DC to DC 2-Way Charger?

[HRTKD]

I too start a diesel in the cold (sub -10°F). Mine is the newer 6.7L and Ford did some magic to make it start quite well in the cold. If I were in your situation I would want something that _keeps_ the batteries charged, not something that can boost them when you go to start it. The latter is hard on your batteries.

The Sterling charger you linked to looks promising. Sterling is a well known brand. If you want something simple, a separate PV panel and controller for just the starter batteries would work. That's how I have my RV toy hauler set up. The generator's starter lead acid battery is isolated from the house LiFePO4. They each have their own PV/controller.

 

[stienman]

I've just stumbled on this product:

2023 Model 12V to 12V DC to DC Chargers 10A | 15A | 20A | 25A | 40A | 70A | 120A models + Remote Control (optional)

12V to 12V DC to DC Chargers 10A | 15A | 20A | 25A | 40A | 70A | 120A models - E-Marking New BB1225 - video - available now New BB1210- video - the benefits of 10A current limiting - charging portable power banks 10A | 15A | 20A | 25A | 40A | 70A | 120A | 200A models available now. We can ship...

sterling-power.com

That's an interesting project - essentially if the voltage on one side is above 13.5, then it's probably being charged from an external source, and this device will pass some current to the battery that's not being charged if it's not full.

Although the OP's batteries are not charged aside from the alternator, it might still work since the operating voltage of the LiFePO4 pack is above 13.5, so if the chassis battery falls below 13, it will pass current from the lithium pack to the chassis battery keeping it topped off. Once the lithium pack drops below 13.5, it will stop, preventing the lithium pack from dying due to trickle charging the chassis battery.

 

[icyroads]

I too start a diesel in the cold (sub -10°F). Mine is the newer 6.7L and Ford did some magic to make it start quite well in the cold. If I were in your situation I would want something that _keeps_ the batteries charged, not something that can boost them when you go to start it. The latter is hard on your batteries.

The Sterling charger you linked to looks promising. Sterling is a well known brand. If you want something simple, a separate PV panel and controller for just the starter batteries would work. That's how I have my RV toy hauler set up. The generator's starter lead acid battery is isolated from the house LiFePO4. They each have their own PV/controller.

I also plan on having a diesel fired preheater for the coolant, which should make starting in the very cold a lot easier. Boosting from the house batteries would be a last resort, like a jump-start.
So, I’m going to take a wait and see approach instead of over de$igning.

 

[dirt diggler]

Right, but I cannot find that statement on the Renogy site or in the manual. And, maybe my mind or eyes are playing tricks on me, but that statement is not on Will’s blueprint page for this.

It is in the manual, mine charges the starter batterys but only when the house battery is at 13.9V for over 20 seconds (IE in float)
from the downloadable manual, pages 16 and 17 are the relivant pages

P16
Alternator/Charging Indicator LED
ON Alternator is charging the service battery
Slow Flashing PV is charging the starting battery
OFF Not charging

P17
If the service battery is in float charge stage, the starting battery will be charged at the same
time. The charging voltage is limited at 13.8V. The charging amperage is limited at 25A
After charging the starting battery for 1 minute, it will disconnect for 30 seconds and
check the starting battery voltage. It will continue to charge starting battery if the voltage
is lower than 12.7V and will stop charging if the voltage is higher than 13.2V

https://store-nkm2yhe9h8.mybigcommerce.com/content/RBC3050D1S-Manual.pdf

 

[RF_Burns]

I have a small motorhome with 200Ahr of LiFePO4 batteries for the house and a standard LA chassis battery for the engine etc. Previously when I had LA house batteries, I had a Combiner which connected both batteries together when either was above about 13.2V (charging voltage) This allowed either the alternator or the house converter/charger to keep both batteries charged.

With the change to LiFePO4 house batteries, I have disconnected the Combiner and I have a DC-DC charger installed. I also have a Samlex EVO converter/charger/inverter/Auto-transfer switch and a solar panel with MPPT feeding into the EVO.

LiFePO4 batteries need the charging current and voltage regulated to proper operation, but the LA batteries just like to be kept fully charged. At the moment, I have the EVO and the MPPT chargers set to float at 13.5V once the charge cycle is complete to carry the 12V loads in the house portion of the motorhome (lights, water pump, fans etc). The LA chassis battery will be happy to also float at that voltage while we are camping. Currently (no pun intended) I have a ATO fuse-holder between the two battery systems. I simply plug in a fuse and the chassis battery will receive a 13.5V trickle charge from the house battery system. The DC-DC charger is only enabled when the IGN key is ON, so it is defeated when the engine is not running. So as long as I remember to pull the fuse before starting the engine, all is fine.

However I know that at some point I will forget to pull the fuse, so I want to automate this. I owned a business dealing in Police/Fire/commercial radio communication systems. We developed and made a Power-Down Timer device that allowed the radio to remain ON after the vehicle engine was shut OFF for a period of time, or until the vehicle battery dropped below a set level. I'm going to re-write the code so that the device's relay will not connect if the engine is running or if the voltage is either above or below the float charge voltage (13.5V). It will come on if the voltage is sitting at 13.5V on the House battery side to maintain the chassis battery charge.

Here is a photo of our Power-Down Timer. They are other's available, but ours had some features that were not available on others, so we made our own.

 

[kite]

I had similar requirements as the OP for my van build. I wanted the ability to charge my starter battery from house LFP battery. I accomplished this with plugs that allow me to reverse the input and output of the dc-dc charger.
This won't give you trickle charge capabilities but it will save you in a pinch.

Details here: https://diysolarforum.com/threads/s...puts-charge-starter-battery.27538/post-342458

 

[icyroads]

I have a small motorhome with 200Ahr of LiFePO4 batteries for the house and a standard LA chassis battery for the engine etc. Previously when I had LA house batteries, I had a Combiner which connected both batteries together when either was above about 13.2V (charging voltage) This allowed either the alternator or the house converter/charger to keep both batteries charged.

With the change to LiFePO4 house batteries, I have disconnected the Combiner and I have a DC-DC charger installed. I also have a Samlex EVO converter/charger/inverter/Auto-transfer switch and a solar panel with MPPT feeding into the EVO.

LiFePO4 batteries need the charging current and voltage regulated to proper operation, but the LA batteries just like to be kept fully charged. At the moment, I have the EVO and the MPPT chargers set to float at 13.5V once the charge cycle is complete to carry the 12V loads in the house portion of the motorhome (lights, water pump, fans etc). The LA chassis battery will be happy to also float at that voltage while we are camping. Currently (no pun intended) I have a ATO fuse-holder between the two battery systems. I simply plug in a fuse and the chassis battery will receive a 13.5V trickle charge from the house battery system. The DC-DC charger is only enabled when the IGN key is ON, so it is defeated when the engine is not running. So as long as I remember to pull the fuse before starting the engine, all is fine.

However I know that at some point I will forget to pull the fuse, so I want to automate this. I owned a business dealing in Police/Fire/commercial radio communication systems. We developed and made a Power-Down Timer device that allowed the radio to remain ON after the vehicle engine was shut OFF for a period of time, or until the vehicle battery dropped below a set level. I'm going to re-write the code so that the device's relay will not connect if the engine is running or if the voltage is either above or below the float charge voltage (13.5V). It will come on if the voltage is sitting at 13.5V on the House battery side to maintain the chassis battery charge.

Here is a photo of our Power-Down Timer. They are other's available, but ours had some features that were not available on others, so we made our own.

View attachment 100575

Why not set up a VSR to only connect the two when the ignition is off and the house batteries are charging?

 

[icyroads]

It is in the manual, mine charges the starter batterys but only when the house battery is at 13.9V for over 20 seconds (IE in float)
from the downloadable manual, pages 16 and 17 are the relivant pages

P16
Alternator/Charging Indicator LED
ON Alternator is charging the service battery
Slow Flashing PV is charging the starting battery
OFF Not charging

P17
If the service battery is in float charge stage, the starting battery will be charged at the same
time. The charging voltage is limited at 13.8V. The charging amperage is limited at 25A
After charging the starting battery for 1 minute, it will disconnect for 30 seconds and
check the starting battery voltage. It will continue to charge starting battery if the voltage
is lower than 12.7V and will stop charging if the voltage is higher than 13.2V

https://store-nkm2yhe9h8.mybigcommerce.com/content/RBC3050D1S-Manual.pdf

Thank you! I missed it. I also searched the text for “starting” (and other variations), but those bits didn’t show up! It is described on different pages in the manual under the product page at Renogy.com, but exactly the same text as you quoted is there, just on pp 12-13 instead.

This answers all but one of my questions: will it charge the starting battery if the service (house) battery is charging from a source other than this device, eg from a separate AC charger?
I’m guessing “no” since the text “Slow Flashing PV is charging the starting battery” seems to hint that only the PV/solar source is used to charge the service battery. Not ideal for me, but it’ll do.

 

[dirt diggler]

Thank you! I missed it. I also searched the text for “starting” (and other variations), but those bits didn’t show up! It is described on different pages in the manual under the product page at Renogy.com, but exactly the same text as you quoted is there, just on pp 12-13 instead.

This answers all but one of my questions: will it charge the starting battery if the service (house) battery is charging from a source other than this device, eg from a separate AC charger?
I’m guessing “no” since the text “Slow Flashing PV is charging the starting battery” seems to hint that only the PV/solar source is used to charge the service battery. Not ideal for me, but it’ll do.

That i don't know but i susspect you may be right that it needs to see solar input rather than just voltage at the house battery ?, my shore power charger charges the starter battery and then the house battery Via the BtoB, the BtoB just sees voltage at the starter battery it doesn't know or care where that voltage is coming from, this way i can use a cheaper FLA charger and the BtoB alters the perams to suit the LiP04 house battery.
TBH if the shore charger had not come free with the amby when i purchased it i would have probably have instaled the charger the way you intend IE charge the house then the starter which IMHO is harder and more expensive than the way i have it now, Lith chargers cost more then i would need to drop the voltage to the starter battery somehow ?

 

[icyroads]

That i don't know but i susspect you may be right that it needs to see solar input rather than just voltage at the house battery ?, my shore power charger charges the starter battery and then the house battery Via the BtoB, the BtoB just sees voltage at the starter battery it doesn't know or care where that voltage is coming from, this way i can use a cheaper FLA charger and the BtoB alters the perams to suit the LiP04 house battery.
TBH if the shore charger had not come free with the amby when i purchased it i would have probably have instaled the charger the way you intend IE charge the house then the starter which IMHO is harder and more expensive than the way i have it now, Lith chargers cost more then i would need to drop the voltage to the starter battery somehow ?

That works, but could take a while for a large house battery as the only way to charge the house battery (solar + alternator will not always sustain extended use for me).
Also, though not a problem for this device at just 50A, but for others gotta be careful not to size some other DC to DC charger(s) so big that it ruins your alternator.

 

[jharrell]

The Renogy Rego seems to be able to do this with “Bidirectional Charging Technology”, but $$$$$ IMHO, and nothing explicitly says without the alternator running, and it looks like it drops solar inputs.

I just picked one up from Amazon for $429, first one that does everything I want, 60A charge to house, reverse 30 amp charge to starter, bluetooth app that can adjust charge current and and all house battery parameters and lithium support ,looks like it even does lithium activation to wake sleeping BMS's.

Just have it powered on the bench with a power supply until I can get it installed, seems to draw about 60 milliamps idle and will power up from either starter or house side, haven't figured out which side it draws from if both connected (I only have one power supply).

App seems fine and can monitor and adjust charge voltage, absorb time etc and current on the house side, starter side seems hard coded 13.2v and 30 amps.

Uses Anderson PP75 connectors and did not come with any cables, so I need to make some.

Will be replacing a Bluesea ACR I installed back when I had AGM's, it worked ok for keeping LFP's charged from alternator, but had no current regulation so could easily draw over 100 amps and I would need to remember to shut it down with manual switch or turn hybrid assist off on my inverter if running roof A/C while driving. Also ACR had low voltage safety which would not wake up LFP BMS if they shut down for over current or under voltage, either does my Magnum inverter, so had to manually jump the house batteries a couple times.

The hope is the REGO will current regulate up to 60 amps and I can lower if I want, keep start battery charged in storage (have solar on house), and wake LFP BMS if need. Will be reverting back to dumb combiner solenoid with push button on dash for emergency jump starting which could also wake LFP's as needed.

https://www.renogy.com/rego-12v-60a-dc-dc-battery-charger/

Here is a pic of the inside: