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Old 07-26-2010
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Wiring question - Echo Charger and Batt Monitor

Hi All...

I think I know the answer to this but I was hoping someone could tell me their thoughts. I am trying to figure out the proper way to add an intelligent battery charger, an echo charger and a battery monitor all together in the same system.

I have two AGM batteries, one for house and one for start. The battery switch is of the "off - on - combine" type so normally, the batteries are never joined. Combine is only used for emergency starting.

The house battery does not share a ground with the start battery. The house battery is connected to a battery monitor shunt on its negative side. An intelligent battery charger is on the house battery, with the positive on the battery post and the negative on the monitor shunt. I'm only interested in monitoring the house battery for current.

The start battery has the engine alternator on positive and the negative on the engine block. Of course the positive is also to the starter.

The negative terminals of the batteries have no connection through any path.

Enter the echo charger. The echo charger wants the two batteries to have their negative terminals tied together. My confusion is, if I connect the two negative terminals, will that screw up my battery monitor? I think I can do this because the house current still would flow through the shunt. The charging current still flows through the shunt for the house battery. I think the only potential problem will be that the charging shunt will measure charge current flowing into the starter battery, which might confuse it.

Are there any issues I missed? Are there better ways to do this?

Thanks...

Jim
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Old 07-26-2010
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First, I'd highly recommend not maintaining seperate ground planes. This is a safety issue more than anything. With two seperate ground planes, you could call one ground (ie engine block) and the other would be considered "floating". The issue is that static charge can build up between these two ground planes and since they aren't shorted together ... then there is nothing to ensure they don't build up a voltage between them. So now, two wires which you think are at 0V are actually at different voltages from one another ... and you could be shocked. Maintain only one DC ground plane, it's the right thing to do.

As for measuring only house battery current ... just connect the shunt to the (-) terminal of the house battery. Connect _nothing_ between the (-) battery terminal and the shunt. The terminal on your shunt that is connected to nothing at this point (the _other_ terminal) can now be considered the (-) battery terminal for this "battery" (which is now a combination of battery and shunt). In this way, you ensure that the current passing through the shunt is exactly the same current passing into the house battery.
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Old 07-26-2010
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Quote:
Originally Posted by scraph View Post
As for measuring only house battery current ... just connect the shunt to the (-) terminal of the house battery. Connect _nothing_ between the (-) battery terminal and the shunt. The terminal on your shunt that is connected to nothing at this point (the _other_ terminal) can now be considered the (-) battery terminal for this "battery" (which is now a combination of battery and shunt). In this way, you ensure that the current passing through the shunt is exactly the same current passing into the house battery.
So I have a negative bus, then the shunt, then the house battery negative terminal. You are saying that the negative terminal of the start battery should go to that bus and the engine block should go to that bus?
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Old 07-26-2010
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That's correct. The battery monitor only requires the shunt to be the first item after the negative post of the house battery. The other side of the shunt can (and should) be connected to a negative bus that is connected to the block at one point.
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Old 07-26-2010
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1- Combine the neg sides AFTER the shunt at a negative BUSS bar. On the battery post side of the shunt there should be NO ground wires of any sort, not even a bilge pump. The jumper from the batt post to the "in" side of the shunt should be the ONLY wire between the shunt and battery post. The neg BUSS is then grounded to the vessels common ground point, usually your engine block.

2- Move the alt output to the house bank with the batt charger. The Echo Charger is not a bi-directional unit and only flows ONE way, FROM the house bank to the reserve/start bank.

3- Install the Echo Charger feeding the start battery FROM the house bank..

4- Do not use the "COMBINE/BOTH" feature when you are in an emergency situation especially with AGM batteries!! If you kill a bank SWITCH to the known good bank. Combining them only forces the good battery to drain into the bad one thus sucking BOTH batteries down to an over all lower state of charge and potentially not leaving enough MCA for the engine. AGM's can charge and discharge at VERY high rates...
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Last edited by Maine Sail; 07-26-2010 at 01:13 PM.
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Old 07-27-2010
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Okay thanks everyone. So basically I need to connect the negative post of the starter battery to the negative bus, then move the ground wire from the engine block to the negative bus.

Let me make sure I understand this correctly.

This means the starting current will pass from the positive terminal of the start battery, through the starter motor, then to the engine block, then through a wire to the negative bus and finally from the negative bus through a wire back to the negative terminal of the start battery. The starting current never passes through the shunt? If so then the wire from the shunt to the house battery need not be sized to pass starting current, just 50A max house current.

The echo charger negative goes on the negative bus.

The alternator line goes on the house bank, so that it can charge both the house bank/devices and charge the starter battery via the echo charger.

Right?
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No. The engine block is last in line. Look at the diagram below. It is from a Xantrex Link 10 manual but shunt wiring is the same for all monitors. The battery shown is the house battery. The wire goes from the negative post to the battery side of the shunt. As Maine posted the shunt has to be the first connection after the battery with no other wires going to the negative side of the battery. The other side of the shunt, the load side in the diagram, goes from the shunt to a negative bus. The negative of the start battery also goes to the bus. The bus is connected to a single point on the engine block. All other negative connections go to the bus which is connected to the engine (earth).
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Thanks Mitiempo, I think that's what I meant to say. I go from the house battery neg terminal to the shunt, then from the shunt to the neg bus. Then I wire the neg bus to the engine block. Then I wire the neg terminal of the start battery to the bus.

The reason I am trying to figure out where the starter current flows is so I know which wires to size for the start current. It seems that with this arrangement no start current actually flows through the shunt. Why is it a 500A shunt then?
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If you make a seperate negative bus ... it should be pretty stout and located in the proximity of the engine. I'd recommend making the engine block itself your "negative bus" for simplicity ... it's the biggest chunk of metal you have anyways.

You could run:
1. a ground cable from the starter battery (-) to the engine block (rated to starting current)
2. a ground cable from the house battery (-) to shunt (rated to starting current)
3. a ground cable from the shunt to the engine block (rated to starting current)
4. a ground cable from the DC switchboard to the shunt (50A max house current)

Size the first three to carry starting current so that you can start the engine from either the starter battery _OR_ the house bank as necessary.

I recommend running the fourth cable to the shunt (instead of the engine block) simply because it would shorten the length of ground cabling for the house loads. Also, ensure you don't skimp too much on this cable size. You want this overdesigned since electronics (presumably you have some) prefer to operate as close to 12V as you can get them ... so minimize voltage drop here.

The echo charger negative can go anywhere on the negative bus, really, except for between the shunt and house bank.

As I describe, starting current will go from starter battery (+) -> starter -> engine block -> cable from engine block to starter battery (-) -> starter battery (-).

Right, on account of the alternator line.

It is a 500A shunt ... because it is. To be large enough to work for currents that large ... if, by chance, you were dealing with them.
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I would use a bus as it is neater. Only one lug on the block. I generally don't differentiate between engine starting and house loads as far as cable sizing. At least not much. If you have a Blue Seas 8023 for example, an 8 circuit panel and their most popular by far, it is rated at 100 amps. They recommend 2 AWG for cabling. The only way to effectively use a smaller gauge and still keep voltage drop to 3% or less is if the panel is very close to the battery. Blue Seas 2 AWG sizing is figured as 5 feet each way.
I also generally use a separate bus for the negatives near the panel, only running a 14 AWG wire to the panel itself for the indicator lighting. It keeps the panel neater.
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