Adding a 24V battery to an existing 12V RV system with bidirectional charging

Question

I recently purchased a new RV with a 12V system that has the following components:

1. A single lead acid battery
2. The usual 12V lights, pumps and other loads
3. A solar charge controller
4. A 120VAC to 12VDC charger (500W) for when the RV is plugged in

I'm adding a 24V LiFePO4 (8S 280Ah with a BMS) battery bank and later a 2kW 24V inverter and I'd like to be able to charge it from the 12V system, but also have it provide power for the 12V loads.

The reason to go with a 24V battery bank configuration is twofold. First, it's to minimize the amount of current going from the battery to the inverter. A 2kW inverter would require 166A which is just a silly amount to me. 80A is more doable, it will keep the conductor size down and have less losses at the terminals. The second is that it's my understanding that it's better for the cells to wire them in series as they will share the load equally and it should keep them balanced better.

Obviously I'll need some DC-DC converters. I've searched for "bidirectional dc-dc" and it appears to exist as a concept, but there are almost no ready-to-buy products available. Ideally I'd like to have a converter that will charge the 24V bank when the 12V side is at 13.2V and above as that means that either the shore charger is plugged in or the solar is producing more power than what's being consumed, and, vice versa, when the voltage drops below 13.0V, it should pull power from the bank to power the loads and float the lead-acid battery.

Could this be accomplished with two DC-DC converters wired in parallel? It seems like there's the danger of a feedback loop. The 12V-24V converter should also only turn on at a preset supply voltage (13.2V in my example) and I'm yet to find a device like that. Anything else I should look out for?

Another option is to separate the loads and the chargers on the 12V side so that loads are only connected to the step-down converter and the chargers to the step-up converter, but it would require running quite a lot more wires due to the location of all of those things.

Answer 1

it's better for the cells to wire them in series as they will share the load equally and it should keep them balanced better.

They don't. They do get unbalanced. That is why you MUST have a battery management system, not only to protect the cells, but also to balance the series string.

Could this be accomplished with two DC-DC converters wired in parallel?

No. I'd be like a dog chasing its own tail.

separate the loads and the chargers on the 12V side so that loads are only connected to the step-down converter and the chargers to the step-up converter

That's the way.

Have three buses:

• 12 V bus connected to the car alternator, non-critical loads
• DC-DC charger from 12 V to 24 V (charger = current limited)
• 24 V battery with a BMS
• 24 V bus connected to the inverter
• DC-DC step-down converter from 24 V to 12 V
• 12 V critical loads

Answer 2

It is possible to use a buck converter in both directions. If the duty cycle is 50% and the batteries have their nominal voltage, there is no energy transfer, just component losses.

Small variations in the duty cycle create an effective current upstream or downstream. Such a system needs a closed loop current control regulation to avoid coil saturation.

The simulator allows playing around with different duty cycles using P1.

This is not a real world schematic, it just shows a concept. Three such buck converters driven with 120° phase shift can deliver 500W without ugly high currents per MOSFET.

There are topologies with better efficiency, but to make them bidirectional is high art. LT has some controllers to build 2 quadrant current sources like this, often used to drive TEC elements.

Transient response is not important in this application, so a simple microcontroller can create the needed wave forms as well.

^{simulate this circuit – Schematic created using CircuitLab}