Lithium and DC-DC Charger?

[Leemom3@verizon.net]

Hi All,

Still in the way far away time for our Escape, but I am looking at selling my truck camper (which I live in). I have 2 Battleborn lithium batteries, and contemplating getting a DC-DC charger installed in my truck camper.

As I (NOT an RV tech or electrician) understand it - the charger is needed to protect the truck alternator from the charging required for the litium batteries. My truck is a 2020 Ram 1-ton diesel dually longbed, something I do not want to damage, as I expect to sell it for something smaller to tow the Escape.

So…what am I missing with the “full package” off-grid package that Escape offers? The package includes the lithium batteries, inverter, solar - but no DC-DC charger. Is there something in how Escape installs things that a charger is not needed?

I ask this question because the answer may help me decide to do with my Battleborn batteries - sell them with the truck camper, or replace them with an AGM for the buyer, and save them for my Escape trailer.

Any ideas? Thank you for your information.

[JeffreyG]

I bought my Escape 19 with the lithium batteries. It was delivered in October 2021 and included a Victron 12/12-30 DC-DC charger on the line from the tow vehicle. The DC-DC was not specifically listed, but was included as part of ETI's package.


Check with ETI to be sure, but I bet all trailers coming with Lithium batteries are equipped with one.

[Leemom3@verizon.net]

Originally Posted by JeffreyG

I bought my Escape 19 with the lithium batteries. It was delivered in October 2021 and included a Victron 12/12-30 DC-DC charger on the line from the tow vehicle. The DC-DC was not specifically listed, but was included as part of ETI's package.


Check with ETI to be sure, but I bet all trailers coming with Lithium batteries are equipped with one.

Good to know! Then it looks like I can also return my unopened Victron charger!

[alanmalk]

Originally Posted by Leemom3@verizon.net

...
- the charger is needed to protect the truck alternator from the charging required for the lithium batteries.
...

No, this is not the purpose of the DC-DC converter/charger.

First, consider that the alternator is fully capable of charging a dead truck lead/acid battery, which probably could accept more than a hundred amps of charging current at the start of the charge. The alternator will supply that up to its rated output - perhaps 120 amps in the average case. A pair of empty BB Lithium batteries could accept the same charge up to the limit imposed by the battery management system (BMS). The alternator would not notice much difference.

The function of a DC-DC converter is to increase the voltage of the truck system - typically 13.8 volts when float charging, or 12.8 when not being charged - up to 14.4 - 14.8 volts that the Lithium batteries need for a full charge. The reason is that a Lithium battery will only reach about 80% of full charge when charged at 13.8. Not a terrible situation but you are not getting the full benefits from an expensive Lithium battery - and possibly shortening the lifespan of the Lithium battery, but I haven't seen any conclusive proof of that.

An added benefit of the DC-DC charger in a tow & trailer combination is that the long run of wire from the alternator back to the batteries, which may be another 20 feet further back from the hitch in the case of a 21C, is to overcome the substantial voltage drop in all that wire. I've seen more than 1.5 volts of loss between my alternator to the battery. That's enough so the Lithium batteries wouldn't charge at all. So, the bottom line is if you want to charge your batteries while towing, a DC-DC charger is necessary.

[Leemom3@verizon.net]

Originally Posted by alanmalk

No, this is not the purpose of the DC-DC converter/charger.

First, consider that the alternator is fully capable of charging a dead truck lead/acid battery, which probably could accept more than a hundred amps of charging current at the start of the charge. The alternator will supply that up to its rated output - perhaps 120 amps in the average case. A pair of empty BB Lithium batteries could accept the same charge up to the limit imposed by the battery management system (BMS). The alternator would not notice much difference.

The function of a DC-DC converter is to increase the voltage of the truck system - typically 13.8 volts when float charging, or 12.8 when not being charged - up to 14.4 - 14.8 volts that the Lithium batteries need for a full charge. The reason is that a Lithium battery will only reach about 80% of full charge when charged at 13.8. Not a terrible situation but you are not getting the full benefits from an expensive Lithium battery - and possibly shortening the lifespan of the Lithium battery, but I haven't seen any conclusive proof of that.

An added benefit of the DC-DC charger in a tow & trailer combination is that the long run of wire from the alternator back to the batteries, which may be another 20 feet further back from the hitch in the case of a 21C, is to overcome the substantial voltage drop in all that wire. I've seen more than 1.5 volts of loss between my alternator to the battery. That's enough so the Lithium batteries wouldn't charge at all. So, the bottom line is if you want to charge your batteries while towing, a DC-DC charger is necessary.

Thank you for your detailed response. My reading - as someone not an electricican or RV mechanic - is that while the alternator can charge the lithium batteries as you stated, there is a possibility of causing damage to the alternator because of the voltage charge level of (or to) the lithiums. While I do not want to throw away money (by any damage to the Battleborn batteries), worse to me would be damaging my truck alternator.

This video from Battleborn speaks to the question: https://youtu.be/VY2b71zoyvg
(Sorry - not sure if it is a hot link. You might need to copy and paste).

This video from Victron addresses it in greater detail. As I understand it, essentially that the higher voltage level of lithium batteries causes the alternator to run hotter, potentially causing damage to the alternator: https://youtu.be/jgoIocPgOug (again - you may have to copy and paste the link).

Again, I am not an electrician or RV tech. I am looking at these points of information asa someone trying to understand things from the manufacturers (Battleborn and Victron).

[Mike Lewis]

I installed a Sterling 30A DC-DC charger in my trailer. It serves two purposes: (1) it steps up the voltage from the tow vehicle's batteries to properly charge the trailer's lithium batteries, and (2) it senses the tow vehicle battery's voltage and stops the charge if it drops below a certain level, thus protecting the truck battery.

I found the Sterling to be hard to configure but it seems to work: at idle it charges my trailer batteries at around 13 amps. I haven't determined how well it works at highway speeds, though. If I had it to do over I would get the 60A charger.

I ran a pair of 6-gauge wires from the truck battery back to the trailer through plastic conduit and made a second umbilical. I doubt if many people would want to make that effort, though.

[JeffreyG]

Originally Posted by Mike Lewis

I ran a pair of 6-gauge wires from the truck battery back to the trailer through plastic conduit and made a second umbilical. I doubt if many people would want to make that effort, though.

Worth a note, for most people you are not going to get a huge amount of charging done while driving even with a DC-DC convertor installed. It's still worth having one in general as it will keep the battery up while powering the fridge etc, but there are some limits.


1) Most tow vehicles are wired with long runs of thin gage wire back to the 7-pin. You upgraded to 6 ga, but for most vehicles, if you are not going to increase the gage you are not going to get an enormous amount of current from the two.
2) Modern tow vehicles often have smart charging systems that will pull back alternator power. Some also have a 'tow mode' switch that will let the alternator run full power while towing, but if not, don't expect the alternator to provide max power all day while towing.

[dstreight]

The Sterling, and maybe other DC-DC chargers too, also serves as an isolator.

[Mike Lewis]

Originally Posted by JeffreyG

2) Modern tow vehicles often have smart charging systems that will pull back alternator power. Some also have a 'tow mode' switch that will let the alternator run full power while towing, but if not, don't expect the alternator to provide max power all day while towing.

The Sterling doesn't seem to work all that well, perhaps this is why. However, I connected that 6 gauge wire directly to my truck's battery, so I assumed that the alternator would see the battery being drained and charge accordingly.

I only tried to measure the Sterling's operation once, by draining the trailer's batteries, switching off the solar panels, then towing the trailer for a few hours. I don't recall the exact result but I remember not being that impressed. Maybe I'll investigate more this winter.

[splitting_lanes]

Hi Jeffrey -

Have you checked the difference in the charging of the trailer batteries when your truck is in tow mode?

Do you run in tow mode all the time when towing? Does it lock out 6th gear?


Whoops, meant to quote your statement…

[JeffreyG]

Originally Posted by Mike Lewis

I only tried to measure the Sterling's operation once, by draining the trailer's batteries, switching off the solar panels, then towing the trailer for a few hours. I don't recall the exact result but I remember not being that impressed. Maybe I'll investigate more this winter.

I left home with my battery sitting at 85% SOC and drove 45 minutes with the solar disconnected and the fridge running on propane. When I got where I was going, the battery was at 87% SOC.


So the good news is that it went up slightly despite a modest load, but the tow vehicle certainly was not charging it right up quick.

[alanmalk]

Originally Posted by Leemom3@verizon.net

...
This video from Victron addresses it in greater detail. As I understand it, essentially that the higher voltage level of lithium batteries causes the alternator to run hotter, potentially causing damage to the alternator: https://youtu.be/jgoIocPgOug (again - you may have to copy and paste the link).
...

I took a quick look at this video and noted a few items of interest.
First, no mention of the voltage regulator on the first test (where the alternator smoked). I am suspicious that the voltage was set higher than a typical car (observing that the latest vehicles are no longer typical thanks to gas saving tricks). Second, note that they used a 300 Ah battery for their tests. But what would have happened if they used a 300 Ah lead-acid AGM battery? My hunch is they would have the same smoke issue. Another thing I noted - the pulley on the driver motor and on the alternator were the same diameter. Correct me if I am wrong but don't vehicles use a 2 to 1 ratio to increase alternator speed?

The most important thing I observed is that they did not use the 60 to 80 feet of undersized wire that many vehicle & trailer combinations have between alternator and battery. This poor wire path acts as a resistor which limits the total current (batteries may have low internal resistance but the wire adds external resistance which adds directly to the internal resistance for a substantial total). My Toyota Tacoma truck is a prime example of undersized wire. Previous posters above have noted that they up-sized their wire to overcome this problem.

Finally, note that is a combination of high amps and low cooling that caused the smoke - not high(er) voltages.

So I am offering the same conclusion as earlier. You need a DC-DC charger(converter) in order to fully charge Lithium batteries in your trailer. Without it there is no significant charging, and no strain on your alternator.

[alanmalk]

Originally Posted by Mike Lewis

...
I ran a pair of 6-gauge wires from the truck battery back to the trailer through plastic conduit and made a second umbilical. I doubt if many people would want to make that effort, though.

Actually today started an installation of 10-gauge wires from the alternator to the rear of the truck. Preliminarily for running a portable freezer but will extend it to the trailer plug. In the trailer I am planning to use an ultra-cheap DC-DC boost converter (one of those scary sub-$20 contraptions directly from China.) The input and output current can be limited so the amperage won't cause more than a 10% voltage drop between the alternator and device as estimated with voltage drop tables and expected currents. I anticipate a charging current in the neighborhood of 10-15 amps at the battery while driving. Lots of fuses for obvious reasons.


Oh-man, the expense I go to to save money.

[Leemom3@verizon.net]

Originally Posted by alanmalk

I took a quick look at this video and noted a few items of interest.
First, no mention of the voltage regulator on the first test (where the alternator smoked). I am suspicious that the voltage was set higher than a typical car (observing that the latest vehicles are no longer typical thanks to gas saving tricks). Second, note that they used a 300 Ah battery for their tests. But what would have happened if they used a 300 Ah lead-acid AGM battery? My hunch is they would have the same smoke issue. Another thing I noted - the pulley on the driver motor and on the alternator were the same diameter. Correct me if I am wrong but don't vehicles use a 2 to 1 ratio to increase alternator speed?

The most important thing I observed is that they did not use the 60 to 80 feet of undersized wire that many vehicle & trailer combinations have between alternator and battery. This poor wire path acts as a resistor which limits the total current (batteries may have low internal resistance but the wire adds external resistance which adds directly to the internal resistance for a substantial total). My Toyota Tacoma truck is a prime example of undersized wire. Previous posters above have noted that they up-sized their wire to overcome this problem.

Finally, note that is a combination of high amps and low cooling that caused the smoke - not high(er) voltages.

So I am offering the same conclusion as earlier. You need a DC-DC charger(converter) in order to fully charge Lithium batteries in your trailer. Without it there is no significant charging, and no strain on your alternator.

Thanks again for your input. I read these replies with interest - and no ability to give an educated response. Early on the learning curve here.

[rubicon327]

I installed a Victron 18A DC-DC charger as part of my lithium upgrade. When hooked up to a 2007 4Runner with stock trailer charge wiring I saw ~10A at idle. Not sure how much this would increase at higher engine speeds if at all.

[bborzell]

When I asked for a list of “full solar/lithium” electrical component model numbers for our build (to be picked up on Wednesday), it included the Victron DC-DC charger.

[richm]

Originally Posted by alanmalk

Finally, note that is a combination of high amps and low cooling that caused the smoke - not high(er) voltages.

Alan is correct. Lithium batteries do not limit current because they have a very low internal resistance. A DC-DC charger acts as a current limiter, and that is why a DC-DC charger is essential to protect the alternator from burning up.


One additional concern to be aware of is the tow vehicle wiring rating. For example, if the tow vehicle has a 40A fuse on the power supplied by the 7-pin plug, make sure the DC-DC charger has a limit less than 40A.

[alanmalk]

Originally Posted by richm

Lithium batteries do not limit current because they have a very low internal resistance. A DC-DC charger acts as a current limiter, and that is why a DC-DC charger is essential to protect the alternator from burning up.
...

I'm beginning to wonder if there is some "LiFePO4 mythology" going on.

I checked the web site of 4 well known Lithium battery suppliers, Battle Born, Dakota, Renogy and Victron. Internal resistance numbers were not to be found! I checked the web site of my (now discarded) Odyssey AGM lead-acid batteries and their specification page listed the internal resistance as 2.2 mOhm. Since I use 2 in parallel, the internal resistance was 1.1 mOhm. Searching further, I found the specifications for a single 3.2V LiFePO4 cell, 100 Ah, as 0.9 mOhm (sorry, didn't save the reference). Finally - because searching began to yield limited results - found this chart of internal resistance for various battery types and sizes VS temperature - https://www.researchgate.net/publica...ctric_Vehicles.

The chart in this study essentially suggests that for the same Amp-Hour size, AGM and LiFePO4 are roughly the same internal resistance. Both are insanely low and could accept 100s of amps when fully discharged. The Lithium batteries of course have a BMS to limit the charge current to 1C as a general rule.

I note that my alternator has not burned up in 7 years of charging my pair of AGM batteries in my Escape 21C. The most reasonable answer is because of the relatively high resistance of long and thin wires between the alternator and the batteries. Had I connected my batteries directly to the alternator with 18 inches of 4/0 cable and run at 1500 rpm, then I would expect a cooked alternator.

Yes, a DC-DC charger does act as a current limiter - because a 40 Amp DC-DC is affordable where as a 100 Amp DC-DC charger is probably too way expensive for simple trailer battery charging. (I think I saw one for many thousands of dollars - but this is a faint memory.)

But is a DC-DC charger necessary? Yes, but not for current limiting. It provides the boosted and regulated (stable) 14.4 - 14.8 volts necessary to actually charge the Lithium battery while driving. If charging batteries while driving is not important then the DC-DC charger is not necessary.

[Centex]

Originally Posted by alanmalk

I'm beginning to wonder if there is some "LiFePO4 mythology" going on.
<snip>
If charging [lithium] batteries while driving is not important then the DC-DC charger is not necessary.

An interesting post, Alan, I always enjoy reading your considered opinions.

Regarding the 'necessity' of a DC-DC charger, can I ask you to expand a bit further ...

• As I read your post, I understand you to be saying that If one is not interesting in [optimally] charging lithium batteries while driving, you see no problem leaving the TV>trailer 12V 'charging' connection intact absent the DC-DC charger. Is that correct?
• IF one intends to allow solar charging via a 'lithium-profile controller' while driving, do you see any problem having the TV>trailer 12V 'charging' connection intact without a DC-DC charger?

I'm wondering about the role of the DC-DC charger as a 'one-way-gate' between the trailer and TV systems when the trailer's lithium battery voltage is higher than the TV battery voltage and the two are connected (or maybe that role is a myth)?

My own plan is to see if I can get by with solar charging of my lithium battery when driving, not installing a DC-DC charger but also severing the TV>trailer 12V 'charging' connection at the trailer's umbilical junction box. Do you think severing that TV>trailer 12V 'charging' connection is unnecessary or lacks any benefit in my scenario?

(Yeah, I realize the sun doesn't always shine when driving and I may wish for optimal charging from the TV at some point. There's always 'room' to add a DC-DC charger if that's the lesson I learn )

[alanmalk]

Originally Posted by Centex

...

• As I read your post, I understand you to be saying that If one is not interesting in [optimally] charging lithium batteries while driving, you see no problem leaving the TV>trailer 12V 'charging' connection intact absent the DC-DC charger. Is that correct?
• IF one intends to allow solar charging via a 'lithium-profile controller' while driving, do you see any problem having the TV>trailer 12V 'charging' connection intact without a DC-DC charger?

I'm wondering about the role of the DC-DC charger as a 'one-way-gate' between the trailer and TV systems when the trailer's lithium battery voltage is higher than the TV battery voltage and the two are connected (or maybe that role is a myth)?

My own plan is to see if I can get by with solar charging of my lithium battery when driving, not installing a DC-DC charger but also severing the TV>trailer 12V 'charging' connection at the trailer's umbilical junction box. Do you think severing that TV>trailer 12V 'charging' connection is unnecessary or lacks any benefit in my scenario?

(Yeah, I realize the sun doesn't always shine when driving and I may wish for optimal charging from the TV at some point. There's always 'room' to add a DC-DC charger if that's the lesson I learn )

Actually, the sun always shines when driving - it starts to rain when you arrive at your destination.

#1 - Based on my experience with driving/charging a pair of 100 Ah AGM batteries, with documented low resistance, I see no problems to the alternator. I know from my measurements that the voltage drop to my batteries is beyond trivial - therefore minimal amp draw on the alternator.

#2 - Solar charging, Lithium profile (14.4 - 14.8 volts). No personal experience, yet. But I would not be totally comfortable - not from a damage to components, but possibly not accomplishing the intended charge. Without proof, I suspect amps could be lost to charging the vehicle lead-acid battery. This would be easy to prove one way or another by using an amp meter in the vehicle charge line on a sunny day.

If you don't want to bother disconnecting the charge line on a regular basis then consider a Schottky diode (or 2 in parallel) in the line. You obtain a one-way current flow into the trailer when needed, but your solar power can not flow into the vehicle. They are small, fairly cheap and easy enough to remove when you spring for that DC-DC charger.