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Old 02-28-2018, 08:20 PM   #61
Kiltym
 
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Default Re: Wiring idea for charging house batteries

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Originally Posted by onemanvan View Post
The B2B I purchased ( DMT1230 ) does not have a dedicated sense wire. IE: You tell it what voltage to output and it compares that to the voltage at the B2B charger.
So if I set the charger to charge Bulk at 14.9, the batteries should receive the 14.7 since the charger doesn't know better. I suppose the concern is that as the amps drop (absorption), the voltage will actually increase (because of less voltage drop on the cable) which is perhaps the issue I was missing as eventually at 10A it will be almost at the higher 14.9 voltage which is perhaps too high for the battery.



And the calcs you did on 10' is 2x the length I am using. I think you would need to enter 4.5' as the length on the calculator "* Please use one-way distance to the load. Not round trip distance.". Or am I missing something.... ? I assumed I could take total length and divide by 2.

If you were just using the 10' as an example, all good, but wanted to be clear that it is longer then what would be needed in our Westy assuming a mounting location around the sink.

I think I will stick with the 6AWG. Should be fine, especially after the revlation that as the amps taper off, the loss is less, and it will push the batteries up to the "correct" voltage by the time it's finished. Yes, some performance loss (which would translate into time to charge) vs a 2AWG cable, but doesn't seem worth it to me. But am happy to be convinced otherwise as I have not ordered the cable yet.
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Old 02-28-2018, 09:14 PM   #62
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Default Re: Wiring idea for charging house batteries

I was just using 10 feet as an example. The key point being - as the battery approaches full charge ( absorption phase ) the current will taper off - such that wire size becomes less critical.

Oftentimes you will see documentation that refers to bulk phase as constant current and absorption phase as constant voltage.

In the bulk phase voltage is not so important - basically anything between 13~14 volts will provide enough 'push' to transfer whatever current is available into the battery.

In the absorption phase you want to bring the battery up to a certain voltage and no higher.

It's sort of like blowing air into a balloon. Where pressure is synonymous with voltage and volume of air is synonymous with current.

When the balloon is empty it takes very little pressure to push a large volume of air into the balloon ( bulk phase / high acceptance rate / constant current ).

As the balloon stretches tight - near maximum capacity - it takes more pressure to push a lesser volume of air into the balloon ( absorption phase / low acceptance rate / constant voltage ).

Too much pressure and POP...
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Old 02-28-2018, 09:49 PM   #63
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Default Re: Wiring idea for charging house batteries

You need to account for a round trip, not just one way. 6 awg charging cable at 50A is grossly undersized. You pay the big bucks for a good charger and then undermine it with sub-par cable.
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Old 02-28-2018, 10:04 PM   #64
Kiltym
 
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Default Re: Wiring idea for charging house batteries

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Originally Posted by calbiker View Post
You need to account for a round trip, not just one way. 6 awg charging cable at 50A is grossly undersized. You pay the big bucks for a good charger and then undermine it with sub-par cable.
I understand a circuit is the round trip, which I wrote out earlier. 9’ total. With a chassis ground, its total for positive and negative cable runs AFAIK.

The calculator referenced however wants only a one-way length, and presumably it does the math for the “round trip”. So to use that calculator, one would enter 4.5’ AFAIK. If I am wrong about this, please someone correct me!

Based on multiple calulcators (and the above understanding), I have to disagree with your assessment about 6AWG on a 50A charger with such a short cable run. If you can point me to something contrary, please do as I would be interested to read/understand it.

The voltage total drop is 1%. I feel this is quite adequate.
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Old 02-28-2018, 10:49 PM   #65
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Default Re: Wiring idea for charging house batteries

I was under the impression you had a longer run. That said...

If you only have 10' round trip then a 6 awg cable will have the following drop:
V = 0.4 mohm/ft * 10 ft * 50 A = 0.2 V
That's a 1.4% drop from 14.7V.

A 2 awg cable has this drop:
V = 0.156 mohm/ft * 10 ft * 50 A = 0.078V
That's a 0.5% drop from 14.7V.

I always advocate less than 1% voltage drop.

What's the difference in cost?

10 ft of 6 awg welding cable costs $19.00, while 2 awg costs $24.50. $5.50 extra cost will give you a faster charge.

In addition, 6 awg cable is close to max ampacity. 60 deg C insulation is rated at 55A. 75 C insulation is rated at 65 A. This rating is when the ambient air is at room temp. I would not be comfortable with those slim margins.

2 awg is rated at 95A with 60C insulation.
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Old 02-28-2018, 10:57 PM   #66
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Default Re: Wiring idea for charging house batteries

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Originally Posted by calbiker View Post
In addition, 6 awg cable is close to max ampacity. 60 deg C insulation is rated at 55A. 75 C insulation is rated at 65 A. This rating is when the ambient air is at room temp. I would not be comfortable with those slim margins.

2 awg is rated at 95C with 60C insulation.
The cable I use would be rated for 105C. ABYC standards (marine), which is much stricter then cars, allows for 120Amps, according to Bluesea. After living many years on a boat, any wiring I do is per their standards. Tinned cable, etc....

“Recommended Wire
AWG 6

Capacity per ABYC Standards: 120 amps.
This wire selection is the result of applying ABYC Standards factors, additional derating factors, and allowable voltage drop.”

Anyway, I think I am fine, but agree, thicker is always better, can’t argue with the math!

For reference, capacity per AWG: https://www.coonerwire.com/amp-chart/

Last edited by Kiltym; 02-28-2018 at 11:00 PM.
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Old 03-25-2018, 03:06 PM   #67
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Default Re: Wiring idea for charging house batteries

I received the Kisae DC-DC charger last week. The new 50A version (DMT-1250) I had been waiting for.

I now have it installed, and is working perfectly.

IMO, finally, the converter and generator can be used for something useful :).

With the wiring from the Kisslinger relay to provide the source DC power, the system works great. Plugged into AC shore power, generator running, or driving the car, all goes through a solid 3-stage charge system, and all happens automatically.

I turned up the converter DC output to about 13.8V.

I disconnected and taped off the DC cable from the alternator/relay that normally charges the house batteries when the engine is running. Can easily be re-connected if ever needed.

I have flipped the breaker for the Westy charger to keep it turned off when on AC power. Otherwise it will try to keep the voltage > 14 when the new charger is in float mode and keeping the batteries at 13.6. This does lose the ability to trickle charge the engine battery, but I am not overly concerned about losing this function. If ever needed, simply flip the breaker to turn off the converter (turns off new charger), and flip back on the breaker for the Westy charger, and you can charge the engine battery if needed.

A few things to note:

1) The AC breaker that controls the Westy charger states it also controls the outlet for the microwave. However, when the breaker is off, the Westy charger is not working, which is good, but the microwave still has power. So something is mislabelled here, not sure it applies to all or not.

2) The little "plug" icon on the front computer comes from the Westy charger. So I do not see that icon with the Westy charger turned off.

3) And the front computer does not "sense" the new charger, so even when charging, the computer displays the battery icon indicating it is not charging (even though it is). If the engine is running, the icon goes away as normal, but when on AC power, it does not.

None of these are of concern, and should not effect anything, just pointing them out for future reference.

Attached is a photo of the install. I put it in the cabinet under the drain near the main battery switch.

Next step is solar panels, which is great as they can be plugged into the same charger and have one complete integrated charging system that is all automatic. I am very happy with the DMT1250 and the features it provides. As onemanvan pointed out, there may be some constraints with Lithium batteries, but for AGM or Gel, all is pretty good.
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