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DC DC charger calculations

saflyer

By saflyer
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saflyer

saflyer

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I installed two lithium batteries in my coach. Following the advice not to charge them directly from the alternator I disconnected the cable for that. I’ve been depending on shore/generator power through the inverter/charger plus solar panels to charge the new house batteries. I’d like to get charging from the alternator again so the best option I am aware of is to install a DCDC charger. What goes into the calculations of what size DCDC charger to install?

Ed             
‘05 HR Ambassador.

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Rocketman3

Rocketman3

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I installed a Victron Dc-Dc charger 30a non-isolated in my rig. I have an ignition fuse block I created (powered when the ignition is on) for a FASS pump, I also use that to drive a relay for the charge signal for the Dc-Dc charger.  (I wanted to make sure I didn’t have a circular charging going on between the two banks).

The one downside is whenever I shut the rig off - like for fuel, the Dc-Dc goes back into absorbing mode. Also, with only 30a going into a 540ah of batteries- it it only another trickle of power. 
 

Good Luck!

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CAT Stephen

CAT Stephen

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Ed,

Here is the process to determine the right DC to DC charging solution:

- Determine if you need visibility to your lithium battery bank SOC (State of Charge).  If you need SOC visibility, then you will need an isolated DC to DC charger. 

- Determine the rated output of your engine alternator by calling your chassis manufacturer

- Determine the maximum chassis electrical load by using a clamp-on amp meter while running the maximum chassis electrical loads such as your Diesel engine  air intake heater, all headlights/ running lights, and the 12v water bay heater (if applicable).

- Determine the maximum DC to DC charger draw by subtracting the maximum chassis electrical load from the alternator rated output then subtract 10%-20% safety factor from the sum.  Finally, subtract the DC to DC charger efficiency from the sum.  

- Select your DC to DC charger not exceeding the final sum determined above.  If you want to keep the DC to DC charging solution under $350.00, your choices are either Victron Energy or Renogy.  If you want the solution to automatically turn off when you are not running your engine, then your only choice is Victron Energy.  

 

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RolandE

RolandE

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I have installed the B2B from Sterling with a charge current of max 120 amps.
However, I do not use the ignition signal to turn on the charge booster but have used a radio controlled relay to turn on the charge booster whenever I need it (and the engine is running). I charge the LiFeYPo4 almost exclusively with solar power or mains power, rarely using the alternator/charge booster.

What charging power is needed also depends on how much power you want to charge into the battery via the alternator. If there is sufficient solar power and/or mains power and the alternator is only needed as an emergency solution, 30 amps should be quite sufficient.

Roland

 

Edited by RolandE
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saflyer

saflyer

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11 hours ago, CAT Stephen said:

Ed,

Here is the process to determine the right DC to DC charging solution:

- Determine if you need visibility to your lithium battery bank SOC (State of Charge).  If you need SOC visibility, then you will need an isolated DC to DC charger. 

- Determine the rated output of your engine alternator by calling your chassis manufacturer

- Determine the maximum chassis electrical load by using a clamp-on amp meter while running the maximum chassis electrical loads such as your Diesel engine  air intake heater, all headlights/ running lights, and the 12v water bay heater (if applicable).

- Determine the maximum DC to DC charger draw by subtracting the maximum chassis electrical load from the alternator rated output then subtract 10%-20% safety factor from the sum.  Finally, subtract the DC to DC charger efficiency from the sum.  

- Select your DC to DC charger not exceeding the final sum determined above.  If you want to keep the DC to DC charging solution under $350.00, your choices are either Victron Energy or Renogy.  If you want the solution to automatically turn off when you are not running your engine, then your only choice is Victron Energy.  

 

You got me at “Determine if you need visibility to your lithium battery”. I’ve never heard the visibility term related to electrical systems. Please explain.

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CAT Stephen

CAT Stephen

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Ed, 

I am presenting a technical deep dive for you because I don't know your technical knowledge level in RV electrical systems.   So please forgive me in advance if I cover details that you already know.

 Visibility to your battery bank state of charge is good to have because you will know how much energy that you have used and how much energy is remaining.   This measurement is called state of charge.   Without this visibility, you will not know how much energy is remaining in your batteries.  Measuring energy requires a shunt between the house negative battery terminal to all house electrical loads. 

Here is an example of the SOC visibility to the house lithium battery bank state of charge using two (2) SOK 206 Amp-Hour 12V Lithium Batteries wired in parallel:

  • If you have installed a shunt on your house lithium battery bank, the shunt will measure the precise amount of energy that is used by your house battery bank as a percentage.  If you run a 12VDC 30 Amp resistance heater in your water bay for four (4) hours, the energy used is 30 amps * 4 hours for a total of 120 amp hours of energy at 12V.  Your shunt will know that this energy was used so it would calculate your state of charge by the following equation:
    • SOC % = (House Battery Bank Capacity in Amp*Hours - Energy Used in Amp*Hours) / (House Battery Bank Capacity in Amp*Hours)
    • Using the example data above, the SOC % = ((412 Amp*Hours Capacity) (120 Amp*Hours Used)) / (412 Amp*Hours Capacity) = 70.8%

You may already have a shunt today for energy measurement, but introducing a non-isolated DC to DC charger will make any shunt very inaccurate because non-isolated DC to DC chargers bypass your shunt through the chassis / house 12V ground. 

So, what should you do now?  Here are your choices:

 

Edited by CAT Stephen
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timaz996

timaz996

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Posted

The easiest and safest route in my opinion is look at the size of your alternator and figure that your coach would pull a maximum of 60 A. I went with a Renogy 40 amp DC to DC charger. As per the Renogy manual The 40 amp model can pull as much as 60 A from your alternator. So in my opinion 40 A was a lot better than 30 from the Victron unit and a heck of a lot less money and simpler to wire in. I do like Victron equipment as most everything I own is from them. But this was just a much simpler unit to install. 

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saflyer

saflyer

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On 12/23/2022 at 10:17 AM, CAT Stephen said:

Ed, 

I am presenting a technical deep dive for you because I don't know your technical knowledge level in RV electrical systems.   So please forgive me in advance if I cover details that you already know.

 Visibility to your battery bank state of charge is good to have because you will know how much energy that you have used and how much energy is remaining.   This measurement is called state of charge.   Without this visibility, you will not know how much energy is remaining in your batteries.  Measuring energy requires a shunt between the house negative battery terminal to all house electrical loads. 

Here is an example of the SOC visibility to the house lithium battery bank state of charge using two (2) SOK 206 Amp-Hour 12V Lithium Batteries wired in parallel:

  • If you have installed a shunt on your house lithium battery bank, the shunt will measure the precise amount of energy that is used by your house battery bank as a percentage.  If you run a 12VDC 30 Amp resistance heater in your water bay for four (4) hours, the energy used is 30 amps * 4 hours for a total of 120 amp hours of energy at 12V.  Your shunt will know that this energy was used so it would calculate your state of charge by the following equation:
    • SOC % = (House Battery Bank Capacity in Amp*Hours - Energy Used in Amp*Hours) / (House Battery Bank Capacity in Amp*Hours)
    • Using the example data above, the SOC % = ((412 Amp*Hours Capacity) (120 Amp*Hours Used)) / (412 Amp*Hours Capacity) = 70.8%

You may already have a shunt today for energy measurement, but introducing a non-isolated DC to DC charger will make any shunt very inaccurate because non-isolated DC to DC chargers bypass your shunt through the chassis / house 12V ground. 

So, what should you do now?  Here are your choices:

 

Thanks. I’m familiar with SOC, just not the term visibility in relation to it. I now see you mean readout or gauge.

You’ve helped me with the issue of isolated versus non-isolated. I have a Magnum Energy ME-ARC remote and an ME-BMK battery monitor kit. The BMK has a shunt, obviously. So are all shunts the same? Can more than one BMK be connected to the same shunt? Can more than one shunt be put in series on the same cable? I’d like to add a Victron BMV-712 monitor to get the Bluetooth capability.

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Rocketman3

Rocketman3

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If your shunt comes off the battery negative and nothing else is on the negative wire then a non-isolated Dc-Dc charger will work just fine - the frame ground is on the load side of the shunt and it’s cheaper. 
 

you can have more than one shunt on the negative line. However, every shunt add resistance and voltage drop to the system (that’s how they work).

I have two shunts - a Batrium for the BMS and a Victron. It’s not ideal but…

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CAT Stephen

CAT Stephen

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On 12/24/2022 at 12:43 PM, saflyer said:

Thanks. I’m familiar with SOC, just not the term visibility in relation to it. I now see you mean readout or gauge.

You’ve helped me with the issue of isolated versus non-isolated. I have a Magnum Energy ME-ARC remote and an ME-BMK battery monitor kit. The BMK has a shunt, obviously. So are all shunts the same? Can more than one BMK be connected to the same shunt? Can more than one shunt be put in series on the same cable? I’d like to add a Victron BMV-712 monitor to get the Bluetooth capability.

Ed, 

Merry Christmas to you sir!!  Thank you for sharing your current equipment. 

 

The differences in shunts are power capacity and interoperability features.  Since you already have the Magnum shunt (That is included within your ME-BMK battery monitor kit) then I recommend that you stay with that Magnum shunt because your ME-ARC remote fully integrates with your existing Magnum shunt. 

 

All Lithium battery systems have a BMS for safety and some have integral shunts built into the BMS, but your ME-ARC remote is not compatible with that BMS shunt.  With that being said, there is no harm or suboptimal state by leaving your current configuration (i.e BMS shunt in series with your Magnum Energy shunt) as is.  This approach is common and standard as almost all lithium batteries are equipped with an integral BMS shunt to prevent overload. 

 

You "might" be able to connect a second Magnum BMK to your existing Magnum shunt.  I recommend that you contact Magnum Energy technical support to get their confirm that you can.  The technical support from Magnum Energy is excellent!!

 

Yes, you can place more than one shunt in series with the existing shunt.  Since your current objective that you shared above is to enable bluetooth connectivity for your battery bank, I recommend that you purchase a Victron 500A Smart Shunt which is available for ~$130 instead of a Victron BMV712 because it is less expensive and offers many additional features over the BMV712 .  One of the best features is the ability to monitor your battery capacity remotely by adding a Victron Cerbo GX in the future at your option.  Here is the purchasing information for the Victron SmartShunt:  

One key limitation to keep in mind with your vision of a shunt with Bluetooth connectivity is that Bluetooth range is very limited.  Since you have a 40' Class A coach and I assume your new Bluetooth enabled shunt is near the rear of your coach 38' behind your position in the drivers seat because the battery bank is located near the rear of your coach, then you will not be able to connect to your new shunt via bluetooth until you are located ~<15 feet from your shunt.  If that limited Bluetooth range doesn't meet your needs, then I recommend that you also purchase the Victron Cerbo GX in addition to the Victron Smartshunt to enable Wifi Connectivity for your smart shunt.  Here is the purchasing information for the Victron Cerbo GX.  Keep in mind that the Cerbo GX is expensive because it does much more than satisfy your use case above.  The Cerbo GX supports a 2.4 Ghz access point, a 2.4 Ghz Wifi Hotspot from your cellular provider, or GSM cellular connectivity on a low monthly cost dedicated cellular connectivity plan.  It also have its own access point built in so you may connect to the Cerbo GX directly via its SSID:

Edited by CAT Stephen
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saflyer

saflyer

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20 hours ago, CAT Stephen said:

Ed, 

Merry Christmas to you sir!!  Thank you for sharing your current equipment. 

 

The differences in shunts are power capacity and interoperability features.  Since you already have the Magnum shunt (That is included within your ME-BMK battery monitor kit) then I recommend that you stay with that Magnum shunt because your ME-ARC remote fully integrates with your existing Magnum shunt. 

 

All Lithium battery systems have a BMS for safety and some have integral shunts built into the BMS, but your ME-ARC remote is not compatible with that BMS shunt.  With that being said, there is no harm or suboptimal state by leaving your current configuration (i.e BMS shunt in series with your Magnum Energy shunt) as is.  This approach is common and standard as almost all lithium batteries are equipped with an integral BMS shunt to prevent overload. 

 

You "might" be able to connect a second Magnum BMK to your existing Magnum shunt.  I recommend that you contact Magnum Energy technical support to get their confirm that you can.  The technical support from Magnum Energy is excellent!!

 

Yes, you can place more than one shunt in series with the existing shunt.  Since your current objective that you shared above is to enable bluetooth connectivity for your battery bank, I recommend that you purchase a Victron 500A Smart Shunt which is available for ~$130 instead of a Victron BMV712 because it is less expensive and offers many additional features over the BMV712 .  One of the best features is the ability to monitor your battery capacity remotely by adding a Victron Cerbo GX in the future at your option.  Here is the purchasing information for the Victron SmartShunt:  

One key limitation to keep in mind with your vision of a shunt with Bluetooth connectivity is that Bluetooth range is very limited.  Since you have a 40' Class A coach and I assume your new Bluetooth enabled shunt is near the rear of your coach 38' behind your position in the drivers seat because the battery bank is located near the rear of your coach, then you will not be able to connect to your new shunt via bluetooth until you are located ~<15 feet from your shunt.  If that limited Bluetooth range doesn't meet your needs, then I recommend that you also purchase the Victron Cerbo GX in addition to the Victron Smartshunt to enable Wifi Connectivity for your smart shunt.  Here is the purchasing information for the Victron Cerbo GX.  Keep in mind that the Cerbo GX is expensive because it does much more than satisfy your use case above.  The Cerbo GX supports a 2.4 Ghz access point, a 2.4 Ghz Wifi Hotspot from your cellular provider, or GSM cellular connectivity on a low monthly cost dedicated cellular connectivity plan.  It also have its own access point built in so you may connect to the Cerbo GX directly via its SSID:

Bluetooth range is an issue. My Victron solar charge controller has Bluetooth. I don’t get connectivity in the living room. Have to step to the bedroom door to check it. I’ll check out the Cerbo GX.

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TomV48

TomV48

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 BattleBorn promotes a battery charge isolator that seems to be bi directional and cycles on and off to protect the charging circuit.  I went with the DC to DC charger which limits how much charge is available and thus how much you can SUCK out of your alternator.   In my case my Renogy, 40 amp DC to DC, will put out 39 -41 amps and seems to draw about 50 amps by my crude measurement,  off the chassis system/alternator.   Between that and 600 watts of solar I am always fully charged when I get to my destination.     AS FAR AS A SACRED formula goes I have no idea.   Without a Residential Frig there is probably no earthly reason I will need 600 amps of LiFePo batteries but that is where I went.   400 would probably have been plenty but I got carried away building them myself a few years ago before store-bought ones came down so far in price.   For our non Res Frig situation, the 40amp DC-DC and 600 watts of solar which puts out in real time between 29 and 39 amps at solar noon gives us more than we need or are likely to ever use.  If they ever go really low the Magnum running on the Generator seems to put about 70-80 amps an hour in them so in a emergency/pinch I can live with a few hours of generator.

On 12/26/2022 at 11:56 AM, saflyer said:

Bluetooth range is an issue. My Victron solar charge controller has Bluetooth. I don’t get connectivity in the living room. Have to step to the bedroom door to check it. I’ll check out the Cerbo GX.

GOOD GOD that make me feel better to hear you say it.   I sit mid coach at the table and have to strain my arm out into the hall to get my shunt or solar charger to read

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