System Schematic, component relationships.

hemmjo · February 2015

I am installing another system for the mission I work with and adding some new components to an existing system. The two systems, in different buildings, will be as identical as possible. All will have the same components, etc.

I am adding Iota Battery chargers to the systems and a MidNite Battery monitor to the systems . I am not 100% sure where they are best hooked into the system.

Attached is a simple schematic showing the basic components, wires sizes, breakes and fuses, etc

Not shown, all components will be grounded with #6 copper and outside ground rod.

Comments specifically on the charger and Battery Monitor are appreciated.

Thanks,

John

BB. · February 2015

Re: System Schematic, component relationships.

I have not checked the details--But your schematic looks pretty much complete and correct.

The few things that jump out at me...

First, the 55 amp battery charger would normally have:

55 amps * 1.25 NEC fuse/wiring derating = 68.75 minimum rated fuse/breaker/wiring

So, the you would use the next larger fuse/wire/breaker for the charger DC wiring.

Fuse vs breakers... Certainly either are fine for safety--But breakers tend to be "nicer" because you can turn off devices instead of needing to disconnect wiring. Also, fuses are surprisingly expensive and need to be "stocked" for emergency repairs. I don't expect you to blow a fuse--But a breaker may be a bit nicer solution.

You have the Midnite monitor connected directly to the battery bank. The shunt will not log the current used by the Midnite monitor--If the installation manual says to connect directly to the battery bank--That is fine, less voltage drop and more accurate battery monitoring. Otherwise, try not to connect other loads to the battery, but after the shunt so that all current is logged into/out of the battery bank.

-Bill

hemmjo · February 2015

I contacted MidNite about the best location to connect the MNBCM. They said to connect it as close to the battery as possible. Their comment was, it draws so little current it will not make any difference to the WhizBang, Jr.

As I get deeper into the details of the charging specifics, and understand more, keep having more questions. The MNBCM uses voltage to determine battery capacity. According to the manual 12.75v is 100% SOC. There is .12v difference for each 10% of capacity with 11.65v as 10% SOC. This represents a 1% voltage drop. In order to limit voltage drop to 1% at 90 amps I need 1/0 cable according to the calculator at http://www.southwire.com/support/voltage-drop-calculator.htm The inverter manual said to use #2 cable up to max of 5 feet. This puts the drop at about 1.37%

So, if the MNBCM is connected to the bus;
At night with the inverter pulling current, there will be a voltage drop at the bus showing the battery discharged more than it really is.
When the sun it up and there is little or no load on the system, the bus be at higher voltage than the batteries so the monitor will indicate a higher charge and might even indicate the batteries met the full charge requirement when they really have not?

I am understanding from all of this that the voltages the Charge Controller sees are not really the voltages that the battery sees, so in order to charge correctly the CC may need to be set a bit higher than the battery really needs so the voltage is correct when it actually "gets to the battery"

Also, if the inverter is pulling current from the bus while the charge is in progress, that will lower the voltage to the battery even more.

Do I have any of this right?

Thanks,

John

BB. · February 2015

Connecting the Midnite MNBCM to the battery bank directly is fine--And more accurate for the MNBCM (as it monitors battery bank voltage). I was just trying to make a point that attaching any device to the battery bank bypasses the Battery Monitor Shunt and can create "errors" with shunt based battery monitors.

Since the MNBCM draws so little current, its contribution to the battery monitor error is not measurable.

I could not find how it works with a quick Google Search. I am sure it has an internal "digital model" of how a battery bank works. It monitors the battery voltage and time (i.e., 12.75 volts at 75F is "full", voltage drops to 12.4 volts for 5 minutes is much different than if it drops to 12.4 volts for 4 hours.

Here is another company that makes a voltage based battery monitor. They have a lot of information on how they use voltage and their "digital model" battery to estimate state of charge. Also has a lot of other interesting information about batteries too:

http://smartgauge.co.uk/smartgauge.html
http://smartgauge.co.uk/technical1.html

-Bill

System Schematic, component relationships.

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