then I will connect the 2 sets together in parallel. That will keep it at 12v but double the capacity to 4-500 amps.

Correct me if I'm wrong. But I think I've got it right.

Larry

You've got it right by my math

[Ex-Calif]

Originally Posted by Lake-Effect

Small correction: you double the positive run when calculating WIRE size, to be within the desired V-drop (eg 3% or 10%) of the wiring run.

.

I suppose I wasn't clear.

The wire run sets the wire size and results in a fuse choice to protect the wire.

And for this run bigger wire is better.

Last weekend saw a failed (corroded) wire from an old schooner. The cable had to be 2 inch diameter - huge mutha.

[hlev00]

Originally Posted by Sailing Drummer

hlev00 - My battery box is the length of a 4D but wider. The fit will be close so I'm not sure if that battery box will work. I may have to go with a different box. As for the capacity - I will have 2 sets of 6v batteries consisting of 2 batteries in each set and they (each set) will be connected in serial. That will give me 12v at the same capacity as each battery (about 200-250 amps depending on which batteries I go with). So in theory each set will be like a 12v battery at about 200-250 amps. Then I will connect the 2 sets together in parallel. That will keep it at 12v but double the capacity to 4-500 amps.

Correct me if I'm wrong. But I think I've got it right.

Larry

Larry, I agree your math is right if you've got the extra space for that capacity 6V batteries. I've been trying to do the same but my battery box is pretty much limited to exactly two 4Ds. When I look at Trojans I can't get 4 to fit in the same space. Or maybe I'm not thinking about the placement correctly. Which batteries you are looking at?

[Sailing Drummer]

Originally Posted by hlev00

Larry, I agree your math is right if you've got the extra space for that capacity 6V batteries. I've been trying to do the same but my battery box is pretty much limited to exactly two 4Ds. When I look at Trojans I can't get 4 to fit in the same space. Or maybe I'm not thinking about the placement correctly. Which batteries you are looking at?

hlev00 - If your battery box is for 2 X 4D and they are side by side you've got plenty of room. A 4D is about 21X8.25 inches. So side by side they'd be 21X16.5 in. A 6v like Trojan T-105 (or a similar sized battery) is about 10.3 X 7.1 . So if you put them side by side and then end to end the space would be 20.6 X 14.2 . If your box is for them to be end to end then I don't think they would fit.

Larry