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Old 21-05-2013, 09:41   #1
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Two battery banks in different locations

Hi,

We are looking at a boat that only has a limited space available for the house battery bank size we want. So, we are wondering about moving the batteries under each of the settee's so that we had a distributed weight on each side of the boat.... two batteries on each side.

First - electrically speaking, is this advisable?

Second - any guidelines on how to run the wire length/size to ensure electrical balance?

-z
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Old 21-05-2013, 10:31   #2
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I would run the feed from the port side and return it to starboard side to help balance use. Instead of feed and return on one battery then the rest connected. With the added distance make sure the wires are big enough. You have to add the wire length both ways to figure line loss. IMO
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Old 21-05-2013, 10:56   #3
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Re: Two battery banks in different locations

That will be some beefy cables, depending on the distance and the current you'll be pushing through. It's not the worst thing in the world but you'll need to get it right. Expect to drop a few hundred bucks on the cables alone. The bigger the cables, the less of a problem you'll have. I'd probably have all your charging sources go to one of the banks, then have a positive bus you draw from on the other bank. At least that way (I think) you'll tax everything evenly.
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Old 21-05-2013, 10:58   #4
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Re: Two battery banks in different locations

Maybe the trem 'balanced circuit' has more than one meaning. To me a balanced circuit is one that dosent affect the compass of other electrical devices when its turned on. To do this the hot and the ground leads need to be next to each other so the magnetic field created by one is off set by the other.
As to the size, you have to know the maixmum amp draw and the length of the + and the - circuits. then you look up the wire size you need.
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Old 21-05-2013, 11:15   #5
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Re: Two battery banks in different locations

z:

I assume that these are house batteries, not starting. If so figure the largest amperage that could go into or out of the batteries. If you have an inverter/charger it is probably the draw of the inverter. If you don't have an inverter, then it is the size of the charger, alternator (both in) or the size of the main breaker on the DC panel (out). The alternator output is almost always the controlling factor in this situation, typically 55-100 amps.

So assume you have a 2,000 watt inverter. That will take as much as 250 amps to run at maximum. Then figure the to and from distance to each set of batteries. Then go into the Ancor web site and use their wire sizing chart. 10% voltage drop is ok for an inverter at max power.

So further assuming that you have 15' to and from each bank or a total of 30', then firgure the wire size to carry that current and stay under 10%. In that case it takes 1/0 wire, not for the 10% but for the ampacity- the ultimate current capacity of the wire independent of voltage drop.

And you will need a fuse at each battery bank of 250 amp capacity.

If you don't have an inverter and the maximum in/out current is 80 amps (typical for most alternators) then with a 30' in plus out and a 5% voltage drop limit (appropriate for the alternator output at max) then the wire size is #3 and you will need an 80 amp fuse near each battery.

And think about how you are going to protect these batteries from engine starting loads. Assuming that you have a 1,2,all switch you will always have to start the engine with the switch on the starting battery only. If it is on all, the starting current will be shared by the three banks and it is likely you will blow the 80 amp fuse. Not so likely if you have 1/0 wire and a 250 amp fuse protecting each bank.

Of course the safest is to wire everything with 2/0, put a 300 amp fuse at each battery and not worry about where the switch is set, except at anchor to protect the starting battery from being discharged.

So it is doable. I don't subscribe to the feed and draw argument of the previous poster- it just doesn't work for two battery banks remotely located and really doesn't make any difference in practise. But it does take some engineering as noted above to do it safely.

David
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Old 21-05-2013, 14:16   #6
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Re: Two battery banks in different locations

Thanks David. 250 amps seems like a lot... I'm confused at how you arrived at that figure.

We would be keeping the starter battery where it is now - close to the engine. The two other sets of batteries (house) would be under the settees. By 'Balanced' I meant having the current drawn in such a way as to not cause impedance imbalance... not sure if that is a problem if the one set of batteries is closer to the load than the other set [in this type of configuration].

I saw that xantrex makes a charger/inverter that can handle three different sets of batteries but I would only still be using two banks... a house bank and a starter.
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Old 21-05-2013, 15:04   #7
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Re: Two battery banks in different locations

Considering something similar, but the banks will be forward and aft.
Big cables is a given, but a question about charging.
Presently there are 3 banks, house, start and bow thruster. Battery charger has three outputs and connected to each bank, alternator output goes to a low volt loss 3 way splitter and then to each bank.

The plan is to to connect house and bow thrust banks as a single bank, but is it OK to leave the charging arrangement as is, i.e., each part of the bank is getting its own charge??
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Old 21-05-2013, 15:07   #8
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Re: Two battery banks in different locations

Nigel:

A 2000 watt inverter theoretically takes 2000/12 = 167 amps DC to operate it. But inverters are about 85% efficient at high power so it really is 167/.85 = 196 amps. So I fudged it a bit to get 250. You could get by with #2 gauge for 30 feet round trip.

"Balanced" is in the eye of the beholder ;-). Ideally the resistance from one set of batteries should be the same as the other so the current draw (or input) will be the same. But a slightly different state of charge, a bit of corrosion on one connector, etc makes more difference than theoretical wire resistance so IMO it doesn't matter.

And if the two battery banks are splitting the load equally, the current will be half of the 196 amp maximum in the above calculation. But I wouldn't want to count on it and I don't think ABYC does either. You have to design for a failure in one of the battery banks so the other one is safe supplying all of the current.

Oh and in the case of the Xantrex inverter/charger that you mentioned, I think "handle" refers to the ability to charge three battery banks and keep them isolated. In inverter mode, I'll bet all of the current is drawn from one set of terminals. It probably was designed to charge a starter batter, a generator battery and the house bank.

David
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Old 21-05-2013, 15:14   #9
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Re: Two battery banks in different locations

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Originally Posted by djmarchand View Post
Nigel:

A 2000 watt inverter theoretically takes 2000/12 = 167 amps DC to operate it. But inverters are about 85% efficient at high power so it really is 167/.85 = 196 amps. So I fudged it a bit to get 250. You could get by with #2 gauge for 30 feet round trip.

"Balanced" is in the eye of the beholder ;-). Ideally the resistance from one set of batteries should be the same as the other so the current draw (or input) will be the same. But a slightly different state of charge, a bit of corrosion on one connector, etc makes more difference than theoretical wire resistance so IMO it doesn't matter.

And if the two battery banks are splitting the load equally, the current will be half of the 196 amp maximum in the above calculation. But I wouldn't want to count on it and I don't think ABYC does either. You have to design for a failure in one of the battery banks so the other one is safe supplying all of the current.

David
OK! That makes sense! For some reason I thought you were talking about 250 AC AMPS at 120 volt!
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Old 21-05-2013, 15:55   #10
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Re: Two battery banks in different locations

Minimum of AWG 1/0 battery cable to connect the two banks, assuming you don't have a 100' boat with a 30' beam :-)

Run all charging sources -- battery charger, alternator, solar panels, generator, wind generator, etc. -- directly to the house batteries. Positive to one side of the boat; negative to the other. Cabling size will depend upon intended use. With a big inverter, you'll need BIG cables.

Use a voltage follower device (best) such as an EchoCharge or DuoCharge to maintain the start battery. ACR's, battery combiners, etc. are OK, too. Do NOT use a diode "isolator" device.

Pay attention to proper CPDs (circuit protection devices...fuses, breakers), per ABYC recommendations.

Separation of the batteries will work fine. My house bank has been separated for some 20 years now (4 Trojan T-105's on the port side, 2 T-105's on the starboard side); my batteries are now 7 years old and doing fine, with higher-than-normal float and absorption voltages.

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Old 22-05-2013, 13:15   #11
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Re: Two battery banks in different locations

Quote:
A 2000 watt inverter theoretically takes 2000/12 = 167 amps DC to operate it. But inverters are about 85% efficient at high power so it really is 167/.85 = 196 amps. So I fudged it a bit to get 250. You could get by with #2 gauge for 30 feet round trip.
It would work but you would truly just be getting by! With a fully charged battery (12.6VDC) the voltage drop for 196A in a 30 foot circuit would be: 7.6% so you would have 11.6VDC at the input terminals to the inverter. And this does not count the resistance from switches, fuses and crimped ring terminals. The performance of the inverter would suffer.

I agree with btrayfors: 1/0 cable is the minimum I would use.
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Old 22-05-2013, 13:58   #12
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Re: Two battery banks in different locations

Overkill.

By all means put the inverter load as close as possible to one of the house batteries and use 1/0 or better cables.

But

Consider the other house battery as a reserve. By definition the inverter loads are (should be) intermittent loads. So expect the local battery to be the source and just use 6 gauge wire to link to the other battery.

As the inverter takes some charge from the primary battery, its voltage will drop and some current will flow through the 6 gauge wire but small compared to the "local" load. However after the inverter load is removed current will flow from the reserve to the primary battery for a number of minutes to replenish the charge and equalize the two batteries. You DON'T need 250 amps available from both batteries.

When charging, even if the alternator could sustain 100 amps, the voltage drop over a 6 gauge wire is not important because as the battery voltages increase the current drops. As they approach full charge the current in the 6 gauge wire will be below 5 amps so the voltage drop across the line will drop to less than 0.01 volts and both batteries will get a full charge.
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Old 22-05-2013, 14:38   #13
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Re: Two battery banks in different locations

Oh, hell....just use plain old house lamp wire. The battery voltages will equalize eventually!

And, if they don't, no matter...the house wire will serve as a fusible link :-)

Just be sure to get the white color kind, not the brown. That's the marine type.

Bill
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Old 22-05-2013, 15:21   #14
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Re: Two battery banks in different locations

Comment #13 +1
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Old 22-05-2013, 15:37   #15
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Re: Two battery banks in different locations

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Comment #13 +1
Heck, Charlie, I advise all my clients to wire things so they can put a C1 load on (some of) their house batteries, don't you?

Don't know what the fuss is all about :-)

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