Best way to dc-dc charge high capacity battery bank in van?

[hipringles]

I am adding a 4p4s* or 8p4s (280ah cells) for a ~14-28kwh system in a van, intended to have enough power for multiple days of off grid AC use. The issue I am struggling with is what type of DC to DC charger to provide a bit of assistance on toping off the batteries when moving/driving long distances? I plan to top off from house power before leaving, but if I have a <many> hour drive from location to location, i'd like to get a bit of power back. The van has 2 250A alternators (ford transit with High Spec Interface Connector).

I understand, charging 2.3k-4.6kah on will take tens of hours.

I am concerned about large voltage drop from the connection to the batteries (~20ft). A stronger DC charger would eliminate this? (say 120ah charger). Any suggestions? Multiple (3x60a?) smaller chargers? Do I need to ones that can communicate to ensure they stay in line voltage wise?

Any general recommendations/suggestions appreciated

 

[MisterSandals]

4p4s or 8s4p 280ah system in a van

4P4S is 12v and would require 16x 70ah cells
8S4P is 24v and would require 32x 70ah cells

Why not use 4x 280ah cells for 12v
or
8x 280ah cells for 24v?

 

[Dzl]

I am adding a 4p4s or 8s4p 280ah system in a van, intended to have enough power for multiple days of off grid AC use. The issue I am struggling with is what type of DC to DC charger to provide a bit of assistance on toping off the batteries when moving/driving long distances? I plan to top off from house power before leaving, but if I have a <many> hour drive from location to location, i'd like to get a bit of power back. The van has 2 250A alternators (ford transit with High Spec Interface Connector).

I understand, charging 2.3k-4.6kah on will take tens of hours.

I am concerned about large voltage drop from the connection to the batteries (~20ft). A stronger DC charger would eliminate this? (say 120ah charger). Any suggestions? Multiple (3x60a?) smaller chargers? Do I need to ones that can communicate to ensure they stay in line voltage wise?

Any general recommendations/suggestions appreciated

Wow, that is a large battery bank for a van!

A note on your terminology:

"4p4s" = 1 x 16 cell 12v battery pack, (16 cells total, 14kWh)
"8s4p" = 4 x 8 cell 24v batter packs (32 cells total, 32kWh)

Just wanted to double check and make sure you know the two options you mentioned are (1) different numbers of cells (2) different voltages (3) different capacities (3) different approaches "parallel first" (single battery pack) vs "series first" (multiple battery packs)

 

[MisterSandals]

Maybe there is a TREMENDOUS sale on 70ah cells? Link please!

 

[hipringles]

Wow, that is a large battery bank for a van!

A note on your terminology:

"4p4s" = 1 x 16 cell 12v battery pack, (16 cells total, 14kWh)
"8s4p" = 4 x 8 cell 24v batter packs (32 cells total, 32kWh)

Just wanted to double check and make sure you know the two options you mentioned are (1) different numbers of cells (2) different voltages (3) different capacities (3) different approaches "parallel first" (single battery pack) vs "series first" (multiple battery packs)

DZL is correct, 280ah cells, 16 or 32 (4 or 8 parallel, 4 sets of those in series for 12v), making ~14-28kwh. (Maybe a 24v system with 4p8s instead)

Edited OP for clarity. (and this one)

And yeah, its a big one. The main goal is:

Home (Fully charged via wall power) --> Out in the backcountry for 3-5 days, where we will run a 12v 75A AC between 4-8 hours a day, maybe with a 1-3 hour drive every day) --> 6-8 hour drive somewhere else --> 2-3 days offgrid again. Want to attempt to re-claim as much power as practical in these drives.

 

[hipringles]

4P4S is 12v and would require 16x 70ah cells
8S4P is 24v and would require 32x 70ah cells

Why not use 4x 280ah cells for 12v
or
8x 280ah cells for 24v?

I butcherd the OP. TLDR 8P4S for 12v, or 4P8S for 24v, based on 280ah cells. AKA ~14 or 28kwh

 

[pollenface]

Redarc make dc-dc chargers with MPPT solar input suitable for lifepo4.

 

[Dzl]

I am concerned about large voltage drop from the connection to the batteries (~20ft). A stronger DC charger would eliminate this? (say 120ah charger).

No, voltage drop is determined by Vdrop = Current x Resistance (the higher the resistance or the higher the current the greater the voltage drop). If you want to reduce voltage drop you either have to increase the voltage, increase the wire size (or decrease the distance), or decrease the current.

Generally, what I've seen in B2B manuals, is that they recommend placing the B2B as close to the house battery as possible. This can't do anything to eliminate inefficiency due to voltage drop, but it does mitigate the low voltage.

Any suggestions?
Any general recommendations/suggestions appreciated

A high amperage B2B
Multiple medium B2B's in parallel
Or look into an external regulator, such as from Balmar

This last option is the most feature rich from what I've heard. Its more money than many want to spend, but its not more money than you would be spending on a high output b2b or 2 or 3 smaller ones, its probably less. With a high output secondary alternator an external regulator may be the way to harvest the most power from it. Its worth looking into. They are most popular in the marine world.

I have a thread started here where I'm trying to keep track of the various B2B charger options

Multiple (3x60a?) smaller chargers?

Multiple in parallel would be an option.

Do I need to ones that can communicate to ensure they stay in line voltage wise?

Probably not, but I would buy ones that explicitly say they can be paralleled. I believe Victron and maybe Sterling fit the bill.

 

[sduser]

DZL is correct, 280ah cells, 16 or 32 (4 or 8 parallel, 4 sets of those in series for 12v), making ~14-28kwh. (Maybe a 24v system with 4p8s instead)

Edited OP for clarity. (and this one)

And yeah, its a big one. The main goal is:

Home (Fully charged via wall power) --> Out in the backcountry for 3-5 days, where we will run a 12v 75A AC between 4-8 hours a day, maybe with a 1-3 hour drive every day) --> 6-8 hour drive somewhere else --> 2-3 days offgrid again. Want to attempt to re-claim as much power as practical in these drives.

Probably the best way to approach this is to create a table for daily energy consumption / production and see how much DCDC energy you need to get to net zero or net positive production. Then based on your daily drive time you'll know how much DCDC power you'll need.

Place the chargers as close to the house batteries as practically possible. Size your charger input cable size for no more than 3% voltage drop, output for as little as possible (<0.5% maybe?).

Is the entire output of the second alternator available for charging? Figure it can support a maybe a third of that amperage rating in load, probably half or more if you only charge while at higher RPMs (while driving, not sitting at idle) to avoid burning up the alternator.

 

[MattiFin]

Or look into an external regulator, such as from Balmar

This last option is the most feature rich from what I've heard. Its more money than many want to spend, but its not more money than you would be spending on a high output b2b or 2 or 3 smaller ones, its probably less. With a high output secondary alternator an external regulator may be the way to harvest the most power from it. Its worth looking into. They are most popular in the marine world.

Seems like "we" would need smart alternator controller that would convert your second alternator to lifepo4 charging profile.
For GM and Ford smart alternators this would be like almost too easy.

 

[Dzl]

Seems like "we" would need smart alternator controller that would convert your second alternator to lifepo4 charging profile.
For GM and Ford smart alternators this would be like almost too easy.

I was too quick to post/distracted. I believe I posted the wrong link originally. I think it is the Balmar MC series.

Multi-Stage Regulators - Balmar

Balmar Multi-Stage Voltage Regulators Optimize Your Charging System Max Charge MC-618 Voltage Regulator 9 Selectable Programs for Marine Batteries (including LiFePO4) 15 Amp Maximum Field Current Advanced Programming Modes Alternator & Battery Temperature Sensing & Control Exclusive Belt Load...

balmar.net

Balmar MC-614H - High Performance External Alternator Regulator

Balmar MC-614H High performance External Alternator Regulator

shop.marinehowto.com

 

[MattiFin]

I was too quick to post/distracted. I believe I posted the wrong link originally. I think it is the Balmar MC series.

Multi-Stage Regulators - Balmar

Balmar Multi-Stage Voltage Regulators Optimize Your Charging System Max Charge MC-618 Voltage Regulator 9 Selectable Programs for Marine Batteries (including LiFePO4) 15 Amp Maximum Field Current Advanced Programming Modes Alternator & Battery Temperature Sensing & Control Exclusive Belt Load...

balmar.net

Balmar MC-614H - High Performance External Alternator Regulator

Balmar MC-614H High performance External Alternator Regulator

shop.marinehowto.com

That is one way to do it but OP would need to replace(or heavily modify?) the existing auxiliary alternator.
Hypotetical "smart alternator compatible" alternator controller would be super easy plug&play if you already have high output 250A auxilliary alternator.
Because the smart alternator does all the "heavy lifting" already the controller won't need any power electronics. Literally 5 dollars in parts (and 20000 usd for software or rabid hobbyist)

 

[Dzl]

That is one way to do it but OP would need to replace(or heavily modify?) the existing auxiliary alternator.

Why would the alternator need to be replaced/modified? I'm not super familiar with external regulators but I have not heard it mentioned that this is necessary. Is the issue with compatibility between external regulators and 'smart' alternators? The newest vehicle I own is 2003, so I'm not at all familiar with smart alternators, even in concept.

 

[HRTKD]

hipringles, is there any chance of implementing solar panels to reduce the need for so much battery capacity? The solar panels could be roof mounted, ground deployed or both.