DC to DC charger for Lithium batteries

[DavidEM]

I have posted comments here and there about this subject, but I thought I would start a thread dedicated to it for others to comment.

The Problem

Lithium batteries, particularly systems with 200+ Ahs of capacity have a very low internal resistance compared to FLA or AGM batteries. When well discharged, that low resistance can pull a lot of amps from the chassis alternator when driving and may damage it from overheating. Don't assume that a big spec like a 200 amp alternator is good to supply that much current for hours at a time. It is not unless it is a heavy duty alternator like is installed in emergency response vehicles and is particularly vulnerable at idle with the resulting low cooling air flow.

The other issue is that the fixed voltage output of a chassis alternator won't necessarily charge the Lithium batteries completely. LFP batteries need 14.4-14.6 volts to fully charge them and with wiring voltage drop you won't usually get that voltage from your chassis alternator.

The Solution- DC to DC charger

DC2DC chargers are a good solution for this, but see below. They take an alternator voltage and increase it or decrease it for delivery to the batteries for optimal charging. They also limit the current to their rated output which is 20,30,40 or 60 amps- you choose. This solves both problems: It provides the right charging profile for lithium batteries for optimal charging and limits the current, thereby protecting the alternator.

Installation Issues

The catch is that almost all motorhomes have a battery isolation manager, either the Intellitec BIRD, the older Precision Circuits BCC or the more modern PC BIM 160. These are installed to connect or disconnect the coach DC system from the chassis DC system when the respective system voltages are correct for charging. They also provide emergency Aux Starting for the chassis engine.

The issue is that you can't install a DC2DC charger in series or in parallel with one of these BIMs. In parallel you won't get any DC current limitation, nor the right charge voltage profile and in series since the DC2DC charger is not bidirectional you won't get any chassis battery charging from the coach DC system. You will also only connect the DC2DC charger when the BIM thinks the voltages are correct which will limit charging performance.

So what do you do?

The best solution is to completely remove the existing BIM and replace it with an appropriately sized DC2DC charger. You will lose the Aux Start capability and the ability to charge the chassis system from the coach DC system due to the non bidirectional nature of the DC2DC charger.

A simple solution is to wire a heavy duty rotary switch in parallel with the DC2DC charger. Turn it on when you are storing your coach to keep the chassis batteries charged up from the coach system. The switch will also work in place of the dash Aux Start switch to boost engine starting. The only problem is that it won't be automatic and if you forget to turn it off, you lose current limiting as well as possibly run down both DC systems and not be able to start the chassis engine.

What is a future solution?

A future solution that I haven't seen yet is a more sophisticated DC2DC charger that combines the capabilities of current limiting with bidirectional connections triggered by appropriate charging voltages. When Lithium batteries become the predominant coach battery system then I think you will see people like Precision Circuits, Intellitec, etc producing them and coach builders installing them at the factory.

Thoughts?

David

[CaptainYARRR]

Very interesting write-up...I keep waffling on whether to replace or go lithium. This is helpful in my evaluation.

[AJMike]

The start issue can easily be addressed by connecting a bypass around the DC-DC charger with a switch so that only one of the two circuits is active at a time. Then, if you need the house battery boost, just throw the switch, start the engine and then reset the switch. It is a rare occurrence anyway so it does not seem like much trouble to have to do that.

Alternately you can do what I did - get one of those small portable electronic battery starters. They cost about $50-$60 and are easy to carry around.

As for the issue about the DC-DC charger limiting the charge to the house battery, well, that is its purpose.

[gpounder]

The solution is simple and for may larger RV's the parts are already installed.

I have a Class A that uses a relay activated by oil pressure to connect both the house and chassis batteries together. That relay is also my boost relay.

I also have a now a amp-l-start charger (replaced the Xantrex echo) to trickle charge the chassis battery when the house is plugged in.

The conversion was not hard. I removed the oil pressure sensor wire from the relay and attached it to the sensor terminal on the DC-DC charger (Victron Orion-TR smart). Now when the engine runs the DC to DC is active and my boost switch is still intact.

The Amp-L-Start has a setting for Lithium batteries.

This drawing shows how it all works and more.

For those that have a dual isolation battery charger more common on a class C, that can be removed. You should already have boost relay and the DC to DC sense wire could be attached to a ignition circuit.

[creativepart]

I have just been through this exact process.

I installed 400ah of LiFePo4 batteries last month.

I agonized over what to do for chassis and new house battery charging. I found very little info about what people did on this topic. Many do nothing and “hope” everything will be ok with their alternator charging the new batteries just like the old.

My motorhome has a BIM. It connects the house and chassis batteries when driving and when parked to keep both charged. Plus it provides an emergency start function, again connecting both house and chassis batteries.

After a lot of thought, I came to the conclusion that the BIM had to go. That seemed daunting but wasn’t all that difficult to accomplish.

But as the OP says after the BIM was removed I needed to replace the most important features of the BIM. I installed a DC2DC charger to charge the new house batteries properly while driving. But then I needed to provide charging for the chassis battery when parked. So I installed a Amp-L-Start to handle this chore.

There is another option… and that is to not delete the OEM BIM but to replace it with a Li-BIM made by the same company. This Li-BIM does everything the OEM BIM does but uses an on/off timer to sort of regulate the alternator’s charge properties. It’s a direct replacement for the OEM BIM which is super easy to install. I’d say the jury is still out on this solution, but it is better than doing nothing.

Thanks to the OP for starting this thread as I know from when I started my battery project last month there is very little actual advice on how to proceed for motorhomes with a BIM-type device.

[Kid Gloves]

LiFePo4 batteries do not require 14.4-14.6 volts to fully charge. They will fully charge at any voltage above their resting voltage, typically 13.6V. It may take a significant amount of time, but 13.7 volts will eventually get the job done.

[gpounder]

Originally Posted by creativepart

I have just been through this exact process.

I installed 400ah of LiFePo4 batteries last month.

I agonized over what to do for chassis and new house battery charging. I found very little info about what people did on this topic. Many do nothing and “hope” everything will be ok with their alternator charging the new batteries just like the old.

My motorhome has a BIM. It connects the house and chassis batteries when driving and when parked to keep both charged. Plus it provides an emergency start function, again connecting both house and chassis batteries.

After a lot of thought, I came to the conclusion that the BIM had to go. That seemed daunting but wasn’t all that difficult to accomplish.

But as the OP says after the BIM was removed I needed to replace the most important features of the BIM. I installed a DC2DC charger to charge the new house batteries properly while driving. But then I needed to provide charging for the chassis battery when parked. So I installed a Amp-L-Start to handle this chore.

There is another option… and that is to not delete the OEM BIM but to replace it with a Li-BIM made by the same company. This Li-BIM does everything the OEM BIM does but uses an on/off timer to sort of regulate the alternator’s charge properties. It’s a direct replacement for the OEM BIM which is super easy to install. I’d say the jury is still out on this solution, but it is better than doing nothing.

Thanks to the OP for starting this thread as I know from when I started my battery project last month there is very little actual advice on how to proceed for motorhomes with a BIM-type device.

I have heard of the timed device, not a fan of it. The problem is alternators are not made to run at capacity for very long and the manufactures do not publish data on that so timing is just a guess. I also have solar so during the day while driving I have plenty of power to charge batteries and run the res fridge.

[HarryStone]

An alternate to the DC to DC charger is the magnum smart battery combiner. It keeps both battery banks charged as we drive, or if we are on shore power.

[gpounder]

Originally Posted by Kid Gloves

LiFePo4 batteries do not require 14.4-14.6 volts to fully charge. They will fully charge at any voltage above their resting voltage, typically 13.6V. It may take a significant amount of time, but 13.7 volts will eventually get the job done.

There is some value to to what you said and also know at a lower voltage they may not draw as much current but you would need a way to monitor that to protect your alternator.

An important feature to lithium is the ability to charge the batteries fast and that does not happen at lower voltages.

[Kid Gloves]

Originally Posted by gpounder

There is some value to to what you said and also know at a lower voltage they may not draw as much current but you would need a way to monitor that to protect your alternator.

An important feature to lithium is the ability to charge the batteries fast and that does not happen at lower voltages.

Agreed. The statement was made in an effort to correct the often cited misconception that a lithium battery requires 14.4-14.6 volts to fully charge.

[RoadTrip2084]

I also just installed a 280Ah LiFePO4 house battery in my rig.

I removed the original factory diode battery isolator, boost solenoid, and battery maintainer and installed a 60A Renogy DC-DC charger in their spot in the rear electrical panel.

For the DC-DC start-detect circuit I tapped into the fuel-enable circuit in the panel, which is only active when the key is on "Run". I'm not super fond of this design from Renogy since I believe it places the full charging load on the alternator as soon as I turn the key on, before I even start the engine. Plus even after the engine is started and just idling up, etc.

My battery has bluetooth BMS so I can disable charging using my phone so I do that when I start if the battery is low, and then re-enable it once we're on the road the the alternator is running at speed to lesson the burden on the idling alternator. Not clear this is at all necessary but it is a precaution.

To maintain my chassis battery while parked I rewired my factory 80W solar panel to charge it instead of the house battery. This seems to be working well, at least in the summer months. Keeps the chassis battery at 12.9-12.6v.

I also have booster cables in case I need to boost the chassis from the house or toad vehicle at any point as a precaution.

[creativepart]

Originally Posted by HarryStone

An alternate to the DC to DC charger is the magnum smart battery combiner. It keeps both battery banks charged as we drive, or if we are on shore power.

Yes, I was not aware of a whole category of battery combiners, which see more common in the boat world. Even Victron has a product line of these.

[ShelbyM]

Originally Posted by DavidEM

I have posted comments here and there about this subject, but I thought I would start a thread dedicated to it for others to comment.

The Problem

Lithium batteries, particularly systems with 200+ Ahs of capacity have a very low internal resistance compared to FLA or AGM batteries. When well discharged, that low resistance can pull a lot of amps from the chassis alternator when driving and may damage it from overheating. Don't assume that a big spec like a 200 amp alternator is good to supply that much current for hours at a time. It is not unless it is a heavy duty alternator like is installed in emergency response vehicles and is particularly vulnerable at idle with the resulting low cooling air flow.

The other issue is that the fixed voltage output of a chassis alternator won't necessarily charge the Lithium batteries completely. LFP batteries need 14.4-14.6 volts to fully charge them and with wiring voltage drop you won't usually get that voltage from your chassis alternator.

The Solution- DC to DC charger

DC2DC chargers are a good solution for this, but see below. They take an alternator voltage and increase it or decrease it for delivery to the batteries for optimal charging. They also limit the current to their rated output which is 20,30,40 or 60 amps- you choose. This solves both problems: It provides the right charging profile for lithium batteries for optimal charging and limits the current, thereby protecting the alternator.

Installation Issues

The catch is that almost all motorhomes have a battery isolation manager, either the Intellitec BIRD, the older Precision Circuits BCC or the more modern PC BIM 160. These are installed to connect or disconnect the coach DC system from the chassis DC system when the respective system voltages are correct for charging. They also provide emergency Aux Starting for the chassis engine.

The issue is that you can't install a DC2DC charger in series or in parallel with one of these BIMs. In parallel you won't get any DC current limitation, nor the right charge voltage profile and in series since the DC2DC charger is not bidirectional you won't get any chassis battery charging from the coach DC system. You will also only connect the DC2DC charger when the BIM thinks the voltages are correct which will limit charging performance.

So what do you do?

The best solution is to completely remove the existing BIM and replace it with an appropriately sized DC2DC charger. You will lose the Aux Start capability and the ability to charge the chassis system from the coach DC system due to the non bidirectional nature of the DC2DC charger.

A simple solution is to wire a heavy duty rotary switch in parallel with the DC2DC charger. Turn it on when you are storing your coach to keep the chassis batteries charged up from the coach system. The switch will also work in place of the dash Aux Start switch to boost engine starting. The only problem is that it won't be automatic and if you forget to turn it off, you lose current limiting as well as possibly run down both DC systems and not be able to start the chassis engine.

What is a future solution?

A future solution that I haven't seen yet is a more sophisticated DC2DC charger that combines the capabilities of current limiting with bidirectional connections triggered by appropriate charging voltages. When Lithium batteries become the predominant coach battery system then I think you will see people like Precision Circuits, Intellitec, etc producing them and coach builders installing them at the factory.

Thoughts?

David

I'd be interested in seeing examples of this, what specific setups were involved. Links if anyone has them. Not interested in the YouTube "test" video, just real life. Thanks.

[dvspl]

I just accepted the reality that I lost the aux start capability. I replaced that capability with one of these small jump start batteries. Works great for starting my rig if necessary and has already come in handy for helping two neighbors at campsites. https://smile.amazon.com/gp/product/B01D42U0VO/?th=1